Debugging an EA Tremolo

[MikeD]

I've perf-boarded an EA Tremolo, but I'm not getting even near unity gain from the output.  The rate and depth adjustments sound like they are working properly.

I've modded the circuit with a Millenium 2 bypass which is what helped me notice the attenuation of the modified signal.  Also, instead of the 22uF electrolytic cap, I am using two 47uF ceramic capacitors in series.

I've attached a schematic with voltage measurements at the nodes.  The values with a large AC components (ie. within the oscillator) are only approximate as I am only using a DMM to measure the voltages.

I'm thinking the problem might lie with Q1.  The voltage drop between gate and source is only 0.45V, not the 0.6V I was expecting.  I was feeding an input from my digital keyboard and listening to the output while measuring the circuit, and when I measured the voltage at the gate of Q1, the output audio would cut out.

[MikeD]

I had become frustrated and given up on this circuit until I had an idea a couple days ago.

Turns out nothing was wrong with the circuit.  After I tested it on my friend's guitar and hearing the gain go over unity, I realized what the problem was.  I had been using low impedance headphones while testing the circuit by myself, and of course, without a high impedance load on the effect output, I was altering the equivalent resistance of the 4k7 resistor.

To get the headphones to work, I constructed a simple AC non-inverting amplifier with unity gain and a bias of 4.5V, routed the effect output through that, and coupled the unity amplifier to my headphones.  Presto! It works!

Time to package this baby up and tackle the next one!