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PostPosted: 16 Jan 2009, 11:47 
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Location: Houston, TX
Update: Here are instructions to build a DIY Lightspeed Passive Attenuator (11 May 2009).

So this guy came up with the Lightspeed Attenuator about 30 years ago. An Aussie. Mark might run into him some day. He is GeorgeHiFi on the forums.
You can find the topic HERE.
He tells the whole story by page 4 and you have all you really need to know by page 10. Dont get caught up in all the baloney in the next hundred or so pages. Its baloney. The circuit is meant to be used the way George shows it and there are many imitators out there selling it for megabucks.
The principle is 4 LDRs. 2 per channel. Special audio LDRs that go from around 30ohm to 1megohm. Use 5V into a potentiometer to attenuate the power, not the signal. This supplies the LED and is not in anyway in the signal. The LED varies its intensity and the resistor is now variable.
One resistor is in series with signal and one shunts. So we basically have a potentiometer.
The following are:
My Power Supply Layout
Attachment:
LSAPower.GIF

My Layout
Attachment:
lsa.GIF

Here is the schematic
Attachment:
lightspeed-attenuator-schematic.png


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PostPosted: 16 Jan 2009, 11:51 
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GND on the bottom of the LS board is actually power ground and on the top is signal ground.
Reviews are incredible. Read up and comment!
I am matching LDRs right now.
Buy them from Allied NSL-32SR2S
However, I paid extra for sorted and they did not come sorted and one was shot. I bought 25, dont know if I have matches or not. You need to match them. I dont know how closely they need to match yet. The trim pot can be used to match and to use as a 3db balance.
Ri

OH the potentiometer needs to be dual/stereo. 100k log.
You tie 1 and 2 on one of them and 2 and 3 on the other. This way they are inverse and will let you adjust two different voltages to move the resistance around like you need.
AND the pot can be a junk Radio Shack pot. Does not matter, its not in the signal. It needs to keep constant contact or you will quickly have a sound problem and the two parts need to track kind of closely. I found a near perfect one but people say since its log we can vary by even 10 percent on the pots.

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PostPosted: 16 Jan 2009, 21:54 
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Location: Winnipeg, CANADA
Hi Ri,

I really like the idea of this. Are you going to try one out?

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PostPosted: 16 Jan 2009, 22:26 
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I think I will have one done this weekend if I get some time to etch.
I think I have some good matched LDRs. Have to throw them in a spreadsheet and check out how they track each other.
Ri

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PostPosted: 17 Jan 2009, 08:48 
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Okay, I measured 24 resistors/LEDs called LDRs. I used a 3V battery and different resistors in series with the + end of the battery then measured the resulting resistance of the resistor part of the LDR.
I used
100R
9960R
22000R
47000R
resistors to test.
The following shows my results. I think I have many pairs to use for my project.
Ri


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PostPosted: 17 Jan 2009, 15:00 
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Great Ri. Looking forward to see how you may out.

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PostPosted: 17 Jan 2009, 15:53 
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Etching the boards right now !
Have to heat the etching solution by bathing the etching container in boiling water as the whole thing is done outside. Not about to bring it in the house. The acid will eat everything and its very dangerous. Just a tiny inhale can bring you to your knees I am sure. I have luckily never inhaled more than a bit of fumes that must have got in my mouth while it was open but I was holding my breath. That will leave you coughing for quite a while. Wind has blown the fumes into my eyes from over 10ft away and trust me it will send you in the house. What I do is start the process while holding my breath and then run in the house for a breath and then back out to fix whatever needs finishing and then back in for a breath and then checking every few minutes but never breathing around it.
Ri

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PostPosted: 18 Jan 2009, 10:53 
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Etched, hand drilled (drill bit inbetween thumb and forefinger), soldered. Need a fuse holder and fuse still.
Here is a link to the board pics
http://picasaweb.google.com/udailey/Lig ... ttenuator#
Ri
Attachment:
LDR-Lightspeed-Attenuator.jpg


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PostPosted: 18 Jan 2009, 12:14 
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Hi Ri, those look really good! Let me know when you will be taking orders. ;) How exactly did you drill the holes?
Cheers

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PostPosted: 18 Jan 2009, 12:38 
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Joined: 06 Jan 2009, 01:16
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Location: Philadelphia Pa
RiGuy wrote:
Etched, hand drilled (drill bit inbetween thumb and forefinger), soldered. Need a fuse holder and fuse still.
Here is a link to the board pics
http://picasaweb.google.com/udailey/Lig ... ttenuator#
Ri

My god, how primitive! :eek:
Wouldn't a dremel mounted in a drillpress stand be much better? That's what I use. :mrgreen:


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