Output DC on a GGG Mini Mixer

[remmy]

I am messing about with the GGG Mini Mixer circuit and I have noticed there is DC on the output, with the stock 2.2uF output cap (C5) there was 45mV, this started at around 50mV and slowly decreased.

I tried a few other values and the most successful were between 47nF and 220nF at around 1.6mV-1.8mV.

Is it possible with this circuit to get it down to 0mV?  Is so what would need to be altered other than the output cap?

[PRR]

At turn-on, the opamp pins jump up to +4.5VDC. C5 and R12 "eventually" bring the box output to "zero" VDC.

"Eventually" really means "infinite time". The C5 R12 time constant is 2.2 Seconds. After 2.2 seconds the 4.5V will be decayed by 63%, to 1.7V; after 4.4 seconds to 0.6V; after 6.6 seconds to 0.23V. 0.011V after 13 seconds.

That's for perfect caps. All caps leak a little. The output DC never really goes to "zero".

The output DC will be less with a smaller R12 resistor, or the loading of the next box in the chain.

NO audio input should mind a tenth-Volt DC on its input. Your 50mV is very typical.

> 2.2uF .... 220nF

2.2u is likely to be Electrolytic; 220n is probably Film. Electro leaks more.

BUT: the value also sets your Low-Frequency cut-off (the point you define to decide Audio from DC). While 220nFd is ample for the internal 1Meg load, you may be wanting to drive modern mixers, which may be 10K load. 220nFd into 10K is a 75hz cutoff, which is not really "full range". Line level outputs usually want caps over 1uFd, and most-often they ARE electrolytic, despite the small DC leakage.

And leave it on overnight. Electro caps are "self healing", and some hours with voltage applied might reduce their leakage appreciably.

If your next-stage is unusually allergic to DC, you should do something about that (small high quality input cap, DC cancellation) rather than rely on your sources to always be DC-free.

[bool]

In other words:

Drop the 1Meg resistor after that cap to something more sensible - like 100K.

The circuit seems choke full of 100K's - that 1Meg seems out of place and is the obvious culprit for too-slow discharge (and hence the DC offset at the output).

[remmy]

Thank you for the information, I have a better understanding of what is going on with the output DC now, I changed the R12 to a 100K and the output DC was down to 0.1mV.