Auto wah type filter with reverse?

[jimilee]

Does a simple filter with reverse exist? The filter would close instead of open when a chord or note is struck.


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[somnif]

Just take an autowah and plug it in backwards!

...that's logical, right?

[reddesert]

Do you mean a filter that sweeps down in frequency instead of up?  One of the common mods for the old MXR envelope filter is to add an up/down switch.  There is a Tonepad project for it, and a schematic at GGG: http://www.generalguitargadgets.com/wp-content/uploads/ggg_mxr_envelope_filter_mods.pdf - the mod is the "reverse sweep." Smallbear sells the tonepad PCB if you want one (old school single sided PCB).

This mod is possible because of the interesting way the MXR does the filtering - it uses pulse width modulation of a clock signal at ultrasonic frequency, and that signal opens/closes a gate to effectively vary resistance and change the bandpass frequency of the filter. The reverse sweep mod  inverts the clock, so the PWM makes the filter go down instead of up. I am building one of these using the Tonepad layout on perfboard (which is a little nuts). I haven't finished it yet, but I do have the clock running, which is sometimes tricky apparently, and modulating the filter.

[alanp]

Jimi, IIRC you have a Rissole, that has an up/down switch

[jimilee]

Jimi, IIRC you have a Rissole, that has an up/down switch

And I love it, I was just looking for something simple.


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[sirbergersworth]

Doesn't seem he wants a reversible sweep (up or down). He wants the envelope to act in reverse. Gotta be something like that out there. Can you do something similar with the slow gear?


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[midwayfair]

Pick a simple design you like and I'll take a look at the schematic and tell you what to change.

[reddesert]

In simplified terms, all of these (autowahs, Slow Gear, compressors, etc) are examples of voltage control. For guitar pedals, typically the control signal comes from an envelope follower, something like a rectifier and smoothing capacitor that turns the input signal into a signal that follows the volume envelope. For synthesizers, the envelope may come from an envelope generator triggered by a key press.

For an autowah, the control is fed into a VCF, voltage controlled filter, that changes the filter frequency.  The filter could be bandpass or low or high-pass. That simulates rocking a wah pedal as a note is struck.  The up/down switch is like choosing to rock the wah back or forward.

For a compressor the envelope is fed into a VCA, voltage controlled amplifier, that changes the volume instead of frequency, to turn down the gain when the envelope is strongest.

The Slow Gear is an extreme variant of the VCA compressor that makes the volume ramp up as the envelope decays.

[madbean]

Pick a simple design you like and I'll take a look at the schematic and tell you what to change.

[reddesert]

Bean, that's cruel.

Here is the mojo schematic. You can practically feel the toan:

[midwayfair]

Pick a simple design you like and I'll take a look at the schematic and tell you what to change.

My first recommendation is to replace all the chips with discrete components.

[jimilee]

Doesn't seem he wants a reversible sweep (up or down). He wants the envelope to act in reverse. Gotta be something like that out there. Can you do something similar with the slow gear?


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No, reversible sweep is the idea.


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[jimilee]

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[jimilee]

Pick a simple design you like and I'll take a look at the schematic and tell you what to change.

^^^^^^^^


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[midwayfair]

I was hoping for even simpler.

I'll give it some thought.

Initially I'd try putting the bottom of P3 to power instead of ground and reversing D3. There might be more to it.