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This is a schematic for the ZX sound module:

ZX sound module schematic

My understanding is that the first part is a high-pass filter followed by a non-inverting amplifier.

But isn't the second op-amp a comparator? It seems to me that it would output a HIGH if V- < 0.01*Vcc, or LOW otherwise. So, I am just wondering how it outputs a "0-5V varying output" as described in the note above the schematic.

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The 1st stage not only amplifies the mic input but rectifies it - you get half wave rectified audio superimposed on the 0V rail. There are no diodes for the rectification - the 1st stage relies on the op-amp being biased at 0V and it's output cannot go below 0V therefore the output is 0V for no signal and as signal rises, the output develops positive pulses equivalent to the half cycles of the input signal.

The 2nd stage is a comparator (like you assumed) with a threshold voltage set at about 72mV (1k and 68k on a 5V rail) BUT I think they have the op-amp drawn the wrong way round so I'm going to look a little further into this one.

Aha: -

enter image description here

Pin 6 and Pin 5 are incorrectly shown in their diagram. The above picture is the proper pin-out and the guys making the module have slipped-up in their diagram showing pin 6 as +Vin!!

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  • \$\begingroup\$ Thanks. So the second op-amp is still a comparator, right? Won't the output be a high/low signal and not "0-5 V varying" as they say? \$\endgroup\$ – M-V Jun 4 '13 at 8:12
  • \$\begingroup\$ Like you said - it's a comparator - when the peaks from the 1st stage break thru the 70mV area, pin 7 goes high and returns low as the peaks drop below 70mV. If the input signal is small the transitions above 70mV are infrequent. As amplitude increases, pin 7 spends more and more time trying to raise its output high. C3 slows down the output and holds some energy so that it looks more like a varying dc level on the output. \$\endgroup\$ – Andy aka Jun 4 '13 at 8:16
  • \$\begingroup\$ OK, that's what C3 is for. Thanks very much for taking the time to explain. \$\endgroup\$ – M-V Jun 4 '13 at 8:17
  • \$\begingroup\$ @Andyaka nice catch +1 \$\endgroup\$ – JIm Dearden Jun 4 '13 at 9:02

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