The preamp/mixer has been working very well, with a small mod to add a clip detector. This is a bidirectional led, which is really two red LEDs in one package, connected via a couple of resistors and a capacitor across the output. They start to flash when the peak voltage at the output reaches 1.8V.
I shunted R5 with a 10uF capacitor to reduce the AC feedback via the DC (biassing) feedback path.
I have reduced the maximum gain of the vocal inputs from 46dB to 40dB. And changed the instrument inputs to use mic’s rather than pickups.
We don’t play anywhere where we have to “kiss the mic” so close miking for us is with the mic around 3 inches from the mouth.
The busking amp can manage around 10V peak so if we turn the busking amp gain up to a gain of around 15 (5.5dB or 35 on the volume pot) then the LEDs indicate clipping on both stages.
But then we found that the Mandolin does not sound near as good with the pickup compared to a microphone. So we decided to ditch the plug ins and go with a fully miked setup for close miking. The vocal mics are an EV257A and a Behringer XM5000. Both have an impedance of 150 ohms, and, from my measurements both have a similar sensitivity.
The instrument mics are WeyMic WM75s, cheap SM75 lookalikes. They have 600 ohm impedance and seem half as sensitive as the vocal mics. They are mounted on the mic stands are around 4 inches away from their instrument.
The lower sensitivity and higher impedance was problematic. Ideally the input impedance should be greater than 10x the mic impedance, or 6K. But that would make the gain lower than the vocal mics and I needed about twice the signal.
To get over 6K input impedance and a gain of around 200 would involve quite a radical redesign, and I would probably not start with this very simple circuit. Anyway I compromised with a 2K2 input resitance. The vocal mics now have a 4K7 input resistance. It seems to work fine.
This is our normal mode of operation. The mics are set roughly 9 inches from our mouths and equidistant to the instruments. We turn the busking amp up full then adjust the preamp gain until it sounds “right” or we get feedback. Clipping is not an issue in this mode.
The mic’s output is around 5mV p-p. The final maximum output of the busking amp is around 20V p-p so we need a total gain of 4000 or 72dB. The busking amp itself provided 32dB so we only need another 40db (or x100) from the preamp. Hence the change.