LED potentiometer

ralf · 26th December 2002, 11:59 AM

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

Mad_K · 26th December 2002, 12:27 PM

Isn't this a digital unit? I'm guessing the volume is adjusted in the digital domain (IC).

GRollins · 26th December 2002, 01:06 PM

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

paulb · 26th December 2002, 01:14 PM

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.

Nisbeth · 26th December 2002, 03:09 PM

CdS cell = LDR (light dependant resistor) - should be available almost anywhere.

/U.

jcarr · 26th December 2002, 03:40 PM

Maybe you could get a resistor with a horrid tempco, wrap a Nichrome coil around it and start passing some power through the coil...

tvi · 26th December 2002, 03:54 PM

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

FUCHSAUDIO · 25th September 2006, 07:36 PM

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.

Nelson Pass · 25th September 2006, 08:50 PM

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.

EUVL · 26th September 2006, 06:12 AM

See also :

Lightspeed Attenuator a new passive preamp

And in particular on distortion :

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

Patrick

26th December 2002, 12:27 PM	#2
Mad_K diyAudio Member Join Date: Sep 2002 Location: Norway	Isn't this a digital unit? I'm guessing the volume is adjusted in the digital domain (IC). __________________ Mads K

26th December 2002, 03:40 PM	#6
jcarr diyAudio Member Join Date: Jul 2002 Location: Tokyo, Japan	Maybe you could get a resistor with a horrid tempco, wrap a Nichrome coil around it and start passing some power through the coil... __________________ http://www.lyraconnoisseur.com/, http://www.lyraaudio.com

25th September 2006, 07:36 PM	#8
FUCHSAUDIO diyAudio Member Join Date: Sep 2006	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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26th December 2002, 01:06 PM	#3
GRollins diyAudio Member Join Date: Feb 2001 Location: Columbia, SC	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

26th December 2002, 01:14 PM	#4
paulb diyAudio Member Join Date: Jun 2001 Location: Calgary	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.

26th December 2002, 03:09 PM	#5
Nisbeth diyAudio Member Join Date: Jan 2001 Location: Denmark	CdS cell = LDR (light dependant resistor) - should be available almost anywhere. /U.

26th December 2002, 03:54 PM	#7
tvi diyAudio Member Join Date: Apr 2001 Location: Moonee Ponds, Vic, Australia	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

25th September 2006, 08:50 PM	#9
Nelson Pass The one and only Join Date: Mar 2001	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.

26th September 2006, 06:12 AM	#10
EUVL diyAudio Member Join Date: Oct 2003	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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