rainbow bridge

MODULES with corresponding design/inspiration

Noise Generators
- MIDI Oscillator - based on MDCO-3 Chip from MIDIMUSO
- FM 'Nintendo' Poly Synth - Arcano Systems Kit with custom wrapper circuit
- CEM3340 VCO - Look Mum No Computer
- Yamaha TX81Z
- Tri-oscillator drone - inspired by Look Mum No Computer
- White Noise - Ray Wilson

Filters
- 3 * CGS30 State Variable BandPass Filter - Ken Stone
- Vactrol Controlled CGS30 Band Pass filter - based on Ken Stone Design
- Vactrol controlled 3-Pole filter - own design Sallen-Key filter
- Vactrol controlled 2-Pole BPF Filter with built in LFO - own design.
- Manual Control 4-pole LPF - own design
- Vactrol controlled 4-pole BPF with built in LFO - own design.
- Manual control Inductor Based LPFown design.
- Vactrol controlled 2-pole LPF with built in LFO cmbtrolling filter and reso - own design.
- MS-20 style LPF/HPF VCF following Moritz Klein design.
- Diode Ladder filter following Moritz Klein Design.

Sound Shaping
- 2*Low Pass Gate - Synth DIY Guy
- LFO/VCA - Nicholas Woolaston LFO in same panel as own design Vactrol VCA
- Attack-Release - Nathan Ramsden
- Reverb - 2 * PT2399 reverbs, 1 own design, 1 Ebay board.
- Sub-Oscillator ; 1 & 2 Octave Sub, based on Thomas Henry Design
- Ring Modulator - Ken Stone

LFO
- 3 * own variant of Nicolas Woolaston LFO design adapted for 'Positive Only'

Sequencer
- Baby 8 Drum Sequencer - based on Look Mum No Computer concept
- Kick Drum Synth - Ken Stone
- KORG SQ-1 Sequencer

Utilities
- 3 channel Mixer / EQ - own design based on Ken Stone
- Inverting / Non-Inverting Amp/Buffer - own design
- Splitter / Buffer - own design
- Uni-Gain Mixer - Richard Jones design
- 2 Channel Performance Switcher - own design based on Keith Emerson concept
- Portamento - based on Jesse Stevens design.

Keyboards
- Arturia Keystep 32
- Modded Casio SA-46
- Alesis Q49


FORMAT :
Voltage - plus/minus 15V
Jack Size - 1/4 inch
Power Adapter - 3 pin header
Audio Signal Level - 5v peak to peak
CV Signal Level - Positive 0 to 10V
Panel Size - Moog style 8.75 inches

In general, my idiosyncratic format has kept me away from kits or finished module purchases. As a result I think I've dived a bit deeper into how things worked with the aim of creating a playable musical device.

Suprisingly there are currently no Rene Schmitz modules in my rig right now, but I have built several of his designs over time. Reference books are Ray Wilson Make:Analog Synth and Horowitz & Hill The Art of Electronics

Synth as of September 2022
Here is my rig as of September 2022. To the right you can see my Behringer bass amp, that I currently use for the synth. The line out is used for direct recording. The vibe is definitely grungy - no attention whatsoever has been given to appearance.

TEST RIG Synth Test Box

Here is my synth test box, featuring a triangle wave LFO, a manually controllable CV with digital voltmeter, a pair of +-15v power supplies, a momentary gate button, a square wave audio output & probe and a white noise generator.


VACTROLS
A word about Vactrols; basically any CV controlled synth module normally requires a form of voltage controlled resistance, which could be a JFET, an OTA, a diode but my preference is very much the Vactrol. as you can see from my module list above.
A Vactrol in it's simplest form is just a Light Emitting Diode (LED) illuminating a Light Detecting Resistor (LDR)....thus as the LED emits more photons, the resistance of the LDR falls. So why do I like Vactrols so much ?
1- Vactrols can be precisely measured & calibrated as to their resistance when a given current is applied, thus they give nice predictable frequency response in a filter situation.
2- Vactrols are very rugged and can switch large voltages and currents, avoiding complex scaling and post-filter re-amplification.
3- Vactrols have an LED and LDR response time that to my ear is quite musical and can help smooth out the response to a jittery CV. Different choices of LED size, color and LDR type will certainly have an impact.
4- The LED circuit can be electronically isolated from the audio path, thereby eliminating any noise.

So I'm definitely a vactrol fan. I typically make mine using Red LEDs, because the forward voltage is low,about 1.7 volts and 5516 LDRs that run from from 5K to approx 800K ohms. IR Leds have even lower forward voltage so I'm about to make a batch of them and see how they work.
Here's an example of one of my vactrols - note identifier E. And also a little vactrol jam. Vactrol - note identifier E Push ME

ON PLAYING IN TUNE
I really like to just plug in and jam but with a traditional CV driven VCO it is just incredibly difficult to maintain good pitch and correct octave tracking. So my rig has a couple of MIDI audio sound sources, my Arcano NES FM synth and a MIDIMUSO MDCO-3 twin oscillator. Both are literally plug and play and the MDCO-3 in particular produces an awesome square wave that is always in tune and tracks perfectly.

However, I also really like the sound of an analog saw wave and portamento does not work with MIDI so I am planning to work on my CEM3340 VCO - Look Mum No Computer design and try to get it stable at a certain pitch with just fine tuning neccessary and maybe add a range switch for bass.

FALL 2022 Projects
I decided to retire my Music From Outer Space LPF from the rack, taking up too much real estate for a comparatively mild 2-pole filter. Instead I have added 2 new vactrol driven filters, a 4-pole vactrol controlled BPF with manual resonance and a 2 pole LPF with vactrol controlled for both filter and resonance.

Push ME
Vactrol - BPF

Vactrol - LPF


Moritz Klein Filter Enhancements.

I made some changes to my Moritz Klein LP/HP Vactrol build, with the intention of making it nice and responsive to trigger generated CV signals from my AR module. To really make that articulate filter vowel sound, the 2 variable resistive elements, the Vactrols need to be as synced as possible, so I built a dual LDR vactrol driven by a single IR LED and replaced the original separate vactrols. I also added a little gain to the second of Moritz Kleins string of buffers. I also made sure my indicator LED that shows the CV intensity goes through a separate current control resistor so there is no interaction by taking current from the vactrols. I really think the result is a nice improvement,more articulate and more resonant.

Moog Ladder Filter with Vactrols via Yusynth and Eddy Bergman.

Push ME

I recycled the components from an old CD/Radio and decided I would make a Moog-style ladder filter. I followed the Eddy Bergman stripboard model based on the Yusynth schematic. The filtering worked fine but I never got the CV control very satisfactory. So I figured that the transistor that goes through R16 to the filter core is actually just a variable resistor. I therefore put a regular potentiometer in its place and measured the ohms that gave a good sweep. Then I made a CV controlled Vactrol and replaced the pot with that. The attached MP3 is what it sounds like.

I think it definitely has that gritty nasty sound, good enough to justify a proper rebuild.

Update - I built another ladder filter, based on the Kassutronics schematic, which is in turn based on the Yusynth design. Obviously I took into account the learning from the Eddy Bergman build which remains intact.
I chose the Kassutronics design because it replaces the output transistor with a TL074 design. I replaced the whole CV section of the Kassutronics filter with a vactrol, that seems to work well. At this point the VCF is filtering nicely
but the resonance is very mild,so I will be working on that. The overall sound is much smoother and less noisy than the Yusynth build so I will try to figure out why that is and maybe tweak the original build.
Full schematic will be up here in due course.

So I made a couple of tweaks. I decreased the current limiting resistor in the resonance circuit to allow more of the signal back into the right hand ladder and I also reduced one of the CV mixer resistors to allow for a hotter CV fromone input. Push ME
Push ME

Those are a couple of sound files from the Eddy Bergman and Kassutronics filters respectively. I think one more tweak, to allow resistance of the 'current sink' vactrol to go lower and thereby let the top end of the filter sweep let more high frequencies through.
When I've tested that I will publish the final schematic.

Vactrol Wasp Filter.

Push ME
I've read about the amazing Wasp filter and I saw that Renee Schmitz had a schematic, so I've built one with some tweaks. I really don't like OTAs and in this design the CA3080s are just acting as variable resistors so I replaced the 3080s with a dual vactrol driven by an IR LED. Also I had some 74LS04 Hex Inverters so i used those instead of the CD4096.

Now the Hex Inverter stages are all really acting as crude inverting Op-Amp elements with the Hi-Pass stage configured as a Differentor and the Band-Pass and Lo-Pass configured as Integrators. In fact the topology of the Wasp filter is essentially the same as the Ken Stone CGS30 or the Polivolks.

The Vactrol I used involves an IR LED illuminating a pair of LDRs and they will go from about 100K Ohms at OFF to about 2K at fully ON.
Now the equation governing the Hex Inverter stages is Fc = 1 / 2pi*RC.
The performance I want for the filter might be to sweep from say 100 Hertz to maybe 5000 Hertz, that is to say across the main human auditory range, and when I plugged in a Capacitor value of .033 muF I got a sweep from 48 Hertz to 2500 Hertz. So thats a little bit more of a tenor than an alto filter but I really like the result. I think .022 muF would be a little bit more aggressive and I might try to make a switchable version.
In general, I'm suspicious of spooky effects, but my initial build was quite noisy. This completely disappeared when I grounded the unused Hex Inverter pins. Well well.
Vactrol - Wasp vcf


Oh, and I did obtain some actual CD4069s. Swapped one in for the 74LS04. Worked immediately with maybe a slightly 'fatter' sound, slightly more distortion.






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