Variable clipping + BMP tone stack... will it work?

[earthtonesaudio]

Hey I've been pondering a variable clipping circuit (inspired by the AMZ saturation and warp articles) so here's my idea.  Probably would have been quicker to breadboard it but I wanted to share with y'all and get feedback.


Notes: Diodes can be anything.  I chose LEDs for more gain, and different colors for different forward voltage thresholds.  Clipping control also controls gain, more gain to the right, but simultaneously less clipping.  Maybe not earth-shattering, but I think it might be cool.


Any suggestions?

Thanks,
Alex

[brett]

Hi
very cool.  I would add a small capacitor (0.1uF) between the collector and the diodes, just in case you get some mis-biasing through the diodes.  But then again.... maybe they'd cause some weird, cool stuff to happen.

Also, you'll need more gain if this is going to clip with an un-boosted guitar signal.  I suggest that you reduce the 390 ohm resistor on the emitter of Q1 to about 100 ohms, for a gain of 100 and moderate clipping.
cheers

[earthtonesaudio]

Thanks Brett, the blocking cap for the diodes is a good call. 
I noticed that without a cap, the diode only clips when oriented in one direction... anode to collector, I think.  Maybe I did something wrong, but that's the impression I get from the "listen" test, along with the fact that the LED only lights up when it's put in one way. I wouldn't be surprised if it has something to do with biasing.

Since posting last, I've changed it and added a couple things... the first stage is now high gain (no diodes), the second stage has one red clipping LED, and I put a BJT buffer on the end.  I haven't found a tone control I like yet; I'm looking for a flexible single knob tone stack.  Still needs some work.

[paperhouse]

Does anyone have a chart of various LED colors and their clipping thresholds?

[rackham]

Are you getting any noise when you adjust the 1m pot?

I tried something really similar last night after trying to get my 'dirty-to-clean' circuit to work.

I ended up sticking the diodes at the back of the circuit and using a 1m pot between them and the output cap.

[earthtonesaudio]

Does anyone have a chart of various LED colors and their clipping thresholds?

I'm afraid you won't find a chart that shows "THE" clipping thresholds (forward voltage) for different color diodes.  It's all over the map, varies diode by diode.  However, you can generally guess that red, yellow, orange and green are about 1.5V, blue, white, purple, pink are usually 2V or more.  There are some oddballs in there also, like high-efficiency blues that conduct 2.0V forward, and 1.5V reversed!  Best bet is to look up the datasheet for the diode you have in hand.

Are you getting any noise when you adjust the 1m pot?

I tried something really similar last night after trying to get my 'dirty-to-clean' circuit to work.

I ended up sticking the diodes at the back of the circuit and using a 1m pot between them and the output cap.

I took it off the breadboard in order to build a Tube Sound Fuzz, but I don't remember any noise as you adjust the pot. 

If I 'board this again, I'd do the clip pot differently: diodes from the left lug to the wiper, and a cap from wiper to the right lug.  I have no idea what that would do, but it seems cool in my mind.  Some more food for thought: I took a lot of inspiration for this pedal from the "Metal Simplex" pedal (schem on diyguitarist.com).  Have fun with it.  I might come back to this later, but the CMOS distortion was much easier to tweak from the get-go, so I'm going to spend some time away from discrete transistors for a minute.