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Has anyone tried a photoconductive cell in place of a potentiometer or stepped attenuator? There's a great article on passive pre's that I found from searching this site on the subject where the author makes mention of this technology used by a commercially available preamp.I'm about to begin my second DIY audio project, a passive preamp for my system (Snell D, Cardas, Acurus A120, Marantz CC67). I plan on using a simple potentiometer for this which, though not ideal, is cheap (and if my system doesn't work with a passive I won't be out a lot of dough). Eventually (if it works) I plan on upgrading to something a little more audiophile friendly.
The stepped attenuators all seem pricey and the parts to make them DIY ain't much cheaper. So I decided (on a whim) to look up prices for photoconductive cells on digikey and they're quite cheap (price escapes me at the moment, but I remember under $10). I figure that one of these in a box with a small christmas light on a rheostat and you got yourself volume control with none of the problems of a pot but with continuous control like a pot. As long as I understand the specs right there are a couple parts digikey sells which should work in such an application.
Anyone tried one? I did a search for photoconductive and turned up squat.
Follow Ups:
The Universal Audio LA-2A leveling amplifier is a recording studio compressor/limiter that is based on a luminescent panel/photocell module; the non-linear nature of the response time of the module makes for a very good sounding compressor.(compressors are used to limit the dynamic range of a signal in a recording studio; this is absolutely NOT what you want to do in your system)
variable caps? My Hammond A-100 organ, (think of a B-3 with built-in amps) has one for volume, and has been working fine since 1963.
they went belly up.This may be what you're looking for -
The Melos Photentiometer sucked. It drifted so the volume wouldn’t go to zero. It was delicate and easily damaged in transit.I believe the photosensitive element deteriorated with age so they all just head south with time. I know MARS (Melos Audio Restoration) can’t fix them and has several options to replace it.
Nice gimmick but not a reliable or durable product.
Gazzman
no text
would it still qualify as a 'passive' preamp if it still needed a power supply to operate? Not trolling, either.BTW, the Hafler Iris preamp from the mid-80s used opto-electronics to control the volume in a bridge arrangement. They sold out to Rockford as this preamp hit the market so it didn't get much attention then and now. It used old TTL control circuitry, too.
Cheers,
David
I would say that it does because the entire chain that the signal sees is deviod of active components. The power is only needed for the light source which is separate from the signal path and the photo-element is just a fancy variable resistor. If you wanted to you could leave the light source out and just use your room lighting to control volume (yikes!).
n/t
And just how do you think to make pot out of one photo resistor that has only 2 leads. Pot has 3. So?
Stick it in the top half of a divider, with a fixed at the bottom (eg 10K)Resistor off - no signal.
Resistor on - full signal (apart from source resistance).
No claims for linarity or anything else, but it does act as a "pot"
Whay you say is not how pot works. He will need some more brain to figure this out.
Your idea may work but does not simulate a pot.
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