QA401 DC voltage at inputs?

In the process of debugging and measuring a circuit of mine, I discovered that my QA401 is leaking DC out of its inputs, which is a bit surprising. The left + input is reading about 0.38V from tip to ground, and the right + input is around -0.19. The - inputs are reading 0. This is with nothing connected to any inputs.

Do I have to send it in, or is this an easy fix I can do here? I’ve had this for quite a few years, so it’s long out of warranty.

Hi @rjmmusic, the input of the QA401 (atten off) is as follows:

BNC to 33uF series cap to 180 ohm series R to 100K shunt, through relay and into + pin on OPA1612.

You can draw out the above on paper and it might help to to understand.

Is the voltage still present if you load the input with a, say, 100K to ground?

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When I turned it on today, I was reading 0.06 on the left input and 0.19 on the right. When I added the load resistor, I saw the voltage slowly fall to 0.

The circuit I’m testing is an early prototype and I discovered a few issues in it at the same time I was trying to debug this DC issue. I have to wonder if DC coming from my project was charging that cap on the input. I’ll look further into that possibility and test again as soon as I can.


What are the capacitors you use for input ? I’m looking for a smd alternative to audio signal path capacitor for Muse ES serie cap.

Hi @Briks, you can swap most any aluminum electrolytic at the input without much issue. Just make sure it meets your needs for DC and input low-pass filter. The two parts used on the QA403 and QA404 for the input caps are Panasonic EEE-1HA4R7NP and Wurth 865250643009. Both are non-polarized 50V 4.7uF.

Hmm, looks like specialized audio cap are not relevant :slight_smile: good news, bye bye muse es, elna silmic etc… :slight_smile: thanks for sharing pn.
What is the reason for bipolar capacitor ?

What is the reason for bipolar capacitor ?

Hi @Briks, generally so that you can probe a positive or negative DC supply to look at noise. If the caps were unipolar, then probing a positive supply would be OK but a negative supply would stress the caps very badly.

Off course, for negative side of signal…

Thanks a lot for explanation!