Ken Stone’s Dual CMOS filters, a.k.a. “Twin Wasp” VCF.

Here’s a link to Ken’s description of the board.

Continuing the squeeze-it-into-1U-with-small-knobs approach, I designed this Front Panel Express panel for the Twin Wasp.  Key to the design is a miniature rotary switch, Mouser part #633-MRK-112A.  It’s single-pole, twelve-position, configurable to limit the number of positions to your needs.  At over twelve bucks a pop, it’s pricey.  But at least the knob is included.  The switch mounts in a quarter-inch diameter hole and even could fit in a slightly smaller hole, the bushing is that tiny.

Compared with the Bridechamber 3U Panel, the only features I had to give up were the inverted CV inputs and the spread CV, which is an external modification.  And mine is 1/3 the size!  I have created similar 1U panel designs for a dual CGS Steiner VCF and a dual CGS Serge 1973 VCF.  But I probably will not make those, because I have so many filters as it is.

The construction pic follows.  You need nimble, steady fingers to lay-solder the five wires to the itty bitty PC-mount legs of the rotary switch.  This filter uses four of the obsolete CA3080 OTA chips, the heart of so many 1970′s synthesizer circuits.  I was happy to finally use some of my stash of 50 that I’ve been hoarding for a few years.

I made one circuit change after taking the photo here. The FREQ pot sets the initial frequency. Ken Stone’s wiring diagram showed the CCW lug going to ground. But it really needs to go to -15V. And the summing resistor should be 270K or 300K, instead of the 100K on the schematic. I just added a 200K resistor in series with the wire going to the center lug on the pot.

I have to learn how to use this filter now!

From Ken Stone’s CGS website:

This is a licensed adaptation of the classic Serge DTG/DUSG

The Serge VCS module released in conjunction with from bananalogue.com is an extremely versatile control voltage generator and audio source. In the early 1970s, Serge Tcherepnin developed the Positive Slew and Negative Slew modules for the original Serge synthesizer. In time these merged into the classic Dual Universal Slope Generator. The Bananalogue VCS and CGS75 are an adaptation of Serge’s original circuit with a few new features.

The VCS is a unity gain voltage follower. The rising and falling slopes are independently and jointly voltage controllable over a wide range.

VC Transient Envelope Generator

A pulse at the trigger input will start the envelope, or a gate input will sustain the level and the envelope will fall when the gate goes low. Rise and fall are independently and jointly voltage controllable, with variable linear and exponential wave shapes.

VC Portamento

Voltage is slewed according to the rise and fall times.

VC LFO

When the cycle switch is thrown, the trigger input is connected internally to the end trigger output, creating a VC clock with variable waveform and independent rise and fall times.

VC Oscillator

While not as wide ranged, or accurate as a dedicated oscillator module, the VCS is still an excellent audio source. The Exp CV input is scaled approximately to the 1v/oct standard. The Output wave can be swept from triangle to saw with linear and non-linear waveforms. End Out also produces a pulse waveform.

VC Non-Linear Audio Processor (Low-Pass Gate)

If an audio rate signal is slewed, the module responds like a VCF, and a rough VCA. The signal is low-pass filtered down to silence, similar to a low-pass gate.

Envelope Follower

Positive and negative peak detection envelope follower.

VC Pulse Delay

Trigger input starts the envelope and a trigger will be produced again at the End Out when the envelope completes its cycle.

Sub-Harmonic Generator

If a series of triggers are applied to the VCS faster than the total rise and fall times, the module will divide the incoming signal by a whole number. In the audio range the output will be the sub-harmonic series.

I used the Bridechamber panel and a 2-pot Bridechamber bracket which fits the board out of the box.  I am especially happy to have these, because I am lucky enough to have used a Serge in the seventies and experienced the Negative and Positive Slew modules and the Dual Univeral Slope Generator.

I took the trouble to gain-match the NPN and PNP transistors in the lag processor, using a simple technique I found in Electronotes.  The 2N3904 pair are matched with each other and likewise for the 2N3906 pair.  Don’t know how important this is, but it couldn’t hurt.

Here’s a photo of my VCS assembly.  I ran a #20 stranded wire out from the power connector to the ground bus on the jacks.  I used twisted pair for most of the inputs and output, grounding the black wire back only at the jack, since there are but two grounds available on the board and those are dedicated to pots.

Quantity: 1 in a 1U-wide panel

From Ken Stone’s website:

This module is a much expanded version of the Psycho LFO, featuring six free-running oscillators, each variable between LFO and audio ranges, two of which can be switched to have triangular wave outputs. Each oscillator can be switched between low and high ranges, as well as off, and also has a rate LED, to allow visual determination of the frequency at which it is running. As per the original, there are also level and glide controls.

This module can be used to create a range of pseudo-random modulation voltages that can be used to generate unusual sequences, control VCFs or or VCAs etc. When run at higher frequencies, it can be used as a series of audio drones, or to generate complex sounds by frequency modulation a VCO.

Panel:

This is the second module I’ve built using the 8×2 MOTM jack spacing and small pots and knobs. You can download this Front Panel Express Panel.  This format works perfectly for the Psycho LFO, which I had never built because I did not wish to devote 2U of panel space to it, which would be required if standard 1-inch diameter MOTM knobs were used.

Modifications:

There wasn’t room on the panel for Triangle/Square switches for the two oscillators with triangles, and because the triangles impart an undesirable DC offset when switched off, I chose to omit the triangle option and make all six oscillators use square waves. I substituted 2K2 resistors for the specified 1K for the LED current. I could have used 5K6, because the LEDs came from an original Blacet Miniwave. They’re too bright! I’ll probably change them.

Circuit comment:

How many modes of operation does the Super Psycho LFO have?  It has six identical oscillators, each of which can be Off, Low Frequency or High Frequency.  Do the math:  There are 21 different combinations, I think, between one Low Frequency oscillator on to one High Frequency oscillator on.

Here’s the construction. The LEDs are not glued, but are held in place by the wiring harness.

The empty spot on the lower right of the PC board is where the IC for the triangle waves is omitted.  You can  see the two jumpers hard-wiring the square wave option.

Here’s how it looks in the cabinet with the Blacet Miniwave on the left and the CGS Slope Detectors in the same panel format to the right, next to the MOTM-650.

The slope detector is an event-driven gate/trigger generating device. It monitors a control voltage, and responds according to what that voltage is doing. If the voltage is rising, the slope detector gives a “gate” output (approx. 5V) on its “rising” output. Likewise, if the voltage is falling, the slope detector gives an output on its “falling” output. The duration of this gate signal depends on the incoming CV and the setting of the sensitivity pot. When the CV is remaining constant, the “steady” output activates.

These gate signals can be used to fire envelope generators dependent on the action of the CV, for example opening a filter when a melody is rising in pitch, and closing it when the melody is falling in pitch. It can also be used to generate gate signals from LFOs, envelope generators, to advance sequencers etc.

The sensitivity pot allows the slope detector to work with either relatively fast events, like the edges of a square wave pulse train, or very slow events, such as the sine output of an LFO being used to drive filter/phaser sweeps.

This is a dual module, housed in a Front Panel Express Panel of my own design. It’s the first of four modules planned in this style, which has sixteen equally spaced positions (8 x 2) where pots, jacks, switches, or LEDs can be placed. The vertical spacing between components is one inch and the horizontal 0.85 inches, which is the same as MOTM jack spacing. The use of 16mm Alpha pots and 3/4-inch Alco knobs allows the placement of pots anywhere. This is the design insight and also the main compromise. The Slope Detector has one pot, three LEDs and four jacks, perfectly fitting two of them into a 1U panel.  The red LED comes in a chrome bezel, Lumex PN: SSI-LXR4815ID.  SSI-LXR4815YD and SSI-LXR4815GD are yellow and green. I use green to indicate positive voltages, so I chose it to indicate Rising.

Modifications:  I changed the voltage divider resistors on the outputs to be 3K3/1K8 instead of the original 1K8/1K to reduce current draw.  I use these values for all of my digital outputs.  RLED is 2K2 ohms.

The 3-jack bracket from Bridechamber allows mounting the PC boards adjacent to each other.  (Two additional holes need to be drilled.)

Here it is in the cabinet, next to a standard MOTM format module. The most unusual thing is the LEDs located in what’s normally the jack region. I think the smaller knob fits in well.

The Power Supply Delay is a support module of sorts. Due to rather heavy usage of bypass capacitors in the synth and diy scene (a good thing) some shortcomings of commercial power supplies have come to light. The most frequent is the failure of one or both power rails to come up at power-on, caused by the initial demand of all the bypass capacitors. The power supplies sense this as an overload and shut down.

If modules were brought on-line in groups instead of all at once, the power-up surges would be limited to what the power supply can handle. Of course, once a group of modules has started, its current demands are less, allowing ample capacity for the supply to start another group of modules.

The Power Supply Delay does exactly that. After a predetermined time, it closes a set of relay contacts, powering the modules connected to it.

I built the CGS-63 with two 5-volt relays, taking advantage of the 5-volt power supply. The CGS-63 receives power input from a 6-pin connector and cable from the MOTM-990 power distribution board.  The purpose is to allow full utilization of the +/-15V supplies, should I encounter the start-up issue with a particular load-out of the cabinet.  It also adds five 4-pin power connectors and two 6-pin connectors, all of which are powered after the delay time, which is adjusted to about 500 milliseconds.

From Ken Stone’s CGS website:

This VCO started out life as a replacement for the original VCOs in my ’73 Serge, so it replicates a lot of the functionality of that module. Of course there have been numerous enhancements to the design as well, making it right at home in a 1 volt per octave synthesizer. The core chosen is the well known Electronotes ENS-76 VCO Option 1 as used in the ASM-1, Modulus, etc. This core was an obvious choice due to the amount of information, lists of suitable substitutes and so on that are available on the web.

Modifications:

• Custom 1U Front-Panel-Express panel
• Sawtooth wave output is +/-5V
• Pulse wave output comparator DC coupled to sawtooth
• High quality parts throughout
• Some resistor value changes

Check out the CGS VCO Project page.

From Ken Stone’s CGS website:

This module is a “tribute” module, based on the awesome Steiner-Parker Synthacon VCF. Those who know me will know I’m not a big VCF fan. Nonetheless, this VCF really appeals to me. Its sound is quite unlike the Moog ladder, has a lot of character.

How to use this module:

Connect the CV input to a voltage source such as a keyboard, envelope generator or sequencer. Connect the output to a VCA or amplifier. Feed the signal to be filtered into the high-pass, band-pass or low-pass input.

Unlike the original, this version allows signals to be fed into all inputs simultaneously. If the same signal is used in all inputs, the result is reminiscent of a phaser. The real fun starts when you feed different signals into each input, then you get a frequency based “interpolating scanner”, where panning between different sound sources is possible, though also subject to the frequency at which they are running. I have never heard an effect like it before.

Modifications:
Front-Panel-Express Panel
Some resistor value changes in the VC input section:

From Ken Stone’s CGS website:

This is a tube based Voltage Controlled Amplifier / timbral gate. While this module basically operates as a VCA, it does add a degree of distortion to the signal. How much distortion depends on how hard it is driven. Add feedback and it begins to oscillate, synchronizing to the incoming signal to some extent. All this while running on a standard synthesizer power supply, with no extra heater supply required.

Modifications:
Custom Front Panel Express panel
Changed output gain resistor to 49K to add oomph.

Each module has two audio inputs with attenuator pots. The second input is normally connected to the output of the VCA for feedback. This module produces a warm “tube” distortion, because it actually uses a tube! Shielded cabling is used for all audio paths.

The mounting brackets are from:
Scott Deyo
The Bridechamber
www.bridechamber.com

DEMOS

These two demos use both the CGS-65 and the MOTM-190 VCA. We hear a sine wave from a MOTM-300 VCO. Both VCA’s are controlled by the same envelope generator. The first demo starts out with 30 seconds of MOTM-190. It fades to the CGS-65 with the input at a low level (IN 1 knob at 1.5), yielding hardly any distortion. As the IN 1 knob is gradually turned up to about 7, the distortion increases. Then IN 2 is turned up, adding feedback, and the distortion goes wild. After about 5 on the dial subharmonics start kicking in. As the envelope sweeps there there is a really nice effect as the subharmonics ring. Then the MOTM-190 is mixed back in, layering the fundamental, which resonates with the harmonics. Finally the CGS-65 is faded out, leaving only the 190 again at the end.

Now double it all by adding a second MOTM-300, CGS-65 and MOTM-190. There’s still only one envelope generator driving it all. Pitches are courtesy of the CGS Infinite Melody quantized by two Blacet Miniwaves.

From Ken Stone’s CGS website:

The idea for this project came from the fabled middle section of the Serge wave multipliers. … The circuit I came up with I would describe as a “reflector”, and is ingeniously simple. Going on it’s performance, I would guess it to be very close to Serge’s original design.

In addition to this multiplier, there is a second simple multiplier created by adding lag to the feedback path of an op-amp. The results are remeniscent of a well known tube modulator.

For the primary multiplier, the “folder”, connect the input to the triangle wave output of a VCO. Connect a LFO, envelope generator or even a DC voltage to the folds input. The result will be a harmonically rich signal at the “FOLD” output. A second input based on a lag circuit allows square waves and other hard-edged waveforms to be used as the signal source.

There is also a “PULSE” output available, with “pwm” inputs to further vary the posibilities.

To use the second multiplier, the “grinder”, feed the input from the output of a VCO. Adjusting the “drive” and “lag” pots will give variation to the output signal.

Obviously both multipliers can also be used to mangle control voltages, the outputs from LFOs etc.

Modifications:

Stooge Panel

Some parts changes:

• At IC4a change 47K to 100K to double the PWM CV sensitivity.
• At IC4b pin 7 use a 3K3/1K8 divider to change the PULSE OUT level to +/-5V.
• At IC2b pin 6 change 100K to 33K for FOLD CV input to increase sensitivity.
• At IC1a add a 100pf capacitor between pins 1 and 2 to eliminate parasitic oscillation in the GRINDER.
• Add a 1K resistor to ground at GRINDER OUT to cut the amplitude in half, as it can clip at the power rails.

From Ken Stone’s CGS website:

This is a strange mixture of a VCO and a switched capacitor filter based on the filter presented by Jan Hall in Electronotes EN92 P14-15, which was in turn based on an idea in a 1974 issue of Electronic Design on a Biquart (twice the order) filter.

A VCO drives a pair of analog switches, switching two banks of eight 0.047uF (47nF) capacitors across op-amps (effectively creating multiple integrators), and this a multi-passband or comb filter is created at f0, f1, f2 etc.. The overall filter configuration itself should be easily recognized. Two controls are provided within the structure of the filter – damping and bandwidth. Not all combinations of these two are actually valid, some resulting in silence, but none the less, quite an array of variations is possible.

Modifications:

Added a CGS-56 Pulse Buffer to make the 4 sub-octaves available to the front panel.

Housed in this this Front-Panel-Express design.

Here’s a demo MP3 of the Bi-N-Tic as a VCO.

You are hearing the Bi-N-Tic ouput through a VCA. No other oscillators or filters are used. The SUB 1 output is fed back to the IN jack through another VCA driven by an LFO. SUB 2 is fed back via the SUB pot. A random voltage generator is making a little melody, using the 1V/OCT input. It’s quantized for scale tones. Over the course of two minutes I twiddled the Damping, Bandwidth and Sub pots to demonstate a variety of timbres.