Attenuator Construction

I have seen a few posts showing attenuators for the QA40x. I found this attenuator and liked the feature that you can select the attenuation level. One person commented that the LEDs should not be used because they can generate noise. Thoughts on this without the LEDs?

Hi.Certainly an external attenuator to the QA403 can be useful on some occasions, but remember that the QA403 accepts (via its internal attenuator) decidedly wide signals. So before putting in an external attenuator, which will surely introduce noise of various kinds, it is good to make sure that you actually need one. Different discussion is the implementation of an attenuator with 3 resistors (on the load) in order to take advantage of the differential input characteristics of the QA403. You can find such an example at this link :

It appears that the QA403 will handle up to 200 watts (32 dBV). Since I will be evaluating primarily vintage tube amps, very few if any will be over 200 watts. However, I might have need to run evaluations on some solid state amps over 200 watts, so I should probably build an attenuator like the one in It seems pretty easy and straightforward.

Really no need for an attenuator IMHO. You just need to tap off your load when measuring higher power levels and correct for it in the software.


Scott, Since you are mainly analyzing tube amps, you’re probably safe with the assumption that the low side/negative/ grounded/black is not driven. With modern amplifiers, especially class D, that is no longer a safe assumption. Quite a few have both sides driven, or even have the positive grounded and the negative driven on the second channel.

Dale Shirk

Thank you so much for the great advice. I am awaiting cables I ordered and then begins the fun.

Hi all,

I am pretty sure that the LED will create distortions depending on the level.
Btw, if you are looking for attenuators blank boards or filters have a look here:
(4) Analog Filters, Notches, Amps, Attenuators and more | diyAudio
Best regards