Another VCO that works
gstopp at fibermux.com
gstopp at fibermux.com
Wed Sep 20 20:03:15 CEST 1995
Hi all,
As you may remember a couple weeks ago I was concentrating on the EN
tri-square VCO as my primary home-built VCO option. Well now that I've
made two and am waiting for the parts for more (and for list members),
I've gone back to look at the runner-ups.
I was able to get the Chroma VCO to work properly. Yes, it was cockpit
error. 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, as pointed out before, 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:
1. Build the VCO
2. De-flux it with a good industrial de-fluxing solvent (available at
electronics stores in spray cans). Be wasteful.
3. 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.
4. 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 (right, Ric?).
Later,
- Gene
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