welcome: please sign in

Revision 9 as of 2020-04-04 00:42:27

Clear message
location: RWhihat

Rhythm Wolf Hi-Hats

Overview

Original schematic

The RW utilizes both an analog and digital noise generator, and the tune doesnt actually tune anything, and rather sets the mix of these two. R118,113 limit the extent of this mix, and the relative volume of each is quite different. The digital noise is around 5Vpp max, whereas the analog is 500mVpp or so. There are two lowpass filters on the analog noise (C19,60) and the digital noise has a resonant lowpass and a regular RC lowpass (C48,54,67). This combined noise source goes to two pretty standard drum 'VCAs' (Q19,39 - one each for open/closed). The envelopes for these are generated with the same gate/accent setup as the bass synth. The envelope's peak value tracks the accent voltage. The decay time is half for the closed hat. The envelopes also go to comparators to shut off the digital noise source. The output of the VCAs go to a sallen-key highpass filter, using a single transistor. The output then goes to a mixer stage and the volume pot.

I find it odd that the digital noise source is connected to digital powersupplies. This means any noise in the digital side will be coupled into the hihat outputs. The amount of current drawn by the HC14 is really low, so i'm not sure it would have been splattering noise onto the analog supplies if it was connected to them. The other thing is that the comparators do not work as intended here. Since there are diodes and capacitors in the VCAs which hold their charge after the envelope is off, the emitters of the envelope buffers sometimes do not get down below the threshold to shut off the comparator. A pulldown resistor is needed at the emitters to ground. i'm not sure this matters much, though. This section could be modified to apply a sort of 'mute' to the voices, so they end more abruptly. This might be good for the closed hat. Another oddity is that as the decay knob is turned down, it loads down the charging circuit, and the envelope actually gets completely eliminated, so the voice pretty much stops sounding at really low settings. R208,202 could be increased to basically set a more reasonable limit on the decay time.

All in all, i like the sound of this, and fixing the decay times is a pretty simple mod. i would like to be able to alter the pitch in some way, and i think the easiest way would be to alter the supply voltage to the HC14. These can run from 2V to 5.5V, and the pitch of the oscillators change with the supply voltage, as the hysteresis increases with increasing supply voltage. A little modulation on it might be nice as well. If i felt that the mute function was needed, then this voltage would be applied to the emitter of Q72, otherwise it would be easier to just remove Q72 and apply it directly to the supply pin of the HC14.

Mods

Schematic

i tried a number of different mods, and ended up using a small set of them. The ones i kept were as follows:

  1. Minimum decay time - change R208 to 22k and R202 to 4.7k. This is very much to your taste, so i would suggest trying a few different values to see how they sound.
  2. Tune knob rework - i eliminated the analog noise, as it didnt sound as good as the digital noise, and repurposed the tune knob to tune the digital noise. This is accomplished by varying the voltage to the 74AHC14 IC that creates the digital noise. To do this, remove Q72 and replace with an NPN transistor. The trace the 3.3V rail does not need to be cut if that pad is no longer used. Instead, run a trace to the +12V supply. i connected up at the comparator on the backside of the board, but any +12V point would do. Remove R363 and place a 1uF capacitor between the base of the transistor and digital ground. Digital ground can be found closest at C177. If you are not planning on doing pitch bending, i would suggest a 10uF capacitor instead, as it will be more stable. Run a wire to the wiper of P10.
  3. To rework the pot, remove R118 and replace it with a 10k to +12V. Remove R113 and replace it with a 4.7k to ground. You can use a smaller value for the 4.7k, but it might not work. i installed a 3.9k, and it sounded great for a while, but then the digital noise started to cut out at low pot settings. The 74AHC14 is only rated down to 2V, and a 3.9k supplied 1.5V or so. i'm not sure if it was the low voltage or something else i had done that messed it up. A 3.6k is the lowest that will work at all. This will allow a wide frequency sweep. You can reduce the 10k value to get higher frequencies, but it seemed high enough to me at 10k.