Synthbox

[bluesdevil]

I'm just now getting ready to build the op amp version, so I'm no experienced expert on this one, but sounds like a capacitor problem. Should be a non-polar 10uf connected to 2nd lug of treble pot. did you use proper cap there? Or maybe another cap near there is bad?

[Keppy]

Actually, I think I found the problem. I failed to notice a damaged trace connecting pin 3 of the treble pot, resulting in the cap from the previous opamp stage being completely disconnected. I'll fix that and see how it goes. Figures that after weeks of wondering I find the problem a few hours after I finally ask for help!

Incidentally, previous posts said it was fine to use a polarized cap on the output, which I can confirm as the components in my build all worked fine on the breadboard.

[Keppy]

Finally got this thing to work all boxed up, so jdub's pcb layout is now totally verified!

[jdub]

Glad to hear it! 

[liquids]

I'd definitely note that the active EQ section in whatever version that is is wonky. 

At the time I mistakenly believed that I could get unique cut/boost frequency response by using unequal value caps, while keeping the eq 'flat' in the middle.  Not so, that just tilts the EQ in a wacky way and the 'core' un-EQed sound is lost.  The Active EQ is good but if you're not familiar with tinkering with active EQ, just use values something like you'd find in the Xotic pedals or Boss FA-1 instead.  It will be more normal sounding and possibly less problematic.  As ususal with circuits, I'd do it differently today than I did it than...and will do so, when I myself finally build one! 

[Keppy]

Yeah, I'm kinda getting that vibe from the EQ. I fixed the treble pot problem, but when I boxed it up last night it reappeared but was less severe. Today it's gone again. I might re-do the pedal someday anyway. It was one of my first attempts to etch and solder a whole circuit board, and two months later I find myself embarrased by how it turned out. It still works, but the traces and solder joints are awful, much less clean than the ones I've done since. I might tinker with it someday when I feel like making a new board for it. It's still an awesome effect, though, so thanks for the design!

[Ronan]

I breadboarded various input stages to feed the Joe Davisson shocktave, it really is a cool idea using it to get synth-like sounds. I couldn't get any sounds as nice as liquids sax type sound, but I don't have the correct transistors which I ordered tonight. Tracking is a lot better than I expected. The Davisson circuit needs a lot of gain to drive it, and doesn't like any load after it. In the end I just used a dual opamp, one side to drive it, with mega-gain, and the other side as a buffer and simple treble cut control.

If anyone breadboards the circuit, a simple way to test the oscillator is disconnect it from the preceeding tranny stage with diode, and connect it straight to 9V, you should get a steady note. I discovered I can get some original guitar signal mixed in with the oscillator by disconnecting the 1uF cap from the preceeding diode/transistor stage.  This makes it more of a straight octave pedal and it sounds good. When I build this into a box I will fit a switch to give the 2 options.

Just thought I'd post this to encourage others who may be sitting on the fence. It's a useable, but wild, and unique effect, thanks liquids, Davisson, and all who contributed.

[egasimus]

This looks very interesting. Haven't had the chance to hear the clips yet, but I'll probably omit the EQ, perhaps add the resonant LP filter from the Drone Lab instead. Also, what is the benefit of using MOSFETs in the feedback loop?

[liquids]

EQ needs tweaking...I'd do it all quite a bit differently now, but the op-amp based circuit works well.

Mosfets in the loop as pictured create a squared signal with greater swing than two/four diodes or even Red LEDs, if I recall (it's been a while), and cliped somewhat less harshly than the other options to my ears, which may help tracking.  Orman has a good page on various clipping options and their results (this is taken right from there); select zeners properly arranged would do just as well I suspect.

Use what you have (multiple diodes in series, etc) but I found that, to a degree, you want as large a squared-off peak to peak swing as possible, mosfets were a two-component solution on the breadboard, and I was into messing with them at the time I did that revision, and have a lot of them laying around.  There are more than half a dozen ways to accomplish the goal of soft square-ish large swing clipping.

[egasimus]

Are there any specific parameters that I should be looking for when picking the MOSFETs?

[liquids]

Are there any specific parameters that I should be looking for when picking the MOSFETs?

Make sure you like how they sound & work on the breadboard? 

[Ronan]

Played some more with it today. Came up with a simpler/lower component count. This is quite giggable and consistent. Love it, thanks liquids and Davisson again. The first opamp section has a low input impedance and this probably helps roll off the highs from the guitar. I tried a more "correct" way of doing it but got a lot of glitches and worse quality sound. The tracking is great. To a large extent I used values on hand, so a lot of this circuit and the opamps is not or may not be theory correct. 2SC2547 and BC550C seem to work OK, 2N5089 in the mail to try. This is a circuit that really needs to be breadboarded first to make it suit your guitar (and taste). The oscillator needs to be tweaked to suit the range you want, by changing the 2 x 0.1uF caps to 0.068/0.047 or whatever you like. I think the 100K resistor pair in the oscillator could be replaced with a dual pot and fixed resistors to fine tune the range required. The oscillator will only cover so much range without shifting an octave up/down when it reaches its lower/upper limits. I can get clean notes from low E string fret 5 to high E string fret 19. Shame I'm 50 and don't gig any more, but my fingertips are sore from playing/testing/tweaking this circuit, I couldn't stop playing it, I'm smiling. I would have killed to have this up my sleeve 20 years ago Probably only use it in 2 songs all night but the "fark!" factor would be worth it

Here's the schem I'm using at the moment.

[pinksoir]

This sounds amazing! Well done.

I have a question about the bipolar power though... How would I power it? For example, I would be running it from a DC multi-out brick and not batteries so I'd need some sort of bipolar power charge pump like this?

Would I hook this up in the same box as the Synthbox or in it's own separate enclosure?
Here's a link to the layout.

Cheers.

[John Lyons]

Is there a current clip of this (v2) out there?
Op amp version or otherwise.
Interested to hear it, thanks.

John

[Keppy]

I'm planning to do a clip once I fix mine and maybe revise the tone stack. So, if you can't find one, there's one forthcoming.

[Ronan]

I would be interested too Keppy the only clip I've heard is this one here which I believe is liquids' opamp version. The simplified circuit I am playing with does not sound as good as that clip. Are you getting that sax-type sound from your build Keppy?

Ivan, if you build jdub's layout on page 11 the charge pump is already in place on the pcb.

[pinksoir]

Ahaaa. Thanks a million. I can't wait to make this now!

There are so many great pedals on this site, I feel like I've been bitten by the bug... quite addictive.

[John Lyons]

I'm planning to do a clip once I fix mine and maybe revise the tone stack. So, if you can't find one, there's one forthcoming.

Great! Thanks.

[egasimus]

Is there a thorough explanation of how this circuit actually works somewhere in the thread?
And can anyone comment on Ronan's (single-supply!) schematic?

[jdub]

I'm planning to do a clip once I fix mine and maybe revise the tone stack. So, if you can't find one, there's one forthcoming.

If I get some time in the next couple of days, I'll see if I can get a clip or two up as well...shoulda done so a long time ago but didn't have the recording capability until recently...