Capacitor in opamp non-inverting audio amplifier

Question

In an opamp circuit for audio purposes like the following, I'd like to know about the capacitor C1:

• Which type is the best in this application (polyester, Al electrolytic, tantalum, bi-polar electrolytic?)
• If polarized, what is the correct polarity in the circuit and why?

^{simulate this circuit – Schematic created using CircuitLab}

The choice of electrolytic capacitor is justified because it needs a large value if you want to keep a good lower cutoff frequency (in the example, about 70Hz) and low resistor values to reduce noise; but I'd like to know if there are troubles with the polarization (and maybe the distortion due to non-ideality.)

Moreover, is it correct that C1 reduces op-amp offset effects at the output?

Answer 1

In principle, a polarized electrolytic capacitor acting as a "decoupling capacitor", can be connected to a supply rail instead to the ground. This solves the polarity problem but the noise will be higher.

Answer 2

The circuit acts like a high pass filter, as only higher frequencies cause a current flow through the capacitor, meaning that the gain is scaled based on the frequency, it seems to be about a 60z high pass filter

at lower frequencies the gain is small, at higher frequencies it tends towards a gain of 50,

The capacitor will need to survive potentially +-15V to cover all use cases, ideally for an audio signal it only sees mV on average

I would recommend a bipolar electrolytic,

It does not remove the input offset of the op amp, but does reduce it, at the trade off of much higher DC distortion, however for audio this is not too bad of an issue.

Answer 3

Finally, I found a very similar circuit here: https://sound-au.com/articles/reverb.htm I wrote to the author, who kindly replied me, addressing my doubts.

Using polarised electrolytic caps only works if the voltage across the capacitor (AC and/or DC) remains below 0.5V (but the desing guarantees 0.1V maximum). This has always been the case, and relies on the voltage being below that which causes any reverse current.

A non-polarised cap can be used, but they are larger and more expensive. The audible difference is zero.