Wasp VCF Questions...

[anotherjim]

So I've built this...

... and it works, LM13700 substituted for the CA3080's pin for pin.
Now some questions....

The 10k log filter Cutoff control pot is not from the original synth - that has the adjustment back behind what looks like a BJT expo converter. The pot cuts off too early at the bass end (about 9 O'clock).
You may notice the missing value of that resistor to 0v. It is assumed elsewhere to be 1k which I've fitted, although that end of the pot is clamped by a diode drop anyway so I don't think the resistor value is too critical.
Since I want to use it in an old volume pedal shell. Would it make better sense to mount that Cutoff pot as a master Range control and fit another feeding the 10k CV input in the treadle pot position for the actual sweeping? Has anyone tried any of this? My first instinct was to use that Cutoff pot in the treadle and since it won't allow full rotation used there, it could be ok.

The Resonance pot shown is 47k which I've used. The effect ranges from negligible to peaky, but no self-oscillation - the Wasp filter can't oscillate in normal use. The sweep of the 47k lin pot feels ok, however, the original synth had a 100k pot. Is it missing anything by not being 100k?

But perhaps the biggest question is...

...should I paint the pedal yellow?

[garcho]

nice project!

that has the adjustment back behind what looks like a BJT expo converter. The pot cuts off too early at the bass end (about 9 O'clock).

that's the allure of exponential current sources for the OTA

My first instinct was to use that Cutoff pot in the treadle and since it won't allow full rotation used there, it could be ok.

I say always try the simplest solution first, especially since you're fiddling with it already.

But perhaps the biggest question is...
...should I paint the pedal yellow?

not all wasps are yellow this one looks like elvis in his vegas period attire

[ElectricDruid]

So I've built this...

... and it works, LM13700 substituted for the CA3080's pin for pin.

Nice work! And a bit of classic from synth history, if I might say so.

The Resonance pot shown is 47k which I've used. The effect ranges from negligible to peaky, but no self-oscillation - the Wasp filter can't oscillate in normal use. The sweep of the 47k lin pot feels ok, however, the original synth had a 100k pot. Is it missing anything by not being 100k?

It's wired as a voltage divider, so the actual value probably isn't that significant. It's worth pointing out that in this style of SVF circuit, the "resonance" control actually works in reverse - it provides negative feedback that damps an oscillation that would be there without the feedback. "Max feedback" will be with the pot's wiper at ground.

That's why there's that diode clipper network in parallel with it. If the filter starts to resonate, the signal will break over the diode threshold, and more damping will be applied, bringing it back under control. This type of limiting is pretty common in synth filters to try and keep the resonance smooth and sine-like, rather than very loud, squawky, and clipped hard against the rails! Though obviously the form it takes depend on the specifics of each circuit.

[bluebunny]

But perhaps the biggest question is...

...should I paint the pedal yellow?

Definitely.  With stripes.   

[anotherjim]

It's worth pointing out that in this style of SVF circuit, the "resonance" control actually works in reverse

True Tom, as I found out the hard way!

As I measured it (running the filter core on 5v as per original), the Cutoff control frequency becomes inaudibly low at about 1.2v. JH may have intended the scheme as drawn to be fitted in a generic volume pedal, but I don't know if it was an idea or an actual build, but it might explain the action of this control if it was driven from a treadle rack & pinion. The pedal I have came from Maplin a loooooong time back.
Anyway, if the control were to be panel mounted, it might be better to use 2 diodes in series to 0v and delete the 1k resistor across that.

Incidentally, the LP path is inverting and the overall gain with low res and max cutoff is very close to unity. So I'm making a x5 input buffer (working off 9v) and an inverting unity output buffer (probably TL072) followed by a volume control.

[ElectricDruid]

As I measured it (running the filter core on 5v as per original), the Cutoff control frequency becomes inaudibly low at about 1.2v. JH may have intended the scheme as drawn to be fitted in a generic volume pedal, but I don't know if it was an idea or an actual build, but it might explain the action of this control if it was driven from a treadle rack & pinion. The pedal I have came from Maplin a loooooong time back.
Anyway, if the control were to be panel mounted, it might be better to use 2 diodes in series to 0v and delete the 1k resistor across that.

Could that be because of the 13700 rather than the 3080? Doesn't one use twice the Iabc of the other? Or am I remembering that wrong? <goes to check>
Turns out both have an absolute max of 2mA Iabc. But the gm is adjustable over 6 decades on the LM13700, and only 3 decades on the CA3080. So the LM13700 might seem "oversensitive" in comparison to the CA3080!

Incidentally, the LP path is inverting and the overall gain with low res and max cutoff is very close to unity. So I'm making a x5 input buffer (working off 9v) and an inverting unity output buffer (probably TL072) followed by a volume control.

Are the integrator stages in this inverting too? They usually are, and the integrator part seems to be here too, but the OTA could be wired up to be inverting so that the stage as whole was non-inverting. But that would mean that the BP and LP feedback were the same phase, and that can't be, so forget it. The integrator stages _must_ be inverting too, otherwise the feedback paths don't work.
In which case, HP and LP are flipped wrt the input, BP and Notch are in phase.

[Rob Strand]

FYI, this guy went through some Daft Punk Sounds using a Korg MS20,

https://www.youtube.com/watch?v=W1cpim5EAqI

He mentions getting the overdrive sound by overloading the filters in the Korg MS20.
The Korg MS20 uses that Korg filter chip.  I've seen the filter chip schematic on line at
some point. It was very simple only a few transistors.

[PRR]

> gm is adjustable over 6 decades on the LM13700, and only 3 decades on the CA3080

I think that is measurement limits and marketing. In this regard, both have the same architecture and even the old '3080 would have more than 3 decades, but not easy to measure.

IIRC, one sits at 0.6V at Iabc pin and the other at 1.2V. This affects rigs that have some voltage-drive component instead of "proper" current drive.

[anotherjim]

IIRC, one sits at 0.6V at Iabc pin and the other at 1.2V. This affects rigs that have some voltage-drive component instead of "proper" current drive.

Thanks Paul, it looks like that's the answer, there are 2 junction drops to get over on the control pins.
So a 13700 build really wants 2 diodes to 0v on the Cutoff pot. I'll wait & see how it sweeps in a treadle mount before modding mine.

FYI, this guy went through some Daft Punk Sounds using a Korg MS20,

https://www.youtube.com/watch?v=W1cpim5EAqI

He mentions getting the overdrive sound by overloading the filters in the Korg MS20.
The Korg MS20 uses that Korg filter chip.  I've seen the filter chip schematic online at
some point. It was very simple only a few transistors.

The best source I've found for the Korg35...
https://www.aleph.co.jp/%7Etakeda/radio/MS20clone2E.html
That has to be taken in with the surrounding MS20 architecture. x1 35 in LP and x1 in HP. Odd. I've only ever used an MS10 which only had one 35 module. I don't remember being too enthralled by the MS10.

Tom, yes the integrators are inverting. Every active stage is inverting. I've only used three of the 4069 inverters, so some could be used as x1 inverting amplifiers to put those polarities right.

[alanp]

Has anyone tried other VCFs with guitar?

I have a suspicion that the Steiner-Parker filter would sound good, it's one of the more aggressive filter's I've heard (especially with high resonance, where it gets unruly.) Yusynth have a PCB layout for this one.

[snk]

I didn't used them with guitar, but both the MS20 (Steiner Parker) and Polivoks filters are rough and overdriven by nature.
The WASP vcf has a nice vibe and a great distorsion, but it won't self-oscillate like the MS20 or the Polivoks do.

Yusynth have a PCB layout for this one.

Beware : the Yusynth PCB must be for a modular (therefore expecting a +/-15V power supply), not the "regular guitar pedal" 9V. Also, the PCB size will be sized according to the Eurorack/Motm format, so it might be hard to find a suitable enclosure for it.

[anotherjim]

Tom wrote...

That's why there's that diode clipper network in parallel with it. If the filter starts to resonate, the signal will break over the diode threshold, and more damping will be applied, bringing it back under control. This type of limiting is pretty common in synth filters to try and keep the resonance smooth and sine-like, rather than very loud, squawky, and clipped hard against the rails! Though obviously the form it takes depend on the specifics of each circuit.

A hack I've seen (which should be obvious to those in stompbox world), is to switch the diodes over to LED's. I don't think I've heard this but I might try it.

[garcho]

Has anyone tried other VCFs with guitar?

https://schmitzbits.de/ms20.html

Sounds great, works with LM13700 just fine.


also, this is on the breadboard now, it sounds great:

https://electricdruid.net/cem3320-filter-designs/

[noisette]

I don´t know if it has been mentioned already but you could get self oscillation by introducing a little bit of
positive feedback from the bp out (additionally to the existing negative one), you´d need another summer/inverter but maybe it would be worth it?
The magic of WASP imo lies in the linearized cmos inverters, ms20 is also cool but then you may need two: hp and lp in series for a kickass variable bandwith filter like the original!

[Rob Strand]

The best source I've found for the Korg35...
https://www.aleph.co.jp/%7Etakeda/radio/MS20clone2E.html
That has to be taken in with the surrounding MS20 architecture. x1 35 in LP and x1 in HP. Odd. I've only ever used an MS10 which only had one 35 module. I don't remember being too enthralled by the MS10.

Cool link thanks.   I've only noticed this stuff in passing over the years.  I can't remember the links but the name Jurgen Haible mentioned in your link rings a bell.

[ElectricDruid]

I've only noticed this stuff in passing over the years.  I can't remember the links but the name Jugen Haible mentioned in your link rings a bell.

It should do. Jürgen produced no end of interesting designs. Although he's no longer around, some of his work still is:

http://jhaible.com

He was an absolute pioneer in Synth-DIY, and a skilled analog designer.

[bluebunny]

Jürgen produced no end of interesting designs.

He was an absolute pioneer in Synth-DIY, and a skilled analog designer.

Hear hear.

[Rob Strand]

It should do. Jürgen produced no end of interesting designs. Although he's no longer around, some of his work still is:

http://jhaible.com

He was an absolute pioneer in Synth-DIY, and a skilled analog designer.

I guess at the end of the day I'm not a synth dude but I do appreciate the analog trickery used by analog synth designers.

[noisette]

I've only noticed this stuff in passing over the years.  I can't remember the links but the name Jugen Haible mentioned in your link rings a bell.

It should do. Jürgen produced no end of interesting designs. Although he's no longer around, some of his work still is:

http://jhaible.com


He was an absolute pioneer in Synth-DIY, and a skilled analog designer.

Random Source has a lot of his PCBs for sale!

[anotherjim]

Back on the Wasp project...

I've found it's worth fitting 2 series diodes to 0v at the bottom of the Cutoff pot and it doesn't need the 1k there.
Found the best pot is 10k log as shown. The filter sounds fully open before it hits the CW end, but my treadle mount suits it so I can get a full sweep without danger of the pot hitting it's own stop at either end.

The 47k pot I'd used for Resonance had to go 'cause it dont have the splined shaft for the knobs I want to use. Nearest linear I had was 22k so I tried that, and it works fine & will probably stay.

One thing I noticed with the original single diode is that at the CCW end of the Cutoff, just before the filter cuts off, I got this big fat giant rubber ball of a bass tone - but it's too fine to work reliably with the foot treadle. Anybody else noticed that?

For added interest, here's the Maplin volume pedal I've used for my Wasp filter...
https://www.muzines.co.uk/articles/volume-pedal/3686