What kind of capacitors should I use to couple audio? I'm dealing with line level 1Vp-p audio, with a 1V dc offset (typ.) I want to convert this into 1Vp-p AC, at up to 20 kHz. My first version used ceramic 10u 10V caps - a simulation showed attenuation by about 15% at 20 kHz.
Update: I eventually went for 10u 6.3V. I did not need Hi-Fi quality and the fact that 10u 6.3V caps were available in 0603 packages was useful for me.
When you are using capacitors to couple a circuit you need to be worrying about the low frequencies. A coupling capacitor is by definition a high pass circuit.
The larger the capacitor value you chose, the lower your cut off frequency will be on the high pass circuit. Wikipedia shows an example circuit as well as how to pick your capacitor value in respect to your desired cut off frequency and resistance.
As for the type of capacitor, since audio is AC you will need to have a non-polarized capacitor. In my experience, I have never found anything better then a ceramic capacitor for this application.
Ceramic capacitors can be microphonic and might introduce distortion, mylar or polycarbonate would be better.
Considering the specific application (1V p-p, 1V "offset" - I'm assuming offset in this case means the output is 1V above the ground reference), there's no reason to worry about any audible distortion or microphonics; these only come into play with high voltage swings and is, allegedly, part of the "mojo" in Trainwreck guitar amps, which have a ceramic in parallel with a poly cap coupling the first two gain stages with about 300VDC at the plate and a signal that can exceed 200V p-p passing through the cap. At line levels, your MLCC should be sonically transparent.
In most line-level coupling applications, anything over 1uF is usually overkill; if your application is anything close to typical, 10uF will pass frequencies down into the decihertz range.
I too am curious about the 20KHz rolloff in your simulation. Perhaps there's some decoupling between the signal and one of the rails that you haven't accounted for, like the output impedance of the amplifier itself? A 1 ohm resistance at the output would give a corner frequency of about 16KHz with a 10uF capacitor...which really isn't a bad thing, since most humans' hearing is also attenuated above 16KHz. If you really needed to pass everything from subsonic to ultrasonic, you probably want something closer to 4.7uF for the coupling capacitor and a higher impedance input for the next amplifier.
