[sdiy] Preamp circuits

[Magnus Danielson]

David G. Dixon wrote:
> Thanks for that, Magnus.  I think I understand the concept of balanced input
> (differential amp to reject common mode noise, etc.), but given that I just
> have a cheap-ass microphone with a standard "mono" phone jack on it (from my
> kids' karaoke machine, as it happens), how would I bring this in as a
> balanced input?  Would the tip be on input, and the sleeve be the other?
> Then I could choose whether or not to hook the ground to a shield?  

Yes. I think you got the idea.

> Also, I take it you are advocating bringing the signal directly into the
> differential amp.  However, I notice that others are sometimes filtering and
> buffering the differential inputs first.  What are your views on this?

Depends on your needs. If your mike isn't too high-impedance, letting it 
see a pair of 47k resistors (and using 470 k resistors for feedback, 
ground) should work like a charm.

Also, this is a cheap-ass design, right? :)

I assume you have a normal dynamic mike, so it should work fairly well.

If you have an electret mike you might need some buffering, you will 
clearly learn by building. :)

Big-membrane mikes have very high impedances. Seeing 1 Gohm resistors is 
to be expected, but for a standard karaoke-machine... no, not even the 
Japanese does this. Standard dynamic mike is to be expected.

Cheers,
Magnus