SVF based Stereo Harmonic Chopper-Tremolo Setup

[Mr. Lime]

Some years ago I planned building a harmonic trem with wah functionalities to recreate an effect I once used on an old Zoom multieffect board. The trem LFO frequency went up when the wah shifts to highs and vice versa - I loved it. When I moved to another PC I lost my final PCB design and documentation but from todays perspective, there was room for improvement for sure.

Now when I think about it again, I would like to discuss with you guys, how the setup could be done.

I would take the highpass and lowpass outputs of a state variable filter and add a variable resistor to each path.
With a on-on-on switch, someone could choose between HP, dry or LP combinations, 2 of them are always selected and run to a stereo output for panning effects. The frequency control of the trem LFO and the frequency control of the SVF would be tied together on a wah rocket pedal or via CV input.
Obviously a MCU should be used for the LFO to add neat features with small space needed. The variable resistors get modulated antiparallel (a control for 0-180° phase shift would be awesome to blend between harmonic and classic trem, but not a must).

I always loved when percussive trems have a very wide frequency range so that they could be used as chopper/pseudo ringmod. LDRs would be the easiest way but probably not the best as they are rather slow. Some bitcrusher designs use jFETs or OTAs for ring mod sounds. The LM13700 seems interesting but I read a lot of problems setting them up without carrier bleeding, otherwise I had seen it as an elegant solution for this effect. Any suggestions appreciated!


If a MCU with high PWM rate output is used, can a CD4040 divider be used to bring the range down to usual trem rates or is that a bad idea?

Here's a circuit to show what I'm roughly looking for with the ringmod/chopper part:



I also wanted to ask about you guys opinion on this SVF which has the frequency control placed untypically in a feedback loop:



For reference, I think the Spaceman Delta II is a close commercial design (without the possibility to control the SVF frequency shift by foot or beeing stereo.

Looking forward to your experiences!

[Mr. Lime]

I recently stumbled across the AS3372E synth voicing chip.
Looks like a neat option for a voltage controlled state variable filter with two VCAs for harmonic panning in one! No LDRs or JFETs needed and and Ring mod kind of sounds should be possible with high oscillation speed chopping. There's even a single VCA and a double VCA input mixer left over. Does someone have an idea what they could be used for in a guitar pedal context?
Resonance could be voltage controlled as well but I don't see the usecase for modulating it. Best case for digitally saving the setting and recall it but that would be overkill.

Tricky part might be the LFOs for the CVs for panning and filter frequency shifts.


 
https://electricdruid.net/product/as3372e-synth-voicing-chip/

[ElectricDruid]

The 3372 won't give you an SVF. It's a simple 4-pole lowpass filter. I mean, it's a nice filter, but if you need the highpass and lowpass outputs simultaneously then a SVF is your only option.
Rod Elliot has a good page about SVFs, and he has an example using it as a crossover, which is essential what you're doing in a harmonic trem.

https://sound-au.com/articles/state-variable.htm

BTW, note the inversion of the LP output in his Fig.2. I can never get this straight in my head, because the integrators are inverting, but the filtering also introduces a phase-shift. If it was just the integrators inversions, HP and LP are inverted w.r.t the input, and BP is the same way up. But the integrators introduce a 90degree shift at the cutoff too, so the LP output is actually 180 degrees out w.r.t the HP...I THINK?!?

That design you showed that uses one of the feedback paths to change the frequency is going to have side-effects. Either the resonance will change when you change the frequency, or the gain will. The reason the main integrator resistors are usually used is because that enables the resonance and the frequency to be independent.

You also need your pair of VCAs for the panning/mixing part too. LM13700 is certainly a reasonable choice. AS3360 is perhaps easier to implement.
Don't get tangled up with AM versus RM. Which do you want to do? A "proper" tremolo is AM and won't go "through zero" and give you inverted output for -ve inputs. Balancing a Ring Mod is worse than stopping a tremolo from having feedthrough in my experience, but both are tricky if you're talking about audio rate modulation and square waves!

HTH

[Mr. Lime]

Thanks a lot Tom,

I've overseen the difference of the multimode lowpass vs. SVF.
Good read on the crossover circuit and the phase shifting! I read several times that the caps seem to cause phase issues but with an inverting stage at either the HP or LP it seems fine.

Thank you too for the explanation of the simplified SVF circuit earlier.

How about using a quad VCA like the AS2164 or is it wise to rather take two seperate AS3360s? Two VCAs for the SVF frequency control and the other two for the L/R panning?
Interesting would be if the AS2164 / AS3360 works fine with microcontrollers like PIC, Attiny85, Raspberry Pico.
Would be cool to use something different than LDRs.

I don't like the idea of using the LM13700 due to the buffers that can't substitue a proper TL72 buffer for each side.

A fast switching VCA trem is probably close enough for me to ditch ring modulation.

[ElectricDruid]

Ooh, yeah, a single '2164 is a nice idea. Two VCA's for the SVF, and two left over for the crossfading. The control is simple with those chips - 3.3V to 0V, no complications, so easy to get from a uP running on either 3.3V or 5V.

Incidentally, I've had some older Dreadbox synths go across my workbench recently, from the era before many cloned synth chips were easily available, and the other options were basically only the LM13700, so the things are full of V2164's, doing basically everything. If you check out their product descriptions, you'll see that they often talk about "Expo VCAs" - no surprise, if you know the chip they're using. Example:

https://www.juno.co.uk/products/dreadbox-eudemonia-filter-mixer-vca-module/1080746-01

Dave Dixon's Intellijel stuff is another testament to the fact that you can do basically anything you need to do with the '2164! It's a great chip.