> Off-topic: Just listened to the "spider in the bag" piezo preamp clip
> too. Sounds very clean and I'm intrigued about the JFETless nature of
> it. I've tried several circuits with "low noise" opamps and all were
> much noisier than JFETs. (My learning curve is vertical too, but it
> tends to get stuck at a lower latitude!)
Zach,
Noise is never off-topic. The idea of "noise impedance" is often
misunderstood. I can't find a simple explanation of "noise impedance" except
that it gives the lowest overall noise if the mic impedance is matched to
the input stage noise impedance. This is a different parameter to input
impedance.
Low noise preamps are not easy to design, hence the cost can be substantial.
It depends on the type of on-chip amplifier used and the feedback circuit.
Mic transformers are out of fashion, but they are another way of matching a
low impedance into a higher noise impedance input.
http://www.maximintegrated.com/en/app-notes/index.mvp/id/3169
calls it the "optimum source impedance" and if anyone can put this paper
into plain English, I'd be grateful. :-)
Dave Brinicombe
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