DC Blocking Cap Values

[Inventor]

Howdee, I'm working on a miniature synthesizer and I want to AC couple all the inputs and outputs. I'm planning to just put a ceramic 0.22uF cap on each of the inputs and outputs.

The system is +5V supply so I'm putting 1M Ohm dividers off of Vcc to Gnd after the caps to rebias them where appropriate, or using artificial ground where appropriate. For outputs I just use the cap with no resistor.

1. Do you like this approach?
2. Is 1M too big? How about 100k?
3. is 0.22uF too small?

I don't want to use electrolytics or tantalums because of the polarization.

4. what is the biggest reasonably available ceramic cap value?

Thanks in advance.

[DrJustice]

I'm no expert on these things, but a quick calculation of the RC filter:
fc = 1 / (2* PI * R * C)
where R = 1 / ((1/Rbias) + (1/Rload))
using Rbias = 1M, Rload = 10k and C = 0.22µ
gives fc = 73Hz.

That's a bit too high cutoff (3dB point) for general audio, and gets worse with more loading. I don't think you get much bigger ceramic caps. If your system is single rail you can use a polarized cap since the output does not swing 'across' ground. If the output swings across ground, you can use two polarized caps back to back (IIRC). You can also get big polypropylene caps - they're said to be good for audio, although expensive (may not matter for a one off synth).

DJ
Edit: typo
--

[blue hell]

Ceramics can get quite large, think uF's, nowadays, but they are not very linear then.

[Inventor]

OK, here's what I'm-a-gonna do: Use back-to-back polarized caps where interface requires, and convert the whole system to DC coupling, eliminating the caps altogether (at Jan's suggestion)! Should work great, thanks guys. EChucK is emerging!