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Before reading this article, we recommend that you read our article:


Before reading this article, we recommend that you read our article:










The PRX150-Pro’s Advantages




The PRX150-Pro is the result of over 10 years of research and development in attenuator technology. Over the years, we have built and tested numerous attenuators using the conventional and the most popular power reduction methods and we analyzed their inherent shortcomings; specifically the negative effect they have on tone, bandwidth and dynamics. In the late 90s we began providing some customers with custom attenuators utilizing our proprietary design. The result of our R&D is a completely new technology that avoids those shortcomings, while providing a new level of transparency.



1.  Proprietary SRT Technology


The PRX150-Pro employs our patented pending "Speaker Reactance Thru" (SRT) technology.   Unlike other attenuation methods, the PRX150-Pro with its proprietary SRT technology, does not load down the speaker's reactance, therefore, the speaker and amplifier are able to react naturally to each other in a transparent manner.


The speaker's reactance and how it interacts with the tube amplifiers plays a very important role in the development of the overall tone. While there has been much discussion on the type of load an attenuator provides to the tube amplifier, there has been very little talk about how an attenuator loads down the speaker’s reactance.  Up to now the focus has been on the amplifier's load, which is only "half" the picture.  How the attenuator loads down or affects the speaker’s reactance is the other half of the equation and an important aspect to maintaining natural tone.  Our holistic design approach focuses on both halves of the equation; maintaining a proper load to the tube amplifier and maintaining the speaker's natural reactance.  



2.  Speaker Reactance


In order to understand the benefits of the SRT technology, it is first important to discuss speaker reactance.   An audio speaker is a highly reactive device, meaning its impedance varies depending on the frequency applied to it; as opposed to a device such as a fixed resistor, which has a constant resistance value and is not frequency dependent. In other words, a speaker's impedance is different at different frequencies.   When we speak of a speaker’s reactance, we are referring to how much its impedance is allowed to vary over the audio frequency band. 


Furthermore, it is important to understand the difference between Impedance and Resistance.  Impedance is a term used when measuring how much a reactive device impedes energy flow.  How much it impedes energy flow varies with the frequency applied to it.  Resistance, on the other hand, is a term used when measuring how much a non-reactive device resists energy flow.  Non-reactive devices will have a constant resistance value and are not frequency-dependent. 


A typical guitar speaker might have a stated impedance value of 4, 8, or 16 ohm, however, the value is only for its impedance at a single reference frequency.  The speaker’s actual impedance will vary, particularly upwards, as the frequency is increased.  


For example, a 16 ohm speaker can have a DC resistance of approximately 13 ohm, however its impedance might vary from 13 to 50 ohm, or more, across the audio frequency band.  There is an interesting spike in the speaker’s impedance value, typically between 80-120Hz, where the value shoots up dramatically, then tapers down to around its rated impedance.  After that the impedance gradually increases in a more linear fashion as the audio frequency applied to it increases.   In the case of a 16 ohm guitar speaker, the spike can be anywhere from around 50 ohm to well over 200 ohm, depending on the speaker model. 


The speaker's reactive range can be found on its impedance curve, which is available from most speaker manufacturers.  The impedance curve is a graph that compares the speaker's impedance verses the applied frequency.  The same graph will usually also show the frequency response of the speaker.  Each speaker model will have its own impedance curve, which influences the speaker's tone, providing its uniqueness.    When an attenuator loads down or alters the speaker’s reactance it also reduces the speaker’s impedance along this curve, sometimes drastically.  Once the speaker’s reactance has been reduced, it can no longer function as normal, which will have a negative impact on the tone and dynamics. 


When a tube amplifier is connected directly to the guitar speaker, the speaker's reactance interacts with the amplifier's output transformer, which in turn affects the impedance across the amplifier's output tubes.  This impedance varies and is dependent on the frequencies generated by the tube amplifier, which originates from the guitarist and guitar.  As the speaker’s load impedance varies with frequency, the amplifier’s output power and frequency bandwidth are affected.  Therefore, the speaker’s unique impedance curve – or colloquially known as “personality” – influences the tone the amplifier produces and its dynamic response.  The dynamic interaction between a tube amplifier and the speaker is one of the areas that sets a tube amplifier apart from a solid state amplifier.  It is the dynamic interaction that makes a tube amplifier sound and feel "tubey".   One of the reasons that different models of speakers have a dramatic effect on the tone from a tube amplifier, is the speaker's unique reactance (personality) has a direct affect in forming the overall tone. As you can see the tube amplifier and guitar speaker work together dynamically and it is the complex interaction between the two that creates the overall tone, therefore, maintaining the speaker's natural reactance is a critical factor in keeping the attenuated tone and dynamics transparent.


3.  Reactive Headroom


In this article we will be referring to a term we coined here at ARACOM Amplifiers known as a speaker's "reactive headroom." The reactive headroom is the difference between the speaker’s lowest impedance value and its highest impedance value across the applied audio frequency band.  An example would be a non-loaded 16 ohm speaker with a reactance range of 13 to 50 ohm, meaning its reactive headroom is 37 ohm.  This can be compared to the same speaker that has been loaded down by an attenuator and the speaker’s reactance range is now only between 5 to 12 ohm.  Its reactive headroom has been significantly reduced to only 7 ohm. We will use this term when comparing the speaker's non-attenuated impedance curve verses the attenuated impedance curve.

Many guitarists feel their current power attenuator can provide a decent attenuated tone down to around a 9 to 12 dB cut in power. This is not so difficult to achieve with most attenuators, since the speaker's reactance, while altered, might have some life to it. However, a power reduction of 10dB is only a perceived audio reduction of half the volume, which is often not enough reduction. Going beyond the 10dB cut, is where many passive attenuators severely reduce the speaker's reactive headroom, which affects the attenuated tone and dynamics in a negative way. We will discuss speaker loading and the PRX150-Pro's no speaker load design next.    


4.  Speaker Loading


Passive, “resistive network” attenuators load down the speaker's reactance, whether they are non-reactive or reactive.  These types of attenuators place a load in parallel with the speaker, which alters, reduces or nearly flat lines the speaker’s impedance curve.


 In many cases, the parallel load will have a resistance value that is equal to or less than the speaker's state impedance.  Based on ohms law, we can calculate how the load affects the speaker's impedance curve.  The graph below compares the impedance curve of a 16 ohm speaker and the impedance curve of the same speaker with a non-reactive 16 ohm load placed across it.



Graph 1



As you can see a 16 ohm non-reactive load in parallel with a 16 ohm speaker nearly flatlines the speaker's reactance and severely reduces its reactive headroom.   Under this condition, the speaker is highly restricted by the load and it is not possible for the speaker to produce its natural tone.   Even if the parallel load’s resistance value is doubled to 32 ohm, the speaker's reactance would still be severely effected by the parallel load.


Additional resistors are typically added in series and parallel to the speaker, in order to provide further power reduction and to maintain impedance.  Each time an additional resistor is added in parallel across the speaker its reactance is further reduced.  This is one of the reasons that some attenuator designs lose tone and dynamics as the attenuation is increased.


Some manufacturers utilize a reactive load so the amplifier will see some reactance.   However, a reactive load across a speaker will still reduce the speaker’s reactance.  The graph below compares the impedance curve of a 16 ohm speaker and the impedance curve of the same speaker with a reactive 16 ohm load placed across it.



Graph 2



The lower curve is the combined impedance of the 16 ohm speaker in parallel with a 16 ohm reactive load.  This is based on a typical case, where the impedance curve of the reactive load is slightly different than the speaker’s impedance curve.  While the reactive load might be an improvement over a non-reactive load, it still reduces the speaker’s reactive headroom, and again, restricts the speaker from producing its natural tone.


Even if the parallel reactive load had an impedance curve that exactly matched your speaker's impedance curve, it would still reduce your speaker’s impedance by half along the curve, therefore, its reactive headroom will have been reduced by half.

As with the previous example, with a reactive load, additional resistors are typically added in series and parallel to the speaker, in order to provide further power reduction and to maintain impedance. Each time an additional resistor or reactive load is added in parallel across the speaker, its reactance is further reduced. If a variable L-Pad is later switched into the circuit to provide a variable "bedroom" mode, the speaker's reactance will be severely impeded. We will discuss more about L-Pads and their negative effects on reactance, tone and dynamics a bit later.


Some attenuators not only have a negative effect on tone, they can affect the dynamics.  Our definition of dynamics is something you “feel.”  It is how the guitar, tube amplifier and speaker respond to the player.  It is the "touch and feel" or "pick response" and touch-sensitivity that is reduced or lost when using some attenuators.  When an attenuator causes reduction in the dynamics, the response or tone can feel flat like a car not running on all cylinders or it can feel hollow or lifeless.  The tone can feel compressed and it does not respond well to the player's pick action.  When an attenuator reduces the speaker’s reactance it also reduces the overall dynamics.


Guitarists often spend an enormous amount of time and money selecting a speaker or combinations of speakers to match their tube amplifier, only to have an attenuator alter or nearly flatline (squashed) the speaker's impedance curve.  It doing so, the speaker’s unique reactance – or personality – is altered and the speaker will behave differently as it reproduces the spectrum of frequencies. The result is the guitarist often feels like the tone and dynamics has been squashed.



5.  The PRX150-Pro’s No Load Design


In conducting our tests, we have found that the typical load value that an attenuator places in parallel with the speaker, regardless of whether it is non-reactive or reactive, has a negative impact on the original tone.  


This is the area that sets the PRX150-Pro apart from the pack.   The proprietary SRT design does not choke down the speaker’s reactance like most other attenuators; it allows the speaker to “breathe” in a natural manner. 


Step Attenuation Mode

In the step attenuation mode, the PRX150-Pro reduces the power level over a wide attenuation range, all while not placing a parallel load across the speaker. The net result is as follows: 


  • The speaker is not loaded down and there is no impact on the speaker's reactance.
  • The speaker’s impedance curve remains intact, and its original reactive headroom is completely maintained.
  • The non-loaded speaker is not constricted is therefore able to operate naturally and reproduce a natural tone.
  • The PRX150-Pro is able to do this while maintaining a proper impedance load to the tube amplifier.   In doing so, the tube amplifier's power level, distortion level and audio bandwidth is maintained, allowing the amplifier to produces its natural tone.


Only the PRX150-Pro with its proprietary SRT technology offers the "no speaker load" design, while maintaining a proper load to the tube amplifier. All other passive, resistive network type of attenuators, whether non-reactive or reactive, place a load across the speaker and therefore affect its reactive headroom. We previously mentioned that a speaker's impedance can spike up to a very high value. Some impedance curves show the spike to be greater than 300 ohm. Since the SRT technology does not restrict the speaker's reactance, the speaker impedance is allowed to spike to its desired value, even if it is 300 ohm or greater. This would simply not be possible with the other popular attenuation methods.

The SRT technology allows your speaker’s reactance to remain intact, undisturbed and therefore, the PRX150-Pro does not incorporate an artificial reactive load circuit (speaker emulator circuit), since your actual speaker’s reactance is used.    For those attenuators that load down the speaker’s reactive headroom, a reactive circuit might restore some of the reactance the amplifier “sees”, however, this simulated reactance would never match the unique (curve) of your particular speaker, which makes up an important part of your speaker’s personality and tone.   Also, from the speaker's perspective, any load placed across it, regardless if it is non-reactive or active, will alter and reduce its reactive headroom.


With active attenuators using the re-amplification technique there are other issues, such as totally isolating the speaker from the tube amplifier, which we will discuss in further detail later in this article.



Variable Attenuation Mode

While some attenuator products do not perform well at low volume levels (bedroom levels), the PRX150-Pro is able to continue providing a constant tone throughout its attenuation range and into the Variable Attenuation mode. In this mode, the PRX150-Pro avoids the use of a generic L-Pad, which is often used as a variable control in power attenuators. These inexpensive variable L-Pads, cost no more that what a quality potentiometer cost and are designed for home audio use. An L-Pad does allow a wide attenuation range, from full power down to no output (load mode), however, there is a penalty when using this type of device, since the speaker's reactance is affected in negative way. The L-Pad is a dual action, non-reactive device that places one variable resistor in series with the speaker and one variable resistor in parallel with speaker. The resistance across the speaker is adjustable from a fairly low value down to zero ohm, which at this point the speaker is shunted to ground. As you can imagine, as you turn down the power level with this type of devices, the parallel resistance across the speaker is being reduced (eventually shorting out the speaker). This quickly eliminates the speaker's reactance and therefore, in our opinion, severely reduces the dynamics (touch response) and the speaker's ability to produce a natural tone. We have also found that this type of variable L-Pad runs extremely hot for guitar amplifier applications, even when the power level applied is well under their specified power specification. Regardless if a reactive load is used in conjunction with this variable design, the L-Pad will still reduce and eventually eliminate the speaker's reactance.  


With the PRX150-Pro, as you rotate the Variable Attenuation knob, the speaker's reactance is not severely affected; therefore the speaker is able to react naturally. It would have been very easy and less costly to utilize a generic L-Pad for variable control, however, for the sake of preserving tone and dynamics we choose not to.  The PRX150-Pro’s uses our proprietary Variable Attenuation method which does not have a severe impact on the speaker’s reactive headroom, therefore the tone and dynamics remains intact and highly consistent throughout the Step and Variable Attenuation modes.  You will also notice the transparent transition between the Step Attenuation mode and the Variable Attenuation mode. As you turn the PRX150-Pro's continuously variable attenuation knob, you will notice that the attenuation is very linear and its action to be quite smooth, unlike an L-Pad. Most importantly, you will probably not find the need to "tweak" your amp's controls as you attenuate with the PRX150-Pro, since the tone remains very natural throughout the attenuation range.


Note:  The speaker's distortion can contribute to the overall non-attenuated "cranked up" tone and with any power attenuation device; the speaker's distortion is reduced as the power to it is reduced. Our customers have found that when using the PRX150-Pro, they are able to reduce the power to under 1 watt and still maintain a very natural "cranked up" tone with the dynamics intact. Under highly attenuated conditions, what you are hearing is the natural "distortion" from your amplifier, with reduced speaker distortion.



6.  Tone, Dynamics, Speaker Distortion and Hearing Response


In our previous article “Understanding Power Attenuators for Guitar Amplifiers”, we discussed in detail the topics of Tone, Dynamics, Speaker Distortion and Hearing Response.  To summarize these topics:


Tone and Dynamics

Tone can be categorized as: "Transparent" or "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. 

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.


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. 


Speaker Distortion and Hearing Response

There are some unavoidable reasons that tone changes as the volume is decrease.  A couple of main reasons are speaker distortion and hearing response.


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.  However the distortion from many tube amplifiers can sound on their own, even without relying on the speaker’s distortion.   Therefore, the amplifier’s distorted tone can sound just as good at attenuated volume levels.


Our ears perceive different frequencies at different volume levels and 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.    


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.    When we refer to the terms “transparent” or “natural” it is taking into account the exception of the reduced speaker distortion and change in hearing response.



7.  EQ (Equalization)


An EQ is often incorporated into an attenuators design.  Some attenuators offer user EQ controls allowing the user to"tweak" the attenuated tone and some attenuators have a fixed internal EQ circuit with no user controls.  Just because you do not see any  EQ controls does not mean that the attenuator does not have an EQ circuit.


Since a parallel load reduces the speaker’s reactive headroom and therefore affects the tone and dynamics of the amplifier, there are what can be considered "band-aide" approaches to compensate for these deleterious effects.  However, once the tone and dynamics have been altered, it is not easy to restore the amplifier's signal back to its original form. A reactive circuit can be added to the fixed load, which adds some reactance to the amplifier’s load, which in turn can be an improvement over a non-reactive design.  However, the speaker is still loaded down by the parallel load and therefore the tone remains affected.  


An equalizer (EQ) built into the attenuator's circuit can also be considered a "band aide".  The EQ is used to "tweak" the attenuated signal to a more acceptable tone.   An attenuator should reduce the amplifier's signal in a linear fashion, across the entire audio frequency band, while not constricting the audio bandwidth, however, many attenuators do not.  In this situation, the EQ is used to in an attempt to compensate for an attenuator's non-linear attenuation. However an EQ cannot be used to restore a reduction in bandwidth, or to compensate for loading of the speaker (reduced reactive headroom).  


A "cranked up" amplified guitar signal is quite complex with harmonics across the entire audio band.  Once an attenuator alters the amplifier’s signal, it is very difficult for an EQ to restore the signal back to its original form. So while an EQ can be used to "tweak" the signal to a more ear pleasing tone, the tone would not be considered to be transparent by most ears.


A commonly used EQ in an attenuator is a high pass filter, which reduces the low-end response of the signal and emphasizes the mids and highs.  Emphasizing the highs is often more "ear" pleasing when a heavily distorted lead tone is desired.  Some players might not notice or might like the loss of "bottom end" (low frequency response).  However, this type of tone is far from being natural and keen players will notice the loss of "bottom end" and loss of dynamics. 


The PRX150-Pro with its "Speaker Reactance Thru" (SRT) technology does not utilize any EQ circuit nor does not require one.  It attenuates the amplified signal in a very linear fashion across the entire frequency spectrum.  The PRX150-Pro is also a wideband device, allowing the full audio spectrum produced by the tube amplifier to pass through it.  The tone you hear is not altered or "tweaked" by an internal EQ. What you hear is purely the tone generated by your tube amplifier, passing through the attenuator, to the non-loaded speaker.  The tone you hear will be very natural sounding with the dynamics preserved.  You will notice that the full bandwidth of the signal is maintained, with bottom end, mids and highs fully intact.


A word on Speaker Distortion, Hearing Response and EQ

With many attenuators, more is lost than the inevitable perceived differences in tone due to reduced speaker distortion or the way the human ear perceives sound frequencies at different volume levels.  While these factors due come into play when the power is reduced, some use these factors as the main reason for the adverse effect their attenuator has on the tone, when in fact, the attenuation technology used is the main factor.  Some of our customers have mentioned that due to these factors, they previously believed the reduction in tone and dynamics was expected with any power attenuator, as they experienced with their other attenuator(s).  However, after using the PRX150-Pro, they have reported that the tone and dynamics remains very natural at attenuation levels much lower then they thought possible.  The PRX150-Pro shows that with the proper attenuation technology, an attenuator can allow the natural tone of the amplifier to pass through it, therefore there is no reason to EQ the signal.



8.  Limitations of active attenuators using the internal re-amplification technique


The discussion above has primarily focused on passive attenuators using a resistive network to attenuate the amplifier’s power. We will now discuss how active attenuators using the re-amplification technique affect tone and dynamics.


With active attenuators that use the re-amplification technique, the tube amplifier’s entire output power is terminated at the attenuator’s internal dummy load.  A portion of the power is then sampled from the load and is inserted into a cost effective, solid-state amplifier, which then drives the speaker. 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.


The re-amplification attenuation technique utilizes modern technology and many feel this is a highly acceptable form of attenuation. However, there are technical reasons that will not allow this method to provide a truly transparent tone.  


One of reasons is the fact that the guitar speaker is totally isolated from the tube amplifier, as it driven by the solid-state amplifier and not the tube amplifier.  We have already discussed how the complex combined interaction between the tube amplifier and guitar speaker make up the overall tone. However, with this attenuation technique, the speaker and its unique reactance are no longer able to influence the tube amplifier's tone, which in itself alters the original tone as the entire output power from the tube amplifier is terminated into a single load.  Regardless if the load is non-reactive or active, the speaker is completely isolated from the tube amplifier and therefore, therefore the tube amplifier and speaker cannot interact. As mentioned previously, the interaction between the speaker and tube amplifier provides the dynamic response and plays a crucial role in developing the overall tone.  Without the interaction between the speaker and the tube amplifier, the tone and the dynamics will invariably be affected.


The Importance of the Reactance Between the Speaker and the Amplifier

It is well known that different speaker models produce their own unique tone when used with a tube amplifier.  That is one of the reasons that tube amplifiers sound and feel “tubey” and are so different when compared to solid-state amplifiers.  Tube amplifiers are sensitive to the different personalities of each speaker model.  What makes up a speaker’s personality or unique footprint is partially due to its unique impedance curve (reactance).   The speaker’s reactance interacts with the tube amplifier’s output transformer, which in turn influences the load impedance across the tubes.  Thus the speaker is directly involved in forming the overall tone. Not just through is audio reproduction, but also through the reactive, interaction with the tube amplifier.   This is the reason that if you isolate the speaker, as in the case of attenuators that use the “re-amplifying” technique, the tone and dynamics are affected.  The audio speaker and the tube amplifier are not able to interact with each other.

In other words, your speaker's unique reactance, which is part of what creates the speaker's personality, is not playing a role in developing the overall tone as it would normally be doing if it was connected directly to your tube amplifier. The dynamic interaction between your tube amplifier and your speaker does not exist with an active attenuator that uses the re-amplification technology. The active attenuator's solid state amplifier isolates your speaker from your tube amplifier. Your guitar speaker in essence, becomes a P.A. speaker for the attenuator's solid state amplifier.


Impedance Mismatch and Its Effect On Tone

Another reason these types of active attenuators affect the tone is because of mismatched impedance between the tube amplifier and the active attenuator.  Some active attenuators use a dummy load whose resistance value is about two times higher than the typical maximum output impedance found in a tube amplifier, of 16 ohm, therefore there will be an intentional mismatch of impedances.  In this case, the designer allows for a range of impedance mismatching, which is often ranges from 2:1 to 8:1, depending on the tube amplifier’s output impedance.  The load’s resistance value directly affects the impedance the output transformer sees, which in turn, affects the impedance the transformer provides to the tubes and ultimately alters the attenuated tone.

While the impedance mismatch might not be harmful to the tube amplifier, it will affect the tube amplifier's power output, distortion level and the frequency bandwidth. Since the input load impedance is fixed and not selectable, the mismatch will become greater as lower impedance amplifiers are used. The greater the mismatch, the more the bandwidth is affected. In particular, there will be a reduction in the lower frequencies (low end).

For example, if you were to compare the tone between the following scenarios:

a) Amplifier set at 16 ohm and connected to a 16 ohm speaker cabinet

b) Amplifier set at 8 ohm and connected to a 16 ohm speaker cabinet

c) Amplifier set at 4 ohm and connected to a 16 ohm speaker cabinet

You would hear a noticeable difference in tone between the matched and the unmatched conditions. Therefore, anytime there is a mismatch of impedance between the tube amplifier and its load there will be a change in the tone, as in the case of certain attenuators, meaning its tone can no longer be considered transparent.   


Many users feel that re-amplification provides a highly acceptable tone, and we are not here to question that. However, we are pointing out that it is not technically possible for the attenuated tone to be fully transparent for the reasons discussed above.  An attenuator using the “re-amplifying” technique will definitely impact the overall tone and the dynamic response.


While the complete loss of interaction between the tube amplifier and the guitar speaker, and the impedance mismatch are the primary areas of concern when considering an active attenuator that uses the re-amplification technique; there are a few other considerations to keep in mind:


  • Setup is more involved, since they require an AC cord and an additional AC outlet.
  • Due to the use of an internal solid-state amplifier, there are more components that can fail as compared to a passive attenuator. 
  • There can be issues with hum due to a ground loop between the tube amplifier and the attenuator’s internal amplifier.
  • The cost effective, solid-state amplifier driving the speaker instead of the tube amplifier may not be desirable.



9.  Additional PRX150-Pro Advantages


150 Watt Power Rating

The PRX150-Pro is designed to handle a continuous 150 watts and therefore, it is capable of handling the actual power out from most quad amplifiers.  A quad amplifier is defined as an amplifier equipped with 4 power tubes.  These amplifiers will often have a manufacturer's rating of 100 watts, however, the measured fully “cranked” power (volume on max) can be up to 140 watts or even more. We have measured some of our in-house quad, 100 watt amplifiers that do put out nearly 150 watts and ran them flat out into the PRX150-Pro for an extended period of time.    Some power attenuators with a 100 watt rating might not be able to handle the actual power and heat generated by some "stout" 100 watt tube amplifiers.

As part of our standard test procedure, each PRX150-Pro attenuator is tested with a cranked up tube amplifier set at 150 watts. The PRX150-Pro can handle its rated power continuously and it is able to dissipate the generated heat properly.


Keep in mind that the PRX150-Pro is not designed for use with amplifiers equipped with 6 power tubes often labeled as 150 watt amplifiers, since some of these amplifiers are capable of putting out well over 200 watts.



Individual Input and Output Impedance Selection

Exclusive to the PRX150-Pro, is the ability to independently select the attenuator’s input and output impedance.  The user can select from the attenuator's input impedance settings of 2, 4, 8, or 16 ohm to match the amplifier's impedance and separately select from the attenuator's output impedance settings of 2, 4, 8, or 16 ohm to match the speaker's impedance. This allows for 16 impedance combinations and proper impedance matching of dissimilar amplifier and cabinet impedances. With other attenuators, the user is forced to match the input and output impedance, without the possibility of matching dissimilar impedances.    Users that own an amplifier with fixed output impedance will appreciate the PRX150-Pro’s individual input and output impedance switches.  With the PRX150-Pro, the fixed impedance amplifier can be mated with 2, 4, 8, or 16 ohm speaker cabinets.


For example; suppose you have an amplifier with a fixed 4 ohm impedance and you want the attenuated power to be driving a 16 ohm speaker cabinet.  With most other attenuators this is not possible, however, with the PRX150-Pro this is not an issue.  You simply set the attenuator’s input impedance to 4 ohm and separately set the output impedance to 16 ohm.  The SRT technology allows dissimilar impedances to be properly matched, providing the correct load impedance to the tube amplifier.  Unlike other attenuators that force you to match the input and output impedances, the PRX150-Pro has the ability to “mix" and "match" the amplifier and speaker impedances.


The PRX150-Pro is also equipped with two parallel speaker jacks that allow the user to run 2 speaker cabinets.  While other attenuators offer two parallel speaker jacks, the PRX150-Pro has the unique ability to match dissimilar input and output impedances.  When you connect two speaker cabinets of equal impedance in parallel, their combined impedance is half of the single speaker cabinet's impedance value.  


For example; when connecting two 16 ohm speaker cabinets in parallel, the combined impedance value becomes 8 ohm.  If your attenuator is fixed at only 16 ohms, it is no longer compatible for the 8 ohm application.   With the PRX150-Pro you simply change the output impedance from the 16 ohm position to the 8 ohm position.   The input impedance selector can be set from 2, 4, 8 or 16 ohm, depending on the output impedance of the amplifier, which allows the amplifier to see a proper load value. This is not possible with other fixed impedance attenuators.


While on the topic of impedance, it is important for the attenuator to provide a load to the tube amplifier that matches the amplifier’s stated output impedance in order to maintain transparent tone.   There are some attenuators on the market that allow for a certain mismatch between the amplifier's output impedance and the impedance the attenuator is actually providing.   Aside from any possible tube amplifier safety issues, the impedance mismatch affects the impedance the power tubes “see” and therefore changes the damping factor between the amplifier and speaker.  The impedance mismatch reduces the frequency bandwidth from the amplifier and therefore alters the natural tone.  The PRX150-Pro precisely matches the tube amplifiers impedance; therefore, there is no reduction in bandwidth due to mismatched impedance. 



Made in the USA, by ARACOM Amplifiers

The assembly of the PRX150-Pro is not outsourced to another local or overseas manufacturer. The product is manufactured by ARACOM Amplifiers - USA. In doing so, we are able to have full control of the quality of the PRX150-Pro and provide accurate lead times.



10.  Summary of the PRX150-Pro’s Advantages:


*          Utilizing the proprietary, patent pending SRT technology, the PRX150-Pro does not load down the speaker's reactance and allows the speaker and amplifier to react naturally to each other in a transparent manner.


*          The PRX150-Pro maintains proper load impedance to the tube amplifier throughout its attenuation range.  By maintaining proper load impedance, the amplifier’s tone, dynamics and audio bandwidth remains natural.


*          The PRX150-Pro allows for a very wide bandwidth and attenuates the frequency band in a very linear method.


*          The PRX150-Pro’s 150 watt power handling is an advantage over those power attenuators rated at only 100 watts, since many so called “100” watt amplifiers are typically capable of putting out much more than 100 watts, when they are “cranked up”.


*          Equipped with individual, 2, 4, 8, 16 ohm input and 2, 4, 8 and 16 ohm output impedance rotary switches, the PRX150-Pro is able to accommodate dissimilar amplifier and speaker impedances.  This feature allows the PRX150-Pro to be compatible with your entire current and future collection of tube amplifiers and speaker cabinets.


*          Made in the USA, by ARACOM Amplifiers



So Why Did We Spend All This Time Writing This Article?

Many attenuator companies make bold statements about how superior and transparent their attenuated tone is to others, while providing little or no justification.  Many guitarists with a keen ear have commented on how the tone from these same attenuators is contrary to their claims. 


However, we have taken a different approach by providing you information on the PRX150-Pro’s technology as compared to other popular attenuator methods.  In doing so, you now have an understanding on what we strongly believe are advantages the PRX150-Pro has over existing attenuation technologies and you can therefore make an informative decision when selecting a power attenuator. We believe the PRX150-Pro's SRT technology speaks for itself. Instead of making bold claims regarding the tone of our product, we prefer to let our customer’s feedback to do the talking.  Some of our customers have done A/B tests with the most popular passive and active attenuators on the market.  In their response, they have clearly stated that they prefer the transparency, tone and dynamics of the PRX150-Pro over the others, throughout the various levels of attenuation.


Engineered for a high level of transparency

We have mentioned that there is a difference between natural or transparent tone and acceptable or good tone and we discussed the drawbacks of the popular attenuation technologies. In most cases, the technology used, restricts it from being truly transparent, however, the product might still produce acceptable tone and that is an important consideration. Tone is highly subjective and what is considered acceptable or good, varies from guitarist to guitarist. When most people talk about transparent attenuated tone, they often actually mean a tone that is acceptable in their judgment and not necessarily transparent in the true sense. Keep in mind there can be a big difference between what is considered transparent and what is considered acceptable. Transparent attenuation will not color the tone, reduce the signal's bandwidth and will not reduce the dynamics. From a technical perspective, offering an attenuator that provides "acceptable" tone is by far much easier to achieve than a highly "transparent" product.


When you actually compare "acceptable" tone against the non- attenuated tone, you will often realize the attenuator has actually "flattened" or altered the tone and reduced the dynamics by a noticeable amount. At the bottom of this page is a link to our procedure for testing an attenuator's transparency. You might find this procedure useful when comparing power attenuators.


At ARACOM Amplifiers, we weren’t satisfied in merely providing our customers with “acceptable” tone with the PRX150-Pro. Through our years of research, learning and understanding the dynamics between a tube amplifier and a speaker, we knew that in order for an attenuator to provide a natural tone while maintaining its dynamics, the attenuator would have to; not alter the speakers natural reactance (a no load design), have a wide bandwidth with linear attenuation across the entire band and maintain a proper load impedance to the tube amplifier.  While not easy to accomplish, the PRX150-Pro with its SRT technology is able to meet the criteria of these key points.  There is no artificial reactive (speaker emulator) circuit in the PRX150-Pro; since your speaker’s natural reactance is maintained and used.  This allows the non-loaded speaker to produce a very natural attenuated tone.  The PRX150-Pro also does not utilize an internal EQ circuit; therefore the tone is not “tweaked”.  The tone you hear is the signal that has been naturally produced by your tube amplifier, passed through the attenuator and converted to an audio tone by your non-loaded speaker.


Many guitarists intend on using the PRX150-Pro with their “cranked up” tube amplifier, which of course is a very common use for it.  However, players will also find the PRX150 very useful in getting more power tube involvement in their cleaner tones, which can add depth to their gritty clean tones and a greater purity to their lightly overdriven tones, while still maintaining the desired volume.   This is where the PRX150-Pro is 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.  This is a perfect application for the highly transparent PRX150-Pro, since it does not color the sound, or reduce bandwidth, the guitarist can maintain the clarity of the desired tone, while adjusting the overall volume as desired.



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



Related Pages:

PRX150 Testimonials

PRX150-Pro2 vs DAG Product Page

Using the PRX150 attenuator with your tube amplifier
* Please read this before purchasing the PRX150-Pro




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.




Author: Jeff Aragaki
Copyright © 2009 ARACOM Amplifiers (rev. 0809-1)
This article may not be reproduced in any form without written approval from ARACOM Amplifiers.



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