Wasp filter clone anyone ?

[Artesia]

Heres a wasp filter clone, should it interest anyone:

http://jhaible.heim.at/tonline_stuff/hj_wasp.html

JH Wasp Filter Clone

(from a message to synth-diy mailing list in Jan. 98)

Hi,

On the weekend I have finally tested the wasp filter clone, and made some modifications. Results so far:

It sounds considerably different than the ordinary SEM-type state variable filter. The maximum Q is lower on the Wasp version. And there is an additional distortion coming from the CMOS inverter nonlinearities. This distortion is gradually increasing with input level, and you can slightly hear it way before the circuit actually clips. The CMOS inverters seem to be the dominant source of distortion; the CA3080 input dividers are rather on the save side. (100k / 1k ; but remember the absolute maximum voltage swing is limited to 5V anyway.) I have replaced the 100k resistors with 51k without noticeable increase of distortion.

Ok, the Wasp Filter sounds different than other state variables. But what is it good? IMO, every filter's overdrive characteristic has its own special applications. The SSM2040 in LP configuration, for example, is unbeatable when you want to process a full chord of buzzy "Jump"-type saw voices. The Wasp Filter works best on Farfisa-type *organ* sounds. I tried various sounds from my OB-8, and really, the best results came with bright organ sounds. My favorite patch is the filter in LP mode with medium cutoff, and then an envelope with slow attack opening the filter (resonance quite low). This certain "edge" that is added by the filter's distortion is hard to describe, but very pleasant.

I had this circuit at the breadboard first, and I was so pleased with its sound that I build it again on a tiny veroboard. The whole filter consists of 3/6 CD4069 and two 3080's, counting the active components. I built a notch filter from another 1/6 CD4069 (add HP and LP), a heavy distortion section (1/6 4069 with 1meg feedback resistor, and 100nF input capacitor without series resistor), and a fixed 2pole, 5kHz LPF from the remaining 1/6 4069 as a speaker simulator for the distortion section. Now use the SVF to preshape the frequency response of some input signal, and then go into the overdrive / speaker simulator. LPF, BPF and even HPF settings sound pleasant thru the overdrive, with or without resonance. For some reason the notch filter doesn't work well in this configuration. (But it makes a great "one notch phaser" without the overdrive, and controlled by an LFO.)

I was really astonished what you can get out of one single CMOS chip (the OTAs just act as variable resistors, so they don't get much credit here.)

I've also made some other experiments with the 4069. You can build a little saw VCO just with one 4069 and a npn pair: One inverter as integrator, two more for the hysteretic switch, another one as opamp that regulates the current in the exponential converter. Use another one for scaling / level shift and drive a pnp transistor with equal collector and emitter resistors, and you have a poor man's version of the VCS-3 variable triangle/saw VCO. Or overdrive another inverter with a triangle wave, and add a DC offset, and you have a poor man's VCS-3 - "sine with waveform control". Ok, these things have limitations. You *always* get a glitch when the Integrator in the VCO changes direction. And the waveshapers work far from ideal, i.e. bend a positive slope more than a negative one, and I have not even tested the accuracy of the expo converter. But for one little CMOS chip plus a few transistors this makes a hell of a utility and modulation VCO.

Later, I decided to build a special ADSR / LFO circuit for this filter. This is surely not the best ADSR circuit around (time range is limited, offset voltages due to single supply ...), but my design goals were a cheap circuit and 5V single supply operation. I don't think it makes much sense to build it as a standalone envelope generator, but together with the Wasp Filter it makes a nice little processing box.

The whole combination of the separate parts has some unusual aspects that one might consider suboptimal design, but which I have chosen deliberately to have something different from my other modules:

    * No expo converter for  the filter. Manual Cutoff is set by a logarithmic potentiometer, and the envelope is added to this exponential CV. This results in a certain interaction of Manual Cutoff and Envelope Depth settings. I wouldn't choose this for my first VCF, but it is an interesting alternative if you already have a lot of filters that "do it the right way".

    * The Distortion is located after the filter. Usually, you create rich harmonics with an overdrive, and then you filter these harmonics with a VCF, a phaser, or something similar. But it is also very interesting to have a VCF before a distortion unit, to pre-shape the waveform that feeds the distortion in a dynamic way. The avoid the very sharp sound of a pure distortion unit, a fixed filter ("speaker simulator") is hard-wired after the overdrive section.

    * The state variable filter normally doesn't self oscillate. However, if you plug in a long cable at it's output, it will. This is because the CMOS buffers have a rather large output impedance which will form an extra filter pole together with the cable's capacitance. This could easily be cured with an extra buffer stage, but I decided not to do it in my unit. Remember, this is a "low fi" device, and didn't even the Floyd create some of their most interesting FX sounds by plugging in a WahWah pedal backwards (;->) ??

    * The Filter and Distortion Schematics

[AndrewMartens]

JH's Wasp Filter clone was on my "to do" project list for about 6 months.  A few weeks ago, I finally breadboarded it.  Alas, my first attempt met with no luck - I'm not sure if my CA3080s were shot, or if I just screwed up the circuit.  The fact that it runs on +5VDC is fantastic, and I know that many people have built this circuit successfully.  IIRC, the cutoff control worked okay, though the filter slope didn't sound steep enough, and all the resonance control did was decrease the volume instead of... well, increasing resonance.  Right now the breadboard is just sitting there, waiting for me to go back and figure out WTF I did wrong with it. 

Since then I tried a few other projects:  Rat clone (success!), a +9VDC single-supply MS-20 filter clone that had an okay cutoff but just squealed noisily at resonance (scrapped that one), the "UBE Screamer" 4049/4069-based distortion (vey noisy, even at low gain,though it did distort nicely).

I'd be interested to see if anyone else here had success with the Wasp Filter.  I followed JH's schematic exactly, even checking the Synth-DIY archives to determine typical values for the unlabelled components in the Cutoff section at the bottom right (I'll go and check what I put on my breadboard later).  I think Cyndustries makes a dual-Wasp commercial version, and the sound samples were quite interesting.  Hopefully I will have a chance to re-assemble this filter and determine what exactly I did wrong.

[Artesia]

be very carful about your chioce of capacitors in audio circuits... especially in filter circuits, the chioce of the right ones or the wrong ones can make a huge difference.

[AndrewMartens]

be very carful about your chioce of capacitors in audio circuits... especially in filter circuits, the chioce of the right ones or the wrong ones can make a huge difference.

Yeah, that's probably the issue.  I'm using mylar caps everywhere in there, with the exception of the 68pF in the notch circuit, and the 10uF at the output.  I'd use polystyrene, but I can't find those for purchase anywhere convenient.  I'll probably just have to go with "inconvenient" and deal with the hassle of getting things shipped across the border from Mouser - at least they carry them at a reasonable price.

But in the meanwhile, I'll probably rebuild my Wasp filter over the holidays and double-check all the components values again.

[cheater]

Just wondering...

what cap types have you guys had the best experiences with?

What prices did you get on those parts and where?

Thanks,

D.