Bluesblast.com - DIY Analog Synth

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.
- 2 * Manual control Inductor Based LPF own design.
- Vactrol controlled 2-pole LPF with built in LFO controlling both filter and reso - own design.
- MS-20 style LPF/HPF VCF following Moritz Klein design.
- Diode Ladder filter following Moritz Klein Design.
- 2 Vactrol Controlled Moog Ladder filters based on Eddy Bergman/Rene Schmitz/Kassutronics/Yusynth plus my own Vactrol controls. - Vactrol Controlled Wasp filter, Rene Schmitz plus own design for Vactrol control. - Manual control Using UAF42 , Texas Instruments design.
- MS-20 Style Filter with LM13700 , based on Rene Schmitz design.
- 2 * LPF filter based on the Korg PE-1000 design, with my own tweaks.

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.
- Sample & Hold - Moritz Klein 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.

Reference books are Ray Wilson Make:Analog Synth and Horowitz & Hill The Art of Electronics

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

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.


OKAWA DESIGN UTILITIES.
Okawa Electronic Design Tools
I think one of the most important steps you can take in Modular Synth Building is to embrace the mathematics of electronics and break away from just building circuits you got from here or there. In fact once you get to trust the math, even modelling in Spice is often unneccessary. So a shout out to Okawa Electronics for publishing this excellent set of design tools, simple to use and a great set of output data and graphs.

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. In other terms Vactrols have excellent dynamic range and support a high signal to noise ratio.
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.

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.







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.



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.


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.


I've read about the amazing Wasp filter and I saw that Rene 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.



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.

The above is the final build schematic. I nixed the High Pass and Notch outputs in favour of a simple switch between Low Pass and Band Pass. I switched from 74LS04 to CD4069, somehow a slightly warmer sound. I came up with a nice way to put a manual frequency cutoff. Instead of having the manual pot define a CV and using a Summing Amp, I simply put a dual-gang 100K pot in parallel with the Vactrol LDRs.

With no external CV the manual pot gives a really full control with lovely vowellike wahs and left at maximum resistance it has virtually no effect on the CV mode. And of course any combination between.

I basically love this filter.....there's no dead zones, virtually any combination of settings, input amplitude and CV gives an interesting output. Very Cool. And now fully productionized.

Hah ! So I went back to my 2 versions of the Moog Ladder and did exactly the same change, putting in this case a 500K Pot in parallel with the Vactrol LDR. OMG, absolutely incredible results from both filters, opening new dimensions of control over the filter voice.

Moritz Klein Sample & Hold - Build and Debug.
I've never tried building a Sample and Hold until now, but I saw Moritz Klein's video and it seems to do some pretty cool stuff, namely providing pseudo random CV to modulate filters or generate kind of ambient beeps and boops.
Having built versions of the MK MS-20 and Diode Ladder (which is actually awesome), I produced a build with only some minor tweaks. Some extra gain on the output buffer and use of MPF102 JFET.
Here are a bunch of sound samples using the KORG SQ-1 as a gate generator and various LFOs and Drones as a sample source.



I think on the whole, the results are encouraging, definitely some interesting sounds, but i found that if I tried sampling an audio file with a frequency much higher than the gate trigger, that a brief snatch of the audio gets through to the output.
And of course that sounds kind of glitchy when fed to the VCO or Filter. I think it might be because MK's design uses a quite large Hold Capacitor of .1 muF instead of what seems to be the norm in these modules of .001 muF or 1 nF. I also think you can hear of a bit of a portamento effect which I suspect is the larger capacitor taking time to charge.
I think I will leave it as is and try some experiments before making any adjustments, but I'm also thinking of trying out Rene Schmitz YASH design with an inbuilt gate LFO and using the LF398 S&H IC.

UAF42 Filter.
I did some clearing up of old recycled components and came across a UAF42AP chip, a Universal Active Filter, I think originally developed by Texas Instruments.
This is esessentially a state variable filter on a chip (similar to Wasp or CGS30 topology).
All you need to do is define the filter cutoff frequencies by supplying an external resistance between certain pins. I found a recycled dual gang 500K pot for that job and also constructed a resonance circuit using the CGS30 design.
It worked right off the bat and makes a nice manual filter for blowing out a rock jam. It would be very easy to make it vactrol controlled but this is a good fun project just as it is. Here is a demo jam.




200mH Inductor Filter

I also found some 200Mh Inductor Coils I bought a while back and used to make some 'Wah' modules with my son, so I tried to construct a LPF and get a better idea of why they are so rarely used in Analog Synth. I found a nice design tool courtesy of Marki Microwave at - Marki Microwave RF Design Tools

. Once I had the basic filter core laid out I put some gain stages in and a simple resonance circuit and this is what it sounds like

It's a reasonably powerful filter and I like the sound particularly in the lower registers, but the useful range in the control pots seems to be very frequency dependent and narrow. You can hear in my playing apparent dropouts where I didn't stay in the playable zone. I think it would be tough to make this a VCF using my standard vactrol, it would need to be somehow frequency dependent.


A Word on Resonance.

In simple terms I look on resonance as using Positive Feedback to increase the effect that a filter module is having on a given waveform, with the caveat that the module must always have an element of gain. A purely passive stage won't exhibit resonance.
Basically every gain stage or amplifier in a synth filter is either inverting or non-inverting and the filtered output signal is either inverted or not accordingly. If we mix a portion of a non-inverted output back into the original signal, then the filter will progressively accentuate its own effect as it 'sees' the merged input signal. Notice how in the Wasp filter the Resonance feedback is taken after 2 inverting stages, thereby putting the output signal back in phase with the input and allowing for positive feedback.

Negative feedback of course has the opposite effect and tends to negate any filtering or distortion in a gain stage. For this reason it may be used in hi-fi amplifier, but not commonly in a synth.

The Inductor Filter above features only non-inverting op-amp gain stages. Therefore if we take the output, marked as the connector R and feed an attenuated copy of it back in before the filter core, the filtering effect is progressively enhanced.
Taken to the extreme, a filter can self-oscillate when any random voltage fluctuation can be amplified through positive feedback into a sharp frequency peak with no external input.

Percussion Corner.

My existing rig had just a simple Baby 8 sequencer with single output and a CGS Kick Module, hence just a kind of simple Techno Thump rhythm possible.
To make for a much more interesting set of options I built a new Baby 8, but with a twist, 3 8-way dip switches to route each pulse to up to 3 outputs.

The implementation is kind of a mess because it turns out each switch needs a diode to prevent signal going back through the circuit, hence an auxiliary board required. But it works.
From there I still have the old CGS kick, but I also built a version of Eric Archer's Mini Space Rocker using 4-way rotary switches to allow up to 64 combinations of voicing capacitors. Here is a demo of the new modules.
And I still have an unused output. My intention is to try and create a real metallic clang sort of sound, maybe using an XOR CMOS approach.

Canonical Form of Vactrol Sallen Key Filter.
In an effort to document a solid starting point for Vactrol filters I set out to make the simplest 2-Pole Sallen Key LPF that I could using a Dual Vactrol as the sole means of modulating the cutoff frequency and a manual resonance control.

In essence, the module accepts an audio input, buffers it and passes the result through a Sallen-Key filter, with 2 variable resistors , which are in fact 2 LDRs controlled by a single Red LED. The LDRs range from just over 1Meg Ohm unlit to about 4K Ohm fully lit. If C1 & C2 are set to .01 mu F that equates to a filter sweep between approx 15 Hz and about 4K Hz. The Non-Inverting Amp in the Sallen Key circuit has a gain of about 3.

The CV is buffered and used directly to illuminate the LED with a 500 ohm current-limiting resistor. No scaling or biasing is built into the audio or CV circuits.

Finally a feed from the output is taken through a voltage dividing pot and a portion of resonance inducing positive feedback sent back through C2.

I built it on a scrap of stripboard and it sound like this.....

The module is quite aggressive, self oscillates freely with high resonance, is very sensitive to external attenuation of the audio or CV inputs. It would be easy to tweak this if desired...
To make a manual cutoff control, connect a dual gang pot, with say 500k across R1 & R2. You can see from the diagram I thought about this but actually left it out. The 500 Ohm current limiting resistor could be varied to align maximum brightness of the LED with the maximum positive value of the CV.
You could use a larger resonance pot or insert a resistor before the pot to limit the resonance.....lots of potential.

FET Variable Resistor Investigation....

As you can see from the above my preferred variable resistor is the Vactrol, but Vactrols have one main disadvantage which is slower speed to light up and go off again. For a really deterministic type of response I've been looking at FET type solutions.
It's pretty well known that FETs have a so-called Linear or Ohmic region of amplification where they exhibit resistor-like functions for a certain range of gate voltages. But it turns out there are four commonly encountered types of JFETs and MOSFETs and each type will need a different pattern of CV to work effectively.

For example ; my Arturia Keystep produces a +5V Gate CV. If I wanted a VCA that passed signal only when the gate is active, my only option with the raw Gate CV is to use an N-Channel Mosfet. The N-Channel JFET and P-Channel MOSFET only respond to -Ve CV and the P-Channel JFET is fully conductive at zero gate voltage, which is the inverse of what I want.
Any other type of FET device would require additional circuitry to invert/bias/scale the CV signal.

Device	Channel	Mode	Conductance at Zero Gate Volts	Max Conductance Volts	CV Type	Example Devices
JFET	P-Channel	Depletion	Max	Zero	+5 (min) to zero(max)	2N5460 2N5461
JFET	N-Channel	Depletion	Max	Zero	-5 (min) to zero(max)	MPF102 J112
MOSFET	P-Channel	Enhancement	Min	Nominal -12 V	Zero (min) to -12 (max)	IRF4905 IRF9540
MOSFET	N-Channel	Enhancement	Min	Nominal +12 V	Zero (min) to +12 (max)	BS107 IRF540

The above diagram (for an N-Channel MOSFET) is similar to those seen in FET Datasheets and it is worth getting to understand how it works.
VGS is the Gate to Source Voltage and in Synth terms is effectively the CV.
VDS is the Drain to Source Voltage and again in Synth terms is the audio signal.
ID is the output Current and therefore within the 'Ohmic' region this is effectively the inverse of resistance - higher current equates to lower resistance.

Each of the curved lines corresponds to the Current-to-Signal (i.e. Id to VDS) at a specific CV (or VGS) voltage. The graph is almost like a 3D diagram in 2D, with the CV defining a surface extending into the page.
What this tells us is that once the Audio Signal level (VDS) exceeds about 2V, that the MOSFET will enter Saturation mode and emit constant current ; in other words for synth purposes
we will need to scale our audio levels to about 2V and use a gain stage to bring back to synth line level after processing.

In a synth situation the CV will be constantly changing and we'd like to get the most dynamic range of resistances, therefore using the standard synth range of about 5V seems absolutely fine.

So my conclusions so far are as follows.
1 - There are Four different FET types that look relevant to Synth makers. Each has a specific behaviour relative to CVs and so DESIGN is the key word to specifying the right FET device for a certain use.
2 - Approximately 5v CV and Audio less than 2V seems like the right kind of scaling.

So far I've built a basic VCA just as a trial run. My intention now is to build a VCA and a Sallen-Key VCF to prove I've got things about right.....watch this space.

Here's my basic VCA schematic - it works like this. R1 & R2 create a voltage divider that attenuates the input signal to prevent the signal driving the MOSFET into saturation. R6 & R7 likewise form a voltage divider on the CV.
Q1 is a BS107 N-Channel MOSFET and per my chart above has maximum resistance at ZERO volts and the resistance then decreases as the CV increases in a positive direction.
The CV does not need to be buffered because the MOSFET input impedance is very high & it does not consume current.
Thus Q1 & R3 form a voltage divider....when the CV is low the effective resistance of Q1 is high relative to R3 and as the resistance of Q1 falls, more and more of the signal is 'seen' by the OpAmps - the resistors R4 & R5 define the output stage gain which restores the signal to synth line level.


The above trace shows how the VCA responds to a varying amplitude signal modulated by a triangle wave LFO. The MOSFET is probably still going into saturation at the top of the LFO signal as you can see a flattening at the top of the curve with the 'soft knee' tube-like distortion that MOSFETs are known for. This gives the module a sound which is rather like a tube compressor. When the signal amplitude gets to the 'knee' in the response curve the input waveforms start to get 'rounded off' leading to a pleasant harmonic distortion and a gentle compression. I found that my Arturia Keystep is producing a gate voltage of +12V and actually the MOSFETs can easily handle this and it gives audio headroom up to I think about 4 volts.
Some trimpots in the design could help fine tune the signal or CV attenuation. When I solder up the final version I will include those tweaks as well as variable output gain.

Having got a working MOSFET VCA I am moving onto a VCLPF Sallen Key design.


This is a tried and true LPF topology, and as with the VCA the concept is to attenuate the input signal, perform the filtering and apply gain to get back to synth line level.

So my basic design consideration for this filter is that I'd like it to sweep between the the upper practical end of human audible frequency (lets say 10K Hz) down to a few 10s Hertz at the low end.
Without putting addiional resistors in parallel or series with the MOSFETs, the conductance of the MOSFETs is essentially a given, for the range of about 0-12V CV. THerefore we are left with a suitable choice of capacitor. We know from the VCA build that the MOSFET Resistance full 'on' must be small relative to 1K Ohms and fully 'off' must be large relative to 1K.
So if we assume 500 OHms and 50K Ohms , we know that the equation for cutoff frequency is F = 1 / 2piRC.
And plugging in some capacitor choices we find that .031 muF gives us 10200 Hrz at fully 'on' and about 100 Hz at fully 'off'.
So that will be my start point in the prototype build but I'll use sockets to make it easy to swap capacitors.

I built the above schematic and started testing, initially on my test rig, but when I realized the sensitivity to both the CV and Audio signal I started testing on the real synth......learning points so far...
1 - the Mosfet gate MUST have a resistive route to ground, because otherwise it stores charge like a tiny capacitor and will tend to stay at a high CV level, effectively always ON.
2 - the synth amplitude appears to be capable driving the MOSFET right out of the Ohmic region and into saturation. This equates to approx zero ohms and cansequently the filter goes to a supersonic high cutoff frequency. Therefore I've put 500 Ohm resistors in series
with the Mosfet Sources and this definitely goes in the right direction, but I'd probably go with 1K in the final schematic. The problem is rooted in that the resonance circuit needs to feed back a reinforcing positive signal and this can easily drive the filter into saturation.
3 - the Mosfet (IRF540) turns out to have a threshold cutoff at about 2 volts where the resistance is Maximum, i.e. Fully Off. I am therefore designing a bias circuit to stop the gate falling below about 2 V, which should mean that the resistive behaviour
of the Mosfet is always under CV control.
4 - As you might expect the Resonance control is not perfect. I'll stabilize the points above and then try to fine tune the resonance as the last step to avoid the problem of pushing the envelope into saturation. 5 - In short the fixes to above push the component count up....it remains to be seen whether the result justifies the effort.


Well, the 2 samples above probably are about as good as i can get with this design. The basic problem with the Mosfet filter, is that the input signal needs to be pretty low, say below 1 Volt and then re-amplified for the output.
This makes the filter quite noisy because any noise also gets amplified. Also, the resonance you want in a good filter is just positive feedback, which increases the signal through the Mosfet, and of course pushes it out of the ohmic region.

The good thing is the responsiveness of the Mosfet to using a gate signal to modulate the sound, which is really its one intrinsic advantage over a Vactrol.
I think that probably concludes the FET investigation.....

MIXER Summing Amplifier Fix.
The oldest module in my entire rig is a 3-channel mixer with a Bass/Middle/Treble powered tone control. It never seemed quite right and a little investigation with hindsight revealed why - the inverting amplifier had a gain >1.....so with 2 strong channels
input the output would be approx 10 volts, with 3 channels it was hitting the rails and distorting. I fixed one resistor, brought the gain to about .5 and all is sweetness and light.....easy.

LM4250 Experiment....
A while back I tried to make a Polivoks clone, with pretty mediocre results, very weak filter performance. Instead of the original Russian programmable op-amp I used LM4250, which features pin 8 as supposedly providing CV control. So having a few LM4250s left over I tried to make a basic
adjustable gain amp.....really no discernable voltage control on Pin 8 at all. Maybe I was sold fakes, LM741 dressed up as LM4250. Anyhow, experiment over, no useful result at all !! From what I understand the Wasp is a comparable topology to the Poliviks and my version sounds pretty good so thats fine.

PE-1000 Low Pass Filter
I'm thinking sbout making a Low-Pass filter based on the Korg PE-1000 polyphonic synth from the early 1970s. The PE-1000 was a truly polyphonic instrument , amazingly with a dedicated tone generator for each key.
The filter section is very interesting to me because it combines 2 variable resistance technologies in one schematic, a nifty twin vactrol with a center tap controlling a Sallen Key type filter and a pair of JFETs controlling the feedback / Q.
Right now I'm parsing the original schematic and will produce my own sub-schematic in due course.
And here is the schematic.


At the heart of the build is a device I call MOD1 - a dual LDR Vactrol with a center tap. I encapsulated it in epoxy and it looks like a little turtle in the middle of the build. The filer processing is directly lifted from the original PE-1000
Low Pass Filter. For the resonance circuit I built a simplified version of the Rene Schmitz MS-20 filter and for the output stage I simply used an Op-Amp, seeing no virtue in the original transistor design.
I also added a CV control as an alternative driver for the Vactrol.

Here is a demo clip, featuring both manually and CV control. The resonance is aggressive and it definitely has a touch of MS-20 about it.
My favorite feature though is that it works great with both manual and CV control. Right now, I think it has my favourite filter and I will certainly build one more to verify that the schematic is correct.
And here is a demo of my second build. This clip has the resonance turned to a very aggressive setting. The filter really has 4 variables...the CV input, the resonance pot, the filter cutoff pot,
which interacts with the CV setting when that is switched on, but it is also sensitive to the input amplitude. I'm not entirely sure of the processing but I think it must relate to the role of the JFET and a lower amplitude input will obviously help it
stay in the resistive zone....very interesting.

Low Pass Filter Voltage Divider Experiment.
Here's a question ; A passive Low Pass filter can be represented as a sort of voltage divider between the signal voltage and a ground voltage, where a resistor and a capacitor act as the divider......the impedance of the capacitor varies with frequency of course hence the output voltage varies by frequency. So the question is what if we varied the ground voltage, for example with a CV from an LFO ?
This thought came to me thinking about the Moog ladder filter because obviously the filter capacitors dont dump signal to ground but to the resonance side of the ladder.
I don't know the answer to this, so lets try and experiment.
And the result is !!! In fact the 'ground' potential made absolutely no difference at all to the filter response. Which kind of makes me wonder why the Moog filter capacitors are where they are between the ladders ?
What if they just went to ground ?

Henry Filter Mark II I-See variant.
So sadly I blew up the Inductor filter i made, see above....but there is a silver line. I found a schematic called the 'I-See Wah' pedal and it was pretty easy to adapt it as a synth module.
The result is actually really good, take a listen.
It filters nicely right across the full control pot, it has 3 different voice capacitors and has an awesome bass tone. Interestingly it leverages an effect called the 'Miller Effect' to create a sort of variable capacitance, which relates to> my voltage divider experiment above. Now I will probably try a CV controlled version.