I'm a bit new to electronics, so this may be a dumb question, though I hope not.
I'm working on a pre-amp circuit which can drive several TDA7492 boards from a single source (I've found they don't enjoy all being connected directly to some sources)
The preamp is a single supply design, with a gentle gain to boost the 1.25v line level from my Pi to "professional" +4dB level (These boards can take up to 3.6v peak-peak) to maximise the signal, followed by the amps themselves and a unity-gain stage to supply an isolated aux-out for my friend's poweramp.
It works nicely in PartSim, though I haven't decided exactly on some of the component values for the gain: http://partsim.com/simulator/#38256
The coupling caps will be polyester, and I've arranged so that I can use low values, which are within budget. Higher value polys get expensive very quickly. I've had all the expected issues with version 1.0 of the amp where I very mistakenly used Tantalums. I understand now why these are very bad - inductance and ESR.
My problem is with C1 in the circuit. What C1 does is block DC and thus sets the DC gain to unity, which keeps the DC offset at 1/2 supply whatever AC gain is applied. Without this, the gain pushes the offset up and can clip the waveform.
I need the feedback resistance to be low overall, as I've read that this is good for low distortion - I assume the distortion would be due to thermal noise in the feedback circuit. This means I need a high value cap here so that the filtering effect of R1+R2 & C1 doesn't lower the gain in the audio range. The best I can achieve is the 220uF you see here which drops the output by 10% between 20 and 50Hz. I can live with this.
But my problem is understanding whether C1 is in the signal path. I can see arguments both ways - the signal doesn't really flow through it, so probably not. But if I used an electrolytic here and it didn't pass the signal cleanly, would this in fact create distortion before the capacitor and pass the noise in to the inverting input, either affecting the gain or even amplifying the distortion?
If I can use an electrolytic, that's great. If I can't, I've got a problem with a high enough value poly cap being extremely expensive (But perhaps I can compromise here - can anyone suggest a more appropriate, inexpensive capacitor type?)
Thanks very much in advance! :-)