VCF Circuit Tests
gstopp at fibermux.com
gstopp at fibermux.com
Fri Sep 29 20:01:59 CET 1995
Hi all,
Being badly bitten by the building bug, I've turned my energies from
VCO's to VCF's this week and have come up with two working filters to
add to my growing pile of pending projects. The first VCF is a
discrete 4-pole Moog ladder, and the second is a state variable as
published in the Electronotes Preferred Circuits Collection.
*********************************************************************
The Moog Ladder VCF:
The Moog ladder VCF is built entirely of discrete components. I lifted
the design straight from the service manual for my Minimoog, which is
as far as I can tell a really old one. The VCF drawing is 1446 Rev. C,
dated 6/14/71. I used all 2N3904's, plus one 2N3906 at the CV summing
node. I picked the Minimoog filter over the Moog modular design
because it uses identical bipolar power supply voltages rather than
the asymmetric +12v/-6v supplies in the modular. The circuit works
fine at +/- 10, +/- 12, and +/- 15 volts. All resistor values are the
same as the original (except for a 91K on the range trim rather than
120K), and all the capacitor values are the same except the
electrolytic values were rounded up to the nearest next available
voltage and capacitance values. There may be a couple other resistor
values to change once I try keyboard tracking at 1v\octave, since I'll
probably be using +\- 15v supplies.
This VCF sounds great. It has the characteristic 4-pole "warm" sound
when no resonance is used, with the decrease in overall signal volume
when resonance is increased, all the way up to the smooth whistle of
self-oscillation.
One mistake I did make during testing - I applied the resonance output
from the output op-amp structure to the top of the resonance pot, and
the wiper of the pot to the resonance input at the bottom of the
transistor ladder, with the bottom of the resonance pot grounded. In
this state the filter acted strangely and the waveform at the output
was very strange. I double-checked the schematic - the resonance pot
is connected as a rheostat, not a voltage divider! I un-grounded the
bottom of the resonance pot and all was fixed.
I would recommend this circuit to any home-builder - it's a definite
classic and it is not really hard to build (no power pins to hook up
on any of the components!).
*********************************************************************
The Electronotes State Variable VCF:
From the Preferred Circuits Collection, VCF Option 1 from EN #71 page
16.
This VCF uses an op-amp CV summer driving an exponential current
source made from a 2N3904 and an AD821 matched PNP pair. Each PNP
transistor in the AD821 drives its own filter pole in the filter guts.
The filter guts has a CA3140 signal input summer, feeding into an
integrator made from a CA3080 and a CA3140 (the first pole). This then
feeds another identical integrator (the second pole). The output of
this goes back around into the signal input summer, completing the
state variable loop. The highpass output is taken from the output of
the signal input summer. The bandpass output is taken from the output
of the first integrator's CA3140. The lowpass output is taken from the
output of the second integrator's CA3140. Each output has its own
buffer op-amp to protect the CA3140s from output loading. A notch
output is created by summing the lowpass output with the highpass
output. The resonance pot is a voltage divider fed by the bandpass
output, with the wiper of the pot feeding back into the signal input
summer. An interesting point is that maximum feedback equals the
lowest resonance, and minimum feedback or none at all equals
self-oscillation, which is the opposite from the 4-pole filter where
higher feedback means higher resonance.
The CA3140's are used due to their extremely high input impedance,
which allows the filter's cutoff to reach well down into the sub-sonic
region, making it useful for control voltage processing (it's D.C.
coupled). This filter also has phase-lead circuitry (a cap in the
dividers at the inputs of the CA3080's) which contribute to the
stability of the filter at very high resonance and high frequency
settings. The CA3140's are also important in their predictable
phase/frequency response. The highest cutoff frequency is around 18
Khz.
This VCF sounds great also. The high resonance settings are
particularly "sproingey" sounding. The lowpass output has a slightly
brighter sound than a 4-pole VCF. The bandpass sounds like a wah-wah
pedal, and the highpass sounds, well, like a highpass filter. The
notch output sounds like a simple phase shifter such as an MXR Phase
90. The best phase shifter sound is when the Q is turned all the way
down. If the Q is increased to high settings, the phase shifter effect
goes away, probably since the notch gets really narrow.
As with all state variable filters I have played with, there is a fine
line between really high resonance settings and oscillation, and
self-oscillation does not happen smoothly but rather gets "triggered"
by something (like a large input transient or rapid change of cutoff
frequency). Once the filter gets "tripped" into self oscillation it
slams hard against the power supply rails and therefore is loud and
clipped (i.e. obnoxious). Perhaps some experimentation should be done
to provide some kind of limiter to the Q feedback to maintain a nice
sine wave.
Anyway this VCF is quite easy to build and performs great. While the
original design uses a tempco resistor and an AD821 matched pair
(which the Analog Devices MAT-03 will replace) in the exponential
converter, I simply used a normal 5% resistor instead of the tempco
and a pair of 2N3906's for the matched pair. In VCF applications this
is much less critical than for VCO's and so I'll probably leave it
this way.
That's it for now,
- Gene
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