Ray's WSG Mods

[eonion]

Hi,

I built a Weird Sound Generator a little while ago and I'm struggling to implement Ray's CV Mods that he describes on his website here. They seem so simple and straightforward that I thought I'd have them finished very quickly, but they don't seem to be working properly and I am curious if anyone else had any issues getting them to work. The mods I am referring to are the ones that use JFETs to apply control voltage to the four oscillators.

It seems like whenever I connect one of the JFET circuits to one of the frequency pots as he describes, the waveform of the oscillator changes and becomes a harmonic of the original frequency. This is before I apply any CV at all. When I do add CV, the pitch does modulate as expected, but the waveform still sounds like it's been altered.

Can anyone offer me any insight as to why this might be happening? I really don't think I'm implementing the mod incorrectly since there are only four components and a few connections. It seems like it should be foolproof.

Thanks!

[cygmu]

Clearly this is an old post but perhaps still worth responding...

I've been taking a look at these mods in the last few days as I learn about electronics and build some synths.

From what I understand about JFETs, even with 0 CV applied the JFET channel will be conducting --- the gate needs to drop to some negative voltage, which varies a lot but could be e.g. -4V (with respect to the lower voltage on the channel), before they switch off. So not applying a CV to the JFET gate will still have an impact on the sound.

In the oscillator mod, the JFET sits in the feedback loop and controls how much
current flows around to charge/discharge the capacitor. I think this mod is going to result in asymmetric behaviour so you won't get square waves anymore but pulse waves.

Suppose your 40106 threshold voltages are 4V and 6V, that is, the output goes low when input (i.e. the voltage on the capacitor) hits 6V and goes high when input drops to 4V. If you apply 0V to the JFET, here's how the oscillator works:

1. Suppose the output just went high. Capacitor has 4V, output has 9V, JFET gate has 0V, i.e. 4V less than lower of the other two voltages. The JFET is just about off so the capacitor charges through the pot just as it did before you modded your WSG.

2. If the output just went low, the capacitor is at 6V, the output is at 0V and the JFET is at 0V. The channel is now on and conducting, with quite a low resistance, so the capacitor discharges through both the pot and the 4k7 resistor that forms part of the mod. It will therefore discharge much more quickly than it charged.

Well, that's my understanding anyway. I should build it.

Something that bothers me about these mods is that if you apply any CV to the gate at all, in situation 2 above you will be forward biasing the gate, which you are not supposed to do to a JFET. The gate is protected from too much current inflow by the 300k input resistor, so I suppose the JFET won't burn out, but the design definitely violates everything I've read about using JFETs.

So my conclusions after thinking about (but not building) these mods for a couple of days are:
- the mods will affect the waveform, giving you a pulse wave with >50% duty cycle even with 0 CV applied
- the mods are a bit of a "quick and dirty" design, very much in keeping with the WSG ethos!

I'd be interested to know if anyone can tell me why I'm wrong in the above or if anyone has other experience with these or similar circuits.