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Old 26th December 2002, 12:59 PM   #1
ralf is offline ralf  Germany
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Default LED potentiometer

Hi, All

hope you enjoy nice xmas days...
Has anyone experiences with led potentiometers like this one in TACT amlifier?

are there kits somewhere? is it difficult to build?
how do they sound?

Ralf
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Old 26th December 2002, 01:27 PM   #2
Mad_K is offline Mad_K  Norway
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Isn't this a digital unit? I'm guessing the volume is adjusted in the digital domain (IC).
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Old 26th December 2002, 02:06 PM   #3
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How Tact does it I don't know, but it is quite easy to build an LED volume control. I've got a Morley volume pedal for my bass rig that uses such a scheme; they've been doing it for years.
At least in the case of the Morley, I think the receiving element is a CdS cell. I've never heard anyone say whether CdS cells sound decent or not. I believe they have a tendency to fatigue when exposed to the same light level for a while, so the volume might not remain exactly where you left it. However, I can say from experience it doesn't change by enough to matter.
If you let your imagination stretch a bit, you can start getting into using phototransistors instead, which are superior in many ways.
I've got some notes on this sort of thing around here somewhere. I was looking into it from another angle a year or two ago but dropped it when a crisis popped up. I remember leaving more detailed notes than usual so that I'd be able to pick up the thread later when things calmed down. It looked--at least for my purposes--to be an interesting avenue for exploration.

Grey
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Old 26th December 2002, 02:14 PM   #4
paulb is offline paulb  Canada
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Isn't the LED just displaying the setting? I agree with Mad_K, I think it's digital. PGAxxxx or something similar.

CdS cells? Can you still buy those things? Sounds like something out of an old Popular Electronics.
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Old 26th December 2002, 04:09 PM   #5
Nisbeth is offline Nisbeth  Denmark
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CdS cell = LDR (light dependant resistor) - should be available almost anywhere.

/U.
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Old 26th December 2002, 04:40 PM   #6
jcarr is offline jcarr  United States
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Maybe you could get a resistor with a horrid tempco, wrap a Nichrome coil around it and start passing some power through the coil...
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Old 26th December 2002, 04:54 PM   #7
tvi is offline tvi  Australia
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I posted this in the "Son of Dork: Attenuator" thread, might be of interest.
<a href="http://www.diyaudio.com/forums/showthread.php?threadid=3626&highlight=dorkus"><im g src="http://www.diyaudio.com/forums/attachment.php?postid=38072"></a>

I think it is similar to the <a href="http://www.melosaudiorestoration.com/article-03.html">Photentiometer</a> used by Melos in thier preamps??

Try searching Vactrol with google

Regards
James
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Old 25th September 2006, 08:36 PM   #8
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Default Photentiometer ?

New to the forum and I've been curious about this circuit and topology for a long time. I was wondering if anyone had technical info, or tried to make their own Phototentiometer ? I thought I'd seen a post by georgehifi about this.

I would appreciate any information out there.
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Old 25th September 2006, 09:50 PM   #9
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I played with the Cadmium Sulfide elements (the ones used
by Randy Smith's Mesa) and while they are workable for
musical instruments, they had too much distortion for use in
"hi-fi", and the tracking you can achieve between 2 channels
was not good.

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Old 26th September 2006, 07:12 AM   #10
EUVL is offline EUVL  Europe
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See also :

Lightspeed Attenuator a new passive preamp


And in particular on distortion :

http://www.diyaudio.com/forums/showt...709#post994709


Patrick
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