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Revision 4 as of 2020-03-14 12:22:02

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location: RWperc

Rhythm Wolf Percussion

[[|Schematic]]

This has three main sections. The first is a standard pinged resonant filter (U13A,B) for the low and high percussion sounds. The second is a noise source gated by a transistor (Q37), and the third is a compressor built around a few opamps (U19,24) and transistors (Q2,17,8). The pinged filters work as they should and sound fine. They respond to higher accents with greater volume. The gated noise source sort of works. It turns the noise on and generally follows the envelope, but it doesnt really shut off. The JFET has a nominal on resistance of 60ohms, so with the 13k drain resistor, that should give -45dB attenuation for "off", which it might do, but that's not really enough. You can definitely still hear it at the ouput, even when the voice is not triggered. This is compounded by the fact that it goes through a compressor, which amplifies it when its "off". If you use the "howl" function, this noise is completely unreasonable. If you have the percussion voice turned up at all, the noise is constantly at the output, and its distorted, so it splatters all across the audible range.

i can not hear the difference between the compressed and uncompressed pinged filters, so i dont think its really worth using the compressor. Its possible the compression depth just needs to be adjusted. This wouldn't suprise me, as its based around a JFET, and they typically have a wide manufacturing spread. The inclusion of JFETs is a bit strange here as they could hvae just used a standard diffpair with current source. The JFET in the noise gate also doesnt really do its job, and they would have been better off using the gating scheme from the hi-hats. If those introduced too much distortion, another diffpair could be used. It suprises me to see a six voice analog synthesizer with no trimmers inside. I have to imagine a bunch of things are just out of spec.

And now on to the electrical oddities:

  1. There is a cap on the mixing pot wiper. Either end of the pot is driven by an opamp, so when its at the extremes it puts a hefty capacitive load on the opamps. This makes them less stable. A series resistor on the wiper would have fixed this.
  2. The gate of Q39 is driven directly by an opamp output. This means that any excursion above 0.6V will destroy the JFET. Normal operation precludes this, but who knows what happens at startup. A series resistor would have fixed this, and reduced capacitive feedthrough on the attack phase. Also the gate resistor should probably go to -Vcc instead of ground.
  3. D54 discharges C126, but there is no return current path, so it only happens once on the first trigger, and then never effects the circuit again. A current drain resistor is need on D54. This pathway is intended to gate the compressor open for the attack phase, but this is already done with the D49 pathway, and could have been replicated with a capacitor in parallel with R34. all in all, U22A is not needed, and the offset for the compressor CV could have been mixed into U19A, along with any combination of envelope/attack only, etc.
  4. U11B is completely redundant, and off in a corner of the board above the digital section. U5A is not used. i'm guessing they originally ran the signal all the way over to the corner and then back to the mixing section, but later decided this was not worth it, and added another opamp (U5), but then did not unroute the old opamp. So you have half an opamp going unused and a signal being double buffered.