The ARP/Rhodes Chroma VCO
By Gene Stopp <gene@ixiacom.com>This article, originally part of a longer post to Analogue Heaven or DIY, is reprinted here with kind permission from the author.
I was able to get the Chroma VCO to work properly. Let me just say that this VCO is a lot more sensitive to solder flux effects than the others for some reason (charge pump maybe?).
This design uses a 3046 NPN transistor array to charge a cap in an integrator which gets monitored by an Exar 4151 charge pump IC which detects the sawtooth peak and dumps a charge back into the integrator cap to reset the sawtooth. A tempco resistor is used in the Chroma design at the input to the exponential converter - I did my tests with a regular metal film type, and as with the tri-square VCO there was no problem with drift in a stable environment. Useable frequency range is from a couple hertz up to over 20 khz. Tracking is quite good over the entire audio range.
The sawtooth shape has a problem, such that the start of the rising ramp has a step in it which starts out small at low frequencies and eats more and more into the ramp at higher fequencies until most of the ramp is gone leaving narrow pulses at frequencies just above 20 Khz. This step is proportional to the charge pump's finite cap discharge time. This waveform glitch means that this VCO is not really useable as the basis of a general-purpose multi-waveform VCO. About the only other wave you can get from this sawtooth is a pulse, however the pulse width modulation will be limited to a range starting at the sawtooth peak and working down in reference voltage to the half-way point on the sawtooth ramp, because if you go lower than this you run the danger of hitting the step and that will cause a jump in pulse width. Also this "danger zone" will get bigger as the frequency goes up, but it will at the same time get less important to the sound since the harmonics will start leaving the audio range.
In a good general-purpose VCO it is desireable to allow the PWM voltage to approach the triangle (or well-formed saw) from outside its amplitude range, sweep through the range, and then go beyond the range once again on the other side. The sound of this is of course silence, followed by narrow pulses, through a square wave, back to narrow pulses, back to silence. This is what you want in modular systems. The Chroma VCO, however, only needs to go from narrow pulses to a square wave in the "protected" environment of the guts of a polysynth.
However the point must be made that this VCO has a very low parts count. In the Chroma schematic there are two VCOs sharing the NPN transistors in a single 3046, plus dual op-amps that share functions as well. I copied this on a vectorboard and ended up with two VCOs that use a total of six IC's in less room than one of the EN tri-square VCOs. I did the rat's nest thing again and plugged the VCOs into my modular Moog. Here are my thoughts:
- The sawtooth sounds fine in the range where waveshape-related timbre is most important (low audio to mid audio).
- The VCOs track very well over the entire tuneable range
- Multiple VCOs (four or more) tracking each other with sawtooth waveforms is a really bitchin sound
Therefore I am thinking of building a module that provides multiple VCOs that track together providing only sawtooth waveforms, kind of like a limited version of the modular Moog's 921A Oscillator Driver / 921B Oscillator group. This module will take up minimal panel space, just enough for tuning knobs, a couple of FM inputs, and the sawtooth outputs.
Regarding the solder flux thing: when I built my first EN tri-square VCO, it worked great even without de-fluxing, except below about ten hertz. I de-fluxed the circuit and it became perfect at all frequencies. When I first built the Chroma dual VCO, I cleaned off the solder side with a pencil brush and isopropyl alcohol. Then I performed my tests, with less than total success. Then when I built the second tri-square VCO it had really rotten waveforms up to about 50 hertz. After checking all components and connections, I decided that solder flux may be the problem so I really blasted it with industrial stuff. It was better, but not perfect. The next day I looked again and it was performing even better than the first tri-square VCO! Knowing all of this I then really blasted the Chroma VCO circuit, then dried it with a heat gun, and sure enough, it straightened right up performance-wise. So here's some guidelines:
- Build the VCO
- De-flux it with a good industrial de-fluxing solvent (available at electronics stores in spray cans). Be wasteful.
- The temperature drop caused by spray can de-pressurization will cause moisture to condense on the circuit, so that it will not perform well right away.
- Either let the circuit board dry completely (about a day) or force the issue with a heat gun, hair dryer, hot sunlight, etc.
Oh yeah before I finish I should mention that I modified the VCOs to add hard sync inputs. I did not use the sync circuit as shown in the Chroma schematics, however. The Chroma uses some additional transistors to provide an additional discharge path to VCO 2's integrator. As I was looking in the databook for the 4151 I noticed that the sawtooth peak is established by a comparator reference pin that is tied to +5 volts in the Chroma, but was created by a 20K-10K resistor divider in my circuit since I am only using +/- 15v supplies. Well I just added a cap to this reference point to allow an external edge to change the 4151 discharge reference, thereby forcing early reset of the sawtooth. Works great.
Thanks to Ric Miller for the 4151's he sent me. I think he said he had access to a whole bunch of these for real cheap. 
