Aracom Amplifiers Crank it up! UNDERSTANDING POWER ATTENUATORS FOR GUITAR AMPLIFIERS

ATTENUATOR 02

UNDERSTANDING POWER ATTENUATORS FOR GUITAR AMPLIFIERS

WHAT IS POWER ATTENUATION?

Power attenuation employs a device or method of reducing a tube amplifier’s output power level, and in turn reduces the speaker’s volume level. The most common use of power attenuation is to get the “cranked up” sound of a tube amplifier at reasonable volume levels.

Methods of Attenuation

Before we dive into the details, let’s first have a general discussion on power attenuation methods and reasons why you would want to use power attenuation.

First of all, power attenuation is a means of reducing a tube amplifiers power before it reaches the speaker. Power attenuation methods include attenuation circuits that can be internal and external to the amplifier.

Internal attenuation methods include: a master volume control or a power scaling circuit. The Master Volume Control typically allows more of the amplifier’s preamp section to be overdriven and the drive to the power tubes is reduced. Preamp tube distortion and power tube distortion sounds different and most guitarists prefer tone when the power tubes are driven hard. The various methods of power scaling, alters the amplifier’s internal DC voltage(s) and therefore reduce the power from the amplifier’s output tubes.

External attenuation involves placing a device between the amplifier and speaker, which reduces the amplifier's power and controls the overall volume level. Common methods of external attenuation include; power load devices and the re-amplification technique. There is also an attenuation technique where custom or modified speakers are used, that allows the user to adjust the flux density (strength) of the speaker's magnet.

Therefore, some methods of attenuation that are internal to the amplifier allow only for preamp distortion and some methods allow for preamp and power tube distortion. External attenuators allow for “whole” amp distortion, meaning the preamp and power amp tubes can be overdriven. In this article we will be focusing on external power attenuators for use with tube amplifiers.

Some like it on 10…

Many guitarists prefer the natural overdriven tone of their “cranked up” tube amp, especially when the power tubes are being driven hard, in lieu of using effect pedals. This means the amplifier’s volume control must be turned up all the way or nearly wide open, which usually means “loud”! While the “cranked up” amplifier may sound and feel good to you, it might not be tolerable by people around you and it could eventually cause you permanent hearing damage. This is where the power attenuator, whether internal or external, comes into play.

A power attenuator can also be a useful tool for jazz and blues guitarist, since they often prefer the tone of their amplifier when the volume is set to a high enough level that allows the power tubes to saturate a bit, providing a lightly overdriven clean tone or grinding blues tone. However, the volume level at this point is often too loud for the situation. A power attenuator can be used to keep the desired tone, while reducing the speaker's volume level.

Even amplifiers equipped with a master volume control or one that utilizes the power scaling technique can also benefit with the use of an external power attenuator. While the internal methods of power attenuation can provide very good tone, the tone can sometimes be different than a fully cranked up non-attenuated amplifier. When using an external power attenuator with internal power controlled amplifiers, the attenuator provides an addition step to control the overdriven tone and volume; therefore you do not need to rely solely on the amplifier's internal power control for the overall volume level. The amplifier’s internal power control can be used to “tweak” the amplifier’s tone and the power attenuator can be used as the “master volume” control, setting the overall volume of the speaker.

Also, some internal power control designs do not offer good tone at low volume “bedroom” levels. However, utilizing a power attenuator in conjunction with the amplifier’s internal power control can often result in satisfying tone at low volume levels.

Save your ears and possibly your speakers…

Another practical use of a power attenuator would be to reduce the power of an amplifier to match the power rating of the speaker. For example, if you have a 100 watt amplifier and a speaker with a 15 watt power rating, an attenuator can be used to reduce the power down to a safe level. Also, if you have an expensive vintage speaker and you want to reduce the chance of damaging it, you could also use an attenuator to reduce the amplifier’s power to a very safe level, below the speaker’s power rating.

Even when the output power of a fully cranked tube amplifier is within the power rating of a speaker, there are cases when backing off the power to the speaker a bit with an attenuator improves the overall tone quality. These are cases when the speaker is being driven too hard and actually backing off a bit improves the tone.

More important, an attenuator can be used to protect our ears from dangerously high volume levels. Cranking up a tube amplifier until tone-pleasing distortion occurs can be addictive and put us in the groove for playing. Under these conditions we are often aware that the volume is loud; however we might not realize that it can cause permanent hearing damage. A good power attenuator can please our need for cranked up tone and save our ears at the same time!

External power attenuators can be split into two primary categories: passive and active. In this article will explain the differences between passive and active attenuators. We will also talk about the types of loads they use, discuss various attenuator features, and finally we will examine the pros and cons of two popular attenuator technologies.

Note: throughout the discussion below, the term "speaker" is used in its singular form; however "speaker" can be a single speaker in a cabinet or a multiple speakers in a cabinet or multiple cabinets used simultaneously.

CATAGORIES OF EXTERNAL POWER ATTENUATOR TECHNOLOGY: PASSIVE AND ACTIVE

Passive Technology: A passive attenuator does not utilize an active electronic circuit to attenuate the amplifier's signal and its attenuation circuit does not require external power.

The most common form of passive power attenuation utilizes a resistive network (parallel and series resistive load) to attenuate and reduce the tube amplifier's signal. The resistive network can be non-reactive or reactive. Attenuators that rely purely on the use of resistive loads (non-reactive or reactive) are often referred to as “power soaks”.

A less common form of passive attenuation has an internal “step down” transformer that is used to intentionally provide an impedance mismatch between the amplifier and the speaker. The attenuator’s transformer affects the impedance the power tubes “see” and therefore changes the damping factor between the amplifier and speaker. The change in impedance causes a portion of the power to be reflected from the step down transformer back to the amplifier, thus causing a reduction in the amplifier’s output power.

The power that is reflected back to the amplifier is known as "flyback" voltage. The attenuator’s transformer has multiple taps which are selected for various attenuation levels. As the attenuation is increased, the impedance the power tubes “see” is reduced. One of the drawbacks of using this technique is the more the impedance is mismatched, the narrower the attenuator’s frequency response becomes. Therefore this technique can significantly affect the amplifier’s tone and distortion quality depending on the attenuation level. The other issue is possible Output Transformer damage if the technique is taken too far.

Passive attenuators can employ a fan or a lamp and still be considered passive, since these devices can be powered via the amplifier's signal. In this case, the fan or lamp is inserted in amplifier's signal path, utilizing the amplifier's output signal to power them. These devices are reactive and therefore they have an impact on the attenuator's impedance.

There are some passive attenuators that do require AC power. Keep in mind we categorize a passive attenuator as an attenuator whose attenuation circuit does not require external power. However, a passive attenuator can incorporate an active AC powered fan to cool the internal devices and still be considered as a passive attenuator, since its attenuation circuit is completely passive.

Active Technology: Active attenuators unitize active devices and therefore require an external AC or DC power source.

A common example of an active attenuator is one that uses a “re-amplifying” technique. This technique typically terminates the amplifier's entire output power into a dummy load, which can be passive or reactive. A portion of the terminated signal is sampled and is then inserted into a secondary internal or external solid state power amplifier that drives the speaker.

There are “all in one" active attenuators with a built-in load and a cost effective, internal solid state amplifier. The solid state amplifier typically incorporates an emitter follower design, which is also known as a voltage follower. These amplifiers are of a simplistic design with unity gain (gain = 1). Therefore, the solid state amplifier's output power will track and be equal to the sampled power level feed into it.

You can also use separate components to “re-amplify”, by plugging the tube amplifier into a passive attenuator with a load mode, or a load box or a speaker emulator as long as they have the line out feature. The line out signal is feed into a separate power amplifier which is used to control the volume of the speaker. The benefit of using a separate power amplifier is you not only can reduce the power of the tube amplifier, but you can increase its power depending on the power rating of the separate power amplifier.

Note: Regarding all-in-one attenuators using the “re-amplifying” technique; the portion of the circuitry that actually attenuates the tube amplifier’s signal is purely passive. Meaning the circuit that does the actual attenuation is not active. Typically a fixed, non-reactive power resistor is used to terminate the tube amplifier’s output and a simple variable resistor network is used to sample the amplifier’s signal from the power resistor. The only active portion of the attenuator is of course, the solid state amplifier.

Another active form of reducing sound level is with a technology that replaces your permanent magnet speaker with a special field coil speaker. DC power is used to control the magnetic intensity surrounding the speaker's voice coil. The user can vary the DC voltage level, which varies the magnetic intensity and in turn controls the sound pressure level from the field coil speaker.

There are other forms of active attenuation; however most are not being marketed as standalone products.

ATTENUATOR LOAD TYPES: NON-REACTIVE AND REACTIVE

A load is used in most attenuator designs, regardless if they are passive or active. The load is used to absorb either a portion or entire output power of the tube amplifier. Loads can be categorized as non-reactive or reactive. The load can be as simple as a single component or multiple components making up a load circuit. Loads internal to attenuators are used to provide different levels of power reduction and for impedance matching purposes.

NON-REACTIVE LOAD: A common example of a non-reactive load is a power resistor, which is purely resistive with no reactive properties. The power resistor is used to convert the electrical energy into thermal energy (heat). The resistance value of a non-reactive load is fixed. As you vary the frequency applied to a non-reactive load, its resistance will not change, meaning it is not frequency dependent.

Power resistors are often configured in a bridge or "T" network, which are resistors in series and parallel with the speaker that provides the desired attenuation level and overall load resistance.

REACTIVE LOAD: The resistance value of a reactive load is not fixed, meaning the resistance value is frequency dependent. In other words, the resistance will change depending on the frequency applied to it. Reactive devices will have an impedance value and not a fixed resistance value. For reactive devices, the term “impedance” is used, which means the device’s resistance is frequency dependent.

One example of a reactive device is an audio speaker. The resistance value of an audio speaker changes depending on the frequency applied to it. An impedance graph can be created for a specific speaker, showing its resistance value verses various frequencies within the audio band. Impedance curves are often supplied or available for guitar speakers.

A common example of a reactive load used in a power attenuator would be an RLC or resonant circuit (Resistor [R]/Inductor [L]/Capacitor [C]) that is made up of a resistor, an inductor and a capacitor, or multiple combinations of these components to simulate a speaker's reactance. The reactive load is often used in a resistive bridge or "T" network (components in series and parallel with the speaker) to provide the desired attenuation level and overall load impedance. The RLC circuit that simulates the speaker’s reactance is also known as a “Speaker Emulator”.

In addition to an RLC circuit, there are a few other reactive types of loads that include:

a) A dummy speaker; also known as a silent speaker or crippled speaker, is a reactive load device that is basically a speaker without the cone. Therefore the dummy speaker produces very little sound. Like a real audio speaker, the dummy/silent speaker converts the amplifier's signal into mechanical energy. However without the speaker's cone, the dummy speaker's reactance can be different than an actual speaker's reactance. A dummy speaker can also be less efficient in converting the electrical energy into mechanical energy as compared to an actual speaker; therefore, it can end up dissipating a large amount of thermal energy much like a non-reactive load.

b) A light or a fan is reactive device that can be incorporated into the attenuator's load circuit to add reactance and provide attenuation. These devices can be actually powered via the amplifier’s output signal and therefore do not require any external power source. If the device is used in this manner, the attenuator would still be considered passive.

CATAGORIZING POWER ATTENUATORS

Now that we have discussed the two different types of attenuator categories and the types of loads available, we can categorize most power attenuators into 1 of 4 types:

Passive attenuation with non-reactive load

Passive attenuation with reactive load

Active attenuation with non-reactive load

Active attenuation with reactive load

ATTENUTOR FEATURES:

Power Rating: One of the most important attenuator specifications is its power rating. The attenuator must be able to handle at least the maximum power the tube amplifier is capable of producing. Attenuator manufactures will provide the continuous maximum power their product is rated for. However the stated power rating of the tube amplifier might not be so accurate. Some amplifier manufactures, will rate their amplifier's power rating in "audio" watts and not the fully cranked "volume on max" power level. The rating in audio watts goes back to when the output power rating for guitar amplifiers was rated similar to audio amplifiers. For example: a tube amplifier with a manufacturer's specified output power rating of 100 watts, like a typical “quad” amplifier with 4 power tubes, can often produce a fully cranked power output of 140 watts or more; particularly if the amplifier is equipped with high power tubes such as EL34s, KT88s or 6550s. It is even possible to design a “quad” amplifier up to the limits of the power tubes, resulting in an output power of 200 watts, although around 140 watts is more typical for a “quad” guitar amplifier. Therefore, an attenuator rated with a maximum input power rating of 100 watts, most likely would not be suitable for an amplifier with a 100 watt rating, since that amplifier might be capable of producing up to 140 watts or more.

You should also be aware that some so-called 50 watt amplifiers that are equipped with 2 output tubes can be capable of producing 70 watts or more.

It is therefore important to select an attenuator whose power rating meets or exceeds the tube amplifier’s actual maximum power output and not necessarily what is labeled on the amplifier.

Also related to the attenuator's power rating is the temperature vs. power dissipation specification of the power resistors used in the attenuator. All power resistors have temperature vs. power dissipation specifications; meaning that as the resistor absorbs power, its temperature goes up, and the maximum power it can dissipate goes down. Manufacturers of well-designed attenuators will take this into account and will select devices that will be within the power dissipation specification when the device is at maximum operating temperature.

As a user it is important for you to know this, so you can determine if the attenuator is a safe product for use with your amplifier. Attenuator designs that do not take into consideration the temperature vs. power dissipation specification might not be able to maintain their stated maximum power rating over its full operational temperature range. Also, if you exceed the power rating of the attenuator, you run the risk of overheating the attenuator's components, resulting is possible damage to the attenuator or worse, to your amplifier.

Note: The important power rating of an attenuator is its Continuous Average Power handling specification. This power specification should meet or exceed the power your amplifier is actually capable of producing. A power specification stated as Watts RMS (Roots-Mean-Square) is erroneously used to describe average power. Power in Watts is always calculated from the RMS voltage, however the correct power rating term is a value in Watts and not Watts RMS. Also power in Watts is never based on the Peak voltage; therefore, a power rating in Watts Peak is also an incorrect term. An audio amplifier or a device with a rated power based the peak-to-peak voltage would be misleading, since the specified power level would be higher than if based on the RMS voltage. When checking the power rating of an attenuator, it is the Continuous Average Power specification in Watts that is important. As mentioned, a tube guitar amplifier is often capable of putting out more power than what is stated on its label. It is important to know what the actual maximum power out is from the tube amplifier and the maximum Continuous Average Power rating of the attenuator. You never want to exceed the Continuous Average Power rating of the attenuator.

IMPEDANCE: FIXED, SELECTABLE, AND UNIVERSAL

The next important thing to consider with an attenuator is its impedance options. There are three varieties which are used:

Fixed Impedance: Some attenuators are designed to be compatible with specific impedances; for instance, 2, 4, 8 or 16 ohm. A fixed impedance attenuator might be acceptable if you are only matching an amplifier and a speaker cabinet with the single common impedance.

Selectable Impedance: The user can select from multiple impedance values, i.e. 2, 4, 8 or 16 ohm. The selectable impedance feature is ideal, since it is not limited to a single impedance. You can use the attenuator with multiple amplifiers and speakers with various impedances as long as they are matched in pairs. For example, a selectable impedance attenuator could be set for: 2 ohms in/2 ohms out, or 4 ohms in/4 ohms out, 16ohms in/16ohms out. Selecting different input and output impedances is typically not available, i.e. 2 ohms in/16 ohms out would not be an option for most attenuators.

Universal Impedance: The impedance value is not user selectable and sometimes the attenuator's exact load value is not specified. Universal Impedance does not mean the device adjusts to the tube amplifier's impedance, it means the attenuator manufacturer allows for an impedance range, such as 4-16 ohms. These types of attenuators are often a compromise, since they purposely allow for a range of impedance mismatch. Some amplifier manufactures do not recommend their amplifiers to be mismatched to the speaker's load. Therefore, if you plan on using an attenuator with universal impedance, it is best to check with the manufacture of your amplifier to get their opinion about using the attenuator product.

Volume Control: User controls include the step method with specific increments of attenuation or continuous variable attenuation control, or both.

Most passive attenuators use the step method, which uses a switch to select between various set attenuation levels. The switch can be a simple 2 position switch or a multi-position step switch. Some attenuators incorporate both a step switch and a continuous variable control.

There are also attenuators on the market that use separate output jacks for each attenuation step or level. Instead of using the rotary step switch, the attenuator uses individual speaker jacks. This type of product requires the user to plug and unplug the speaker cable into these individual jacks.

Some passive attenuators also include a continuously variable attenuation control. A continuous variable control is typically employed at the highest attenuation level (lowest volume level). The lower volume mode is often referred as the "bedroom" mode. A "bedroom" mode may or may not be important to you depending on your application. If you do plan on using the attenuator down to very low volume levels, then it is important you check the sound quality at these levels. Some attenuators in the "bedroom" mode can have a negative impact on the tone.

Active attenuators typically provide the continuously variable type of control. It does not really matter if an attenuator incorporates a step switch or variable control, what is important is that the total attenuation provided suits your needs.

Additional attenuator features:

Equalizer (EQ): Depending on the design of the attenuator, an EQ circuit may or may not be incorporated. If the circuit does incorporate an EQ, the attenuator might be equipped with user adjustable controls, such as pots or switches that allow the user adjust the attenuated tone or the EQ can be fixed with no user controls. Some attenuator designs do not incorporate an EQ circuit. Cost effective attenuators might not utilize an EQ circuit in order to keep their design as simple as possible, even though there product might benefit by having an EQ. Some high end attenuators might purposely not want to EQ the amplifier’s signal, since their attenuated tone is highly acceptable un-equalized.

Single or multiple speaker jacks: The attenuator may be equipped with a one or two speaker jacks. If two jacks are provided, they are normally wired in parallel. If you plan on simultaneously using 2 speaker cabinets with an attenuator, then an attenuator with 2 speaker jacks would be important, unless at least one of your speaker cabinets already incorporates an extra parallel speaker jack for this purpose.

Load Mode: Some attenuators feature a Load Mode setting, which means the attenuator can be operated as a load box. In this mode, the speaker can be disconnected and the attenuator alone will provide an appropriate load to the amplifier. The Load Mode setting is often used when servicing an amplifier. This mode can also be used in conjunction with a Line Out feature as discussed below.

Line Out jack with/without level control: The Line Out feature samples a portion of the amplifier's signal that can be then sent to an external device such as a recording system, sound system or slaving to another guitar amplifier or to a power amplifier. If the Load Mode is not offered, then the audio speaker must always remain connected to the attenuator. When using the line out jack to an external device, the speaker connected to the attenuator is not providing the tone to the external device. The tone is purely from the tube amplifier. Often a Line Out level control is also provided.

Some attenuators feature a Load Mode along with the Line Out jack. When the attenuator is operating in the Load Mode, the audio speaker can typically be disconnected from the attenuator. Using a separate power amplifier, the attenuator can be used to re-amplify the tube amplifier’s signal, as discussed previously this article. In this case, the Line Out jack from the attenuator is fed into a separate power amplifier which drives the speaker.

Want the volume level of your Fender Champ to be equal to or greater than a 100 watt Marshall stack? For example, suppose you like the "cranked up" tone of your low powered practice amp, but the volume level is not loud enough for gigging. One solution would be to plug your practice amp into an attenuator with the Load Mode and Line Out feature. You can then use the Line Out to drive an external power amplifier, which then drives a speaker cabinet with an appropriate power rating. You can select the power of the external power amplifier to whatever suits your requirement. For example: plug a 6 watt practice amp into the attenuator, take the attenuator's Line Out into a 100 watt power amplifier, which is then plugged into a 4x12 speaker cabinet. Now you have a 100 watt Champ!

TONE, VOLUME AND POWER

Now that we have covered the various types of power attenuators, the technology used and their features, we will now discuss the topic regarding tone, dynamics and volume level.

Tone and Dynamics:

Of course one of the most important aspects of a power attenuator is how well it maintains the tone and dynamics of the tube amplifier and speaker.

What is the difference between Tone and Dynamics?

Tone is what you hear and dynamics is what you feel.

Tone:

Tone is a personal and a highly subjective matter. What is considered as good tone varies from person to person and therefore it is up to you to decide whether the attenuated tone is good or not. The attenuated tone depends on many factors such as; type of amplifier being used, the amplifier’ settings, the type of speaker(s)/cabinet, the attenuated volume level, the style of music; blues, rock, metal, speed, a bit of science, as well as a lot of other factors. The type of tone also depends on what the end result the user is looking for. The guitarist could be searching for transparency in the attenuator or if they want the attenuator to equalize, shape or filter the signal.

Since external power attenuators are located in between the amplifier and the speaker, they can affect an amplifier’s tone; some attenuators affect tone more than others. Tone can be categorized into "transparent" and "acceptable".

Transparent tone means the sound is very natural, but at a reduced volume level. The signal’s frequency response (bandwidth) remains the same and it has been attenuated in a linear fashion. There are unavoidable reasons that tone changes as the volume is decrease, which we will discuss shortly. Even considering these unavoidable reasons, it is possible for an attenuator to provide a very transparent and natural tone at a wide range of attenuation levels. However, a majority of attenuators do alter or color the tone.

Acceptable tone means the natural sound has been altered, however the altered tone is considered “good” or “acceptable” by the user. The frequency spectrum has been altered; its bandwidth has been narrowed, either the lows, highs or both have been reduced or the signal has not been attenuated in a linear fashion and it is skewed or slopped with an emphasis on the highs or lows. While altering the signal can lead to bad sounding tone, some attenuators alter the tone in an acceptable manner. Some users prefer the “tweaked” attenuated tone as compared to the non-attenuated tone. Like in the case of when an attenuator skews the spectrum in favor of the highs, while reducing the lows. This is a popular tone for playing leads. However, attenuators that do alter the original tone can be quite far from being considered transparent.

Dynamics:

Dynamics, used within the context of this discussion, is something that you feel. Dynamics is the touch response, the pick action, the nuances; it is how the amplifier and speaker respond to the player’s technique. Some attenuators can reduce or even “kill” the dynamics. Reduced dynamics can make the amplifier’s tone “feel” compressed, flat, less driven or less responsive. Experienced players know very well the meaning of dynamics, which is the interaction between their player’s hands, guitar, amplifier and speaker. They can also judge how much the attenuator affects this dynamic interaction.

Speaker Distortion:

Also when discussing “transparent” tone, you must consider how much the speaker’s distortion plays a part in the overall tone. When a speaker is driven fairly hard, a phenomenon known as cone breakup takes place. When the cone breaks up, it adds distortion to the tone, meaning the speaker is contributing its own distortion characteristics to the overall distorted tone. Different models of speakers will have cone breakup at different power levels; therefore the point where a speaker begins to distort is model dependant.

Most likely the non-attenuated, overdriven sound is a combination of the tube amplifier's overdriven tone, plus additional distortion from the speaker. However, as the attenuator reduces the amplifier's power, the speaker is driven less and the speaker's distortion is also reduced. Therefore, the overall distorted tone will change as the power level is reduced and the speaker's distortion contributes less to the overall tone. As the power is reduced, the amplifier’s overdriven tone is contributing more to the overall tone and the speaker’s distortion less. This reduction in speaker attenuation would be true for any power attenuation technique, internal or external, and cannot be avoided. However, if the tube amplifier provides very good overdriven tone on its own without having to rely on the speaker’s distortion, then playing through a well designed attenuator will maintain the amplifier’s tone qualities at a lower volume level.

Fletcher and Munson (Audio Perception)

Also related to tone and volume level is our hearing response. A well known study on how our ears perceives different frequencies at different volume levels was conducted in 1933 by Harvey Fletcher and W A Munson, in which they provided contour curves of the human hearing response. These curves are known as equal-loudness contour curves and they have been updated as defined in the international standard ISO 226:2003. Basically, the result of these tests show that as volume level is decreased across the audio frequency band, the ear perceives the lows and highs to drop more than the mids. However, as the volume level is decreased towards our hearing limits, our ears perceive the lows, mids and highs back to a “normal” response.

What does all this mean? Even if we were to listen to the tone from an “ideal” attenuator that offers very linear attenuation across the entire audio frequency band, played through an “ideal” speaker, your ears might hear subtle changes in tone as the power to the speaker decreases, due to our natural hearing response. As the volume is decreased to the limits of our hearing, our ears perceive the tone to be approaching “normal” again.

Two factors come into play, which cannot be avoided when reducing the tube amplifier’s power to the speaker. One being less speaker distortion and the other is our aural perception of different frequencies at lower volume levels. Taking these two factors into consideration means the attenuated tone will be affected, not necessarily in a bad way, however affected.

Loudness VS Power Level

Differences in power levels are often referenced as differences in dB (decibels). Doubling an amplifier’s power or reducing its power by half would be a difference of 3dB. For example decreasing a power level of 100 watts to 50 watts would be a -3dB reduction. Reducing 100 watts to 25 watts would be a -6dB reduction. While reducing the output power by half seems to be a lot, it might be hardly noticeable by ear. The reason is, decreasing the power by half (-3dB) does not provide an audio perception of half the loudness level. In order to perceive a 50% drop in the audio loudness level, the power would have to be cut by 90% or -10dB. Therefore, a 100 watt amplifier would have to be reduced to 10 watts in order for an audio perception of a sound level to be cut in half.

Keep in mind that 10 watts can still be too loud for some applications. Some people even consider even one watt to be too loud for “bedroom” use.

The speaker’s efficiency determines the Sound Pressure Level (SPL), how “loud” the volume will be. The speaker’s efficiency is rate as a sensitivity specification, which is based on the sound pressure level measured in db the speaker produces with 1 watt of power applied and measured at a distance of 1 meter from the speaker. A typical 12” guitar speaker will have a sensitivity rating around 100, however this rating does vary and is specific to each speaker model.

For example, a speaker with a sensitivity rating of 100 will produce 100db at 1 meter (3.3’) with 1 watt of power. A speaker with a sensitivity rating of 97 will produce 97db at 1 meter with 1 watt of power.

An amplifier providing 50 watts of power into a speaker with sensitivity rating of 100 will produce the same SPL as an amplifier providing 100 watts of power into a speaker with a sensitivity rating of 97. Therefore, the volume level depends on the sensitivity of your speaker and the power applied to it.

The following chart compares examples of loudness VS power level. The SPL is based on a speaker with a sensitivity rating of 100. The approximate loudness is based on the subject standing one meter (3.3’) in front of the speaker.

POWER VS. LOUDNESS CHART

Watts	SPL (db)	Loudness

0.0078	79	Passenger car at 10 (60-80dB)
0.0156	82
0.0312	85	Vacuum cleaner
0.0625	88	Major Road Noise (80-90dB)
0.125	91	Noisy factory
0.25	94
0.5	97
1	100	Jack hammer at 1m
2	103
4	106
8	109	Accelerating motorcycle at 5m
16	112
32	115	Hearing Damage (short term exposure)
64	118	Rock concert
128	121
256	124
512	127	Jet at 100 meters (110-140 dB)
1024	130	Threshold of pain

As you can see, based on the above chart, if your amplifier is cranking out 32 watts of power, into a speaker with a sensitivity rating of 100 and you are standing 3.3’ in front of the speaker, hearing damage due to short term exposure can occur.

The distance you are from the speaker of course affects the SPL. If you are in a space where there is no reflected sound, such as outdoors, then each time the distance from the speaker is doubled, the loudness of the sound pressure level is reduced by 6 dB.

Feet From Speaker dB-SPL

------------------------- --------------

3 94 (reference)

6 88

12 82

24 76

50 70

100 64

COMPARING PASSIVE AND ACTIVE ATTENUATORS

We will compare a couple of the more popular types of power attenuators.

Passive Attenuator: Resistive Network Design - "Power Soak "

A typical passive, “power soak” type of attenuator will use a resistive network to reduce the amplifier’s power. The resistive network can be purely non-reactive with only power resistors or it can be reactive with an RLC circuit or utilize a dummy speaker. A step switch is often used with this type of design. The switch is used to select various points in the resistive network, which varies the attenuation level, while maintaining the desired impedance. Often a variable L-Pad is switched in for the “bedroom” mode. An L-Pad is a resistive device that connects to the speaker and provides continuously variable attenuation, while keeping a fairly constant resistance value.

Benefits of passive, “power soak” attenuators include; they are typically rugged and very reliable with no active parts to fail and they are easy to setup since they do not require an AC cord or a power outlet, unless the attenuator incorporates an electrical fan. There is also a wide choice with a large number of passive attenuator products on the market with a range features, design techniques, tonal qualities, specifications and prices.

The drawback to the resistive network attenuators or one that incorporates a variable L-Pad, is the fact that they do load down and reduce the interaction and reactance between the tube amplifier and speaker, which causes a reduction in tone and dynamics. While a reactive load might offer an improvement, they still affect the natural reactance of the speaker, therefore, a reduction in tone and dynamics is often still noticeable.

Some passive attenuators provide acceptable tone at the initial attenuation steps and then a noticeable decrease in tone quality as the attenuation is increased. Many do not perform well at the highest attenuation levels, i.e. “bedroom” mode.

Active Attenuator: Re-Amplification Technique

With the re-amplification technique, the tube amplifier’s output power is terminated into a dummy load provide by the attenuator. A portion of the power is sampled from the load and is inserted into a solid-state amplifier, which then drives the speaker.

You can use an “all in one” active attenuator that provides a load to the tube amp and has a cost effective internal solid state amplifier to power the speaker or you can use a passive attenuator, load box or a speaker emulator that is equipped with a Line Out feature to feed an external power amplifier.

The benefits of using the re-amplification technique include: the sampled signal can be "re" amplified to whatever power level the solid-state amplifier is capable of producing. The re-amplified tone should remain constant over the entire power (volume) range. Therefore, if the tone of the sampled signal is acceptable to you, then tone should remain consistent as you adjust the volume out of the solid-state amplifier.

The drawbacks of using the re-amplification technique include: requires an AC cord and an extra power outlet, and since the technique uses a solid-state amplifier, there are a lot more components that can fail. Some users comment on hum or squealing issues, which can be caused by a ground loop between the attenuator’s internal amplifier and the source (tube) amplifier. Their starting prices are a lot higher than some passive attenuators. Another drawback is the fact that 100% of the tube amplifier's power is terminated into a load. The reactance the tube amplifier sees “if any” is determined solely by the attenuator’s artificial load and not by the actual speaker. Furthermore, the speaker's reactance (unique impedance footprint) does not play a part in shaping the overall tone, since the reactance or interaction between the speaker and the tube amplifier is non-existent. The speaker and tube amplifier are isolated from each other, separated by the solid-state "re-amplifier".

Also the load value provided to the tube amplifier is often fixed, which is intended to accommodate a range of tube amplifier impedances. Typically the load value is fixed at a resistance higher than the amplifier’s impedance, therefore there is an intentional mis-match of impedances. The load the attenuator provides to the tube amplifier affects the impedance the output transformer provides to the tubes.

While the impedance mis-match might not be harmful to the tube amplifier, it will cause the frequency bandwidth from the tube amplifier will be reduced. Since the input load impedance is fixed and not selectable, the mis-match will become greater as lower impedance amplifiers are used. The more the mis-match, the more the bandwidth is affected. In particular, there will be a reduction in the lower frequencies (low end). Therefore, the sampled signal from the attenuators load is not the same as if it were sampling the signal from a tube amplifier connected directly to a speaker.

Some users also do not like the idea that their tube amplifier is no longer powering their speaker and that the active attenuator's internal, cost-effective, solid-state amplifier is actually driving the speaker. For example; let’s say you have a vintage or boutique amplifier worth several thousands of dollars, with an active (internal re-amplifying) attenuator, you end up having a cost effective solid state amplifier driving your prized vintage or high-end speaker and not your tube amp. Your guitar speaker in essence, becomes a P.A. speaker for the attenuator's solid state amplifier.

SELECTING A POWER ATTENUATOR

Summarizing the discussion above, your selection of a power attenuator should be based on the following:

1. Power Rating

2. Impedance

3. Tone and Dynamics

4. Attenuation Range

5. Additional features

6. Cost

Power Rating

When selecting an attenuator, it is very important its power rating meets or exceeds your requirements. You will need to know the maximum continuous power the attenuator is capable of handling. Also you will need to know what the actual maximum power your amplifier is capable of producing. Keep in mind the actual power out can be significantly greater than what is marked on the amplifier.

Impedance

You must select an attenuator with an impedance that is compatible with your amplifier and speaker. If your collection of amplifiers and speaker cabinets all have one common impedance or can be set for a common impedance, then an attenuator with a fixed impedance value can be adequate. However, if you have multiple amplifiers and/or speaker cabinets that vary in impedance, then an attenuator with user selectable impedances would be an important feature. You might be wary of a product that can "universally" accept a wide range of impedances. These units might compromise the amplifier's load to a level that is not considered safe by the manufacturer of the amplifier.

While on the topic of impedance, keep in mind that if you are planning on simultaneously using 2 parallel speaker cabinets (with equal impedance values), you must set the amplifier’s and attenuator’s impedance half the stated single speaker cabinet impedance value. For example if you are running (2) 16 ohm speaker cabinets in parallel, you must set or use an amplifier and attenuator with an impedance value of 8 ohms.

Tone and Dynamics

After you are confident that the attenuator's impedance and power rating are safe for your amplifier, then the product's tone and dynamics is the next important factor to consider. How the attenuator affects the tone and dynamics of your amplifier and speaker should take precedence over any other features (bells and whistles) the attenuator might have to offer.

When evaluating an attenuator product, listen to how it affects the tone and dynamics in following areas:

Reduction in frequency bandwidth: loss of highs or lows.

Reduction in drive or distortion: the tone is less driven or less distorted.

Flat tone: the tone sounds dull, not lively.

Loss of dynamics: loss of responsiveness; the amplifier does not respond as much to "pick attack" or to the players touch.

Attenuation Range and Type of Level Adjustment

Volume level can be controlled by step increments or by continuous variable control (or both).

Passive attenuators often use a step control followed by a variable control which is often referred to as the "bedroom" mode, since it controls the lowest volume level. Active attenuators are often equipped with the variable type of control. Step switch attenuation is not necessarily a disadvantage, since the user will normally want to change attenuation levels in increments of 3dB or more, therefore fine adjustment is not so critical. After all, in order to hear a perceived 50% drop in volume level, you need to drop the power down by 10dB.

The "bedroom" mode may or may not be important to you. It depends on how low of a volume level you want to achieve and your starting reference point, which is the maximum output power of your amplifier.

Knowing the overall attenuation range of the attenuator is important. Some attenuators have an attenuation range from 100% power (attenuator bypassed) down to 0 watt (no sound). While other attenuators offer a smaller attenuation range. It is important for you to know the attenuator's range and if it provides the attenuation you require.

How to calculate the attenuation range for your application:

a) Decide the lowest power level you would like to achieve. Refer to the Power vs. Loudness chart and determine the attenuated SPL you would like to achieve.

b) Determine your amplifier’s maximum output power. Remember this must be the power your amplifier is actually capable of producing, which might be different than what is labeled on the amplifier.

c) Subtract the amplifier’s maximum output power from the SPL you would like to achieve (all units in dB). This is the minimum amount of attenuation you require.

d) Verify that the attenuator you are considering will provide at least that amount of attenuation.

For example; you would like to have a maximum attenuated level of 1 watt (100dB) and your amplifier is capable of producing 64 watts (118dB), based on the SPL stated on the table. Then from the table, we have calculated that an attenuation range of 18db or more will satisfy your sound level requirement.

Additional Features

There are other, additional features that you may want to consider in selecting an attenuator. These include:

User Adjustable Equalizer (EQ): While user adjustable EQ controls might seem like a good feature, you may or may not feel a need for one depending on the overall attenuated tone. In some cases an EQ is desired if the attenuated tone is no longer natural sounding and needs to be “tweaked”. An EQ is sometimes provided to adjust the tone to compensate for the effects of lower volume levels or just to provide an additional equalization step between the amplifier and speaker. Some attenuators have an EQ built into their circuit that is not user adjustable; in this case the designer has “tweaked” and fixed the tone as needed.

However if the attenuator is fairly transparent and does not significantly alter the tone, then an EQ might not be desired. For some users, the purpose of the attenuator is to maintain the tone and dynamics of the tube amplifier and speaker as true as possible and not to further equalize the amplifier’s signal.

Load Mode and Line Out: While not always mandatory, these features can come in handy, particularly if you have requirements as stated earlier.

Cost: Last but not least is the cost. There is a whole spectrum of prices for power attenuators, ranging from under $50 to well over $700. A power attenuator can be as simple as stuffing an L-Pad or a handful of power resistors into a project box, or it can be much more elaborate, with innovative technology incorporated into it.

Regardless of your budget, your first priority in selecting a power attenuator is to ensure the power attenuator is safe for your tube amplifier and that the power output from your amplifier is within the design limits of the attenuator.

Secondly you should consider that an attenuator might be with you for a lifetime, since it is a universal piece of equipment. Therefore, you might want to invest in an attenuator that allows you to select from various impedances, so the attenuator will remain compatible with amplifiers and speaker cabinets that are later added your collection. Also, you might want to let your “ears” select the attenuator and not the price tag, in doing so the attenuator might just be with you for a lifetime.

ATTENUATOR SAFETY ISSUES

We suggest that you first consult the manufacturer of your tube amplifier to see what their recommendations are on using an attenuation device and also check with the attenuator manufacturer to get there "statement" regarding safe and proper use of their product.

Some attenuator manufacturers state that their product is just as safe as running your amplifier directly into a speaker. This might be true for a well-designed attenuator product that is properly matched to your amplifier’s power and impedance. However an attenuator that is not properly designed or not correctly matched to your amplifier can add additional stress to your amplifier, especially when played “wide open”. Also the impedance of some attenuators can vary as the attenuation level is changed.

If you normally play your amplifier flat out into a speaker for an extended period of time without any issues, then running your amp flat out through a properly designed attenuator will probably not add any additional stress to your amplifier.

You should be aware that there is additional stress on your amplifier, when playing the amplifier flat out, with or without an attenuator. When your amp is played wide open, maximum current will be flowing through the amplifier's circuitry, power transformer, output transformer, and the output tubes. A byproduct of the maximum current flow is a maximum amount of heat generated by; the tubes, the power and output transformer, and certain parts of the amplifier’s circuit. There is a maximum amount of stress on the amplifier's components when operating at full volume. Some tube amplifiers will blow a fuse when operated flat out, with or without the use of a power attenuator. If you have never run your amplifier flat out straight into a speaker cabinet for an extended period of time, then you do not actually know if your amplifier can handle this type of stress. Keep in mind, not all tube guitar amplifiers are designed to be played wide open.

Some vintages amps have output transformers that were not designed for the amp to be fully turned up and played for an extended period of time. For example, some vintage amplifiers used output transformers that are underrated for the power the amplifier is capable of producing when it is fully cranked up. Many vintage amps used output transformers that were designed to handle only their "audio" (clean) power levels. A lot of vintage amps were designed to produce clean tones and were not specifically designed to be turned up for maximum distortion. For example; a vintage amp might have an output transformer designed to handle 10 watts, since this specification would be more than enough to handle the amp set for a clean tone. However, when fully cranked, the amplifier might be capable of putting out 15 watts or more.

If the vintage amp is run at or near its maximum power capability for an extended period of time, the underrated output transformer could overheat and fail. The same could happen if a boutique amp is using an output transformer built to vintage specifications.

Also, running an amp flat out with or without an attenuator will normally reduce the life of the output tubes, since maximum current is flowing and maximum heat is being generated during this condition. If your amp should happen to have a marginal power tube, the tube might operate fine at lower power levels and then could fail with the amp cranked up with maximum current flowing through the tube. The same could be true for any marginal component in the power tube section of the amplifier.

If a fuse blows or if an output tube, output transformer or other components fail with the use of an attenuator, it may or may not have happened without the attenuator. Running an amp flat out with or without an attenuator will normally reduce the life of the output tubes, since maximum current is flowing and maximum heat is being generated during this condition.

If your amplifier is equipped with a Master Volume Control or a Power Scaling circuit, you can use these controls in addition to the attenuator. By using these controls to back off the amplifier’s power a bit, you can reduce the stress on the amplifier’s components.

Other safety issues include how well the attenuator maintains its stated impedance over the various attenuation levels and its ability to safely handle the power and heat within its circuit. Some amplifiers can generate higher than normal temperatures when their output impedance is not matched properly. This can add to additional stress when running the amplifier flat out through the attenuator, unless proper impedance it kept.

In general a properly designed attenuator, correctly matched to the amplifier, should not add any additional stress to your amplifier.

While we are on the topic of safety, remember to always use speaker cables and not guitar cables when connecting in and out of the attenuator. Never play a tube amplifier without the speaker jack terminated into a proper load. Also, carefully read and follow the instructions provided in the attenuator’s Owner’s Manual.

OTHER POWER ATTENUATION DEVICES:

Related to power attenuators are; Attenuator Speakers, Speaker Emulators, Speaker Isolation Cabinets and Load Boxes.

An Attenuator Speaker is an attenuation technique, where the speaker itself provided the attenuation. It involves the use of custom or modified speakers that allow the user to vary the flux density strength of the speaker's magnet. Varying the flux density strength will vary the output volume level (SPL) of the speaker. This can be accomplished by using a field coil speaker along with a variable field coil power supply and the user controls the speaker's magnetic strength via the variable power supply. Another method that allows the user to vary the flux density strength, involves a custom electromagnetic speaker that allows the user to mechanically adjust the distance between the magnet and the speaker's voice coil.

A speaker emulator is a reactive load box device which is used to terminate the tube amplifier’s power. The speaker emulator will have a Line Out feature. The Line Out will normally have a level control, used to reduce the signal level to the Line Out jack. The signal from the output jack is typically fed into a power amplifier (re-amplification) or sound system mixing board.

A Speaker Isolation Cabinet is a sound-proof enclosure that surrounds the speaker and microphone and prevents sound leakage into the outside environment, enabling the amplifier to be turned up without excessive listening volume.

A Load Box typically replaces the speaker with non-reactive or reactive load, which is used to terminate 100% of the amplifier’s power and does not provide variable attenuation. They are often used for testing amplifiers when no sound is desired. The Load Box may or may not provide a Line Out feature.

Thank you for taking the time to read this article, we hope that you have found it useful.

Check out ARACOM’s Power Rox Power Attenuator:

the PRX150 Line of Attenuators - “A Revolution in Power Attenuation Technology”

Also be sure to read the following ARACOM articles:

The PRX150 Attenuator Advantages

TESTING THE TRANSPARENCY OF A POWER ATTENUATOR

GLOSSARY OF ATTENUATOR TERMS

Active: a device that requires a source of energy for its operation. Examples of active devices include power supplies, transistors, LEDs, and amplifiers.

Bandwidth: bandwidth is the width of the range (or band) of frequencies that an electronic signal uses on a given transmission medium. In this usage, bandwidth is expressed in terms of the difference between the highest-frequency signal component and the lowest-frequency signal component. Since the frequency of a signal is measured in hertz (the number of cycles of change per second), a given bandwidth is the difference in hertz between the highest frequency the signal uses and the lowest frequency it uses.

Capacitor: a device consisting of two parallel plates separated by an insulator, called the "dielectric". The capacitance is proportional to the area of the plates, and inversely proportional to the distance between them. Capacitors are used to block DC while passing AC. They are a frequency-dependent device, which means that their capacitive reactance, or "effective resistance" to AC increases as the frequency gets lower. This makes capacitors useful for tone controls, where different frequency bands must be passed, or for bypassing AC signals to ground while passing DC through for filtering purposes.

Impedance: a complex quantity containing both a resistance and a reactance.

Inductor: a circuit element consisting of a coil of wire would on a core material made of ferrous or non-ferrous material. An inductor resists changes in the flow of electric current through it, because it generates a magnetic field that acts to oppose the flow of current through it, which means that the current cannot change instantaneously in the inductor. This property makes inductors very useful for filtering out residual ripple in a power supply, or for use in signal shaping filters. They are frequency-dependent devices, which means that their inductive reactance, or "effective resistance" to AC decreases as the frequency gets lower, and increases as the frequency gets higher. This property makes them useful in tone controls and other filters.

Line Out: as used in power attenuator and load boxes, is a low level output, sampled from the tube amplifier’s signal. Typically a Line Level Control is provided. The Line Out signal can be used to drive an external device such as a recording system, sound system or slaving to another guitar amplifier or to a power amplifier.

Load: a device or the resistance of a device to which power is delivered.

Load Box: a device where the entire tube amplifier’s power is terminated into. The load can be non-reactive or reactive. The Load Box may or may not provide the Line Out feature.

L-Pad: an L pad (meaning Loss or Losser Pads) is a special configuration of rheostats used to control the volume of a loudspeaker while maintaining constant load impedance.

Power Soak: a term originating from an attenuator product called the “Power Soak” offered by Tom Scholz of the band Boston. The term has become more generic now and it applies to attenuators that rely purely on the use of resistive loads (non-reactive or reactive).

Passive: a device that does not require a source of energy for its operation. Examples of passive devices are electrical resistors, electrical capacitors, diodes, cables, and wires.

Resistor: a circuit element that presents a resistance to the flow of electric current. A current flowing through a resistance will create a voltage drop across that resistance in accordance with Ohm's law.

RLC: an RLC circuit (also known as a resonant circuit, tuned circuit, or LCR circuit) is an electrical circuit consisting of a resistor (R), an inductor (L), and a capacitor (C), connected in series or in parallel.

Speaker/ Attenuator: an attenuation technique that involves the use of custom or modified speakers that allow the user to vary the flux density strength of the speaker's magnet. Varying the flux density strength will vary the output volume level (SPL) of the speaker. This can be accomplished by using a field coil speaker along with a variable field coil power supply and the user controls the speaker's magnetic strength via the variable power supply. Another method that allows the user to vary the flux density strength, involves a custom electromagnetic speaker that allows the user to mechanically adjust the distance between the magnet and the speaker's voice coil.

Speaker Emulator: a load incorporating a circuit that is designed to emulate the reactance of a speaker. It can be designed to emulate a generic speaker, a specific speaker model or speakers in a cabinet, i.e. 4x12 configuration. The speaker emulator circuitry can utilized in a power attenuator to provide reactance. It can also be offered as a standalone product which the speaker emulator provides the load to the tube amplifier and a Line Out is provided to drive other external devices.

Speaker Isolation Cabinet: is a sound-proof enclosure that surrounds, the speaker and microphone and prevents sound leakage into the outside environment, enabling the amplifier to be turned up without excessive listening volume.

SPL (Sound Pressure Level): sound level is usually defined in terms of something called Sound Pressure Level (SPL). SPL is actually a ratio of the absolute, Sound Pressure and a reference level (usually the Threshold of Hearing, or the lowest intensity sound that can be heard by most people). SPL is measured in decibels (dB), because of the incredibly broad range of intensities we can hear.

Tinnitus: the perception of noise, such as a ringing or beating sound, which has no external source. The most common cause for tinnitus is noise-induced hearing loss. The condition is often rated clinically on a simple scale from "slight" to "catastrophic" according to the practical difficulties it imposes, such as interference with sleep, quiet activities, and normal daily activities. A good reason to use an a power attenuator with guitar amplifiers.

Related Information:

For more information on Power Attenuation, Master Volume Controls and Power Scaling, we highly recommend that you read “The Ultimate Tone, Volume 4” by Kevin O'Connor.

Note: The content in this article is provided for informational purpose only. Use a power attenuator with your amplifier at your own risk.