On 7/19/06, Dave Tweed wrote: > 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. > It really depends on what's in the preamplifer in the microphone. > 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. I bet it's been done -- those little microphones are used in some cost-sensitive applications. > 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. Can you elaborate on this? This would be related to the op-amp's input offset current? Or are you referring to "motorboating" when there is also a power amplifier on the supply? > 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. His first circuit used a much lower value. Do you remember how many volts DC were across your mic? Regards, Mark markrages@gmail -- You think that it is a secret, but it never has been one. - fortune cookie -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist