VCF Four Pole 24dB/Oct With VC Resonance
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Features
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Looking for an awesome pre-built filter module?Synthasonic can help. Synthasonic is fabricating fully built modules for this project. So if you like diy but diy with a lot more convenience...
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Important Update
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Thomas White did an excellent job on his Low Pass filter. He even commented "I genuinely enjoy the filter."
He made it feel right at home in his MOTM modular. See more
of Thomas White's gear and projects at his website: natural rhythm music dot com
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Introduction
Voltage Controlled Filters are, in my opinion, one of the coolest modules in any synthesizer. They can subtract or add harmonics from the original signal and dynamically change the harmonic content as they are swept by external control voltages or track the keyboard voltage used to control your oscillators. I love the sound of a couple of oscillators tuned low and close to unison put through a low pass filter with a fair amount of resonance. As you modulate the cut off frequency (also in the low range) it has the sound of an airplane passing overhead or a hoomy singer modulating the frequency content of their droning chant by formant filtering with the mouth and throat. And of course white noise through a slowly modulated resonant filter sounds like you're out on the frozen tundra. This filter includes voltage controllable cut off frequency and resonance (or Q) that gives you more variables to play with as you experiment with sounds from your imagination. Additionally it provides inputs for a three-signal mixer.
VCF Four Pole 24dB/Oct With VC Resonance Sample MP3sInput was two low frequency square waves.
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| VCF 24dB/Oct With VC Resonance PC Board (5.5" x 3.5") |
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| (1) VCF 24dB/Oct With VC Resonance PC Board $20.00 | |
VCF 24dB/Oct With VC Resonance Schematic Page 1 PDF
Resistors changed from 1k to 100 ohms (R27, R29, R30, R32)
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Up to 3 signal inputs can be applied to circuit points AIN1 thru AIN3. The .1uF input caps into
1 meg input resistors reduces the high pass effect that a smaller input resistor would have since
pin 2 of U5-A is a virtual ground. Square waves of relatively low frequency will not differentiate too badly
thus preserving their low frequency content. Signal levels of +/-5V are expected. If you have higher signal levels
then reduce the value of R43 to insure that U5-A is not clipping when you feed in your signals. The opposite would be
true as well (lower levels increase value of R43 to get adequate signal to noise ratio).
U5-A acts as an active mixer with a gain of .39 (with values shown). The output of U5-A is fed into the filter via R22.
The suggested panel layout shows jacks and three pots used as adjustable voltage dividers (level controls) to provide an
input mixer for the unit. The jacks and mixer pots are not shown on the schematic.
This VCF uses LM13700 transconductance amplifiers as voltage-controlled integrators. There are four of them in a chain and they operate in the same way. U2-B's transconductance is controlled by current flowing through R18 to Q3 to ground. It acts like a voltage-controlled resistor that in conjunction with C2 is an RC filter. Since the output of U2-B is a current the signal is actually integrated onto C2. The filtered (actually integrated) signal is buffered by U4-B and fed to the next stage (a portion is also fed back to the input of U2-B via R26). R14 is used to bias the LM13700 linearizing diodes on (which is advertised to reduce distortion through the amp). R52 compensates for the positive offset applied via R14 (yes it goes to -12 but its applied to the inverting input). The goal is to keep the signal path of the system as close to operating about ground as possible. In practice you will see as high as +/-200 to +/-300 mV of offset at any of U4's outputs but that's fine. If things were operating near the rails we would have a problem. The output is finally capacitively coupled to the output so the output signal operates precisely about ground. Each filter section contributes 6dB/octave filtering (thus the four together result in 24dB/octave). The cut-off frequency control voltage inputs (CV1, CV2 and CV3) are applied to U1-A via the 100K input resistors R7, R10, and R11. Control voltage of between -5V and +10V are expected. Initial Cutoff Frequency control (R3) is used to set the initial cut-off frequency. R1 and R6 limit the range of voltage available at the wiper of R3 in the extreme positions. The summed control voltages are inverted by U1-A (inverting amp gain of .02) and fed to trimmer R5. The 20mV per volt output from U1-A can be trimmed to the requisite 18mV/volt by R5 that drives logging transistor Q1 to achieve a 1V/oct response in filter cut-off frequency. Current through Q1 is mirrored by Q2 that controls the current flowing from collector to emitter in Q3, Q4, Q5, and Q6. These transistors (Q3, Q4, Q5, and Q6) control the current flowing from the "amp bias input" pins of the LM13700s to ground via current limiting resistors (R18, R19, R20, and R21). The result is that all four integrators are controlled simultaneously. Since all four of them are tuned to the same cut-off frequency very little signal above the cut-off frequency gets through. This is a very effective filtering technique. The output of the last integrator (U4-A pin 1) is fed via C7 to U5-B (non-inverting gain of 2) amplifier that acts as the output buffer. R39 couples U5-B pin 1's output to FOUT the filter output.
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VCF 24dB/Oct With VC Resonance Schematic Page 2 PDF
Point RA pg.1 (U5-B pin 7) connects to corresponding point RA pg.2 (R61 pin 1) that is the input to gain of 4 inverting amplifier U8-A. U8-A's output is fed into U7-A (LM13700 transconductance amp) that controls the amount of negative feedback applied to the input of the filter (point RB pg.2 connects to corresponding point RB pg.1). Adding negative feedback results in the characteristic low-pass filter ringing that adds interesting harmonics to the original signal. You will also notice that the output signal level will decrease as the feedback is increased. Simultaneously the amplitude of the ringing will increase. This control is referred to as resonance because the harmonic content of the original signal is accentuated when the control is advanced. At maximum feedback the circuit will produce a very pure sine wave that can be used as a pitch source or control voltage. When the resonance is at about mid level and noise is used as the input signal the output will produce pitched noise. The resonance control circuit simply applies from ground to -10 volts to R47 that controls the "amp bias input" of U7-A and thus its transconductance linearly. The resonance control signal XRES is expected to be -5V to +5 volts.
Approx. Current Consumption +12V 32mA -12V 32mA +15V 40mA -15V 45mA
You can use just about any general purpose BIFET opamp for the TL084 (quad) and TL082 (dual) and you can sub any of these (LM13600, NE5517, AU5517, NTE870) for the LM13700. General purpose NPNs can be used for Q1 and Q2 and their PNP counterparts should be used for the PNPs.
As usual I must disclaim any credit regarding the invention of these concepts. The Robert Moogs and Bernie Hutchins of the world figured all of these concepts out. I am merely implementing my version of it. Acknowledgements Page
VCF 24dB/Oct With VC Resonance PCB Parts Layout (Parts Side Shown) PDF
Resistors to change from 1K to 100 ohms (R27, R29, R30, R32)
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The latest rev boards accomodate an 8 pin DIP dual transistor
package or two discrete transistors for Q1 and Q2.
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VCF 24dB/Oct With VC Resonance PCB Bottom Copper (Parts Side Shown)
VCF 24dB/Oct With VC Resonance PCB Top Copper(Parts Side Shown)
VCF 24dB/Oct With VC Resonance PCB Top Silk Screen
VCF 24dB/Oct With VC Resonance Front Panel and Wiring PDF
The jacks and mixer pots shown in this suggested layout are not on the schematic. Use whatever jacks you are using in your system. The suggested mixer pots are 100K linear taper (log taper would also work fine). This suggestion is provided merely as an example or wiring guide.
VCF 24dB/Oct With VC Resonance Project Parts List
I specify all resistors as 1% but 5% will work. Metal film 1% will give you better temperature stability. Capacitors can be film, ceramic, or silver mica. The 100pF integrator caps should be high quality and well matched for best results. You can use just about any general purpose BIFET opamp for the TL084 (quad) and TL082 (dual) and you can sub any of these (LM13600, NE5517, AU5517, NTE870) for the LM13700.
| Qty. | Description | Value | Designators |
|---|---|---|---|
| 3 | LM13700 Dual gm OpAmp(s) | LM13700 | U2, U3, U7 |
| 4 | TL082 Dual Op Amp(s) | TL082 | U1, U5, U6, U8 |
| 1 | TL084 Quad Op Amp | TL084 | U4 |
| 1 | 1N914 Sw. Diode | 1N914 | D1 |
| 2 | 2N3904 NPN Transistor | 2N3904 | Q1, Q2 |
| 4 | 2N3906 PNP Transistor | 2N3906 | Q4, Q6, Q5, Q3 |
| 10 | Ceramic or Film Capacitors | .1uF | C9, C8, C6, C7, C10, C14, C11, C15, C16, C12 |
| 5 | Ceramic or Film Capacitors | 100pF | C4, C5, C3, C1, C2 |
| 2 | Tantalum Capacitors | 10uF @35V | C13, C17 |
| 5 | Linear Taper Potentiometer(s) | 100K | R3, R48 & Mixer Pots |
| 1 | Cermet Trim Pot | 100 ohm | R5 |
| 2 | Resistor 1/4 Watt 1%(s) | 49.9K | R54, R49 |
| 19 | Resistor 1/4 Watt 1%(s) | 100K | R22, R24, R25, R33, R23, R41, R40, R31, R7, R11, R10, R4, R26, R28, R50, R46, R55, R51, R44 |
| 5 | Resistor 1/4 Watt 1%(s) | 10K | R18, R19, R20, R21, R9 |
| 2 | Resistor 1/4 Watt 1%(s) | 120K | R60, R45 |
| 1 | Resistor 1/4 Watt 1% | 150K | R53 |
| 1 | Resistor 1/4 Watt 1%(s) | 1K | R57 |
| 4 | Resistor 1/4 Watt 1%(s) | 100 ohms | R29, R27, R30, R32 |
| 4 | Resistor 1/4 Watt 1%(s) | 1M | R42, R34, R37, R13 |
| 1 | Resistor 1/4 Watt 1% | 200K | R52 |
| 4 | Resistor 1/4 Watt 1%(s) | 20K | R14, R15, R16, R17 |
| 2 | Resistor 1/4 Watt 1%(s) | 2K | R12, R39 |
| 4 | Resistor 1/4 Watt 1%(s) | 30K | R58, R47, R56, R61 |
| 1 | Resistor 1/4 Watt 1% | 33K | R1 |
| 1 | Resistor 1/4 Watt 1% | 390K | R43 |
| 3 | Resistor 1/4 Watt 1%(s) | 39K | R6, R62, R63 |
| 1 | Resistor 1/4 Watt 1% | 4.7K | R59 |
| 1 | Resistor 1/4 Watt 1% | 4.7M | R38 |
| 2 | Resistor 1/4 Watt 1%(s) | 43K | R64, R65 |
| 1 | Resistor 1/4 Watt 1% | 475 ohm | R8 |
| 1 | Resistor 1/4 Watt 1% | 47K | R2 |
| 8 | 1/4" Phone Jacks | Phone Jack | Panel Jacks |
Miscellaneous
- (1) 1/16" Thick aluminum plate for mounting the pots and switches.
- Unit is typically mounted in a synth case with other synth modules.
- Assorted hardware 1" 6-32 nuts and bolts, 1/2" #8 wood screws, etc
- Knobs for potentiometers, wire, solder and typical assorted electronics hand tools.
- Digital Volt Meter and a Signal Tracer or oscilloscope for testing.