Precision Fuzz: TL431 based fuzzes

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dylan159
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Post by dylan159 »

Do you know what I like to do in my free time when I'm not making new circuits? Abusing for audio parts not meant for the purpose! Turns out the ubiquitous TL431 precision programmable reference (I have to thank that for how many I've salvaged so far) answers the "can it fuzz?" question with a surprisingly bold YES.
The idea came to me from the datasheet itself, showing a test circuit for voltage amplification. That looked surprisingly close to a common emitter amplifier, bias and all, which might seem surprising from the equivalent circuit. Of course the test circuit couldn't be used as it was because of the extraordinarily low impedances required to run the device at the minimum recommended current, the goal being precision and not at all fuzz. Given the similarity to a regular BJT stage though, coming up with a functioning one was surprisingly easy:
431fuzz.PNG
Here's 4 versions for you to experiment with, given as starting points but also a working circuit already. In all versions I used DC-only feedback because of the very stable biasing, but without the drawback of AC feedback shredding my input impedance to pieces. That resulted in a very weak output no matter what. The 2.2k load resistor is chosen because it's a good compromise for quiescent current, but 10k starts to be too much and it sounds sputtery.
The first version, probably my favorite, has no buffer and a somewhat low input impedance. The input cap is to taste in all versions, but here it works well to keep everything a bit tamer.
The second version is the same, just with higher input impedance and some tweaks to keep the bias right. This one is more buzzy, unruly and gated, both because more signal gets through and also more bass.
The third and fourth versions feature a DC-coupled buffer biased the same way as above, with two different input impedances. This is to show that this arrangement works and quite well.
Finally, I was surprised to find out that the usual way to adjust gain on a single stage without ruining your input impedance, which we might still call "emitter degeneration", works here too. You can hear in the demo below the effect of a 500R pot between anode and ground. AC coupling the pot in parallel with a resistor should work too.
Here's the demo of all versions, featuring also an earlier one with a standard AC-coupled emitter follower:

This IC packs a lot of gain in a small TO92 (or smaller) package. Noise isn't the best, especially with the buffer, but the datasheet already puts our hopes down in that regard. Still, given how cheap and readily available these things are, I think it makes a good alternative as small and simple fuzz.
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Post by Ichabod_Crane »

Are you talking about this component?
http://www.banzaimusic.com/TL431CLP.html

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Post by dylan159 »

Ichabod_Crane wrote: 25 Oct 2021, 08:05 Are you talking about this component?
http://www.banzaimusic.com/TL431CLP.html
yes. there are many different grades and packages but that isn't a concern here. The best source of them is broken switching power supplies for me, you almost always find one.
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Post by george giblet »

I noticed that example circuit in the datasheet a few weeks back. I was designing a TL431 reference with some tempco tweaks for test purposes (turd polishing using the parts I have).

Good work on the bias point tuning. Also, interesting addition of the emitter follower. Does adding a resistor from the emitter to ground change the tone?

At the time I was pondering adding a resistor or diode in the anode and/or cathode.

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Post by dylan159 »

george giblet wrote: 26 Oct 2021, 02:39 I noticed that example circuit in the datasheet a few weeks back. I was designing a TL431 reference with some tempco tweaks for test purposes (turd polishing using the parts I have).

Good work on the bias point tuning. Also, interesting addition of the emitter follower. Does adding a resistor from the emitter to ground change the tone?

At the time I was pondering adding a resistor or diode in the anode and/or cathode.
I didn't have to do much for the bias. probably because it's meant as voltage reference, it just biases, the only difference seeming to be from bias current.
I tried adding a resistor to ground and it doesn't change much. This was literally trial and error to see if the dc path to ground was enough.
I tried resistor on anode (described as gain control), resistor on cathode is just the load. Diode on cathode should work for clipping, unless you mean for bias. Diode on anode might change the bias.
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Post by george giblet »

I didn't have to do much for the bias. probably because it's meant as voltage reference, it just biases, the only difference seeming to be from bias current.
I tried adding a resistor to ground and it doesn't change much. This was literally trial and error to see if the dc path to ground was enough.
Yes, it's a bit like that. It's a cool idea just the same.
I tried resistor on anode (described as gain control), resistor on cathode is just the load. Diode on cathode should work for clipping, unless you mean for bias. Diode on anode might change the bias.
The cathode version I was thinking of was to put a diode or resistor in series with the cathode but have the ref/feedback not at the cathode but at the other side of the series resistor, where it is now. It would mess with the way the TL431's internal circuit works. For good or bad I'm not sure.

I've been tempted to put some of this into spice but I want to finish off my non-audio projects.

Another funky chip application which might came to mind is to use the diff-amp from a LM723 regulator as an audio amplifier. Normally the chip needs 10V but if you don't use the reference it will probably work from a 9V PSU. Running from batteries might no work so great as the chip's internal biasing could misbehave at low supply voltages. I've been playing with LM723's lately as well, more junk box voltage references. The amp idea was definitely inspired by the TL431 circuit.

It was a real surprise to see your post the other day.
All good stuff.
Last edited by george giblet on 27 Oct 2021, 20:57, edited 1 time in total.

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Post by dylan159 »

The cathode version I was thinking of was to put a diode or resistor in series with the cathode but have the ref/feedback not at the cathode but at the other side of the series resistor, where it is now. It would mess with the way the TL431's internal circuit works. For good or bad I'm not sure
Honestly I think even with a diode it would just affect bias. you're just tapping off at a different potential for the adj pin. The current flow is unidirectional so the diode would always be forward biased (although it saturate earlier with the diode). worth trying.
Another funky chip application which might came to mind is to use the diff-amp from a LM723
Let me know how that goes!
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Post by Ichabod_Crane »

I tried on breadboard the first version, exactly like your schematic, except I didn't use the 1N5817, and the power filter cap it's a 22uF (just for the rush).
I used for mistake a 100nF instead the 10uF C4, that made it more sputtering and a bit too much weak on bridge pickup.
I got a lot of volume, but I used a 10k linear, I suppose a log will be better, but probably I'll reach the unity volume at 9.00 o'clock or so.
Original schematic sounds ok: fuzzy, dinamyc, just a little tiny bit sputtering, and it responses well to the volume guitar.
I tried it with battery and with my 9v PSU without any problem. I found it not so noisy, considering it was on breadboard, no shield and clip alligator, etc...
I didn't try any gain pot, for the moment. Should I put a 500R between TL431 anode and the ground, right? I have a 500R linear, casually. I think about to test some low pass filter control, too, though it sounds fair open but not so much, not so... how do you say? Icy peaks? Maybe a Big Muff tone control it will be ok. (Do you think it's necessary an LPB-1 in the end of the circuit after the tone section?)

I bought for this TL431 for a friend's project and now I don't need it, anymore. If I like it I could to build a small fuzz pedal.

Thanks!

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Post by george giblet »

Let me know how that goes!
I'm not sure when I'll get back to it. At the moment I'm bogged down with my non-audio life.

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Post by dylan159 »

Ichabod_Crane wrote: 27 Oct 2021, 13:35 I tried on breadboard the first version, exactly like your schematic, except I didn't use the 1N5817, and the power filter cap it's a 22uF (just for the rush).
I used for mistake a 100nF instead the 10uF C4, that made it more sputtering and a bit too much weak on bridge pickup.
I got a lot of volume, but I used a 10k linear, I suppose a log will be better, but probably I'll reach the unity volume at 9.00 o'clock or so.
Original schematic sounds ok: fuzzy, dinamyc, just a little tiny bit sputtering, and it responses well to the volume guitar.
I tried it with battery and with my 9v PSU without any problem. I found it not so noisy, considering it was on breadboard, no shield and clip alligator, etc...
I didn't try any gain pot, for the moment. Should I put a 500R between TL431 anode and the ground, right? I have a 500R linear, casually. I think about to test some low pass filter control, too, though it sounds fair open but not so much, not so... how do you say? Icy peaks? Maybe a Big Muff tone control it will be ok. (Do you think it's necessary an LPB-1 in the end of the circuit after the tone section?)

I bought for this TL431 for a friend's project and now I don't need it, anymore. If I like it I could to build a small fuzz pedal.

Thanks!
Thanks for the feedback. Reducing C4 affected it so much because you were cutting AC feedback only at high frequencies then. and you get the same signal loss I had in the beginning because input impedance is very low indeed then.
Volume is probably the same for both of us, limited by the supply voltage. Definitely use log and unity will be easy to find between 9 and 12.
yes, the "gain" control goes between anode and ground. I think 500 is enough to keep a decent control but 1K works just as well. Of course you can try the non scratchy version by putting the pot in series with a cap, both in parallel with a resistor.

You can use any tone control you like, this thing has low output impedance. Since there's not any volume to make up for, I'd maybe add an output buffer if that's necessary, because tone controls left there at the end without a buffer belong to the past century.

Last thing. When I meant noise, I meant circuit generated noise, not power supply or interference. Those will be as good as you can make them for it all being a single ended design.
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Post by Ichabod_Crane »

I'm emulating the schematics with Tina TI. For some reason I don't know C4 work doesn't match with the reality. Lowering or jumper it in Tina TI I got a low cut, but when I do the same thing on the breadboard I got a very dull and muddy sound instead, with almost no treble and sputtering effect.

So, I can't trust on the software too much. But I'm trying a gain control like you suggest. On Tina TI a 1k could be ok, but rev log. My 500R is linear, so... I don't know if it's ok, too. I don't think so.
Your suggestion remember me the gain control of the Fuzz Face, the "non-scratch" tricks you said is something like that? Then, do I have to add a resistor in parallel from the anode to the ground, too? What value? I don't know what should do that resistor.
Thanks

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Post by dylan159 »

Ichabod_Crane wrote: 28 Oct 2021, 10:41 I'm emulating the schematics with Tina TI. For some reason I don't know C4 work doesn't match with the reality. Lowering or jumper it in Tina TI I got a low cut, but when I do the same thing on the breadboard I got a very dull and muddy sound instead, with almost no treble and sputtering effect.

So, I can't trust on the software too much. But I'm trying a gain control like you suggest. On Tina TI a 1k could be ok, but rev log. My 500R is linear, so... I don't know if it's ok, too. I don't think so.
Your suggestion remember me the gain control of the Fuzz Face, the "non-scratch" tricks you said is something like that? Then, do I have to add a resistor in parallel from the anode to the ground, too? What value? I don't know what should do that resistor.
Thanks
Shorting C4? I think i get the problem now. You're only looking at the output AC analysis, which is less than half the picture. That cap is there so there's feedback only at DC. if you short it you throw off the bias completely, while if you reduce it you in theory get a low shelf as you said, but it also means that the input impedance at those frequencies is severely reduced, so that not much signal gets through at all.
The fuzz face control saves parts, but it's not completely scratch free. The better way is to have a resistor, the cap in parallel, and the pot wired as variable resistor in series with the cap only.
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Post by Ichabod_Crane »

Thanks.

I hope this schematic will help us to understand:

Image

Testing the C4, with the original 10uF the circuit is fine, I got a well balanced sound. The 100nF makes it a bit sputtering. Shorting the cap the sound is with no treble.
While Tina TI said to me a different thing. Look at this:

Image

If the circuit worked like that was useful, I could put a potentiometer and get a sort of bass filter control. But it isn't so. Too bad.

About the Fuzz/Gain control. Do you mean wiring like that? I'll try it soon, if I have a 1k pot.
Meanwhile, Tina TI said me a weird thing. The cap cut me the low end, it have to be exaggerating big (470uF), and it still cut the peak and some low end, starting around from 300Hz. The resistor seems not so critical. It could be 22k or 1k. A 470R starts to give me some more db.

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Post by dylan159 »

Let's remind ourselves that the spice model wasn't really made with this in mind, but this is pretty much what I'd expect. It's worth checking bias too and you'll see that it will be completely off with the cap shorted. Also probably better to limit the frequency range to something close to 20khz because looking up to 1M is a bit misleading.
The input cap is better suited for a bass control so that input impedance isn't decreased further. it will still resonate with the guitar pickup but that's well known and not a bad thing.
the gain pot wiring seems good to me. I've set an arbitrary limit for the total useful resistance being 500R and that's the case here but it's not too strict. worth checking the sweep by checking if the mid point of a rev log pot (15% wired this way) is close to halfway. Also check how much the series 1K changes the bias.
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Post by Ichabod_Crane »

I don't know how much Tina TI can help be with that bias thing that probably I can't understand well.

I don't have 1k pot, so I tried the 500R, and we definitely need a reverse log.
I tried to put a 22uF in series with the 500R pot, but I lose a bit of... grit, and it farts at low frequency. I don't like it. I put a 1k resistor in parallel with the pot and the cap, but it doesn't change the things. Anyway, I don't have scratch issue using just the pot.
The 500R already cut a lot of gain, and cut even high frequency. The same thing happend to me with the Fuzz Face. I don't know if I want to control the gain like that, lowering the gain with the Volume guitar sounds much better.

Changing the input cap I can change the low end. I replaced the 100nF with the only cap I have under my hands: a 1nF. Of course it cut totally the bass and makes the sound very sparkly. But the trick works, of course.

This circuit seems have no "natural" cut in the audible high frequency. Using a Tone pot can help to do it. And the last stage of the Big Muff with its Tone cuts frequency too much high to be useful. (The Volume is placed in the end of the circuit),

I played with Tina TI. I tried to add a small cap 220pF to the ground after the Tone pot. It works fine at flat Tone setting starting to cut at 10kHz. But no much effect at low setting, nor at high setting, where we get no cut of the highest frequency.
I tried to place a low pass filter (R/C) between the fuzz circuit and the tone + recovery stage part, I used a 1k and a 4.7nF. It works fine well enough, and maybe the 4.7nF could be even a bit larger.
I tried to place the same filter in the end of the circuit before the Volume pot. Here it cuts too much, a smaller cap could be better.
Where should be the better spot to put the low pass filter without change too much the original fuzz circuit sound?

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Post by dylan159 »

you can try any kind of filter at the output you like, mine is more of a proof of concept although it's fine as it is. Variable cutoff RC, shelving RC, muff, bridged T, even my old idea, the celeriac control, are all valid. buffer the output after a passive control.
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