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GREAT ELECTRONIC AUDIO ITEM JUST CAME AVAILABLE Answered

The Electronic Goldmine  has a rare to find component.
"Tiny audio optocoupler is about the size of a pencil eraser and has 4 leads. These are used extensively in audio compressors, audio level controls, audio limiters, expanders/noise gates, guitar tremolo effects, guitar amplifiers and music effect boxes. Inside the black epoxy case is a 2 lead photocell (not a photodiode or photo-transistor) and a 2 lead LED. This is why the device finds extensive use in audio applications (photocell instead of photodiode). The LED requires about 10-16mA current and the resistance of the photocell varies from 150K up to megohms when the LED is off to under 400ohms when the LED is on."

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iceng (author)Antzy Carmasaic2014-01-27
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iceng (author)2014-01-22

So most will think this is an add for the Goldmine and your probably correct !

BUT I'm not related to this electronic store and this component is the

Answer to a recent Question and as an EE this is a good deal even if

you need to buy 50 for $20 that is 40 cents each for your parts bin.

This post needs an At-A-Boy or Cool or I wont bother next time.

I guess if I can buy a UV laser for under $3 this isn't so exciting and I'm just an old EE who keeps a good spare parts bin

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gmoon (author)iceng2014-01-23

Not exciting, per se, but useful. Thanks for the link.

I've got a handful of LDRs I ordered a few years ago. They were once in vogue (as noted) for use in amplifier channel switching and for tremolo circuits. Some are difficult to replace--the old ones used tiny filament light bulbs--and LEDs work fine, but don't replicate the rise/fall of the LDR exactly.

Used to make my own with CdS photoresistors and a light bulb or an LED. Stick 'em in a piece of heatshrink tubing. Great for those old Craig Anderton compressor circuits...

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iceng (author)gmoon2014-01-24

Perhaps you may also like this supplier

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iceng (author)gmoon2014-01-23

Thanks, the true audio compressors did use the rise to glow and fall of an incandescent integrating filament to work the gain structure of an amplifier.

I know I'm always threatening to design and build one for our main TV because of those annoying higher volume over-modulated commercials that blare at me.

Now that I'm thinking ! ? ! ... I could try to add pi RC or even an LC front end to the LED to simulate the beneficial integral effect of a filament using that LED=>LDR device.

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iceng (author)iceng2014-01-23

Feel free to disagree if you don't agree :-)

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gmoon (author)iceng2014-01-24

If a starting point would help, here's a schematic for the old Anderton compressor. It's an LED type LDR. It's from Guitar Player mag, looks like 1970's vintage.

The "original" schematic I used "way back" was in his book "Electronic Projects for Musicians" (1975) and uses two opamps, not three (the first schemo) but still has the LED LDR (CLM6000).

I think CdS photoresistors might be so slow that there aren't too many negatives using LEDs vs filament bulbs, beyond a certain frequency. Maybe at average audio frequencies it's not bad.

I have an Acoustic G60T amplifier (60 W tube, early 1980's) that uses filament LDRs for channel switching--there isn't any delay circuity for the switching, but it's very soft. If I had to guess, I'd say the channels switch in 200 to 400 milliseconds. From clean->gain it creates a short "swell." LED LDRs would need additional components to replicate that "organic" delay.

I'm sure with an opamp and a well-placed cap or two you can make a little "sample and hold" circuit that keeps the LEDs on for a few milliseconds...

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