Expandora with photodiodes in feedback loop

[c.stoffel]

Hey guys, I've been reading and learning a lot from this forum, so thank you all!
I'm thinking about building something similar to the Bixonic Expandora, which, as far as I understand, is basically a Rat with an expander, correct? I like the idea of the expander, it should give a nice "responsiveness" to the circuit, right? Well, would it be possible to replace the optocoupler in the feedback loop with 2 photodiodes? I suppose LDR's are too slow to this application, so I measured some photodiodes here and the resistance when exposed to a white LED is close to 200R. So I breadboarded a simple circuit with a non inverting amplifier, using two photodiodes in the feedback loop (as in the picture attached) and the gain changed quite quickly with the change in light. But I couldn't find a single example doing something similar to this, so I'm a little lost... do you guys think this is a good/bad idea? Thanks!

[rockhorst]

The Expandora uses a FET optocoupler for speed. I repaired a clone a while ago (honestly, the clone omitted the opto, I had to put it in...a bit shocking that my friend was charged €220 for it by the cloning company without actually delivering what was agreed). The expander part didn't really do it for me, but it's audible.

I've tried both the h11f1 and the original PC419. The latter seemed to give a more audible effect, but if you mess around a bit, both should give good results. The PC419 sold at SmallBear is an SMD part, bit it's workable. I put it on a bit of stripboard and made some longer leads for it.
Don't know where you're from, but if your from Holland, send me a PM, I ordered some extra PC419s as a spare, but don't think I'll ever use 'em.

As for your original question: no idea  

[c.stoffel]

Hey, rockhorst, thanks for your help! Wow, €220 for a clone that's not like the real deal, that's tough... I'm curious: the clone omitted the expander circuit altogether, or just the optocoupler? Actually I've never heard the Expandora in person, as they are really rare here (I'm from Brazil). In fact, I've never heard of anybody using one around here. I was thinking the expander could give you some nice dynamics, at least considering it is a fuzz/distortion pedal. But you say you didn't care for the expander? Could you elaborate on this?

Thank you very much for offering help on the PC419, but I suppose it would be difficult to ship one of these little guys from Holland to Brazil?

I'm trying to find alternatives for the optocoupler because here they were trying to charge me R$18.00 (around U$36.00) for a H11F1 Talk about overpricing?
I tried some LDRs  but I think the response time is too slow for an expander, you can "hear" the resistance changing, especially on the low-to-high resistance transistion. So I measured those photodiodes and used two in opposite directions, for equal resistance on both sides. Well, it seems to work, but I don't think photodiodes are supposed to be used like this? Maybe somebody who has tried something similar (maybe with phototransistors?) will jump in and give us some light on the subject? Maybe I could try a Photofet coupled with a LED?

[wilrecar77]

Why not use an envelope detector?  If something like that is fast enough for an optical compressor it ought to be fast enough for a dirt box.

I've had a similar idea but haven't breadboarded it yet.

I apologise for the lack of a schematic, but this shouldn't be too hard to visualise. Or at least conceptualise.

Split the guitar signal into two paths. First path goes to an envelope detector of sorts (use what you like here. I'd probably use a precision rectifier). You'd smooth the rectified signal with a capacitor and then use the rectified signal with an LED so that the light intensity roughly approximates the guitar signal volume.

The second path of the signal goes to the dirt box.

After the dirt box, put an LDR as the top resistor of a voltage divider and a normal resistor as the bottom of the voltage divider. The dirt box output is at the junction and the signal output of the whole contraption is taken from the other end of the LDR. The bottom resistor is grounded.

With this sort of setup, you don't have to worry about too much speed. Basically, it *should* work like this.

Signal high = LED bright
Signal low = LED off/dim

LED bright = LDR resistance low
LED dim = LDR resistance high

Basically, the effect of this should be that when the guitar signal is large, the output is large. When the guitar signal is small, the output signal is smaller. You would probably want to put some value resistor in parallel with the LDR so that the output signal can be less "naked". I'm not sure if that is the right term, but hopefully this should give you something to play around with. This is similar to the expandora but I believe the approach I described ought to be more modular (send and return for the dirt box?) and easier to play around with.

[c.stoffel]

Hey wilrecar77, thanks for joining the discussion. The envelope detector sounds like a cool idea to experiment with, and your explanation was very clear. Like you said, it *should* work! I guess I'll have to build something along these lines to test if the LDR response is fast enough for the application (and try different LDRs too). Thanks!