Gruntbox - a tentative bass fuzz

[Mark Hammer]

People have asked for bass-oriented fuzzes, so I figured I 'd take a shot at one.  The tentative result can be seen here: http://img.photobucket.com/albums/v474/mhammer/Gruntbox.gif

I say "tentative" because it is essentially an untested theory-based design on paper.  I'm perfing it tonight once the football game is over.  If it sounds okay, I'll try and whip up some sound samples and post a schem+sounds package at my website.

The premise is that a bass fuzz wants a clean bottom and some lower-order harmonics.  Too much sizzle and the bass loses its role and place in the ensemble.  Bass is allowed to sound "angry", but it still has to generally keep its cool if playing behind others.  So, what I've done is split the signal into a clean and fuzz channel, with the fuzz channel strategically filtered and mixed back in with the clean.  The schem provides a great deal of technical info, a sort of circuit walkthrough in a schem.

The clean channel has a gain of 10 applied and can be full bandwidth or be rolled off.  I used the SWTC (Stupidly wonderful tone control) as a variable lowpass filter for the clean channel.  The output volume control allows this to be attenuated for effect/bypass level-matching.

The ultra-low stuff from the bass is filtered out before fuzzing, as well as filtered out between stages.  Essentially 80% of the fundamental you hear will come from the clean channel.  The corner frequencies are sheer guesses.  Once I perf and test with my bass I'll know if they're suitable or not.  The basic design may hold but component value changes may be needed.

The quad op-amp (LM324/TL074 pinout shown) is used entirely in inverting mode.  In order to assure the harmonics are in phase with the fundamentals, the fuzz chennel inverts then re-inverts.  Just for the hell of it I figured I'd split the clipping over two stages, as a kind of "affordable experiment" (i.e., tried out something different where it couldn't hurt).  One half-cycle is clipped in one stage and the other half cycle is clipped in the subsequent stage.  The diode is oriented the same way because the signal is inverted.

The cascaded gains of 10 and 22 give a total gain of 220 at the first clipping stage.  This is multiplied again by 10.  Because the one half-cycle is clipped by the first diode, I wouldn't consider the total gain applied to be exactly equivalent to 2200.  That being said, by the time the other half cycle hits the second diode, it's hot enough.  Normally, that much gain would get you over-the-top fuzz.  However, with the bottom significantly cut, I'm betting that the resulting signal is just hot enough

The two clipping stages restrict the high end significantly such that mostly lower order harmonics are generated.  The highpass filtering after the Harmonics Level control should result in that control introducing mostly a gurgly edge to the bass tone.....or at least that's the hope.

There are only three controls, but quite a bit of flexibility in those three controls.  The Clean Tone and Harmonics level controls allow you to dial where the mids and highs come from.  Because there is greater sensitivity to the fuzz signal than the clean signal at the mixing stage, rolling off the clean tone and goosing the harmonics should get you a nice bark.  Alternatively, you can shut off the harmonics, roll the tone control up to full treble and use the unit as a clean booster if you feel like it.  The rolloffs at both the input and output stages shuld help to keep hiss at a minimum.

Once again, this is a hypothetical circuit, to be verified by a build.  It should "work", but whether it sounds as intended using the component values shown is another thing.  Even if it sounds as intended, this will not sound all that hot for guitar.  But that's fine, guitarists have enough fuzzes of their own I should think.  Bass players need a little TLC too.

I kind of like the way the schem is done, and encourage others to include similar sorts of information in the future.  It wil be very helpful for newbies, and those trying to jog their thinking.

[moosapotamus]

Very cool, Mark... one for the bass players, oh yeah!

I'll be very interested to see/hear how your design turns out, for several reasons... one, I play bass... two, I'm coincidentally toying with another bass fuzz idea too, based on the blue clipper circuit.

My design goals for bass fuss have been slightly different. IMO, the clean blend has to be there, but the rest is wide open to individual preferences.

Personally, I've never been able to get a lot of use out of a tone control on the clean side. Others may like it alot, and that's totally cool. I guess I can see how it could be useful. But, to me it feels more natural to have the bottom stay in the same place with the fuzz washing into the upper registers when you step on the switch. 460 to 10k is quite a wide range for your lowpass filter. I'm curious to hear how that works out.

I'm not trying to roll off any high end at all from the fuzz. I'm trying to get as much treble content into (and out of) the fuzz side as possible. It's suprising how much the clean signal can fill in. I also like the sound of a really grinding, screetching, buzzing fuzz blended into the clean signal at a really low level.

Another reason for not rolling off any high end in the fuzz is that the majority of bass amps & cabs already do quite a bit of that already. I would say that EQ is crucial to getting a great bass fuzz sound, but more from the perspective of boosting certain frequencies instead of cutting or rolling off... But, that's just my theory. :wink:

The main criticisim I have of the majority of bass fuzz pedals I've heard is that they sound tame... they just don't get really fuzzy enough for my tastes, especially in a live setting. So, my current theory is that having more treble content in the fuzz will make it cut through a bass cab with 15" speakers much mo better... maybe. Anyway...

Cool scheme. Interested to hear how the proto sounds.

Thanks
~ Charlie

[Gabriel Simoes]

Nice idea mark!
The project looks superb, let's see how it sounds ...
I will try to put it all on a protoboard and tweek the component values to see how it gets ..

In my opinion it only lacks volume controls for both channels ... since sometimes I like to use the fuzz just on the top end ... and in a lower volume, just to make the bass sound more agressive, but with the same punch of the clean ... I hope you understand what I'm saying, my english is not that good, ... and right now I'm really asleep hehehe
I think the tone control on the clean signal is good, sometimes you just dont want to mix the high end of the clean signal with the fuzz signal ... versatility! ... so... if you dont want it .... roll it all the way and let the signal flow as there was no tone control there ...
I just didnt get why using 2 stages for clipping, just to make it a little assimetrical because of the difference in the in the gain ?
Lets see how this project will go ... right now I'm not that genius but I'm happy that in 2 months I can understand a lot of things from the project .... maybe in some time I will understand everything hehehe .... dreaming ....
It's time for the bass players to show that 6 strings are not really necessary for anything ..... well guitar players ... maybe to play alain caron fusion songs hehehe
Gabriel

[Mark Hammer]

Well, I finished it, and it works pretty much as planned.  My bass sounds a little bit more like a tenor saxophone.  Of course, I don't sound like either a bass player OR a saxophone player, but that's another story! :lol:

I need to change a few resistor and cap values to provide better balance between straight and fuzz.  Right now, moving the harmonic level control up just a tiny bit drowns out the straight signal.  I want to include that possibility, but not until the harmonics level control is up about 3/4 of the way or more.  I also need to try it out with a 12" speaker tomorrow - an 8" speaker may not wake up the family, but it doesn't wake up the bass either.  In general, though, the gain of the clipping stages and the filtering seems to work just right.

Why two clipping stages?  Because if I used one non-inverting stage that would be 3 op-amps.  I went with a quad op-amp for layout purposes and am glad I did.  The pinout made things easy - it's not often I whip up a perf project in one night and it works just like that.  Since I had 4 op-amps to work with, I used two inverting stages.  By inverting twice that made the fuzz in phase with the clean signal at the mixing stage.

But you are right.  I *could* have used one inverting stage and done all my clipping with a back-to-back pair of diodes, then simply used another inverting stage as a unity-gain buffer to reverse phase before mixing.  I just wanted to solve my phase problems while tinkering with something else.  And you are right.  This does introduce some asymmetrical clipping.

Listening to it (and sound clips will follow tomorrow when I have it tuned just right), it occurred to me that it would be great to combine this with an envelope controlled filter.  In this scheme, the input stage would feed the mixer with clean signal, the fuzz channel, and a rectifier stage to drive a filter on the fuzz path.  The reason why I mention this is because bass doesn't normally have a lot of harmonic content so there isn't much to filter.  As well, sweeping a bandpass filter robs the bottom.  If one has a parallel fuzz channel that could be independently filtered, you'd get to keep the bottom and at the same time have something to filter.  As you well know, having a harmonically rich signal always makes filters sound like they are working harder.

[Gabriel Simoes]

Hey ..... so you are almost reaching a kind of bass synth / fuzz in the same pedal! hehehe and in a much simpler schematic than most synths hehehe ....
I hope you get the values balanced pretty soon, and if it's not a problem I hope I can find a pcb of perf layout of it too, because thats my biggest problem right now, I'm worse than bad drawing pcbs ....
But about the volumes, do the 2 channels sound balanced or there's a need to use individual volume controls ?

[Fret Wire]

I'd be curious to hear the clips. I've got an EB-O, but don't take it seriously, just fool around a little. One of my favorite dirty bass sounds is Cream's live version of "Sitting on top of the World" off the "Goodbye" album. Especially during the intro. Check out that specific cut if you never had. Jack Bruce using an EB-O with Marshalls. I'd like to emulate that at home.

[Mark Hammer]

Hey ..... so you are almost reaching a kind of bass synth / fuzz in the same pedal! hehehe and in a much simpler schematic than most synths hehehe ....

Well, I wouldn't call the suggested circuit a "synth", but if it worked as intended it would give a more usable autowah sound, even if your bass pickups are dull sounding and your strings old.  Lying in bed last night, I realized that since there is a gain of 220 at that point I could simply run a diode from the output of the first clipping stage and I'd have a decent envelope signal level to drive a filter stage like the Dr. Q filter.  That would only require one more op-amp.  

Alternatively, since the fuzz channel signal is so hot, maybe the absolutely bonehead simple thing to do is to stick a fixed resistor after the harmonics level control, parallel it with one half of an optoisolator, run a cap to ground for a variable lowpass filter of the harmonics, and use the LED half of the optoisolator as the diode from clipping stage 1.  How's that for simple?  I may try that later today.

I hope you get the values balanced pretty soon, and if it's not a problem I hope I can find a pcb of perf layout of it too, because thats my biggest problem right now, I'm worse than bad drawing pcbs ....

Actually, I find the circuit ridiculously easy to perf.  Unless you have no perfboard, just go straight to perf and don't even bother with a PCB.  Most of the caps are small values, so between that and the vertically-mounted resistors and diodes, it's a snap.  The use of inverting op-amps makes it easier still - fewer components to tinker with and having all '+' inputs facing each other simplifies wiring up the Vref.  I'll post a picture of my perfboard (front and back) with the soundclips when it's "done".

But about the volumes, do the 2 channels sound balanced or there's a need to use individual volume controls ?

I left the clean channel fixed in terms of level.  Because there is an overall gain in the circuit from the input stage, it is a simple matter of using the output level and harmonics level controls to get more clean than fuzz and more fuzz than clean at levels above bypass.  Although, I suppose one could always set up the output stage like a mixer with two level controls, and skip the master volume.  Personally, I think the setup I used is easier to use, but that's a matter of opinion rather than any absolute.

[rocket]

I think the input impedance is a little low ( it's only 10k as the opamp is inverting)

[Bernardduur]

I would like to hear this one; am now happy with my bazz fuss, but am always looking for something better.

[jmusser]

OK Mark, I may as well ask this question, because someone will, will there be some mod notes with this circuit to adapt it to guitar, or is this circuit so geared for bass, that it wouldn't make a decent guitar pedal? I know I've read in the Anderton book, that there were mods you could make to his bass fuss that would make it usable for guitar.

[puretube]

 jogging here...  

[Mark Hammer]

OK Mark, I may as well ask this question, because someone will, will there be some mod notes with this circuit to adapt it to guitar, or is this circuit so geared for bass, that it wouldn't make a decent guitar pedal? I know I've read in the Anderton book, that there were mods you could make to his bass fuss that would make it usable for guitar.

Nope.  This is pure 100% bass content.  Bass players deserve something that is theirs and theirs alone.

On the other hand...... :wink: ......it strikes me that the little sidechain with two consecutive op-amps handling clipping of each halfwave is an interesting idea that maybe folks should play with.  We keep blathering on about symmetrical and asymmetrical clipping here, but tend to do so by applying the SAME gain to an imbalanced pair of diodes.  Splitting the clipping across two stages with disparate gains is another way to tackle asymmetry.

[Bernardduur]

Good answer!

[robotboy]

Sounds awesome. I can't wait to hear a clip. I have a few bass player friends who keep nagging me to build them a pedal, and this might be just what they've been looking for.

[Mark Hammer]

Unfortunately, I won't be able to supply them (soundclips) until much later today, and perhaps not until tomorrow, but the nearest comparison I can think of is the tone that Larry Graham gets in the old Sly and the Family Stone nugget "Dance to the Music" during the little solo showcase in the middle.

[Bernardduur]

That is one cool sound, I must agree..... I once searched to get this sound.... I'll try to build it also tomorrow.

[Mark Hammer]

If you do, make the following changes:

1) Change the cap on the second clipping stage from 1nf to 1n5 or 1n8.  That will provide better tonal balance in the harmonics.

2) Insert a 68k fixed resistor between the output of the 2nd clipping stage and the input of the Harmonics-Level pot.  That will attenuate the harmonics signal a bit and make more of the pot's range usable.  You will still be able to get more fuzz than clean, but will be able to dial in a broader range of subtle sounds as well.

[Mark Hammer]

Okay.  Mods done.  Soundfile posted to hammer.ampage.org

The zipfile contains the soundfile and revised schem.  Nothing terribly impressive in the playing but the tone is clearly illustrated.

Make a point of looking at the waveform if you have a chance.  This is nicely asymmetrical clipping and it works extremely well in this context.  Clipping that "knows its place" and politely bows out before it obscures things.

[soggybag]

Can not download the file thee is a permissions problem.

[Mark Hammer]

Fixed.  Sorry about that.