Mark Rages wrote: > On 7/19/06, Dave Tweed wrote: > > Because the gain on the two paths (inverting vs. noninverting) isn't > > anywhere close to being equal. > > > > Isn't that obvious? > > Ah, not to me. > > If you can make the noise at the mic half the noise at the supply > (with judicious selection of R6, perhaps Jinx's trimpot) then the > noise at the output is still half the noise at the supply. It doesn't > get amplified at all. OK, I see where you're heading with this. I'll grant you that each opamp stage will amplify common-mode signals with a net gain of 1. And that if you can balance the dynamic resistance of the microphone with its load resistor under all operating conditions, you should get the supply noise as a common-mode signal at the input of the first stage. But any imbalance at the input of the first stage (which is what I was alluding to) will get amplified just like the desired microphone signal, and I can't believe anyone would actually rely on this in practice. It also leaves you open to positive feedback through the power supply, especially at high frequencies, which can cause all kinds of hard to find problems. I once built a circuit very similar to this -- an electret microphone capsule feeding a sequence of about 6 opamp stages (some were filters, some were straight gain), combined with some digital processing (CMOS logic and a microprocessor). I got the best results in terms of noise and stability by filtering both the opamp bias node (just like Olin suggested here) and the microphone supply (by splitting its load/bias resistor as someone else has suggested). As a side note, the OP should double-check the value of that bias resistor relative to the specs of his microphone capsule. I remember using a value about an order of magnitude less (i.e., about 10x the operating current through the microphone). I was using the cheap Panasonic capsules from Digi-Key, with generally very good results. -- Dave Tweed -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist