Acoustic 360 Bass Fuzz

[moosapotamus]

I've tinkered with this circuit quite a bit over the years, and I've always had some version of it on my pedalboard. I've tried different higher voltage power supplies, different types of clean blend circuits, and a few other mods. But I think my latest revision could maybe be the last. So, here it is...



The clean blend circuit is based on the EH mole/hog's foot, and I've reduced or eliminated some resistors to make it sound better with a 9V power supply (as opposed to the original's 24V supply). I also came up with the Decay control, which enables some really cool gated fuzz sounds. There is some DC on the Decay pot, so it scratches a bit when it's adjusted, but I still really dig it. Here's a short video of how it works and sounds....

There's more info and build docs posted on my blog... http://moosapotamus.net/ideas/

~ Charlie

[John Lyons]

Very nice. I dig it.

[karter2000]

Awesome. PCB ordered!

[Gus]

http://www.diystompboxes.com/smfforum/index.php?topic=87908.0

I am not a fan of the emitter follower(EF) biasing used.  If you want to keep close to the same input resistance of the EF, I would make R16 a 1.8meg and add a 2.2meg from base to ground this also gives a little offset for the Vbe drop.  I would also use a high beta transistor for the EF

Have you tried a 470uf cap in series with the decay control to ground to get rid of the control noise?  This is not the same because it is not DC coupled to the collector.

The beta/hfe looks like it needs to be selected for the Q2 and Q3 transistors.

Clip sounds nice.

[kingswayguitar]

cool as acucumber

[Mark Hammer]

The "Decay" control is an interesting feature.  Given that the basic fuzz circuit structure is essentially the same as that if the Shin-Ei FY-2, the Orpheum, and the Mosrite Fuzz-Rite, it would seem that this could be implemented on any of those.  Perhaps not as-is, but with little fuss.

[moosapotamus]

Thanks for the comments and suggestions.

Gus - I just adjusted (or eliminated) those biasing resistors all by ear. I liked how it sounded, so that's what I went with. When I tried a 100u cap in series with the Decay control, it effectively disabled the Decay control. I couldn't figure out how to make it not scratchy. But I liked the gated sound so much, I just went with it anyway.

BTW - This circuit doesn't seem to perform as well on a 9V battery as it does with an external power supply. As soon as the battery voltage starts to drop off a little, the fuzz quality really starts to suffer. Conversely, it gets even more aggressive sounding if you increase the power supply. I've gone up to 18VDC with no issues. Maybe adding a charge pump would be a good mod.

~ Charlie

[PRR]

> battery voltage starts to drop off a little, the fuzz quality really starts to suffer

Not sure about the fuzz, but bias on Q1 is *very* sensitive to supply voltage, and is liable to crap-out if battery falls much.

[R.G.]

Likewise. It would be a good idea to provide a local regulator for that stage, and set it to work at something like 6V if you can. An LM317L will do a nice job of that.

[Gus]

Some ideas for more experiments
I noticed you did not turn the clean gain up all the way
I did a DC sim of a lower gain circuit it is about x10 with a "perfect" input and the .1uf lowpass filter caps removed
and I simmed the collector voltage with the decay at two different settings
and added another sim idea with a bias control for the first stage of the fuzz not the same as the decay control it changes the collector voltage
I don't know how close the 2n2222s are to the transistors you used

[moosapotamus]

Thanks for the comments!

...bias on Q1 is *very* sensitive to supply voltage, and is liable to crap-out if battery falls much.

Interesting. That's the first time I've heard that said about the Mole/Hog's Foot.

RG - "...for that stage"... You mean 6V for only the clean blend part?

Gus - What do you think your sim ideas would do for the sound?

Thanks
~ Charlie

[R.G.]

Whichever stage is touchy about voltage.

Older discrete stages with a side effect of good sounds may be worth the added complexity of tinkering with the power supply to keep them happy. Like providing a sheltered workshop for a musical genius that can't balance their own checkbook.

If it's not possible to get it running at a lower regulated voltage like maybe 6V from a 9V battery, it might even be worth adding a charge pump to make 14-18V from a 9V battery then regulating that down to 12V. Sometimes circuits don't sound the same if modified for different voltages.

[moosapotamus]

That is another possibility for others who might want to build their own (and use a battery), thanks. That's really the only reason I mentioned it. I actually stopped using batteries in my pedals a long time ago. They're wasteful and it's a PITA to have to worry about unplugging everything. I find it much more convenient and reliable to just use a proper power supply. I highly recommend it.

Thanks
~ Charlie

[Gus]

The change to the clean amp was to use more of the volume control range I don't think I saw you turn it up more than about 1/2 way.

The two decay sims  show what the collector voltage is.
You are forming a voltage divider with the two resistors and the center is connected to the collector
so the amp is loaded more (DC coupled loading to both +9 an d ground)
And the output of the collector is high resistance(look up open collector)

The bottom right sim is an idea to vary the bias(and collector voltage) with minimum change to the input resistance of the stage.

A question is what are you doing with the decay is it the collector voltage shift and/or the load on the collector?

[moosapotamus]

I actually like leaving some room on the clean amp to be able to get a nice level boost.

I tried your bias idea for Q2 (replace 680k with fixed 47k and a 1M pot). It had no affect at all on the sound.

I also checked the Q2 collector voltage when varying the decay control. Using a wallwart putting out 8.9V, I get 2.8V with decay set at 4.7k, and 3.4V with decay pot at max (9.85k).

I don't know what is technically going on with the decay control. But might it also have something to do with the feedback around Q3?

~ Charlie

[karter2000]

I bought one of Charlie's awesome PCBs, and got it finished the other day.  I have to say, great job Charlie!  It sounds great, just like I remember the Viva Analog 360+ I built awhile back.  The clean blend makes it really usable with tons of low.  The only issue is that the decay doesn't seem to work as it should, but it may be an issue with my build.  The decay seems to only lower the amount of gain, with no gating.  Other than that, I love it.  I didn't have 2N5210s, so I used 2n5089s, around 700hfe.  I tried some other trans with gains from 200 to 500, but I preferred the higher gain trans.  It also sounds great at 12-18v, so I'm going to use an unused Madbean road rage charge pump with it.

Again, great job Charlie!

[moosapotamus]

Thanks, karter. That's a bummer about your decay control. I love that gated fuzz sound! I can't imagine that has anything to do with your transistors. Wiring, maybe? But yes, it does respond very nicely to higher supply voltage.

~ Charlie

[karter2000]

I went through the circuit again, and it appears that the decay is working as it should.  The fuzz sounds pretty close to your demo video now, so it's all good!

[pinkjimiphoton]

hey charlie,
have you tried gus's ideas for q1 to get it to bias more stable-y?

i built something similar, it's unusable on a power supply, but sounds KILLER on a battery.

and strangely, on a power supply i can run single coils no problem, but with a battery it's just noise.. have to use humbuckers.

the bias seems to even get affected sometimes when i really whack it hard... almost swells in sometimes.

i'm wondering if maybe a real small resistance...47-100r maybe, or a 50r trimmer or whatever between q1 e and ground may make it more stable?

i'm gonna try it on my "swinger box" today hopefully and see if it works out.

great demo!! i've been looking at this one for a while, thanks for posting brother!!

[moosapotamus]

Cheers, jimi!
You mean Q2? Q1 is the clean blend part of the circuit, and Gus' idea for that was to lower the gain to be able to use more of the control range. But I like the range as is, able to get a good boost if you want. I just neglected to turn it all the way up in the video demo.

I did try Gus' ideas for Q2 (first stage of the fuzz) and just gave it a listen and checked the voltages to compare to what Gus got from his sims (see above). Maybe my brain was on standby, but it never occurred to me that his sims had anything to do with the difference between running on a PSU versus battery. It seemed to me like he posted them but never said why. I'm kinda slow sometimes (sorry, Gus). I'll tinker some more soon as I have a chance.

Thanks
~ Charlie