How to choose a capacitor for filtering an audio signal?

Question

I've recently become interested in analog synthesizers and have successfully built this VCO from MFOS.

The VCO outputs a signal that varies from 0 to about 10 or 12V DC at 28mA. I want to bring this down to (roughly) line level so I can record it without blowing up all my stuff.

1. Using Ohm's law I figure that a 20Ω resistor should bring the voltage down to about 0.5V. Does that sound right?

2. I'll need to use a capacitor to filter out the DC bias of the signal (right?) How do I calculate what capacitance value I need?

Edit:

Here's a link to the schematic for the main part of the oscillator. The output I want to tap (at least at first) is the sine wave output next to R49 (near the bottom right.)

Answer 1

For a DC blocking cap, the impedance of the cap should be small relative to other impedances over the intended frequency range. For example, hi-fi audio goes down to 20 Hz. For some headroom, it's a good idea to design your circuits with high pass filter rolloffs at 10 Hz or so.

The impedance magnitude of a capacitor is 1 / 2πfC, with f in Hz, C in Farads, and the result in Ohms. When this roughly equals whatever impedance is in series with the cap at 10 Hz, then it will pass 20 Hz and up well enough to be considered hi-fi.

For example, if 1 kΩ will be in series with the cap, then it needs to be about 16 µF.