Voltage controlled resistance

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oksure

Member
Joined 2022
#1
Hi,

I've recently started looking into build analog synth modules. The biggest challenge for me is to control resistance with a CV. For example, if I wanna build a simple RC low pass filter and want to control it with a varying voltage, I would need something like a Vactrol. I understand I can build my own, or I could buy them (very expensive). But something tells me I'm looking the wrong way.
Is a Vactrol really the only way to control an analog low pass filter? The fact that this part is hard to find, does it mean that everything is just digital now?

and if I do go to digital, I see that 12bit is quite common for an ADC and DAC. 24bit is hard to find and usually only available in high end chips. So does this mean that most DIY are done with 12bit? Is that acceptable in most cases? I know this is a subjective question, but I just wanna know if "most" people just go ahead with 12bit or if it is very unpopular.

Thanks.
 
#2
My favorites were P-channel J-FETs, but nothing is very accurate except digital. One trick is to PWM a transmission gate in series with a resistor so that 100%=R, 50%=2R etc. LDR and LDR couplers are nice and clean but slow and not accurate. I think, if you really want to stay analog, then you have to do a lot of parts selection and calibrating. Have a look at multiplying DAC chips. There are log chips but, on a project that I did some years back, I ended up using a linear multiplying DAC and choosing ~30 values (+/-15dB) out of 256.
https://www.analog.com/media/en/technical-documentation/data-sheets/DAC08.pdf
https://datasheets.maximintegrated.com/en/ds/MAX5460-MAX5468.pdf
Today, I think that I would just learn DSP programming and do everything in software. There is no comparison of the cost vs capability.
Also, have a look at the I^2C IO chips that can be run from Arduino or Rpi.
https://www.electromaker.io/blog/article/4-amazing-raspberry-pi-synthesizer-projects-67
 
#7
Thanks for the info everyone.
Yeah, digipots could work. But it's painful when you need 8 of them in a circuit. I might just build a small board with a microncontroller and 8 digipots. The port will take a CV and adjust the digipots. Then I'll be able to reuse that board in many other places. It would be nice if I could find 8 channels digipots, or even 12 channel

Never heard of the AD633, will look into it.
As for the LM13700, This is a lot of work. But it's a good option


As for going digital, what about my question regarding 12bit DACs ? Are those regarded as poor? or can I still get a decent output for a synthesizer or effect module? My question really is: Should I completely disregard 12bit DACs or are they very useful in many applications?
 
#8
IF one end of that resistor can be grounded, a FET is cheap and easy to use; if floating things get complicated.

Making your own Vactrols is dirt cheap but since CDS cells (actually its cadmium content) were banned in the EU manufacturers ceased using them so they became scarce.

But there must still be millions in warehouses.
 
#9
oksure said:
I've recently started looking into build analog synth modules. The biggest challenge for me is to control resistance with a CV. For example, if I wanna build a simple RC low pass filter and want to control it with a varying voltage, I would need something like a Vactrol. I understand I can build my own, or I could buy them (very expensive).
Click to expand...
New Vactrols aren't cheap. There are some clones designed especially for the DIY synth, audio compressor, and guitar pedal market that are affordable. There are also some surplus parts that are usable. You don't state where in the world you are, but if you are in the US you can get 12 couplers for $3 here. Sale ends Sunday. I have used them. They seem to be rejects from some line somewhere since every one is different, so I bought a box full and matched up some pairs for a Vactrol based ladder filter.
https://www.goldmine-elec-products.com/prodinfo.asp?number=G15396D
Cabintech Global sells new production Vactrol clones and lots of other tasty chips for the DIY analog synth builder. They also have some pre-built boards for some chips. I have bought, and used their chips and Vactrols, but never tried their boards.
https://cabintechglobal.com/semi
As stated Vactrols are somewhat slow and the release time is longer than the attack time, so their use in a filter is not always optimum, good for audio compressors though. There are lots of other ways. It would be wise to study the circuits used in the old analog synths of the 70's and 80's. The Moog Ladder Filter is the classic VCF, and at least one should be included in every DIY synth build. It uses the variable conductance of a bipolar transistor for the variable resistance component breaking against a fixed capacitor to create a VCF pole. Four pairs are stacked on top of each other Ladder style to create a 4 pole filter whose cutoff frequency is controlled by the current through the transistors. The whole thing is wrapped in an opamp style differential amplifier circuit that is built with mostly discreet parts. Due the phase shift of any filter pole near resonance the negative feedback becomes positive near the corner frequency, so the filter becomes unstable as the feedback is increased (the resonance control). This creates the classic MiniMoog sound.

There are dozens of classic opamp filter designs in textbooks and all over the web. Most of them can be made voltage controlled by using a variable transconductance opamp for the active element. The previously mentioned LM13700 is the most common, as was the now obsolete LM13600.

Many of the analog synths from the 80's used custom chips for the VCO, VCF, and VCA's. These went extinct in the 90's, but are being remade today by several upstart chip makers. These parts are available to the DIYer from several sources like Cabintech Global, Synthcube, Modular Addict and others. Some chips are SMD only, but breadboard friendly adapters are available from lots of places. Amazon is the cheapest for stuff like this. If you are just starting out, choose a VCF chip and build a VCF on a breadboard in a day or so.

It is also possible to do a complete digital synth in a microcontroller chip or "makers board." Arduinos and Rpi's are common. I tend toward the TEENSY boards for lots of CPU power in a small form factor. They have a "drag and drop" audio library that includes things like oscillators, filters, DACs, ADCs, and other synth building blocks. Their forum has several users that are building synths and other musical devices as does Youtube.
https://www.pjrc.com/
To interface the analog and digital worlds DACs and ADCs are needed. 12 bits are the bare minimum needed for accurate pitch information. 8 and 10 bits can be used for filter and amplitude control, but I prefer at least 12 to eliminate "zipper noise" when sweeping a VCF or VCA. I also tend to use at least 14 bits of DAC to generate pitch accurate CV since there are inaccuracies in the analog circuitry that add to the pitch error. Whenever converting audio information to and from digital and analog formats at least 16 bits are needed. 24 are usually preferred for a listening to a complete musical work (song), but 16 seem to be fine for a single synth.

This forum is mostly about DIY amps, speakers, and other equipment for listening to music. There is an "instruments and amps" section dedicated to making music, and one thread for the few of us who tinker with synthesizers.
https://www.diyaudio.com/community/threads/modular-synth-build.258400/
There are several complete forums just for the DIY synth builder out there. Youtube is also another place to see how others build modules and synths.
https://electro-music.com/forum/
 
#11
Tubelab_com said:
New Vactrols aren't cheap. There are some clones designed especially for the DIY synth, audio compressor, and guitar pedal market that are affordable. There are also some surplus parts that are usable. You don't state where in the world you are, but if you are in the US you can get 12 couplers for $3 here. Sale ends Sunday. I have used them. They seem to be rejects from some line somewhere since every one is different, so I bought a box full and matched up some pairs for a Vactrol based ladder filter.
https://www.goldmine-elec-products.com/prodinfo.asp?number=G15396D
Cabintech Global sells new production Vactrol clones and lots of other tasty chips for the DIY analog synth builder. They also have some pre-built boards for some chips. I have bought, and used their chips and Vactrols, but never tried their boards.
https://cabintechglobal.com/semi
As stated Vactrols are somewhat slow and the release time is longer than the attack time, so their use in a filter is not always optimum, good for audio compressors though. There are lots of other ways. It would be wise to study the circuits used in the old analog synths of the 70's and 80's. The Moog Ladder Filter is the classic VCF, and at least one should be included in every DIY synth build. It uses the variable conductance of a bipolar transistor for the variable resistance component breaking against a fixed capacitor to create a VCF pole. Four pairs are stacked on top of each other Ladder style to create a 4 pole filter whose cutoff frequency is controlled by the current through the transistors. The whole thing is wrapped in an opamp style differential amplifier circuit that is built with mostly discreet parts. Due the phase shift of any filter pole near resonance the negative feedback becomes positive near the corner frequency, so the filter becomes unstable as the feedback is increased (the resonance control). This creates the classic MiniMoog sound.

There are dozens of classic opamp filter designs in textbooks and all over the web. Most of them can be made voltage controlled by using a variable transconductance opamp for the active element. The previously mentioned LM13700 is the most common, as was the now obsolete LM13600.

Many of the analog synths from the 80's used custom chips for the VCO, VCF, and VCA's. These went extinct in the 90's, but are being remade today by several upstart chip makers. These parts are available to the DIYer I have found myself spending a lot of time thinking of simple analogue synth concepts, very simple.
Click to expand...

I am using an 8038, dont go there, it works but is a pain.
Then I switch to simple 555, which is way better, though I can use only pulse or triangle waves.
CA3080 is one old skool solution, as is LM13600/700.
The CEM chips are still produced for this niche market, and that will be my next port of call. Since I really want a 7 osc unison monophonic bass synth

However, in the meantime I have become distracted with uC again, and in particular Rpi Pico
 
#13
oksure said:
Yeah, digipots could work. But it's painful when you need 8 of them in a circuit.
Click to expand...
KDonadio said:
Microchip has quad digital potentiometers so you would only need two of them and an 8-bit microcontroller. The ADC on the micro could take the analog control voltages and talk to the digital potentiometers over I2C or SPI.
Click to expand...
Most digipots are 8 bit. That allows only 256 possible values between fully off and fully on. For simple changes that do not happen in real time during a musical note that are OK and I have used them for applications like range changes on a VCO. In a VCF or VCA situation where the filter is swept while sound is playing, or the amplitude of a sound is varied during play, you will hear each individual step unless the change happens very quickly. This is often called "zipper noise."

I learned a long time ago that it is far too objectional for use as the VCF or VCA in a synth where the timbre or amplitude is being changed dynamically by an ADSR, LFO or MIDI controller. It is a cool effect for some sounds, but really annoying when you always have it.
 
#14
Lots of obscure and very useful data in this thread, I am starring it for own use. 🙂

Not into synthesizers but interested in Audioprocessors ... and I do NOT DSP things, I hate digital with a passion.

A dinosaur? ... may be .... over 50 years of Analog thinking and problem solving does that to people 😉
 
#15
Back in the day I used the CML6000,

https://pdf1.alldatasheet.com/datasheet-pdf/view/91638/ETC/CLM6000.html
see the compressor/limiter project in Craig Anderton's book.......

http://www.adjutojunior.com.br/elet..._PROJECTS_FOR_MUSICIANS_by_Craig_Anderton.pdf
Sadly they are out of production now, Else you need a transconductive opamp stage, CA3080's were used back in the day, Nationals LM13600 is now the modern version,

http://vakits.com/sites/default/files/LM13600.pdf
It is to my understanding the 4 quadrant analog multiplier can do it as well, I think I read in to this a while back.

Else these days I choose to use a 12 or 16 bit multiplying DAC, but again that would be MCU controlled but still pure analog.

One last option is Microchip has several electronic pots and variable rheostats on a chip to choose from as well.

As mentioned FETs are probably still the cheapest and cleanest method of gain control.

They Are basically a Voltage control resistor, the gate voltage sets a particular source to drain resistance no matter what the voltage is across it.

I have tested this it remains the same, at least pretty much so, it does change slightly as it is somewhat non linear but it doesn't venture far from its value set by the gate voltage as referenced its source terminal.

FET's are pretty amazing and versatile once under stand them !! 😉

Cheers !! 😀

jer 🙂
 
#16
Here is one more reference I remember from way back.
It uses a exar2206 signal generator and the chip requires a resistor to ground to set the frequency.
This document shows a method to replace the resistor with two opamps to create a voltage control input to the set pin that typical would be a resistor to ground, cool technic, I have seen this published in many Hobbyist Electronics and Radio Electronics magazines throughout the 80's and 90.

https://www.edn.com/function-generator-has-variable-frequency/
Sorry about the terrible quality of my screen shot.

jer 🙂
 

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#17
Get the ARP 2500 manual. Digest it. Takes a year or two. No sloppy photoresistors, no naked BJT junctions, all wonderfully precise VCAs.

Also study the early MOOG. The MOOG cascode filter looks like a bad dream but can be very musical.

Look at the several generations of PAiA synths. John could do more with one UJT than Perlman could do with a dozen.

And of course a RaspberryPi can do all of the above without temperature drift.
 
#18
If you will have a microcontroller in the circuit, the 8 channel CS3308 or 3318 will be a perfect solution for you.
It will run rings around any other solution, for high dynamic range, very low distortion and noise and down to 0.25dB steps. And its 8 channel.
I've used it in my 6-channel level/balance control for the DCX2496 and dozens have been build with total success.
Jan
 
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#19
PRR said:
Get the ARP 2500 manual. Digest it. Takes a year or two. No sloppy photoresistors, no naked BJT junctions, all wonderfully precise VCAs.

Also study the early MOOG. The MOOG cascode filter looks like a bad dream but can be very musical.
Click to expand...
The OP stated in post #1 that he wanted to use a VCF in analog synthesizer modules. I stated that the most well known VCF is the MOOG ladder filter. The Moog ladder has gone through several changes since it first appeared. Most of them were done to use parts that were currently available at minimum cost. The basic topology remained the same. The schematics for the versions used in the MiniMoog Model D can be found on the web.

A few years ago German music electronics company created a very low cost clone of the MiniMoog and simply called it the "D." I bought a "B stock" return from an online music store for about $200 and ripped it apart. After learning most of its secrets, it was reassembled and now resides in my Eurorack. The "D" uses a clone of the Moog ladder built with all SMD discrete parts. It could also be built with leaded parts. There is a bootleg copy of the "D" schematic floating around the web. It can be found by Googling "P0CQJ Schematic" or "P0CQJ_Schematic" then selecting "images." It should be required reading for those venturing into analog music synthesis. The VCF section is on the right side of sheet 3.

It has been stated that these schematics are not correct. I have not seen any evidence to support that claim, but I did not attempt to verify them. I can state that I have seen a "clone of a clone" filter work on my breadboard using 2N3904's for the NPN's and a 2N3906 for the PNP. Best results are obtained with parts matched for beta using one of those cheap Chinese transistor testers. Parts with a higher beta than the 2N3904 do give a sharper resonant sound. My last attempt used ON semiconductor NSS40301 matched pair transistors, but they are SMD since I need to stuff 6 identical filters into a small space. Panasonic DMC20601's (SMD) work good too but have a different footprint.

The old Curtis Electro-Music CEM3320 chip was the VCF in many analog synths of the 80's including most of the Prophets. That chip went extinct long ago. Nwe and improved versions are being made by TWO different manufacturers today. The ALFA-RPAR AS3320 and the Cool Audio V3320. Cool Audio chips are used in several modern analog synths today including the Neutron and the Deepmind. They are another place to start.
 
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