Hi everybody,

I recently was showing a friend a version of the EH Pulsar tremolo pedal that I had built....he agreed it was a great sounding effect with a really cool mechanical sounding chop, but he thought it would sound cool with a slowly sweeping  high-pass or notch filter after the effect.  So we ran it through a rackmount Filter Factory to check his theory, and it was a cool addition to the sound.  We set it for a triangle waveform, high pass filter, resonance was off, and a nice slow sweep speed.

My friend suggested I add such a filter to my pedal, and I thought that sounded like a great plan!  Then I realized I had no idea how to do that!

So my question is, how would one go about adding a sweeping filter to the end of a circuit?  A very simple one....with a knob for the LFO speed and maybe a knob for depth....it doesn't need a resonance knob or a tap tempo feature or selectable waveforms....a triangle wave sounded best on the rackmount filter, with it set to high-pass.

Maybe there's no simple way to do that and I'm asking the impossible, but if anyone has an idea or could maybe point me in a direction where I could do some reading and learning on this idea it would be much appreciated.

-Ryan

Give a read to "The Technology of Wah Pedals" at GEO for some ideas.

You have two routes here. You can do a sweeping filter similar to a wah pedal by using a variable resistance like the JFET in the EA trem, or you can go voltage controlled with any number of synth filters. I have some designs coming out soon based on OTA state variable filters.

Thanks for the suggestion R.G.!

If I want the filter to just continuously sweep automatically that suggests I need to use a voltage controlled style circuit, as from a synth?  

I'll check out the GEO site before I ask any more questions!

Thanks again,

-Ryan

Quote from: RJIf I want the filter to just continuously sweep automatically that suggests I need to use a voltage controlled style circuit, as from a synth?

As much as I love OTA circuits, it's pretty easy to hook up a Twin-T type filter to your LFO.  You can use a FET (I use MOSFETs) as a variable resistor.

There's one here in this list from Munky!
havn't tried it, but looks OK.
http://www.geocities.com/munkydiy/schematics.html

Actually, any wah circuit could be modded easily, by replacing the variable resistor by a light dependant resistor   & using a flashing led to 'sweep' it.
If you want to get seasick, use TWO slightly out of phase led flashers shining on the LDR  

QuoteIf I want the filter to just continuously sweep automatically that suggests I need to use a voltage controlled style circuit, as from a synth?

Nah, there are several ways of continuously sweeping a filter shown in the articles at GEO. The "one resistor to ground" wah circuit adapts itself to a single LED/LDR or JFET variable resistor, as does a twin T filter circuit. There are articles there on both.

I didn't built it too, but if you do a search in this forum you will find a topic about it with some reviews. And Gez did this mod, to make it more versatile: http://www.geocities.com/munkydiy/autowahmod.gif
Here is the link for the stock version: http://www.geocities.com/munkydiy/autowah.jpg

Thanks for all the links and ideas, very much appreciated!  Those websites are great resources....I've got a lot of reading and breadboarding to do.  I've never used LDRs before, definitely have to give that a shot!

Thanks again for the guidance,

-Ryan