Distortion with optocoupler (?)

[TejfolvonDanone]

Hey there!

I was wondering if it is possible to build a distortion with an optocoupler with 3 lead transistor output (like the 4n25).
My idea is to create an common emitter type amp from the transistor and a half/fullwave rectified signal would drive the LED part.
Do you have an suggestions or do you know any other projects similar to this? Is it even worth to try?

Wish you the best,
Tejfol von Danone

[amptramp]

Always worth a try.  I have a box of what looked like TO-39 transistors painted black but they turned out to be SCR's.  But you could make a good distortion out of them.

[dschwartz]

I'm not sure about that, ldrs are too slow

[PRR]

> ldrs are too slow

No, a 4N25 is a Photo-Transistor (plus LED). Propagation Delay with reasonable resistors is under 100uSec (even before you bias the base terminal). It will do audio.

> Is it even worth to try?

You may be in totally un-charted waters.

To a first approximation, light (LED current) is the "same" as base current. So it is just a 2-input amplifier.

But that's a pretty rough approximation. CTR actually varies from 0.1 to 1.5. So some distortion is likely. Whether it is sufficient, or musical, I just can't guess.

[TejfolvonDanone]

So it is "to the breadboard!" time again. I'll post a first draft of the schematic as soon as i get home.
From the first few simulation it looks like an expander or a really dynamic distortion depanding on the biasing of the transistor.

[slacker]

The Tone God did a fuzz face type thing with optoisolators  http://www.diystompboxes.com/smfforum/index.php?topic=55795.msg430165#msg430165 might be worth looking at.

[thehallofshields]

"Thunder Alley" huh? Very cool design.

Did anyone get to hear it?

[TejfolvonDanone]

I haven't seen the Thunder Alley so thanks for it.

https://drive.google.com/file/d/0BzGRfrmHSjb4Qm1Cd2l5RnJvTWM/view?usp=sharing
I came up with this schem. There is room for improvement so i need your opinions on it.
My ideas behind it: OP1 is a buffer for the envelope detector. If the input of the OP2 is higher than the output of it forward biases the diode which allows the cap to charge. OP3 just separates the cap from the LED. As i said transistor of the optocoupler forms a common emitter amplifier.
Although it looks good in the simulations i haven't tried it on a breadboard yet (i'm really busy with my work unfortunately).

[noisette]

check this out
http://www.xqpaudio.net/xqp-545-schematic-2.1-sc.pdf

[amptramp]

check this out
http://www.xqpaudio.net/xqp-545-schematic-2.1-sc.pdf

What is not a good idea in this schematic is having the LED's in parallel fed by one resistor.  If you used double the resistance in series with each LED, it would force current sharing.  Therefore, is one of the optoisolator LED's connected in the opposite direction to the first?  That way, one resistor would make sense.

[noisette]

This afaik is meant average out optocoupler response (as drawn).
there´s some video footage of the xqp guys (no fools) talking about exactly that...

The scematic I posted is actually the 500 series version I meant to post this(pedal version):
http://triphazardpedals.net/th-2-schematic-1.4.pdf
I have built something similar and I really like it though it is not what a metalhead might call distortion
It is great for bass, long waves are affected more by slow vactrols (vtl5c1 is faster than most, that´s why it´s in there probably)

[PRR]

Let's remember the different types of "opto couplers".

Tejfol is taking about TRANSISTOR output couplers. Not photo-resistors. Resistors and transistors can do similar things but are quite different.

[iainpunk]

i know this is kind of an old tread, but i am ordering some parts from my local electronics shop so i can pick them up tomorrow, so i was wondering if there have been transistor-opto coupler based fuzzes.

the 2nd pedal i build was a Bazz Fuss. when fooling around with the circuit on a breadboard, i replaced the Diode with a LED, i found that a LED lit up when playing.
i came across an optocoupler page in their catalog and thought back at this, thinking the LED probably flashes as a function of the input wave-shape, so i wondered if it could drive an opto coupler at audio rate. maybe even in a ''harmonic percolator'' kind of topology... i bet it gets extra MOJO points because its based roughly on the topology of the Harmonic Percolator...

cheers, Iain