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The purpose of this procedure is to test a power attenuator for transparency.   Many attenuators claim to be transparent, but just how transparent are they?   The test will allow you to make a side-by-side audio comparison between the non-attenuated and the attenuated tone.   You will be able to judge how much the attenuator is coloring or flattening the tone, reducing the audio bandwidth or decreasing the dynamics (touch sensitivity).


The idea is to setup your tube amplifier at a comfortable pre-attenuated volume level and compare the non-attenuated tone against the attenuated tone.  To conduct this procedure, you will be switching the amplifier’s signal between non-attenuated and attenuated.  The test is easier to accomplish with an attenuator that has a bypass switch.   The bypass switch will need to be of the “True Bypass” type.  If the attenuator does not have a bypass switch, then you will need to manually bypass the entire attenuator, each time by plugging the amp directly into the speaker cabinet.   By setting the amplifier’s non-attenuated volume at a tolerable level, you are able to quickly switch between and compare the non-attenuated tone and attenuated tone, without blowing your ears out.   With this method any differences in tone between bypassed and not bypassed will become obvious. 


The problem with “cranking up” your amplifier and going straight into the attenuated mode, like most players do, is your ears quickly become accustomed to the attenuated tone, and you have no reference to really judge if the tone is transparent or not.  You will only be able to judge how well you like the attenuated tone and not know the attenuator’s transparency level.  However, by conducting this test you will be able to judge the attenuator for its transparency.


The test can be done with a tube amplifier in a “clean” mode or an overdriven mode.  If in an overdriven mode, then preferably with an amplifier that has a master volume control, so you can set the overall non-attenuated volume level.   You will be comparing the audio bandwidth of the non-attenuated and attenuated tone, therefore, you should ensure that you amplifier is adjusted to cover the frequency spectrum from lows to highs.  Keep in mind, you are conducting this procedure to test for transparency and not (yet) the actual “cranked up” playing conditions.   The “bypassed” volume should be at a comfortable level and not at an extreme volume level.  At high volume levels, our ears will naturally try to compress the high volume levels and your ears will be less sensitive to conduct this test.  Testing the attenuator under actual playing conditions can be done later. 




The Procedure:


1.      Connect the tube amplifier, attenuator and speaker as usual.  Set the attenuator in the bypass mode.  If the bypass switch is separate from the attenuation controls, then also set the attenuator to so that there is between 3 to 6 dB of attenuation. If the attenuation control is a rotary step switch and if the first step is less than 3 db of attenuation, then selct the next level of attenuation. If the attenuator only has a variable attenuation control, then rotate the control so that there is a fairly noticeable amount of attenuation.  


2.      Adjust the amp for a clean tone or overdrive tone.  We suggest you start with the clean tone first.  You can try the opposite tone once you have completed the first test.  If your amp has a master volume control, you can use it if desired.  Set the amplifier so that it is at a comfortable volume level.  Ensure that you amplifier is adjusted to cover the widest frequency spectrum from lows to highs. 


3.      While still in the bypass mode, play some chords and riffs and listen to the overall tone.  Pay particular attention to the extent of the highs and lows.  Also notice how dynamically the amplifier and speaker respond to your touch.


4.      Stop playing and quickly switch the attenuator from bypass to attenuate.  Then play the same chords and riffs.  Compare the non-attenuated tone verses the attenuated tone.   Ignore the fact that in the attenuate mode, there will be a reduced volume level. With many attenuators you can immediately hear the impact the attenuator has on the tone and dynamics. 


Keep in mind, that you should stop playing when switching between the modes (no sound from the amplifier), in order to reduce the stress to the attenuator’s bypass switch. 


Play in intervals and switch between the bypass mode and the attenuate mode.  The more transparent the attenuator is, the closer the bypassed and attenuate tone will be, with the exception of the volume change.  Also with a highly transparent attenuator, you will not notice much or any difference in the dynamic response (touch response).  However, with a majority of attenuators you will hear a difference in the tone and in the dynamics.   The bandwidth can become narrower (less highs and/or lows) or shifted to one side of the spectrum, the tone also might feel flat, duller or less driven and you will often feel a reduction in how the amplifier responds to your touch.  


5.      Now that you have tested the attenuator for transparency at the low attenuation mode (high volume mode), you can now check the tone and dynamics at various attenuation levels. You will probably have to turn up the amplifier's volume a bit, when you get into the higher attenuation modes.    As you switch through the different attenuation levels, carefully listen for any differences in tone and dynamics.  Play chords and riffs that allow you to check the extent of the highs and lows.  See if the amplifier has become less responsive to your touch.   You should to go back to the bypass mode once in a while in order to refresh your ears on what the non-attenuated tone sounds like.  Some attenuators will further reduce the quality of the tone and dynamics as the attenuation level is increased (power is reduced). With a lot of attenuators, going beyond the -10dB level is where the noticeable change in tone and dynamics occurs.





1.         As mentioned earlier, this is easier to accomplish with an attenuator that has a bypass switch, where you can immediately compare the bypassed and attenuated tones.   You will need to check to see if the bypass switch is a “true bypass” that completely bypasses the attenuator’s circuit. 


An attenuator that does not have a bypass switch makes this test a bit more difficult, since it takes time to manually bypass it.  There is no issue in doing the comparison this way; however, it is sometimes not easy to completely remember the previous tone.


2.         Many of you want to conduct this test with an overdriven tone, however your “cranked up” non-master volume controlled amplifier is just too loud in the bypass mode, as would be most cases.  You can conduct this test by using a distortion pedal to create the overdriven tone and then back down on the amplifier’s volume.  Just ensure the pedal is not restricting the frequency bandwidth.  If it does you will not be able to hear the effect the attenuator has on bandwidth reduction.



Related Pages:

Related Pages:

PRX150 vs PRX150-DAG Product Page

The PRX150's Advantages

DRX Attenuator Product Page





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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