Using Vactrols

[StephenGiles]

I can't get my brain around this so I'd be grateful for some help please. I built a few "What" compressors and because I had no VTL5C9s as prescribed in the circuit, I used NSL32s. I got these to work by decreasing the resistor into the LED from 1k to 33ohms. I've also messed about with the compression ratio since then, but that's una parva distinta de pescados.

Now, given that each type has a different light and dark resistance, does each type react by a different law according to the amount of current fed into the LED?
many thanks
Stephen

 

[ebartlet]

Check this out: Vactrol info

 

[StephenGiles]

Mant thanks Eric, that is very informative.
Stephen

 

[PRR]

> does each type react by a different law according to the amount of current fed into the LED?

Sure. That's the main reason there ARE different types. At the same illumination, one LDR may be 10K and another may be 200Ω. There are a half dozen basic recipes, and each blend can be applied thick or thin.

In the paper Eric cites, Fig 1 page 8 will have a different shape for every model-number.

> I got these to work by decreasing the resistor into the LED from 1k to 33ohms.

That's pretty drastic. What is the fixed series resistor to the LDR? Try increasing it until the LED resistor can be a few hundred ohms and you get decent response. However if you are using an LDR that is so very different from the one the plan was designed for, the speed of response may be very different. If the plan relies on LDR speed to get audio-acceptable attack/release, a very different blend of LDR may give unpleasant results. Getting the response speed "nice" is the hardest part of a good LDR limiter. If too slow, inititial transients pass-through to clip later stages. If too fast, gain rises between cycles and bass is distorted. It is pure serendiptiy that LDRs can work as audio limiters, and many blends don't work at all well.

P-E Photocell and Analog Optoisolator Data Book page 60, VTL5C9 data

Siliconex datasheet listing

NSL-32 data, no curves given, only spot-tests

 

[StephenGiles]

Thanks PRR, a couple of VTL5C9s have now come my way, but I'll try out your suggestions anyway.
Stephen