POWER ATTENUATORS FOR GUITAR AMPLIFIERS
WHAT IS POWER
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
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
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
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
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!
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
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
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
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
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
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:
attenuation with non-reactive load
attenuation with reactive load
attenuation with non-reactive load
attenuation with reactive load
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
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
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.
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.
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
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,
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.
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.
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
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
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
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.
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.
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
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 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, 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.
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
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
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,
Loudness VS Power
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
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
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
car at 10 (60-80dB)
Road Noise (80-90dB)
hammer at 1m
motorcycle at 5m
Damage (short term exposure)
100 meters (110-140 dB)
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)
COMPARING PASSIVE AND
We will compare a couple
of the more popular types of power attenuators.
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
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
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
discussion above, your selection of a power attenuator should be based on the
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.
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
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
Reduction in frequency
bandwidth: loss of highs or lows.
Reduction in drive or
distortion: the tone is less driven or less
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.
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
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
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.
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.
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.
that the attenuator you are considering will provide at least that amount of
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
There are other,
additional features that you may want to consider in selecting an attenuator.
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
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.
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.
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
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.
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
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
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
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.
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
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.
PRX150 vs PRX150-DAG Product Page
The PRX150's Advantages
DRX Attenuator Product Page
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.
Author: Jeff Aragaki
Copyright © 2009 ARACOM Amplifiers (rev. 0809-1)
This article not be reproduced in any form without written approval from ARACOM Amplifiers.