Re: A passive opto compressor

[quote author="Viitalahde"]Here's an idea for you brainiacs to catch.

I've been thinking of something like this for years. It'd be a nice, small box with XLR in and out, nothing else. You control the amount of compression with the input signal level.

The circuit would be dead-simple. The photoresistor would be between pins 2 and 3 of the XLR, and at the input you split to the sidechain with a simple 1:4 transformer or so, whatever it takes to drive the LED.. A cheap one would probably do. Then just a rectifier (could be a full wave after the transformer), a cap for smoothing, a couple of resistors before and after the cap for your basic time constants and the LED part.

A box like this could be tweaked to the maximum taste by the choice of R's and C and could be built for peanuts in a palm-sized box (with a small transformer.. I bet you could find something small from telecommunication transformers) .

Insertion loss could be restored with your favourite amp and so on.

:guinness:[/quote]


Simple opto compressor functions can be easy indeed.

I've modified an analogue Echo unit (Amdek Delay Machine) with such an opto to get real infinite repeat without buildup over a certain level. I just attcked an inverting opamp stage, connected the LDR of a VTL5C4 across its feedback resistor, and connected the LED to the output of the same opamp. (With 1k resistor and a capacitor in series, and 1n1418 in parallel with the LED, for AC drive.) Works great, and would certainly also work in
a passive attenuator.

BUT for this you need a real low driving impedance. Otherwise, the nonlinear load of the LED and its surrounding components will cause distortion in your audio signal. (The way passive VU meters do.)

JH.

driving the LED will load the signal too much. maybe old school 600 ohm gear with those hefty output stages can do it. here's a similar idea:

http://sound.westhost.com/project45.htm

i have a compressor with no amps in the signal path, just the a series resistor and the LDR and just an LM386 driving a small 5V bulb (yes, no LED). i use other line amps for gain make up if necessary. pretty transparant. one could probably make it run on P48 and insert the LDR in the mic path, like the Manley VoxBox. just an idea

Jaakko, are you actually talking about making this signal-powered?

llamas in a thread mill? solar power? using the magnetic poles? firewire/usb? rein deer? many ways to get power

I'd venture a signal-powered compressor won't be very happy using opto techniques. Although, some LEDs have gotten to be astonishingly efficient nowadays. And the photoresistor does intrinsically solve so many other problems.

A fairly simple FET-based one would be feasible, if you were willing to take a huge attentuation. Beyond that, a very elaborate scheme could use the fact that most musical material is very low-duty-cycle, and store the energy for a low-power amp to get levels back up. To reduce distortion one would have to use power-factor correction techniques :shock: