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LDR Volume Control Hot-Mod
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jonclancy View Drop Down
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    Posted: 06 Apr 2010 at 10:49am
I'll kick off by outlining an idea for integrating a Light-Dependant Resistor volume control upgrade for the Solo series of headphone amplifiers.  I'll flesh out the details over the next few days when I've committed my ramblings to notepaper and edited from gibberish to English! Wink

This will work a treat with the new PSU1 as well.  Why?

1. There is room in the Solo case for the LDR board.
2. There is already a volume pot in the Solo.
3. There is room to for a small regulator board to power the LDRs.
4. You could either draw power from the secondary supply on the new PSU1, or tap into the pre-regged 24VDC as it enters the Solo (or after the second reg for less voltage drop).

Parts are easily available and the LDR circuit can be made up on perfboard or even etched as a simple PCB.  The whole mod should only take a couple of hours to put together and fit.

Discuss!! Tongue
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Points: 16314 		Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 06 Apr 2010 at 11:54pm
Can't Wait! Confused
That none should be able to park up and enjoy the view without a smartphone and the knowledge in how to use apps
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Points: 16314 		Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 07 Apr 2010 at 10:10am
What I would envisage if asked how I thought it works is...

An LED as light source (or array of LEDs) requiring the regulator board such that the light intensity didn't change with power supply fluctuations.

A fixed resistor in series with a light dependent resistor, input and ground either end and "wiper" taken from the junction of the two.

Dark resistance is usually in Meg Ohms. Lit resistance 50k - 100k Ohms according to this data sheet (the only one I could easily find that is RoHS compliant).

Are there quite a few in parallel?

Jon, it will be interesting to see and read about this novel idea.
That none should be able to park up and enjoy the view without a smartphone and the knowledge in how to use apps
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jonclancy View Drop Down
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Points: 105 		Post Options Post Options   Thanks (0) Thanks(0)   Quote jonclancy Quote  Post ReplyReply Direct Link To This Post Posted: 07 Apr 2010 at 2:25pm
Hi Graham,

You're very near! Star  Yes, a separate regulator board for the LED side of things.  I use an ALW Super-Reg at 5V in mine.  Overkill, probably, but in theory a clean supply is IMHO essential.

Fixed resistor in series, but also a trim-pot to adjust balance.  I use the O'scope for getting mine completely balanced in real time, at the listening volume position.

The best way is to use the circuit based on George Stantscheff's Lightspeed MkII attenuator - i.e. two LDRs per channel in series / shunt configuration.  Details in a nutshell in Uriah Dailey's contribution here:

http://diyaudioprojects.com/Solid/DIY-Lightspeed-Passive-Attenuator/

Uriah can also supply matched sets of LDRs for this project.

George's unveliling of his product to the DIY world here (first 5 pages are most relevant):

http://www.diyaudio.com/forums/analog-line-level/80194-lightspeed-attenuator-new-passive-preamp.html

RoHS wouldn't matter for the hobbyist.  I tried my Lightspeed with the SCHA when I was comparing with the SRG II.  I noticed that the Lightspeed appeared, during the short time I listened, to be very similar in quality to the £100++ TKD stepped attenuator I was also testing.

Having created an EAGLE LDR device, and laid my own attenuator out on perf-board, it struck me that a really compact board could be built to go in the Solo.  Desolder the pot and use the holes for signal etc from the LDR board and use a suitable 100K dual pot (inverted for access) for controlling the 5VDC supply.  I was thinking of something like a Teddy Regulator board for the 5VDC, as they are very compact, but boast good performance.

http://www.pinkfishmedia.net/forum/showthread.php?t=39990

If you have a spare Solo board at Slee towers, I can try this out and return it to you for comparison with a stock unit.  Doesn't matter if it's an older version, as it's just the difference in SQ that we're looking for.

George holds the IP for this idea, but for an individual to mod their own gear, I think it presents an interesting alternative to other pots or stepped attenuators.

Cheers

Jon
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jonclancy View Drop Down
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Points: 105 		Post Options Post Options   Thanks (0) Thanks(0)   Quote jonclancy Quote  Post ReplyReply Direct Link To This Post Posted: 07 Apr 2010 at 2:29pm
Re-reading my original post, it might appear that this is my idea.  It is NOT!  I'm enjoying this vol-co, though, and thought it sounded great through a headphone amp.  It's compact and simple nature immediately stuck me as being a good candidate for modding commerical headphone amps.  Sorry if it appeared I had invented something... Ouch
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Points: 16314 		Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 15 Aug 2010 at 2:20pm
Just to kick this thread off again, Jon kindly sent me his prototype ages ago and only just today did I get round to putting it in the "Slee Towers" system.

Will report back when I get a long enough listen.
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Points: 16314 		Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 16 Aug 2010 at 6:17am
Whilst waiting for the lightspeed attenuator to warm up I uncovered something quite interesting...

European RoHS makes it impossible to sell such a product in European Union countries and other versions of RoHS in other countries probably have the same effect. This is possibly why it became "open-resource"???

But why? One of the hazardous substances RoHS identifies is cadmium and the light dependent resistor elements (LDRs) which the lightspeed attenuator uses are CdS - cadmium sulphide. All LDRs are.

Such a pity as so far I find this attenuator completely "invisible", revealing musical information I never heard before. I was very surprised!

So where from here? The only other way I can think of making such an attenuator is using analogue transistor output opto-couplers. I'll take a look into this and see what I think is possible.
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