I have an audio streaming device I bought that has analog audio output that is way too low (it’s supposedly line level, and I’m not the only one who’s complained about this).
I’ve been using this as an excuse to learn about TOSLINK and DACs, but that isn’t the question here. As an alternative to an external DAC I decided to see if a simple pre-amp would work.
And it does… for a little while. But after a short time (minutes or hours), some sort of bias voltage starts to appear on the positive input pin of both amps (this is inside the DC blocking cap - on the positive input pin of the chip itself). This, of course, gets amplified and biases the output so much that there's no room for the audio. This happens with or without input or load and seems to degenerate over time.
The output load of this preamp is 10 kΩ - it's a power amp I also built that has been in service for many years. Its input starts with a log taper 10 kΩ pot across the input with the wiper going to the input of the amplifier circuit itself (one each with a common shaft for each channel).
At no point do I see any anomalies in either the positive or negative supply rails. So far as I can tell, the circuit draws negligible supply current.
All of the caps are 0805 ceramic X5R with a 10V rating. Could that have something to do with it?
Again, right after construction, the circuit sounds just fine.
What's going on?