modulated filter suggestions?

[blackieNYC]

I want to build a LFO modulated filter.  Not a phaser or flanger, but something like a wah, or an MS-20, some other HP or LP, or maybe Jack Orman's version of the Anderson wah anti-wah.  Probably of the pot-swap-for-LDR kind of thing. (Or a LDR replacing an expression pedal) Any recommendations?  I don't even know - were there commercial pedals like this?
I really like square wave and random LFOs. I plan to try Electric Druid's new 8pin LFO chip.

It occurs to me that, with the exception of the standard wah wah pedal, these circuits are designed to have a filter adjustment pot.  You set it and forget it.  Or maybe you're down on one knee twiddlin'.  Maybe some of these circuits wouldn't sound good if modulated slowly.  Don't you think some circuits will seem to have dead spots?  In a standard wah circuit, the range is "perfect". So, I'm hoping to adjust my LFO depth or LDR resistance to sit in that zone.Will I be able to control the frequency range adequately with an LDR?  Think any of the filters will give me trouble?

Anyway, if you know of something that would be a good place to start, or if this wheel already has been thoroughly over-invented, please share.  Thanks.

[Rixen]

I like Fredriks 'Sentient Machine'

http://www.diystompboxes.com/smfforum/index.php?topic=113535.0

[Keppy]

For random filter, the Maestro FSH-1 is popular.

For LFO, have you looked at the Lightwah?

[ElectricDruid]

I've been doing the same thing myself!

I've pretty much decided to use the Mutron filter as the basis, since you can feed the PDM output from the new 8-pin StompLFO direct to an LDR without any filtering or anything, so it makes a pretty minimal-parts solution.

http://www.generalguitargadgets.com/pdf/ggg_neutron_sc.pdf

I've also looked at 13700 state variable filters(SVF) too though, but I had trouble with the thing thumping at the low end. This is caused by a combination of DC offsets (which you can trim out, but I hate trimmers) and the output varying with Abc, which you can't avoid except to reduce the Iabc you use to the minimum necessary. Rick Holt used one in his Funky MF and I haven't heard any complaints about that, so either I'm being fussy or his design improves on mine in some way (haven't compared closely, but they're bound to be very similar)

http://www.diystompboxes.com/smfforum/index.php?topic=73021.180

I like the SVF for these jobs, since you get the option of highness, bandpass and lowpass outputs.

The Sentient Machine filter is a PWM-Controlled SVF, and I've wondered about releasing a PWM version of the StompLFO to make PWM phasers and filters really simple to do. The current PDM output isn't really suitable (frequency too high). The other thing about the Sentient Machine is that it's a dual filter, so it's a more complicated circuit.

HTH,
Tom

[Mark Hammer]

I want to build a LFO modulated filter.  Not a phaser or flanger, but something like a wah, or an MS-20, some other HP or LP, or maybe Jack Orman's version of the Anderson wah anti-wah.  Probably of the pot-swap-for-LDR kind of thing. (Or a LDR replacing an expression pedal) Any recommendations?  I don't even know - were there commercial pedals like this?
I really like square wave and random LFOs. I plan to try Electric Druid's new 8pin LFO chip.

It occurs to me that, with the exception of the standard wah wah pedal, these circuits are designed to have a filter adjustment pot.  You set it and forget it.  Or maybe you're down on one knee twiddlin'.  Maybe some of these circuits wouldn't sound good if modulated slowly.  Don't you think some circuits will seem to have dead spots?  In a standard wah circuit, the range is "perfect". So, I'm hoping to adjust my LFO depth or LDR resistance to sit in that zone.Will I be able to control the frequency range adequately with an LDR?  Think any of the filters will give me trouble?

Anyway, if you know of something that would be a good place to start, or if this wheel already has been thoroughly over-invented, please share.  Thanks.

The old MXR Auto Q envelope filter included an LFO, whose impact on the filter could be blended with the envelope.  The EHX Blurst modulates a filter.  And Chase Bliss's new Condor pedal is also capable of modulating a lowpass filter.  I'm assuming that the Auto Q uses some of what was in the Envelope Filter pedal, but I have no idea how those other two pedals modulate filters.  The PAiA Synthespin was a modulated bandpass filter, intended to mimic, albeit crudely, a rotating speaker.  It used a FET to provide the control element that would vary the filter center frequency, driven by the standard phase-shift oscillator we see on the EA tremolo, and so many other circuits.  No reason you couldn't adapt the Synthespin to op-amps.  No reason you couldn't have two bandpass filters being swept independent unsynced LFOs, and mixed together.

Modulating a filter strikes me as something that bears a resemblance to using vibrato pedals.  That is, it's something the user probably wants  to have increase or decrease in effect strength, by intention, rather than simply leave on all the time.  More palatable as a continuous effect than a ring modulator, but not as palatable as a chorus.  It's sort of the kind of thing you'd like to be able to hit a momentary switch for a one-shot ramp-up in intensity.

[PMowdes]

What about deadastronauts Tremshifter???

Sent from my F5321 using Tapatalk

[Aph]

The PAiA Synthespin was a modulated bandpass filter, intended to mimic, albeit crudely, a rotating speaker.  It used a FET to provide the control element that would vary the filter center frequency, driven by the standard phase-shift oscillator we see on the EA tremolo, and so many other circuits.  No reason you couldn't adapt the Synthespin to op-amps.

Actually, the Synthespin used two diodes as the control element and a VCO as the LFO. It was opamp based (though with the LM3900). PAiA used the VCO design in other products they produced.
No matter what I tried, I could never get rid of the ticking. The schematic is still on PAiA's site, I think.
I reworked the Synthespin schematic with a better LFO and common opamps and added a 4-way switch to get progressively deeper wah sounds.
I actually would just go with a two way switch. The first position has a nice "airy" sound and the second one has a more traditional "wah" sound.
The diodes can distort and at the heavier wah positions, it's probably too much.



Edit: You're right Mark... the first version was a transistor design. But, I think it was an article in Popular Electronics? The kit I put together in the 70's was a Synthespin MK-II.

[Mark Hammer]

No problem.  I was working from the PAiA build manual, posted on the PAiA Talk web page.  They used a FET, rather than a bipolar, like so many other swept bandpass filters of that era (including Anderton's Motion Filter that they sold).

[blackieNYC]

I think the Lightwah is right on the money!  I'll give myself some alternate LFOs, but this is perfect. Thanks!