Limiting the sweep range on edp wasp filter

[aloupos]

I've built the edp wasp filter found here http://www.modular.fonik.de/Page20.html

I'm trying to use it in a wah shell, but using the 10K pot specified produces to little usable range and the rest sweeps either higher or lower. 

I've tried putting another pot in parallel, hoping to control the range, but it didn't work as I expected. 

How can I dial in the correct sweep range ?

Thanks

[slacker]

The "easy" way to limit the range of the pot is to put resistors in series with it. To limit how high it goes increase the size of R17. To limit how low it goes add a resistor between the other outer lug and R18. If you do this you'll probably need to play about with the values a bit to get the sweep you want because changing one of the resistor values will then affect the sweep so each time you change one you'll probably need to adjust the other one to compensate.
I'd temporarily use a couple of pots and then replace them with fixed resistors once you've got it where you want.

You might be able to set one of the limits just by adjusting the position of the pot once it's fitted to the wah shell, because normally wah pedals don't sweep the whole of the pot's rotation.

[Mark Hammer]

Keep in mind that, as a synth filter, it is intended to provide note-appropriate filtering.  That is, the practical cutoff frequency "floats" with whatever the note is that is being processed and the CV accompanying that pitch.  As such, the 10k cutoff pot shown is supposed to work in tandem with the control voltage coming in from the far right, where it says "CV".  The 10k pot is really intended to be more of a compensation than a control worked in real time.  So don't expect its taper or range to be suitable as a guitar wah, no matter how great the filter is sonically.

It may be the case that you need to replace the 10k with a 5k pot and suitable resistors on each of the outside lugs to produce the total 11k resistance shown and focus control on a portion of the range most suitable for guitar.  For example, maybe it needs to have 2k7 on one side and 3k3 on the other (added to the 5k of the pot).

[aloupos]

Keep in mind that, as a synth filter, it is intended to provide note-appropriate filtering.  That is, the practical cutoff frequency "floats" with whatever the note is that is being processed and the CV accompanying that pitch.  As such, the 10k cutoff pot shown is supposed to work in tandem with the control voltage coming in from the far right, where it says "CV".  The 10k pot is really intended to be more of a compensation than a control worked in real time.  So don't expect its taper or range to be suitable as a guitar wah, no matter how great the filter is sonically.

It may be the case that you need to replace the 10k with a 5k pot and suitable resistors on each of the outside lugs to produce the total 11k resistance shown and focus control on a portion of the range most suitable for guitar.  For example, maybe it needs to have 2k7 on one side and 3k3 on the other (added to the 5k of the pot).

Interesting.  So if this were used in a synth, you would send the CV for the note to the filter?  I was always under the impression that you would send CV from an LFO or an ADSR envelope generator to it.   Under those circumstances, even though I wont be able to produce a CV for the guitar note(s), should I build a manual CV control, sending between 0-9 volts to control the filter rather than using the cutoff?  I'm not really so interested in producing a wah, but more of a synth like filter. 

In the meantime, I'll give slacker's approach a try, using trimpots. 

Thanks guys

[Mark Hammer]

Interesting.  So if this were used in a synth, you would send the CV for the note to the filter?  I was always under the impression that you would send CV from an LFO or an ADSR envelope generator to it.   Under those circumstances, even though I wont be able to produce a CV for the guitar note(s), should I build a manual CV control, sending between 0-9 volts to control the filter rather than using the cutoff?  I'm not really so interested in producing a wah, but more of a synth like filter. 

Well, you are correct. That is why whenever you see a schematic for a synth module, you will often see 4 or more CV inputs/jacks.  One is for the note so that whatever the unit does is note-appropriate.  Another is for an LFO or other modulation source.  Another is for an envelope generator.  And usually a 4th one is the "default" value which the other three add to and take away from.  Some modules will have more CV inputs than that if there is any reason to anticipate other sources (e.g., 2 LFOs).

It IS possible to produce a CV that corresponds to pitch, but then that assumes you are playing only one note at a time.  My sense is that you really only want this to be controlled by your foot and maybe an envelope follower, and that you want to be able to filter chords as well as notes.  In which case, a pitch-tracker is of no practical use.

[aloupos]

Exactly, so, does it make more sense to use build a circuit that uses a pot to vary 0-10 volts (CV), or use the cutoff? 

[frequencycentral]

As a modular synth user/builder, and former owner of a EDP Wasp, I find it endlessly amusing to read about guitarists' takes on filters!

It is possible to get a filter to track the pitch of the oscillators in a synth, but one of the big secrets of olders synths was that this wasn't that accurate. Some synths have filters with continuously variable tracking of the keyboard CV, while some notable synths from Moog just had 0%, 50% or 100%. The Korg MS20 had no filter tracking of the keyboard CV without it being patched in - which is part of the sound of that synth. Filter tracking of the keyboard is very natural sounding - but who wants a synth to sound natural?

In practice THE great analogue synth sound is the minimum to maximum to minimum filter sweep with resonance just short of the point of self oscillation. The Wasp filter never did quite get to self oscillation though. Still an excellent filter. I sold my Wasp for £500 to buy my daughter a Gibson Les Paul Studio which she lost interest in within a year. Heyho.

[frequencycentral]

Exactly, so, does it make more sense to use build a circuit that uses a pot to vary 0-10 volts (CV), or use the cutoff? 

They are the same thing.

The key, as slacker said, is to get the wah pot to use the whole useable range of the filter.

[Mark Hammer]

As a modular synth user/builder, and former owner of a EDP Wasp, I find it endlessly amusing to read about guitarists' takes on filters!

It is possible to get a filter to track the pitch of the oscillators in a synth, but one of the big secrets of olders synths was that this wasn't that accurate. Some synths have filters with continuously variable tracking of the keyboard CV, while some notable synths from Moog just had 0%, 50% or 100%. The Korg MS20 had no filter tracking of the keyboard CV without it being patched in - which is part of the sound of that synth. Filter tracking of the keyboard is very natural sounding - but who wants a synth to sound natural?

I suppose you're right.  Wise to keep in mind though, that the tracking needs of mono synths are somewhat different than those of polyphonic synths.  In a sense, poly synths are obliged to having tighter tracking of individual note pitch in order to keep notes more distinct when chords are played.  But you are certainly correct for the synths you mention, which are all classic instances of mono.

[loss1234]

don't forget...as long as you set the highest and lowest points of the cutoff to be where you like them, you can then use the CV ins to plug in the outputs of lfos or env followers, etc with GREAT effect.

i used a CV based synth filter in my AUDIO ARK and just because a filter CAN track notes (1/v oct) on a synth, doesnt mean the cutoff knob in conjunction with some realtime modulation (like envelope follower) cant sound just as good.

and the nice thing is that really, you can put ANYTHING into the cv inputs...even audio.

[aloupos]

Actually, I think my wasp circuit has found its home, as a live filter for vocals.  I gave it to our singer for xmas   i put it in a stomp enclosure with one huge filter knob and a bypass switch -- she hangs it from the mic stand

I did try the suggestions on limiting the sweep range, but never really got a usable sweep from a foot pedal. 

Thanks for all the suggestions!