Tychobrahe Parachute Pedal SCHEMATIC puhleeeze

[Mark Hammer]

The Parapedal is simply a parametric EQ section, which is simply a state-variable filter.  This is no different from what is found on the EPFM Super Tone Control, the Mutron, and the Lovetone Meatball, to name but a few circuits.  It can be found in virtually ANY circuit book on filters.  What made the Parapedal different as a product is that, where other manufacturers stuck parametric EQs in a box with rotary knobs, Tycho Brahe stuck it in a foot-treadle operated box.  The use of parametric EQ sections as wahs in post-production (i.e., making a guitar "wah" after the it was recorded by manually moving the frequency control around on the EQ-ing for that channel/track) is well known, and sometimes preferred for wah-ing.  This is because, unlike the bandpass nature of wah-wahs, that trim high and low end, parametric section essentially provide a sweepable resonant boost, with treble and bass largely unaffected.

Quite frankly, the Anderton Super Tone Control is a better unit in terms of flexibility, and can probably replicate what the Parapedal does quite easily.  I'm not dissing the Parapedal, merely noting that its reputation comes simply from sticking that dual pot under foot control, rather than any property of the circuit that pot is attached to.  Note that unlike many parametric EQ sections, the Parapedal does not allow you to adjust Q or resonance, where other designs will.

[GFR]

I would not do it with LEDs and LDRs, because of the tracking and other difficulties in such a setup.

I would replace individual resistors with the switched-capacitor equivalents, using the dreaded digital circuits. That can actually be quite workable.

Your foot pedal just sets the duty cycle on a PWM chip by varying the input voltage. The output of the PWM chip directly controls the pulse width of a switched capacitor using a section of a CD4053. The number of controllable parts is literally unlimited. The SG3524 and its ilk will do this well.

Or just use a MF10 (Dual Universal Switched Capacitor Filter) - Dual, biquad 2nd-order universal switched-capacitor, state-variable filter. You just need to change the clock frequency with a pot (like in a chorus/flanger "manual" knob). No need for external caps or any matching.

http://www.maxim-ic.com/quick_view2.cfm/qv_pk/1462/ln/
http://www.national.com/pf/MF/MF10.html (check the AN-307.pdf)



There's two in the chip, so if you want to, you can do a talking wha, or sweep a pair of notches like a phaser (I saw a magazine project like that once). Fun.

Another interesting chip for state variable filter building is the BB UAF42AP Universal Active Filter:

http://focus.ti.com/docs/prod/folders/print/uaf42.html
http://focus.ti.com/analog/docs/techdocsabstract.tsp?familyId=673&abstractName=sbfa005

[Paul Marossy]

The Parapedal is simply a parametric EQ section, which is simply a state-variable filter.

Actually, it's an opamp based dual parametric EQ (two in series) in a wah pedal shell, isn't it? Obviously a different animal than the good old inductor based CryBaby type wah pedal. Anyhow, I think it makes a fine and unique sounding wah pedal.

[Mark Hammer]

Nope, I'm pretty sure it's a SVF, which is why you need to vary two resistances at once.  Don't let the dual pot throw you.  The original Anderton Super Tone Control varied the series resistance that shows up as a 220k resistor and LDR in parallel in the Mu-Tron.  Later on, in DEVICE (as always, check the scanned issues on my site), he noted that an even wider sweep range could be achieved if the dual-ganged 100k series resistance was subbed for a dual-ganged voltage-divider exactly as shown in the Parapedal.  There ARE a couple of things that are different between the Parapedal and other parametric/SVF sections discussed or that you've seen, but not enough to make it NOT a SVF as far as I'm concerned.  The test is to take an audio probe and hear what comes out at points A and H.  In theory, it ought to be a highpass and bandpass signal, respectively.

I've been wrong before, so I might be wrong now, but there's a reason they called it the Parapedal.

[Paul Marossy]

I'm pretty sure it's a SVF, which is why you need to vary two resistances at once.  Don't let the dual pot throw you.

Ah man, that threw me about ten feet! I don't really know anything about SVFs, so it looked to me like it was a dual parametric EQ type thing by looking at the schematic...

[Mark Hammer]

It probably would have helped most folks to recognize it or what it is if they had retained the variable Q and boost/cut controls normally found in a parametric section along with frequency tuning.  S, you are more than forgiven, Paul.  Those bums made it too tricky.

[freakshow__]

so....I'm trying to build a parapedal as well and I too do not know what the value of C3 is or what diode D1 is. Can someone please shed some light on this for me??? I apprechiate it very much!