>Ok, I've been reading up and planning. Just a bit of a recap, I'm
>trying to design a compact, single supply audio mixer with one
>balanced microphone and one line level input. I'll duplicate the
>circuit so I end up with a stereo mixer. This isn't super hi-fi. It'll
>be used for some live recordings, but nothing for an album, mainly
>just for archiving. I can tolerate some noise, as long as it isn't
>overly noticable.
>
>I've broken the project down to be able to deal with things a bit
>easier. I'm workin on the balanced microphone input. I was looking at
>the SSM2135, and in the datasheet
>(http://www.analog.com/UploadedFiles/Data_Sheets/527742802SSM2135_0.pdf)
>there is an example circuit of a balanced line receiver on page 10.
>I've been looking at it more, and it mentions a line impedence of 20K.
>Standard balanced microphones are more along the lines of 600 ohms.
>So, I wonder if it's possible to modify this circuit for operation at
>that lower impedence.
>
>Another option is to use an SSM2135 as a standard differential
>amplifier (like fig 4.18 page 185 in Art of Electronics). The circuit
>in the datasheet mentions something about input impedance changing
>with drive levels in a standard differential amp. Is this something I
>need to worry about?
>
>

> But do have a look at the classic 3-amplifier "instrumentation amplifier"
> circuit. It has some advantages and you can also vary gain with a single
> resistor. Generally speaking, it is easier to play with gain, passband
> shaping etc after the input stage and optimise things in the input stage
> which are unable to be well addressed elsewhere.
>
> Note that the 2 amplifier version usually has inferior common mode rejection
> performance across frequency due to unbalanced  delays in the two signal
> paths. The 3 amplifier version does not have that particular problem.
>
> Here's a good introductory tutorial - this plus the next few pages take you
> from resistive summers to the traditional 3-amplifier 'instrumentation
> amplifier". looks like the rest of the course may be worthwhile as well.
>
>         http://www.allaboutcircuits.com/vol_3/chpt_8/8.html
>
> Note that varying Rgain on the cct on page
> www.allaboutcircuits.com/vol_3/chpt_8/10.html
> gives you single resistor gain control. Remove Rgain and the overall
> amplifier gain is unity - which can be useful. The circuit is in fact often
> drawn without Rgain present At that stage it becomes VERY familiar - it's
> the one amplifier instrumentation amplifier with two input buffers - obvious
> but brilliant :-). (Note that it can have gain with Rgain removed if the
> resistors per side are unequal - just as the single amplifier cct has gain)
>
> Also note that the input buffers remove the dependence of gain on input
> impedance.

> Typical gain you will need is 20 to 40 db.
>
> Do not use a potentiometer where you suggested. Run your differential amp
> at 20 db fixed gain, put the pot afterwards as a voltage divider, and
> follow that with a single opamp with 20 db more gain.
>
> Using input coupling capacitors gives you a small measure of protection
> against DC into your opamps.
>
> You do need to set a DC level on the inputs of the opamps - pull them down
> with resistors. 10K is fine.
>
> And load the mic with 1 or 2k.

> 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.

> 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.

> Just well over 1.0 load for a few days :-)
>
> You can run more gain if you like. If you are recording quiet things you
> will need it.
>
> I usually like to put something in to protect the inputs from static
> discharge, and also something to roll off RF.
>
> Someone posted a PAIA mic preamp schematic a few days ago that had a
> couple of caps and four diodes. That's the kind of thing you need to put
> in.
>
> I notice you have a 22 pf cap from inverting to non-inverting input on two
> opamps. Unless your opamp needs it for stability, stay away from stuff
> like that.