I have a little mic + pre-amp board and a little class D amp board. mic output -> 20nF cap -> audio + input on the amp and audio - input is tied to ground.

Where do I introduce a potentiometer to control the output volume?

If I put the microphone output on a voltage divider to ground, if it ever goes 100% to ground would that short out the pre-amp?

Both the mic pre-amp and the class D amp have little tiny gain trimpots, but I would like something more user accessible.

[in the diagram below I used op-amp symbols for both the pre-amp and the class D amp, and class D amp has differential output.]


simulate this circuit – Schematic created using CircuitLab


enter image description here

The best place to put a volume control is before the class d type amplifier

  • \$\begingroup\$ @m salim. Good circuit in terms of the 2 capacitors to isolate possible DC components from active devices to ground. Possible concern as to who chose & why, the 20nF amp input capacitor. That original cap could indicate a Hi-Z amp input if amp maker chose the cap value, or if it's J Halcres choice because combined with a lower Z amp input it becomes a component of a Hi-pass CR filter ("R" is the input Z of the amp) to remove boomy mic sound. J Halcres well-advised to keep with 47K pot and if amp output sounds "tinny" then increase value of only the added cap at output of preamp to fix \$\endgroup\$ – Stan H Oct 12 '17 at 15:50
  • \$\begingroup\$ @m salim. I upvoted you because the pot value was a sure bet to be suitable and the 2 caps as you know completed a truly competent circuit. I blathered on about the original 20nF cap in my first comment because its purpose was unclear (DC decoupling or frequency shaping or both) because I didn't know "who, why & how" it got into the original circuit. Its purpose does not detract from my comment on your circuit in the slightest. \$\endgroup\$ – Stan H Oct 12 '17 at 16:07
  • \$\begingroup\$ As the original coupling capacitor was 20nF and you now have two in series, you should double the value of each, at least. \$\endgroup\$ – user207421 Oct 12 '17 at 17:25
  • \$\begingroup\$ the totally honest answer is the cap is there because i tried a bunch of different caps in different places and found that the circuit seemed to work better with that cap there. ¯_(ツ)_/¯ \$\endgroup\$ – J Halcres Oct 13 '17 at 4:07

The best place to put a volume control is probably on the output of the preamp stage before the coupling capacitor. You would connect the preamp output to one side, the other to ground and the wiper would connect to your coupling capacitor.

Value wise, a 10k logarithmic pot would be a pretty safe choice but you could probably use anything from 1k-500k. Lower values will require more current from the preamp and higher values will have more noise.


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