Wavelord R7 and Optical Tremolo Volume Normalization

[benny_profane]

What is the purpose of the resistor in series with the optocoupler resistance element (R7)?

I'm revisiting a Cherry Pie (Twin Peaks) harmonic tremolo build to retrofit the TAPLFO3 module and noticed that that circuit does not include the series resistor. Since the different modes have different volume levels, I was considering making the level trim a panel control. While I have the build disassembled, would it make sense to include a series resistor? If so, what should I consider while choosing a value. I currently have VTL5C1 optocouplers installed.

[madbean]

Since most opto devices have a range of "on" resistances under full brightness, R7 guarantees some threshold for the maximum gain allowed from the IC1B output stage (rather than relying on whatever on resistance a particular opto might have). It's not strictly necessary. If you do use a series resistor there you just don't want to make it too large to ensure you can get enough volume output @ IC1B and also the largest pulse possible when the trem depth is max. To large and some signal may creep through in-between the pulses.

[benny_profane]

Okay, thank you—that makes sense. So, the 5C9 has considerably lower ON resistance than the 5C1 at comparable current levels and that must be taken into consideration. I know that the circuit architecture is slightly different between the two projects since the Cherry Pie has two opto-resistor lines for the different frequencies (i.e., Bass and Treble) and a different biasing scheme for the two LED elements. I'm having difficulty getting unity volume for the full-frequency mode (i.e., normal volume tremolo) with the DEPTH set at 0 (like the Wavelord doc describes for the level trim procedure). With the Cherry Pie, is there any way to have the normal mode not have a volume drop without either boosting the signal substantially in the treble- or bass-modulated modes or using a panel control for the level trim?