70's EH Deluxe Octave Multiplexer working right?

[Processaurus]

Hi, I have the 5 knob version of this rare-ish effect, and was wondering if anyone who has one of these can tell me if mine is working typically,  the octave sound is really neat and organic (seems to be an altered version of the original signal, rather than a filtered squarewave like most octavers), but I'm hearing this buzzy overtone that is covering up what would be a very nice timbre octave.  I can get it to go away by rolling its tone knob down, but then its back to that muddy generic sound, like the boss OC-2.  The buzz isn't in the direct sound.  This is with the Fuzz knob all the way down as well.  Same sound with the sensitivity pot in any position.  This is most noticeable/objectionable with the subharmonic knob all the way down (sounds like that control is just a pan between 1 octave down and 2 down).
Stephen Giles' Dirty Ole Schematic

I see what at a passing glance looks like the boss OC-2's basic method of synthesizing the octave in the lower left corner of the schematic, using FETs as voltage controlled switches to invert phase on the original signal, being controlled by the sub octave square wave.  If that is the case maybe the buzz is an unavoidable byproduct of the synthesis, the pointy edges of the audio when it suddenly flips its phase?  Could the FETs switch slower to avoid/minimize this high end crud?

Off the buzz topic, but its seems like a choice analog octaver.  All its complex analog tracking circuitry and it still can't stick that open E, though.

[Rectangular]

I've been meaning to clone one of these bad boys, but the schematic is one of those ugly EH hand-me-downs with a 20 years of xerox degradation. Hopefully the buzz you're describing can be suppressed and/or fixed, otherwise I'm not so sure if this is a hot build. isn't this is one of  Howard (polyphase) Davis'  designs  ? maybe you could email him and ask if your pedal is working 'right'

-rec

[StephenGiles]

What you have here is a really good fundamental extractor!! I might just redraw this circuit, then we can have a 500 post thread of complaints about possible errors

[Processaurus]

I emailed Howard Davis and he was really nice but couldn't say if this was normal or not.  Anyone here have one who can describe their octave sound they get with it?  I've never heard another one so its hard to guess what the typical sound is.

Stephen, I actually got it out of curiosity because of the interest you created here in the fundamental extractor part (which does seem to work a bit better than the other analog stuff I've used (OC-2, BLue box, Octron, EBS octabass), but still not perfect).  The octave sound was interesting enough to me, though, that I'm wanting to see if I could get rid of the buzz and maybe actually use regularly it for playing music.  If that doesn't work, then it'll turn into a science project.

[StephenGiles]

If you look at the output of the 3094 on the right on a scope, you will see an almost perfect sinewave!

[Meanderthal]

 So how doable IS the fundamental extractor? I keep getting more curious...

[Processaurus]

http://media.putfile.com/Deluxe-octave-Multiplexer

here's what it sounds like right now with the knobs set how they were described earlier.  I can see on the waveform a little anomaly (crossover distortion looking) every cycle that must be the buzz.  I'll take it apart sometime soon to see what it looks like on the oscilloscope.  It seems amiss, but I've never heard another one or a sound sample to compare.

[Meanderthal]

 After listening to the clip, I have to agree. It wouldn't make much sense to go to all that trouble designing and building such an elaborate circuit just to mix a nasty buzz with the clean. A simple contrafuzz does a better job of that.  For what it's worth, the buzz seems to track very well... but I'm just listening, not playing through it...
No, I don't know for sure what it's really supposed to do, but it don't sound right.

[Processaurus]

No, I don't know for sure what it's really supposed to do, but it don't sound right.

It sure don't. 

Here's some pictures I took when I popped the hood today for those who're curious, a little more in depth than your average fuzz box as you can see.





some more innards:

http://i4.photobucket.com/albums/y114/processaurus/P1050029.jpg
http://i4.photobucket.com/albums/y114/processaurus/P1050031.jpg
http://i4.photobucket.com/albums/y114/processaurus/P1050032.jpg


One note, the blurry thing in the box by the transformer on the schematic is just a full wave bridge rectifier package, nothing special.

[Processaurus]

a better schematic:



I had a chance to recently look under the hood of my buzzzy tormentor, I believe the problem is a design issue.  The fets indeed switch the phase of the input signal at half the input frequency of the input (for B, C is 2 octaves down, 1/4 the input freq).  Thats where the sound gets the buzzy characteristic, the sudden transition in the waveform from theses phase changes.   Look and see, in the left picture the pink is the direct input signal, the blue is the logic signal B at the gate of the fet, and yellow is the waveform at the output of that opamp, and the top of the blend pot:



Troubleshooting?  One curious thing is that the logic square wave signal changes phase relative to the input signal over the range of a guitar's notes, so the transition happens at different points in the direct signals cycle, depending on its frequency.  Above was at 1kHz (and visually looks more octave like), below is at 550Hz, about a 90 degree difference .  Could this be something up with mine in the fundamental extractor?  Actually, looking at the 550Hz picture now I'm noticing another curiosity, it doesn't seem like its switching phase all the way.  hmmm:


Octave down tone mod attempt:  The picture below is my attempt to slow down the phase switching transition by putting a cap on the gate of the FET.  The result is on the right.  The sound was much less buzzy.   For some reason it had lost some of the liveliness to the octave sound (though it was more acceptable sounding).  Also the higher notes on the neck were getting choked, and a smaller cap made it buzz on the lower notes.  After I took the picture I broke the connection where logic signal B feeds the gate of the FET, and used an opamp integrator to slope the edges of the logic signal a little.  This was a little better.  Then the combo of integrator and a cap to ground on the gate.  That seemed like the best compromise in terms of reducing the buzz byproduct and leaving some of the coolness and of the octave sound.  It still lost some of the higher register's purity in the octaveness.  Not totally happy with the mod nor the original sound, hard to say which is better, I was thinking of making it switchable...



Boss, using almost the same phase switching to synthesize the octave down, got rid of the buzz by putting a sharp filter after the phase switching section.

If you look at the output of the 3094 on the right on a scope, you will see an almost perfect sinewave!

Sine wave output: the green squarewave is the input signal, the pink is the sinewave output of the 3094 (chip's marked EH 1048).  It was cool to see on the scope, it changed the square wave into a sine wave over a good amount of audible frequencies.  If you plug in a guitar and listen here, its super compressed and muddy, a lot of booming fundamental happening.  Not exactly a great tone on its own but it could be used to make an:

Octave up: I tried outputting this point in the circuit into a Foxx Tone machine, it sounded pretty cool, though the octave up was still more apparent in the middle of the neck, the hope was that the octave would be as obvious low on the neck.  The bridge and neck pickup sounded close together though.  This thread  got me thinking it would be worthwhile experiment to put an AD633 multiplier chip in there to get an octave up from this point in the circuit, thats got a good amount of fundamental happening, to get a cleaner analog octave up using squaring rather than the more usual full wave rectifier type octavia.

[StephenGiles]

Now do you believe me? It's a simple build which I did on veroboard over 25 years ago.

[Rectangular]

what software is all the fancy oscilloscoping from ?

[StephenGiles]

Aren't they waveforms from the Deluxe Octave Multiplexer?

[snap]

according to Boscorelli
the phasechange is maybe
not a timing problem
but a DC offset thing.

[Processaurus]

Aren't they waveforms from the Deluxe Octave Multiplexer?

Yep.

what software is all the fancy oscilloscoping from ?

Its not software, its the the bling tektronix scope at my workplace.  Hot, right?

according to Boscorelli
the phasechange is maybe
not a timing problem
but a DC offset thing.

Thats interesting, but how do you mean? DC offset where?

[snap]

see fig.4 & top of next page

[analogguru]

Maybe you like to have a look at the schematic of the dbx 120 subharmonic synthesizer ?:

ftp://ftp.dbxpro.com/pub/PDFs/discontinued/Schematics/120XP%20Schematic.pdf

analogguru

[StephenGiles]

That is a very interesting link, I wonder how it needs to be changed to get into the current product schematics?

[Mark Hammer]

Maybe you like to have a look at the schematic of the dbx 120 subharmonic synthesizer ?:

ftp://ftp.dbxpro.com/pub/PDFs/discontinued/Schematics/120XP%20Schematic.pdf

analogguru

A-HA!  Thanks for that link.  I bought one of those 5 years ago and it sits on my shelf, unused.  Took it apart to see what was in there, but never really used it moe than a few seconds to see if it worked.  Nice to know what's in there.  Much appreciated.

And yes, it is but a humble 4013 used in a more polished manner.

[rockgardenlove]

http://ehx.com/ehx2/Default.asp?q=f&f=%2FCatalog%2F24%5FSynthesis%2F19%5FOctave%5FMultiplexer
Here's the EH reissue.  Not quite the same, but it sounds similar to yours.