Three-legged Quadrafuzz

[Mark Hammer]

The PAiA/Anderton Quadrafuzz project has interested me for a long time.  Since 1983, to be specific.
I borrowed Dino-digi2t's Yamaha MBD-100 to get a rough idea of what it might sound like, since the MBD-100 splits the signal up 3 ways before clipping and blending them.  I wasn't thrilled with it, though.  Didn't like the mixing.

The actual Quadrafuzz, though, was bigger than I really wanted.  I didn't really need all of the options it included.  So I thought I'd shrink it down a bit to something I could perf and stick in a pedal.  So here is what I did.

There are a number of changes.

• I dropped it from a 4-way split down to 3, and ditched the variable lowpass filter.  That let me build it with two quad op-amps.  I used LM837 chips, for no particular reason than that I had them.  Omitting the highest band loses the sizzle, but then I wasn't really keen on having any.
• I adjusted feedback caps in the clipping stages to complement the passband.  The original used identical cap values for all stages  I also added a feedback cap to the output/mixer stage, and upped the lowpass filtering of the input stage a bit more than stock.
• The original offers two switch-selectable Q-settings for each bandpass filter.  I went for a single Q value between the two choices.
• I reduced the value of C22, because the stock value seemed like overkill.  The changed value rolls off a bit of bass at highest gain, but nothing drastic.
• I omitted the individual output jacks for each band, and the electronic switching.

Using 1/8w resistors, 3mm red LEDs, and small ceramic caps, it all fits neatly and easily in a 125-B box.  My only concern is power.  I used a battery +/-9V supply.  The original calls for a bipolar +/-5V regulated supply.  Mine started to howl a bit after a little while, so I gather the current draw is enough to produce an unstable circuit with batteries.  I'm going to have to find out more and come up with a workable solution.

How does it sound?  Decent, though not spectacular or life-changing.  The individual passband volume controls doesn't really provide as much EQ toneshaping as you'd think, though one can get some different-sounding voices.  Certainly more than enough drive.  I guess most noticeably, distorted chords sound a little clearer with notes/strings interfering with each other a bit less.  I may add a BIg Muff style tonestack before the output control.

Will it bump anything off your board?  Given what's out there, I doubt it.  But, using quad op-amps with the more standard LM324 /TL074 pinout, it's actually a fairly easy perf build.  I lined up a pair of quads end to end, such that there were four op-amp stages on each "column".  I put the input stage and filters on one side, and the clipping stages and output mixer on the other.  With the exception of the input stage, it's all inverting op-amps, that allowed for easy connection of the ground pins underneath the chips.

Took me 37 years, but I finally made it.  Working my way up to the Hyperflange. 

[Mark Hammer]

Realized the op-amp sections were mislabelled.  Or at least not changed from the original PAiA drawing that was cropped and re-arranged.  I corrected that, such that the op-amp labelling allows for the columnar arrangement I noted earlier.  IC1 has the input and output stages, and the hi-band filter and clipping, while IC2 had the filter and clipping stages for mid and lower band.

[jmasciswannabe]

Hey Mark! You probably have seen this but if not, I believe it is Von Rutter's take. Might be something in there that helps.

https://www.madbeanpedals.com/projects/_folders/VFE/pdf/VFE_Triumvirate.pdf

[idy]

The other thing that comes to mind is deafbutpicky's "Thrice" which used cmos invertors. I think I ended up giving each band its own gain and volume so I could balance things.

If you only have three bands tempting to make the mids "sweepable." But another opamp...

Interesting that Triumvirate used different clipping elements in each band.

[Mark Hammer]

Hey Mark! You probably have seen this but if not, I believe it is Von Rutter's take. Might be something in there that helps.

https://www.madbeanpedals.com/projects/_folders/VFE/pdf/VFE_Triumvirate.pdf

Aye caramba, that's a lot of trimmers!    In theory, the Triumvirate does exactly what the three-legged thing I posted does.  The difference is that the VFE circuit provides for fine-tuning the passbands and gain for each passband.  Is all that likely interesting to play with?  Probably.  Is it all necessary?  If one was aiming for some sort of "ultimate" overdrive, I suppose.  But I have enough drives of different forms that I didn't need for it to be highly tunable.  I just needed it to work, and relied on the original design properties for the most part.  I may tinker with the low-band properties on my build.

As for the use of different clipping elements in each section, I have to wonder if one is really going to hear it.

But thanks for the replies and input, folks.  Multi-band overdrive is not dead and buried.

[jmasciswannabe]

A lot of trimmers indeed. I haven't gotten around to building one up yet as I have a couple of boards, but looking forward to it based on some build reports. Hyperflange would be cool, too. Not sure if Armdnrdy ever finished a board of it.   

[soggybag]

I notice the LEDs used in the clipping and remembered the Craig Anderton interview where he mentions the "magical properties of red LEDs". Here they are again.

I have always wanted to build this myself. But I never wanted to own a rack mounted "pedal". I almost bought the kit from Paia a couple times.


I like your version better. Three clipping/distortion bands seems like a good choice for practical uses.

[davent]

I built a Quadrafuzz back in the day and think i built, used a +/- 15v supply for it, haven't found an online fuzz schematic and no idea where to even start looking for my own copy.
dave

[jimbeaux]

I built a Quadrafuzz back in the day and think i built, used a +/- 15v supply for it, haven't found an online fuzz schematic and no idea where to even start looking for my own copy.
dave

https://www.paia.com/talk/viewtopic.php?f=6&t=89&sid=d10b6e77769dd0084c099299af6466aa

[davent]

I built a Quadrafuzz back in the day and think i built, used a +/- 15v supply for it, haven't found an online fuzz schematic and no idea where to even start looking for my own copy.
dave

https://www.paia.com/talk/viewtopic.php?f=6&t=89&sid=d10b6e77769dd0084c099299af6466aa

Thanks, had no luck yesterday searching for the Quadrafuzz schematic, kept leading to a PIAI dead page... 7805/7905...
dave

[anotherjim]

I wonder if the band filter can be built into the clipper amps? Also, the output mix could be fixed passive with the 3 mixer controls between the input buffer and the band inputs?
One band short of a Quadra would be a Delta?

[Mark Hammer]

By means of a well-chosen input and feedback cap, yes one could build the filtering into the clipping stages, but the filtering would be crude, since it could only be single-pole.  Reducing overlap between the bands would require moving them farther apart.