This Week on the Breadboard: Fun with the TL431

[Chuck D. Bones]

First off, I want to thank CyberCow for putting me onto this strange and unusual application for the TL431.
The TL431 is a "programmable zener." It has three leads: Anode, Cathode and Ref. By connecting a resistor between Anode & Ref and another between Ref & Cathode, we can dial-in any zener voltage we want between 2.5V and 36V. This device is intended to be used as a shunt regulator, not as a clipping diode or amplifier. But we pedal designers are not easily deterred from doing something crazy.

Inside the TL431 looks like this:


In some ways, it behaves like a BJT with a Vbe = 2.5V and a lot more gain than a typical BJT. I put it in a Fuzz-Face circuit, to wit:





Q1 is there to buffer the input and make the FAT knob more effective. A typical fuzz face has a gain between 70dB and 80dB. The 431 FF has a little over 90dB gain. It's kind of a fun experiment. There are other ways to get that much gain from a FF.

I shot this pic before I added the input buffer.
Knobs (L-R): LEVEL - TONE - BIAS - GAIN - FAT

 

[gtt]

How does it sound?

 

[Chuck D. Bones]

Like a Fuzz Face with more sustain. It's ok, but nothing special.

 

[HamishR]

But you don't know until you try. I wonder what else we'll see with a TL431 in soon...

 

[jcpst]

Nice- I have a stash of these left over from building dylan159’s TL431 Precision Fuzz.

 

[Ginsly]

Ha, I've been kinda waiting for the inevitable 431 Boneyard post, Chuck - very cool!

The only one I've breadboarded so far is the original that CC shared - it's a fart machine for sure, but man is it noisy, especially for a "gated" fuzz like it's labeled (below)... I'll have to try your FF and the other ones he posted containing a 3904.

 

[Chuck D. Bones]

I expected the TL431 to be quiet and the one I plugged into my breadboard was. Full disclosure: I'm using a vintage LM431 I bought from a surplus dealer. I built a version of the circuit above. It looked like this. Sorry for LTSpice schematic, I didn't draw a nice schematic. The BIAS pot is B10K with 18K from pin 3 to Vcc and 15K from pin 1 to GND.