Simple DOD FX17 wah circuit spinoff.

[Ronan]

Simple compared to the original that is, this uses a 10K lin pot so you can use it in a normal wah pedal. I really liked the outcome of this one so am sharing it. No point in sharing the ones that didn't turn out so well. Its quiet and clean, with good range, and is adjustable for resonance, a mix of BP and LP out, and unity volume, although in this one I just used a fixed 10K for the vol trimpot and a fixed 120K for the resonance trimpot. Why the CA3080 instead of LM13700? I wanted to use TL072 for buffers, and the CA3080 made for an easier layout.

Here's the circuit for those who like schems, I've tried to keep the essence of the original circuit, so it should sound very similar. The main objective was to simplify the control of the wah by using a pot instead of the original's capacitive scheme.


A pic of the PCB and another of the PCB in a wah shell for testing.

Sound clip into the standard PC soundcard with minor compression and some delay. Clean, and then with dirt. It didn't come alive with the dirt like it does through an amp, sounds a bit washed out.
https://dl.dropbox.com/u/61170900/CA3080%20wah%20cct%20sound%20clip1.mp3

I have some PCB's left over from a batch of 10, if anyone is interested pm me, $10 shipped to USA or wherever. They are better off going somewhere rather than sitting around here. Anyway, hope you enjoy.

[kaycee]

Sounds really good, and nice neat little board too, great work   Sounds like your getting a good range of sweep from a standard lin pot too, thats an advantage.

[B Tremblay]

Very cool!

[joegagan]

sounds great, and the overall project is nice. thank you.
it sounds really radical when distorted, i like it.

[Paul Marossy]

That's really cool. 

[Earthscum]

Pretty cool!

Here's the notes I have on the FX-17 (did the same thing... at least we know it works, and is repeatable, and in different ways!) I left the transistor in just to isolate it a bit, and because I had some other ideas that I haven't got around to yet. Also, check out the switching I came up with... might give you some mod ideas.

http://www.diystompboxes.com/smfforum/index.php?topic=97120.0

[pinkjimiphoton]

me wantee!!

nice!! real mouthy and transparent...classic.

gonna have to work up a vero!

[Ronan]

Here's some docs to help build the PCB's, and maybe check the junk box for some parts.
Still need to write up a setup procedure for the trimpots, in a few days.

parts list

component overlay

PCB tracks

[wavley]

I'm a pretty big fan of the FX-17 both as a wah and a volume pedal.

Great job!

[Paul Marossy]

Cool. Looking forward to putting one together when my PCB arrives.

[wavley]

My only real problem with the original is the switching, I use it more as a volume pedal than a wah because I don't use wah a lot, I want it to switch back and forth between wah and volume without having to press the switch on the side, it seems silly that the stomp switch doesn't just toggle between the modes. I guess it's so you can use it as a voltage controller too when it's in bypassed wah mode and not effect your signal. maybe I should just move the mode switch to somewhere that I can step on it.

[joegagan]

ha wavley, that is funny. one of my very early pedal mods was putting an extension on that silly side push switch on the FX17 for live use, 1994. it only took a week till i broke the switch internally.

[wavley]

ha wavley, that is funny. one of my very early pedal mods was putting an extension on that silly side push switch on the FX17 for live use, 1994. it only took a week till i broke the switch internally.

I'm glad you said that... was thinking about trying that same thing.

I'm thinking that I should just replace the switch entirely with something stompable and put it on a side pod or something, although it kinda defeats the small footprint of the pedal and that's one of the things I like so much about it.  Maybe a hole drilled through the treadle like on a colorsound wah fuzz would work.

[pinkjimiphoton]

ian...beauty, mate.

thanks so much for the work! i can't wait to build this bloody thing, i have all the parts needed except the board. as soon as it arrives, i am on this like white on rice!!

thanks again bro!

[Paul Marossy]

Quick question for you Ronan. I'm populating my PCB right now and got to the Res. pot and it says you can use a 100K resistor or a trimpot/pot there. But the PCB is marked "220K", so does that mean I need to use a 250K trimpot, or how should I interpret that?

[Ronan]

You can use a 220K or 250K trimpot, or just fit a fixed resistor across the 2 holes circled in red. I used a fixed 100K resistor as in the photos in the first post, so the resonance is not variable on my build.

[Ronan]

Did the PCB arrive already? Seems quick! Here'a pic:



I put the trimpots in there for people who like to tweak a circuit. For the standard circuit, you can use a 100K resistor  for the RES position and a 10K resistor for the VOL position, like in the pic here, which is what I used to make the sound clip. The MIX trimpot is very useful though and worth fitting if you have one.

[Paul Marossy]

You can use a 220K or 250K trimpot, or just fit a fixed resistor across the 2 holes circled in red. I used a fixed 100K resistor as in the photos in the first post, so the resonance is not variable on my build.

OK, thanks for the clarification on that. It looked like it might need a 250K pot, that is what I assumed. But you know what they say about assuming. 

Yeah, I got that PCB this morning. It did seem to get here quite fast.

[Ronan]

I found that over 180K in that position took it to the edge of oscillation on the breadboarded circuit. But it seems a 180K trimpot is not that common, and 220K a lot more common, and 250K is available at Jaycar (a local store here). So, whatever you can get your hands on will work
Now before someone notices, you can see two 20K trimpots in the corner of my PCB, but I ended up with an extra 10K resistor tacked to the wah pot to make the outer trimpot effectively 30K, (20K alone was not enough), hence why I called for a 50K in that position.

Here's some more info to help set up the two trimpots in the corner, this is the bit of info I hadn't written yet since I thought no-one would have a PCB for another week:

There's 2 methods to set the two trimpots next to each other in the corner of the PCB, these trimpots adjust the upper and lower frequency limits of the pedal travel.

Method 1: With NO IC's fitted to the circuit, power it up and measure the voltage on the middle lug of the 10K wah pot to ground.
Put the footpedal in the toe-down position and adjust the trimpot closest to the corner of the PCB for 3.0V.
Then put the footpedal in the heel-down position and adjust the other trimpot for 0.74V. Recheck the toe-down voltage as each trimpot affects the other. Then recheck heel-down voltage and so forth until the voltages are close.
Then remove power and refit the IC's. The voltages measured at the center lug of the wah pot should drop to around 1.2V for toe down and 0.68V for heel down with the IC's fitted. This should get you in the ball park. After that, tweak to get the sound you like.

Method 2: If you soldered the IC's directly into the PCB, then unsolder the wire from the middle lug of the wah pot, power the PCB up, and set the voltages to 3.0V and 0.74V as in method 1. Then remove power, resolder the wire back on the middle lug of the wah pot and check the voltages again for around 1.2V and 0.68V.

If the trimpots aren't set up like this, there is a possibility of damaging the CA3080E's with too much voltage on the bias pin.

[pinkjimiphoton]

hey ian,
the board came today, thanks!! gonna start on it sometime in the next few days, i'll keep you posted.
thanks so much bro!

peace!