This is the Resonant Gate designed by Craig Lee, based on the original low pass gate Don Buchla circuit.  The complete discussion is on Muff’s message board.

There have been many variations on the low pass gate.  All share the central concept of using a Vactrol in the signal path as an amplitude controller.  Various options provide for different levels of feedback, or resonance.  Most low pass gates make us of a manual switch to select the mode of operation.  This one is different in that it features a voltage-controlled feedback, which is labelled Mode.  The mode is variable from no feedback at all to feedback large enough to make the circuit self-oscillate.  The Mode pot sets the initial level of resonance, which can be added to or subtracted from by an external Mode CV through a reversible attenuator.  The initial frequency is set by a pot I labelled ‘FR’ for frequency.  (This is labelled ‘Offset’ on the schematic diagram.)  External CV of frequency also uses a reversible attenuator.

My panel uses the ‘small knob’ MOTM grid format, which allows me to fit two identical resonant gates into a single 1U wide panel.  This choice required me to omit the second CV input that the PC board provides.

My Modification

A problem was reported about the resonance circuit.  It turned out that due to a characteristic of the Vactrol response, the resonance level could fall off over time.  (See the full discussion, linked above.)  I solved this for myself with a simple modification that set the minimum resonance to greater than zero.  This means that the plain VCA mode is no longer available, and there is still some resonance at the lowest setting of the Mode pot.  That’s OK with me.  I like some resonance in the module at all times.  Here’s the backside of the completed module:

My modification consisted of determining a parallel resistor for the resonance Vactrol.  I described the procedure for finding the value in the post listed above.  The end result is a resistor soldered across the Vactrol leads on the top of the board.  Detail:

Finally, here’s a short demo of the resonant gate. It is filtering a triangle wave. A MOTM-800 envelope generator is hitting the CV. The demo starts out with the lowest resonance with the gate all the way open. Then I bring up the CV and play around with the controls a little.

Be sure to look at the Toppobrillo page that describes the module.  Here’s a photo of my build.

This is my build of the J3RK Buchla 258J VCO board, as discussed on Muff’s message board:

Board thread

Build and mod thread

I put it into my famous 1U 16-position MOTM grid panel.  Here’s the Front Panel Express file.

Buchla 258J VCO FPE panel

This is a music VCO with 1V/octave tracking over several octaves and a temperature compensated exponential voltage converter.  I would say its most notable features are a voltage-controlled wave shape, and a very nice linear FM.  The schematic diagram provided by Mark Verbos matches pretty closely to the PC board.

My build and modifications

• The builder decides between mixing a square or sawtooth wave with the sine wave in the wave shaper.  The choice can be made by a selection of resistors at build time or by adding a rather complicated switch.  For simplicity I chose the sawtooth version.
• The PC board offers three outputs:  the main wave shape output, a triangle and a square wave output.  These outputs are roughly 5 volts peak-to-peak.  I added a quad op amp to the board to buffer these outputs and also the sine wave output.  The buffers have gain to bring the levels to a standard 10 volt p-p level.  (I’ll add the buffer schematic later.)
• The PC board has frequency control voltage summing for a coarse tune pot, two 1V/octave inputs, and one reversible CV input (which can easily be doubled to two inputs).  My exponential CV inputs include a 1V/octave input jack, one attenuated FM input, one reversible CV input, and also a fine tune pot for which I had to add a 4M7 resistor to the board.
• For the linear FM section I selected a 1uf polyester film capacitor with a 39K series resistor and a 50K audio taper attenuator pot.  These values work well for me, giving a good depth of FM, and control over the lower levels of modulation.  Low frequencies pass quite well, allowing for slow linear tremolo.
• I added some 100nf bypass caps to the underside of the board.
• As to the much debated choice for switching transistor (a rare 2N3638 is called out), I substituted a BC560C, which works fine.  I always print out a data sheet when installing transistors to insure correct lead placement.  The PC board marks E, B, C, for the transistor leads on the top of the board, which helps.
• I added a 100K linear pot to attenuate the wave shape CV in and left off the 150K resistor from the board.

First Impression

The initial frequency trim pot lets you set a bias point for the coarse range.  At first I set the lowest frequency to 20 Hz.  As this produced a high end of 35 KHz, I decided to set the high end to about 25 KHz and let the low end fall where it might, which turned out to be under 1 Hz!  A very nice range.

I was able to set the 1V/octave tracking for at least three octaves of accuracy and perhaps more.

Adjusting the wave shape trim pots, I had a surprise.  The VC wave shape affects both the sine and the wave outputs!  I was very glad that I brought out the sine wave (which is tapped from the drain of the J201 FET and buffered to bring up the level).  Nicely, the sine output maintains a constant amplitude as it changes shape and becomes edgy.  The wave shape output that mixes in a sawtooth increases in amplitude as it shifts from ‘sine-like’ to ‘saw-like’ in shape.  The ‘sine-like’ shape yields low harmonic content, but is not as smooth as the sine output.  (I’ll post some demo recordings.)

Pictures

Board

Behind the panel

Buffer wiring on the underside of the board

The bracket that I made from thin aluminum stock from Lowes, shown before bending.

This is a circuit of my own, based on the XR-2206 VCO data sheet examples.  Features:

• Coarse and fine frequency controls
• Low/High range switch
• Sine/Triangle switched output
• Square wave output
• Linear, AC-coupled FM input
• Internally normalized for cross modulation

My intended use of these VCOs is as modulation sources for linear FM of other VCOs, such as the Cynthia ZerOscillator and the Fritz Teezer.  I was looking for a sine wave VCO for quite a while, but a 1U sine wave VCO in MOTM format is not commonly found.  I happened across the XR-2206 in a discussion on Muff Wiggler’s forum and found this hobby store that sells a PC board.

Electronics DIY XR-2206 PC Board

Realizing that I did not need 1V/octave tracking or other high-quality music VCO features, I bought two boards and two XR-2206 chips.  (Get the XR-2206 from Mouser for under $5.00 each).

After I got the chips I started breadboarding.  Although I used the PC boards, I didn’t follow the original circuit but built my own.  I added an FM input, set a fixed amplitude for the sine/triangle output, and selected capacitors for the low and high ranges.  The XR-2206 is powered between ground and +15V, so the output is all above ground.  The triangle is much larger than the sine wave.  So I designed a buffer circuit, using a TL072 on a Tellun MUUB-2 PC board to provide +/-5V sine and triangle waves, and a +/-5V square wave obtained with the comparator circuit that I often use.

Here is the Dual XR-2206 Front Panel Express Panel.

Here is the XR-2206 Circuit Diagram, and the Buffer Circuit Diagram.

There is a trimpot on the XR-2206 board for the sine shape, and trimpots on the MUUB board for the sine and triangle levels.  The sine/triangle switch is DPDT.  One side switches the wave selection and the other switches between trimpots on the MUUB, so that the output levels of the sine and triangle can be made the same.  The FM input is AC-coupled by necessity, because the initial frequency is set by the combined resistance of the two frequency pots from pin 7 of the XR-2207 to ground.  DC-coupling would be tricky to do, but not necessary in this application.

The sine/triangle output of each VCO is connected behind the panel to the switching lug of the other VCO’s FM input to facilitate cross-modulation without need for patch cords or multiples.

Here’s a demo recording of the module.  It starts out showing the two frequency ranges of one oscillator, using the sine wave.   (The low range is 8 Hz to 400 Hz, the high range 87 Hz to 4300 Hz.)  Then the second VCO is brought in.  The second then modulates the first, then the first the second, and then each other simultaneously.  Then both are switched to triangle and knobs are twiddled a bit.

Description:

The Trapezoid VCA is a DIY project from Jurgen Haible, renowned modular synthesizer engineer.  It emulates the EMS VCS3 and Synthi A Trapezoid Generator.  I am not familiar with the EMS module, and I wasn’t quite sure what to expect of this one.  Now I’ve built and used it, it is indeed everything I had hoped for in a combined Envelope Generator/VCA.

Download the FPD Panel

The Envelope Generator (EG) part is unique among envelope generators in that it produces a trapezoid shaped waveform with an Attack-On-Decay pattern.  When triggered, the output rises linearly to maximum at a rate determined by the Attack pot, holds for an amount of time determined by the On pot, and then decays at a rate determined by the Decay pot in combination with the Decay CV input.  At settings less that fully clockwise (max time) the Off pot determines how long the output stays at the minimum value (i.e. off) before automatically re-triggering.   I would in fact call it a Trapezoid LFO with variable rise rate, on  time, fall rate, and off time.

Be sure to check out the Dragonfly Alley page for the Trapezoid.

VC Decay

The VC Decay input has its own reversible attenuator (D CV) pot, which controls the amount of CV applied positively when right of center, and negatively when left of center.  A more positive CV increases the decay time, like turning the Decay pot clockwise.  An immediate application is to patch the minus Out envelope back to D CV, which results in an exponential decay curve more like you get with a regular EG.  This is so useful that I made it a jack-normal connection.

Trigger Modes

When Off is at max then you have three ways to trigger a cycle of Attack-On-Decay.

1. Press the handy Manual button
2. Patch a gate into GATE IN
3. Enable Auto Trigger mode

In Auto Trigger mode the EG is triggered by the audio input to the VCA reaching a threshold, which is set by a pot or a trimpot.  I chose to use a trimpot.

Envelope Outputs

The envelope is available as four different outputs from the PC board (more on this later).  I brought out the ‘S’ outputs and made a modification so that the positive output goes from zero to +5V and the negative from +5V to zero.  The ENV LED indicates the OUT + envelope level.

The VCA

Although the PC board provides many different input and output choices for the builder, I chose to use one buffered input and the buffered output.  There is no separate CV input for the VCA available; it is driven by the internal envelope.  It has trimming for zero and initial gain internally.  The GAIN pot establishes the Initial Gain level when you wish to allow some of the signal through when the EG is at zero.  Unlike the initial gain on other VCA modules I’ve used, turning up the initial gain does not lead to a higher output level when the envelope is at maximum.  That is, it does not sum with the envelope.  In fact if you turn GAIN all the way up, the envelope will have no effect!  The envelope merely increases gain up to the maximum.

Construction

I used the MOTM power connector with the on-board 12V regulators.  I made my own mounting bracket using aluminum stock from Lowes.  It worked quite well.  Also for this project for the first time I used 0.1″ MTA connectors with headers on the board.  This turned out to be really useful, since I had to remove the board to add some trimpots.  The PC board uses a clever design for pot connections, so that the builder can choose to install either a trimpot or a 3-pin connector for an off-board pot.

Modifications

I added three trimpots, which are seen in the prototyping area of the board in the photo. A 100K trimput in series with R70 adjusts the output gain of the VCA, but it turned out that it wasn’t needed.  The full-on gain using the buffered input and output is about 1.1 with the unmodified circuit and did not need to be increased.  The other two trimpots adjust the levels for the plus and minus envelope outs.  The 100K gain trimpots were inserted in series with R38 and R42 and allow attenuating the envelope levels, which from the S outputs is roughly 10V out of the box.  Using those in combination with the offset trimpots I was able to set the plus out to be zero to +5V and the minus output to be +5V to zero.

This is a DIY mixer designed to save space in a portable cabinet.  It is based on my 2U DIY Matrix Mixer.  The circuit is almost identical, except that this one has DC coupled inputs and is unity gain.  Like that one, the pots are audio taper.  As I reflected on the panel design, I realized the benefit of making the fourth input be a non-attenuated auxiliary input, a feature not present on any of my other mixers.

Internally it is two completely separate mixers, each built on a Tellun MUUB-2 board.  (You could use one MUUB-4  instead.  I happened to have two MUUB-2 boards lying around.)  The circuit is about the simplest possible.   Download the  Small Matrix Mixer Circuit.

The panel configuration maximizes utility.  It is labeled L and R (left and right), as a convention when using it as a final output mix.  Each of the left inputs is normally connected to ground with no plug inserted, and each of the right inputs is normally connected to whatever is plugged into the corresponding left input.  As an example use of the AUX inputs, imagine the two outputs from a Blacet Mixer being patched there.  Now you have a five input stereo mixer, DC coupled, with a bias pot for each channel.  Patch L OUT to AUX R IN and you have a six input mono mixer.  Or patch the output from a buffered attenuator to the AUX L IN, and you have a fourth channel, equally mixed to both outputs.

Download the Small Matrix Mixer FPE Panel.

Construction was straightforward, using a shortened Bridechamber 3-jack bracket to mount the two MUUB-2 boards. It was time to purchase more hookup wire, so I pinged George Mattson for the part number of the wire he uses.  For a 100 foot roll of Belden 24AWG UL1061 BLK it’s Mouser 566-9984-100-10.  Pricey, but excellent quality.  The pots are Mouser 652-91A1A-B24-D20, a Bourns make with very nice feel.  These have pins rather than lugs, but were easy to lay-solder to.  I bent the pins back for easy access and soldered to them after the pots were mounted.  Power to the second board is run underneath from the first board’s power connector.

Quantity:  2 in 1U

A pair of Flight of Harmony Plague Bearers, built into this small-format 5U panel:
Dual Plague Bearer

These are sold in a variety of packages.  See the Flight of Harmony Plague Bearer page for details and to download the manual.  I bought two barebones packs.  They will customize the order as you like with power connector, pots, connectors.  I didn’t order the pots, because I use panel-mounted Alphas.  What does it do? From the manufacturer:

It is designed to infect, corrupt and pervert a signal beyond recognition. It contains a Voltage-Controlled Resonant Bandpass filter that is designed with the goals of maximum signal alteration and maximum parameter controllability. Each filter has controls for varying the high and low corner frequencies of the passband, as well as variable gain and an input attenuator. Each section of the Plague Bearer can be used as a filter, an oscillator, a noise generator, or even as a resonance oscillator! Depending on the settings, the filter can be a lowpass, highpass, single-bandpass, or multiple-bandpass. Adjusting the resonance point can cause the filter to self-oscillate in many ways. By adjusting the controls to just below the point of oscillation, and then applying a pulse, square, or other abrupt-edged input signal waveform will “ring” the filter. A simple “click” on the input can give a percussive output sound – from bass drum to bell to a harsh metallic clang; Crank the controls up and this beast will scream like a banshee! The oscillation can be damped or continuous. Chain a couple in series and get accumulating feedback – the build-up can be slow, fast or instantaneous. White Noise is easy to do – you don’t even need an input signal! Turn the input all the way down, and then slowly turn the gain up. You can adjust the “color” of the noise with the frequency controls. Daisy-chaining multiple filters can give a “comb” or multi-phase noise. About phase-shift characteristics: A single filter can give a STRONG chorus effect if so desired, useful as a sub-oscillator or just to “fatten” a sound.

Construction

I butted the two PC boards together and mounted them to a Bridechamber 3-jack bracket.  It’s a nice, long bracket that left plenty of room for the jacks in front.  The P.B. boards each have four mounting holes, two on one end and two on one side.  I had to drill the bracket to match these.  The wiring uses headers on the board for all connections.  I like this, because if there were to be a problem, I could easily return a board for repair without unsoldering wires.

Although this model of the P.B. has a CV Gain input, I had to omit that in order to fit two of them into the 1U panel.  This is a little unfortunate, because the gain modulation produces interesting timbre changes.  Here’s a photo:

You can barely see the tiny surface-mounted chip!  These small boards are also low-cost.  Notice the two MOTM/Blacet/Oakley style 4-pin power connectors.  I made up a special power cable that jumpers them both.

Usage

I made a patch that uses them in the filter position in a nominally standard patch.  It’s called ‘Plague Birds’.  Look for it in the music section.

This module is derivative of the Buchla 292c Lopass Gate.  The PC board and assembly instructions were made by Thomas White, described on his site on the Lopass Gate page.  The circuit is similar to the Cyndustries Quad Low Pass Gates, that I also built, and which is also based on the Buchla 292c.  I have always loved the sound of the LPG.  Although I had four of these in the Cyndustries module, I wanted to build Thomas White’s design for several reasons.  The QLPG has but one pot for each gate, which corresponds to the Offset pot on the Resonant LPG.  It sets the initial frequency.  The RLPG adds an attenuator pot for the control voltage input, plus a second CV input, and features a resonance booster with pot.  With two RLPG modules I have two lopass gates in a 4U space with more panel features.  I also decided to use VTL5C4 Vactrols, which have a slower response time than the VTL5C3 used in the QLPG.

Construction Notes

The Alpha pot part numbers in Thomas White’s part list are for the PCB-mount versions.  Unfortunately those are the wrong part numbers, because they are audio taper, and the circuit calls for linear taper pots.  I was building with the 1U BrideChamber panel and bracket, so I ordered linear Alpha pots, mounted them on the panel (using #5 O-rings on the bushings) and wired over to the board.

I found an interesting knob for the rotary switch, Mouser #506-PKGP-70B-1/4, which though a bit pricey at $3.75 USD each is really well suited.  It is a smaller diameter than the standard MOTM know, but has a pointer indicator that fits the fingers nicely.

Since I built the 1U panel with one signal input, I used the direct audio input option on the PC board.  I made use of the control voltage mixer to mix the CV and Control inputs.

How does it sound?  Very much like the QLPG, naturally, since it is based on the same Don Buchla design.  Of course the resonance (filter mode only) can be turned way up!  Do take the time to view Thomas White’s video demonstrations.

This module is a sawtooth-based VCO capable of frequency modulation (FM) extending past zero frequency into the negative-frequency regime. Thru-zero FM provides a much wider and richer variety of sounds than “ordinary” (positive frequency only) FM.

The negative-frequency version of a waveform is simply a time-reversed replica of the original waveform.  When a VCO is modulated through zero frequency, the waveform slows down to a stop and then speeds back up in the reverse direction.

The Teezer’s output waveforms also include triangle and sine waves, for producing sounds with fewer high harmonics, as typically used for bell sounds, train whistles, and so on. The unit also features a variable synchronization control that can be adjusted over a range of settings from hard sync to a fairly loose soft sync.  The module can also serve as a highly accurate and stable “ordinary” VCO (i.e., without the deep FM), with upramp, downramp, triangle and sine output waveforms.

A really cool, inexpensive through-zero VCO!  The Bridechamber panel is nice quality.   Its quirky layout with the offset pots and jacks is non-conformist, in keeping with the module itself!

Construction comments

I got the panel, PC board, and a kit of some special parts from Bridechamber.  High quality pots are used throughout.  As shown in the photo, I was able to mount the PC board to a Stooge 4-pot short bracket I had lying around.  The PC board was not designed for the bracket.  I could use only four of the six mounting holes in the board and had to drill holes in the bracket to match.  There was just enough room.  The power connector sits on the back of the board, cantilevered over the end of the panel.   I wish Mr. Fritz had provided more ground lands on the PC board; there is only one included in each of the two header-style connectors.  I used twisted pair for all the IO wiring and glommed all the grounds together onto a solid wire soldered into the one hole.

Circuit comments

I did the 15 volt power supply modification, as described in the instructions, using zener diodes in place of ferrite beads.  Five separate internal power busses are used!  Prior to soldering in the integrated circuits I connected power and verified all the supply voltages were as expected.  Good thing, because I had installed one zener diode backwards!  That was easy to fix.  As recommended, I had carefully matched the zener diodes D12 and D13 to within 0.01 volt.  Despite this, the triangle wave had a glitch that could not be trimmed out.  I modified the circuit to add a small resistor and got an improved waveform.  Still there is a small glitch that is audible as a harmonic.  You can hear this in the MP3 below.

Performance comments

This VCO tracks very well over at least six octaves.  I didn’t notice any problems.  I compared the thru-zero FM with the Cyndustries ZerOscillator (ZO).  The two VCOs do FM quite differently.  The Teezer has a simple-to-understand Initial Frequency pot that is a manual input to the linear FM.  In the mid position, the frequency is theoretically zero.  To the right is positive frequency (and a rising sawtooth) and to the left is negative frequency (falling sawtooth).  The Coarse Tune has an extremely wide range with audio frequencies being to the right of center.  Very low frequencies are possible for LFO use.

Through Zero shootout

I found that comparable effects are obtained from the ZO with the Range in HIGH and Bias switch in the LOW position.  For this test recording, I used a MOTM-300 sine wave as the modulation source.  The ZO and Teezer were set to the same initial frequency and the triangle outputs of each VCO were sent to a mixer.  You first hear the ZO unmodulated triangle, followed by a manual twisting of the LIN FM pot up and back.  This fades out and fade in to the unmodulated Teezer.  Here you notice more harmonics on the triangle than the ZO.  The LIN FM pot is twisted up and back.  This is a log pot on the Teezer (vs a linear pot on the ZO), so my jerky fingers are more noticeable.  You hear a slight beat also, which could be tuned out with the fine tuning.  I haven’t yet understood how to make the best use of the relationship between the initial and coarse frequency pots.

I designed this module mainly for the purpose of increasing the resonance of the Cynthia/Buchla Quad Low Pass Gates.  There are quite a few low pass gates circuits around the internet.  I was looking over Scott Stites’s Mutant Vactrol Filter project.

One variant of Scott’s filter is organized as two Buchla Low Pass Gate circuits in series, combined with an input mixer and a feedback circuit for resonance control.  It occurred to me to build just the input mixer and feedback circuit as a separate module that could be patched to the low pass gates. Here’s the schematic.  As you can see from the photos, I built two of these circuits on a MOTM prototype board on a Stooge 4-pot bracket behind a custom Front Panel Express panel.

To patch a couple of LPG circuits in series with the mixer in the loop:  route the signal inputs to the mixer; the output of the mixer to LPG input one.  Then LPG out one patches to LPG input two and LPG output two is patched to RES IN on the mixer, using a multiple to take this as the output signal.

I tried using the new module not just with the LPG, but with all sorts of filters and effects.  It adds increased resonance effects to the MOTM-410, which is also a vactrol-based filter with a fixed resonance.  I also found it useful with the Encore Frequency Shifter.  It worked with the MOTM-440, but didn’t do much that the internal resonance control already did.  When not in service as a feedback mixer, this module makes a nice two-input audio mixer for any purpose.  It’s AC-coupled and features audio taper pots.

Here’s a demonstration of resonance enhancement on the Low Pass Gates.  In the following recording you hear the LPG first in the both mode, sort of bland, then in the low pass mode, and finally in low pass mode with the resonance turned up to about six.  This cycle is repeated two times.  The clip switch is turned on, which decreases the strength of the feedback for more control.