Trem Fuzz

[soggybag]

I have been thinking aout making a tremolo that varied the amount of fuzz rather than the volume. Here's a first attempt.

I took an LFO from the Commonsound site and combined it with the Liquid Drive from Fuzz central.

The idea was to vary the amount of clipping from the clipping diodes by using a FET as a voltage controlled resistor.

I'm not sure if this will work as is. Is there any way to calculate the amount of resistance across the D S of the FET from the LFO?

I also see a potential problem with a change in volume.

[sfr]

I've actually been getting an effect somewhat similar to what you've described using a standard trem, a boost and a tube amp driven fairly hard.  The boost brings the levels up enough that the tube amp is getting that nice, tubey "compression"... at the levels it's being driven, the volume drops in the trem cycle really just back off the distortion more than the volume.  It's a nice effect.  Seeing it implemented in what could be a more "drastic" fashion sounds cool.

My gut instinct from circuits I've worked on is that there may be a volume drop with the roll back of the distortion, at least if you go all the way to "clean".  Whether or not that's a good thing is up to you.  It seems that with diode clipping, once you get below the clipping threshold, the output begins to drop.  On the other hand, working with boosters in front of Diode clipping distortions, I've realize once you get to the clipping threshold, pretty much no matter how hard you hit it, the volume is constant, so there should be a fairly large "window" of constant volume, it's just that that window is all in the pretty well distorted range.

Something that might be cool could and might work well would be if the tremelo effected a "blend" knob, akin to that in an ampeg scrambler, that mixes the clean and distorted signals.  I've found I get pretty consistent volume levels out of that pedal across the sweep.  My original idea was to apply this idea to a different distortion, but with the "texture" (octave) knob down, the scrambler makes a relatively nice straight up distortion, so this could work relatively well.  It's a fun, quick build.  There's a fellow here who built a scrambler with an expression pedal, to foot-control the "texture" knob (which controls the character of the fuzz between straight out distortion and the crazy screamy octave box.)  If you find his post, he's got sound clips, it's a cool sound, and if one was to add the trem to the blend control, it would be worth it to add it to the texture control as well.   Adding an envelope follower could also be cool.

Now my interest is piqued - great, thanks for giving me yet another project to add to my list!

[gez]

Firstly, by direct coupling the FET it may well always be on...or not:  1st problem!  Would probably be better to AC couple (take a look at the EA trem and get some ideas from that).  Personally, if I want simple I prefer to use MOSFETs DC biased from a trimpot across the rails (look at ROG's input stage for the EA) and AC couple LFO signals to the gate.

If this were me, I'd precede the modulated distortion stage with another op-amp clipping stage (use a dual for the audio and a separate amp for the LFO) so that you already have a moderately clipped signal. Then I'd position the FET in parallel with the 500k drive pot instead - use a fixed resistor or a trimmer at least - as with the set up you have, once the path to the diodes is cut off, so to speak, there's nothing to limit the op-amp's gain (no clipping threshold) so you're going to get amplitude modulation and the signal will probably clean up too much (until it hits the rails and then you'll get sudden harsh clipping as well as a huge output).

[soggybag]

Thanks for the input Gez. I was thinking that there would be a problem with the FET. I was thinking about using an LDR here, but I thought there might be some interesting sounds to be had at higher LFO frequencies.

It sounds, in your second paragraph, that you are describing a clipping section similiar to a TS? I thought of this but couldn't quite put it together. So I settled on the idea here since it was much more simple.

[gez]

It sounds, in your second paragraph, that you are describing a clipping section similiar to a TS? I thought of this but couldn't quite put it together. So I settled on the idea here since it was much more simple.

Just two clipping stages cascaded, the second one being modulated.  If you used the set up in a TS, i.e. diodes in the feedback loop, then you could directly couple the output of the 1st stage to the +ve input of the next.  The first stage could use back-to-back diodes to ground, but everything would have to be AC coupled (slightly more parts).

[gez]

Firstly, by direct coupling the FET it may well always be on...or not

Actually, it would definitely be on: the gate of the FET is being held positive of its source so the gate-source junction is conducting (the sweep of the LFO doesn't swing down low enough to start closing off the channel).  With only a 1k current limiting resistor in there you may well fry the FET (been a while since I used them and I can't remember what this junction can take in terms of forward current).

[varialbender]

Why not just create a general blender? That's probably the sound you're thinking of, and you wouldn't have problems with volume, and it would be kickass on other effects.

[soggybag]

The blender idea is a good one. The LFO modulated blender could be used in a number of ways.

[sfr]

I was thinking the same thing about an LFO controlled blender - I spent a good chunk of time the other day trying playing around trying to figure out how to hook an LFO up to something like the final schematic in the "Rock'n'control"  article over at The Tone God's website; seemed like the easiest way to go, although it uses two vactrols.  (Not a problem for me because for some reason I've got almost a dozen VTL5C3's.) 

I'm not quite sure how to go about doing this though - any ideas?  Seems like you should be able to use something like the Trem Lune LFO to control the voltage into the transistors at the begining of the begining of the Rock'n'control?  OR am I totally off basse?  I'm still trying to learn enough about transistors to figure out what values to use in the rock'n'control.

(R'n'C is here:  http://www.geocities.com/thetonegod/rockncontrol/rock.html, looking particularly at the last schematic)

[gez]

I'm not familiar with Andrew's article, but you could do this with a LM13700.

Invert the LFO output so that you have 2 outputs 180 degrees out of phase from one another.  Each distortion circuit is fed to each half of the LM13700, both halves acting as  VCAs; one LFO output controls the Iabc pin of one half, the other LFO output the Iabc pin of the other half.

This would give you stereo panning, but if you mixed the signals with an op-amp mixer you'd get a mono output.

[sfr]

Hrm, I'll have to look into that.  I'm not very familiar with OTA's, but I guess that's what datasheets and research are for!  Seems like a more elegant solution for the "trem blending" idea, although kludging an LFO onto the circuit in Andrew's article seemed more appealing to me, because you could end up with a box capable to controlling *any* pot.  Although the more I think of it, the less reasons I can think of to trem any control that would really require the transistor/vactrol solution over the OTA one.  (Although I still want to wrap my head around that article because I can think of plenty of times external *foot* control of a three-lug connected pot could be handy.)

[soggybag]

Think there's a trick you can do with two LEDs being fed off an LFO where as one is being lit the other darkens. One LED shows the opposite the other. I read this in a post here I think. I for get exactly how thi was set up, but I do remember it was not complicated. With this idea you could use two LDRs to mix two signals.

[handwire]

also you can try putting a trem before a fuzz that cleans well with the volume knob