Ok, I've posted a PDF of my current efforts at
http://joshy.redmartian.org/opamp.pdf

I'd love any and all comments on my design choices, component values,
etc. Some things in particular I'm unsure of: I believe I've set both
amplification stages to 10x. A signal running through both stages
would be amplified by 100x then, correct? I haven't added the input
series caps. Nigel's calculations show a value of 1uF. Would I want
this to be polarized or non polarized? Do the 22pF caps across the
opamps need to be present on all the opamps, or just the two leftmost
ones? Does the mic loading resistor look ok?

In regards to the question about my input source, this will vary. I
would guess that for the most part I will be using dynamic mics,
professional ones, but low output level. There is the chance for a
higher output model at some point in the future. The mics I will
likely be using won't neccesarily be designed for ambient micing, so I
might have to boost their signal quite a bit. Would it make sense to
bump up the gain a bit more? I was thinking of taking the two stages
up to about 12x each.

Thanks!

Josh
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On Tue, 24 Aug 2004 09:13:03 +0100, Nigel Orr
<nigel@axoninstruments.co.uk> wrote:
> You still haven't said what sort of mics you are using and exactly what you
> are expecting to record, but I don't think that's enough gain, unless they
> are super-cheap high impedance or super-cheap electret mics.  Can you
> connect one into a 'scope and see what the typical signals are?  Or test it
> through an existing mixer?
>
> If these are cheap high-output mics, or you only need to get up to -10dB
> consumer audio recording level, and the source is fairly loud, 40dB may be
> adequate.
>
> > If I use DC blocking series caps, how do I figure out the value? I
> > will also have a 2K resistor across the put (in parallel with the
> > 22pF cap).
>
> The 2 C's go along with the two 10K resistors (ignoring mic impedance) to
> make a filter with rolloff frequency (3dB down) at
> f = 1/(2*pi*R*C)
>
> So a 1uF cap would set a rolloff at about 8Hz.

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