TRANSPARENCY OF A POWER ATTENUATOR
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
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.
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.
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
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.
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.
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
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
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.
PRX150 vs PRX150-DAG Product Page
The PRX150's Advantages
DRX Attenuator Product Page
UNDERSTANDING POWER ATTENUATORS FOR GUITAR AMPLIFIERS
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.