VCOs for home-built modulars - update
gstopp at fibermux.com
gstopp at fibermux.com
Thu Aug 31 03:03:44 CEST 1995
Vacation's over, back to the salt mines....
NATIONAL APP NOTE #299 VCO:
I gave up on this one. Too funky.
Went to the eee-lectronic store today and picked up some CA3140
MOSFET-input op-amps, then stuck 'em into the Elecronotes VCOs as the
The sawtooth VCO runs real nice but won't go below about 9 hertz no
matter what I do. Looks like a good audio range VCO however. High end
is around 60 khz. The sawtooth waveform is perfect.
The tri-square VCO now oscillates from 1/15 hertz to 58 khz in one
sweep of the course frequency pot. The waveforms are picture perfect
all the way except for the sawtooth, which due to the way that it is
derived gets funny lookin' starting at about 18-20 khz. This however
is no big deal and it's real pur-dee from there on down to the lowest
THE CHROMA VCOs:
I got the Exar 4151's in the mail (thanks Ric!) and made a dual
sawtooth VCO per the Chroma schematic. This dude has a sweep range
from around 60 hertz (try to go lower and oscillation stops) to
just above 30 khz. Waveshape ranges from a very jittery
rounded-off rising saw at the low end, to a fairly normal-looking
saw in mid-frequencies, to a half-sawtooth half-square-wave mutant
thing around 15 khz, to a train of narrow pulses around 25-30 khz
before they dwindle away into thin spikes. Now the jitter at the
low end is probably due to the VCOs glitching each other's power
rails since I have their CV inputs ganged together and no bypass
caps at all (yah I know bad design practice). Also the Chroma docs
talk about waveshape being funny and I guess they don't think it's
a big deal since it's a subtractive machine anyway. No tracking
tests were done.
What do I think about this design? It's so similar to the
Electronotes sawtooth VCO that the slightly lower parts count does
not justify the drop in performance and waveshape. This VCO may be
good enough for the bowels of a polysynth, but I'm after
studio-quality modular specs here. Besides if I'm gonna spend the
time, energy, and all that to sit down and burn my fingers with a
soldering iron making some analog monster from hell, at least I
want a QUALITY WAVEFORM analog monster from hell.
Besides I personally believe that waveform precision from
sub-audio to hypersonic has an effect on the sound. Yes this is
subjective and therefore I can get all cranky and argumentative
about it because hey - after all, the less evidence you have for
something the stronger the personal belief, right? Look at
religion. I mean if it were casually proveable on the test bench
by any Tom, Dick, or Harry with an O-scope then who would be
That being said, my vote falls to the Electronotes triangle-square
VCO. To recap, this VCO consists of a TL082 dual op-amp acting as
the CV input summer with a tempco resistor element and exponential
converter reference amp, driving a volts/octave trimpot into an
Analog Devices MAT-03 precision matched PNP pair, which drives a
CA3080 integrator. The cap in the integrator is buffered with a
CA3140 ultra-high input impedance MOSFET op-amp, which then goes
into a Schmidt trigger made from another CA3080. The output of this
Schmidt trigger feeds back into the integrator 3080 to act as the
triangle direction control. The 3140 buffer is itself buffered with
another half-TL082 amp, to form the triangle output buffer. The
Schmidt trigger 3080 output drives a 748 comparator to form the
square wave output buffer. The triangle and square also go to the
sawtooth function generator, which uses an invert/don't invert
method to make a sawtooth wave. Another 748 is used to create a
pulse wave, with another half-TL082 as a pulse width control summer
for pulse width modulation. A "Symmetrized Ramp Modulation" feature
is implemented so that the pulse width control voltages also mutate
the sawtooth shape for a unique timbral modulation. The builder can
make the PWM and saw shape independent to comform more to "normal"
VCO design practices if desired.
The MAT-03 may be replaced with hand-matched 2N3906's. I have not
tried this and therefore can't dictate a procedure to anybody at
this time. However there is a procedure somewhere in Electronotes
and I will try to look for it. Doing this will get you around the
$8.50 or so that these parts cost. Personally I would rather use
the expensive bugger because frankly it's worth it to me.
The tempco resistor can be replaced with a normal resistor during
construction. Eventual swap-out is recommended because then the
VCO will be totally perfect in all respects and heck that's a good
The CA3080 is readily available.
The CA3140 has really high input impedance. This means that when
used as a buffer on the integrator cap the VCO will go down to
really low frequencies. You can use something else but it sure is
neat to have a real low bottom end for modulation purposes.
The 748 is an old, old, uncompensated 741. You may wonder what such
an old non-ideal device is doing here but when you make a Schmidt
trigger with hysteresis out of it I dare you to see the vertical
edges on the square wave it produces with a scope. Once again here
goes my personal belief that the extra extra huge slew rate which
makes for the extra extra sharp edges on the square waves makes a
really profound hollow-sounding square wave. Just my opinion.
The TL082s are real common and better than 1458s. Besides I steal
them from work.
It may seem redundant to buffer the buffer to bring out the
triangle wave. This is done since the 3140's output is used
elsewhere in the VCO and won't drive a short to ground through a
1K resistor and so it should not be available to the outside
world. See the act of plugging and unplugging patch cords in
modular systems causes temporary shorts in the tip to ground and
if you did this with the raw triangle output the VCO guts could
get glitched. My motto is "buffer everything".
The parts I will order and then offer for resale to anybody who
wants them are:
MAT-03 Precision Matched PNP Pair - Analog Devices
CA3080 Transconductance op-amp - Harris, National (LM3080)
CA3140 MOSFET input op-amp - Harris
MC1748 Uncompensated 741 op-amp - Motorola
MPF102 Small signal FET - Motorola
And of course the tempco resistors, for which I am still awaiting
samples for testing. I will be ordering 2 K ohms +3300 ppm/C' since
that is the part specified in the EN schematics. I know that some of
you out there might like 1.87 K ohm values because those are
replacement parts for ARP VCOs among others, but I'm not fixing ARPs
here, I'm building new modules.
More information about the Synth-diy