Voltage Controlled Sallen/Key Filter using JFETs - distortion not wanted!

[LaserSteve]

Here, have a dose of Thomas Henry's really good synth DIY...

http://www.birthofasynth.com/Thomas_Henry/Pages/VCF-1.html

Steve

[mikeg]

I would look at using optical parts. Like an optical compressor. Easier to do. The control voltage drives the LED light source which changes the resistance of the photo resistor.

[DrGonzoDK]

I would look at using optical parts. Like an optical compressor. Easier to do. The control voltage drives the LED light source which changes the resistance of the photo resistor.

That's a really good idea. Also since photoresistors generally decrease their resistance with increasing luminous flux, and an increase of control voltage increases the cutoff frequency (at least, that's what people would expect).

There is a question of linearity - or should i say, exponentiality (i'd generally want it to be somewhat stable with 1V increase corresponding to a doubling frequency); but it's an interesting idea nonetheless. However, it'd require quite a few optically coupled parts.

I like the originality of it though. :-D

[DrGonzoDK]

Here, have a dose of Thomas Henry's really good synth DIY...

http://www.birthofasynth.com/Thomas_Henry/Pages/VCF-1.html

Steve

I'm having quite a lot of fun with the LM13700's.

With a circuit equivalent to the attachment (drew the schematic up really quickly in LTspice, it's kinda ugly...), there is voltage control going on (by varying the voltage at V4 to vary the current into input 1. Obviously the resistor can also vary this, but keeping that constant...) of the simple R/C filter. Both in the real world, and LTspice. ;-)

So the OTA route is quite obviously a very beneficial one. Here, it's just a single OTA being used as a simple voltage controlled resistor feeding into a capacitor. It'll sure come in handy elsewhere...

I still need to get the diode biasing right to improve the linearity and such. But what a fascinating little circuit ^^

[T3sl4co1l]

That's a really good idea. Also since photoresistors generally decrease their resistance with increasing luminous flux, and an increase of control voltage increases the cutoff frequency (at least, that's what people would expect).

There is a question of linearity - or should i say, exponentiality (i'd generally want it to be somewhat stable with 1V increase corresponding to a doubling frequency); but it's an interesting idea nonetheless. However, it'd require quite a few optically coupled parts.

FWIW, photoresistors are quite nicely resistive.  Compare to photoFETs, which are worse than JFETs (the linear range is quite narrow indeed, no matter what the illumination level).  The light response is notoriously quirky though: it has multiple time constants, or diffusion effects, and possibly dependency from illumination level (i.e., the time constants themselves change with intensity).  So, using them for a precision filter or VCO might not be such a great idea.

(They're also not RoHS, though that doesn't seem to have impacted the supply and availability much. )

Tim

[DrGonzoDK]

That's a really good idea. Also since photoresistors generally decrease their resistance with increasing luminous flux, and an increase of control voltage increases the cutoff frequency (at least, that's what people would expect).

There is a question of linearity - or should i say, exponentiality (i'd generally want it to be somewhat stable with 1V increase corresponding to a doubling frequency); but it's an interesting idea nonetheless. However, it'd require quite a few optically coupled parts.

FWIW, photoresistors are quite nicely resistive.  Compare to photoFETs, which are worse than JFETs (the linear range is quite narrow indeed, no matter what the illumination level).  The light response is notoriously quirky though: it has multiple time constants, or diffusion effects, and possibly dependency from illumination level (i.e., the time constants themselves change with intensity).  So, using them for a precision filter or VCO might not be such a great idea.

(They're also not RoHS, though that doesn't seem to have impacted the supply and availability much. )

Tim

It's a fun idea, and it has precedent. Old-school wah wah guitar pedals (basically, a low-pass, all-pass or band-pass filter) used optocouplers to determine the cutoff. Obviously, your foot is not really /that/ much of a precision controller, but it could probably work just fine.

There's also precedent as the original replier wrote - optocoupled compressors and limiters were all the rage in studios in the 70s because of a smoother response than FET VCA-based compressors. Obviously, they did /not/ use LEDs but other lamp types (such as incandescent) which respond somewhat slower to current. But still...

[DrGonzoDK]

So - I finally got down to bodging together a simple LM13700 test today (non-resonant -6db/oct. low pass filter) and, i'm pleased to report, it works 

It's being fed from a single lab power supply; the negative voltage rail is being generated on the left breadboard using 4 parallel LM7660's (I've found that even with very large output capacitors, the voltage stability is much, much better when they are parallelled...) - both some National-branded and TI-branded parts. I prefer the National Semiconductor logo ;-). The control voltage is being generated using the ground and negative rail to the trim potmeter, and then buffered by a (not shown) unity gain buffer, then fed through a single 3k8 resistor and into pin 1 (Iabc) of the half LM13700.

For some reason, I hadn't put in the styroflex bypass caps on the LM13700 when I took this photo - but they're there ;-)

All in all, this is great. Currently working on a proper two-pole resonant filter, as well as getting the control voltage scaling "just right". I gotta say i love the operational transconductance amplifier. It's an underappreciated circuit element - just look at the absolutely tiny amount of content on Youtube on it (yep, looking at you, Dave ;-)).

[floobydust]

For all those wanting to build an analog synth, the main IC's have risen from the ashes

The obsolete SSM2044 4-pole VCF, used in Korg, E-Mu, PPG, Kawai etc. synths is re-issued by Sound Semiconductor as the SSI2144. Price was very good for a rail.

Curtis Electromusic is also remaking the CEM3320 4-pole VCF and CEM3340 VCO.

I think only a good VCA is left needed but Sound Semiconductor is working on a reissue of the SSM 2164/V2164 as the SSI2264 Quad VCA.

Just mentioning it because these analog functions are very hard to design, they made it into IC's, then went obsolete and it's almost lost art

[DrGonzoDK]

For all those wanting to build an analog synth, the main IC's have risen from the ashes

The obsolete SSM2044 4-pole VCF, used in Korg, E-Mu, PPG, Kawai etc. synths is re-issued by Sound Semiconductor as the SSI2144. Price was very good for a rail.

Curtis Electromusic is also remaking the CEM3320 4-pole VCF and CEM3340 VCO.

I think only a good VCA is left needed but Sound Semiconductor is working on a reissue of the SSM 2164/V2164 as the SSI2264 Quad VCA.

Just mentioning it because these analog functions are very hard to design, they made it into IC's, then went obsolete and it's almost lost art

I've had CEM and SSM-based synthesizers, but sonically, these filters don't beat the Moog ladder filter or the Roland Jupiter/Juno filters - so that is why I'm taking the trouble.

I do find it fantastic that these old parts are being reissued, though. The world needs more analog ICs 

[DrGonzoDK]

So - while doing this project, I chanced upon some old early 1970's diagram of non-OTA using, modified Sallen/Key filter designs. They did mention you have to be "imaginative" to acheive it, but if you aren't that concerned about linearity (which I'm not) and more concerned with that ephemeral "sound" - of the resonance especially, reverse biased NPN's do the trick. Essentially, they are configured as current-controlled resistors.

I'm still iterating on this design (hence the horrible OA input and output hi-pass sections), but it sure does acheive what was needed - voltage control of cutoff. And without a large degree of distortion.

To add even more nonlinearity, one could throw antiparallel diodes in the feedback path. All in all, an interesting topology which is straight outta normal Sallen/Key territory (see the attached diagram).

I'll get around to implementing it in a non-bredboard form probably today. Might post some audio examples. Also, I'm thinking of controlling the feedback resistance with a photoresistor, since that bit doesn't seem as amenable to voltage control. But, VC of resonance is a lesser concern :-)