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by n-eight_Archive
Evanc521 wrote:A few things:I don't understand the purpose of the voltage divider in the power supply... nothing appears to be connected to the half point.Ground (technically the zero-volt reference) is connected to the "half point." This effectively creates a +-4.5VDC bipolar supply for the 4558. Fairly common in pedal designs. The alternative is to bias the signal to +4.5VDC at the input of each opamp with a high-value resistor and run the opamp at 9VEvanc521 wrote:I'm sure there is a good reason for it, but I don't understand the purpose of the parallel resistors (circled in red). A non-standard value of approximately 27.7k must have been needed there. Nowadays, that value is readily available, but certainly wasn't in the 70s when this thing was designed. You could easily substitute if you have a 27.7k resistor sitting around.Evanc521 wrote:The volume control is fairly un-orthodox (circled in green)It's not a typical attenuator circuit. It's more of a variable gain circuit.Evanc521 wrote:Not sure of the value of the "mystery pot{ (circled in blue).It's serving a very similar function as the "volume" pot for the previous gain stage, but in a variable resistor configuration. A multi-turn 50k or 100k trimmer would probably be appropriate to dial in the desired amount of gain. For instance, you could set it to unity when "volume" is fully CW, though a few dB of boost would probably be more useful.