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Old 2nd August 2010, 10:49 AM   #31
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Originally Posted by jackinnj View Post
As long as you have the "control law" you can use the equations to determine how any implementation will work.
That is not the problem that I am interested in at this point.

Currently, the individual devices themselves are not manufactured consistently enough to implement a "control law" that does not involve the nasty job of hand-matching components for two channels and very great difficulty in matching more than two channels at a time. This is the problem I'm interested in -- to make it possible to implement a multi-channel system with consistent attenuation response across channels without hand-matching individual devices. At this point, I'm mostly interested in the variation between parts and how to deal with that variation, not in designing a working system.
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Old 9th August 2010, 04:05 PM   #32
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Update -

For the moment, I've switched from a T pad to an L pad style attenuator, and initial testing of the control suggests that I can achieve a .25~42dB attenuator with a constant Z of 5K with good accuracy with unmatched devices. Not as good as the T attenuator, but it should be adequate.

Switching to an L pad allows me to use 4 devices (comfortably loaded on a 24-pin header without bending wires) for a two-channel board that is configurable as a dual mono or stereo (volume + balance) instead of the eight devices for the T pad.

Design permits through-hole potentiometers on-board, or wired separately. Jumpers to allow multiple boards operating in 'mono' mode to be connected to a master volume control for surround sound or sound board applications, providing the lack-of-matched-sets concept works out OK.

I'm planning to use a socket for the 24 pin header, and extend the LDR resistor wires to solder directly to pads. That will allow flexibility in removing the header if needed for servicing, and yet ensure a fully soldered connection between the LDR resistors and the input/output pads.

Added pads to permit installation of capacitors (tantalum) across LEDs if desired.

A week of company this week, won't be doing anything for a while . . .
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Old 13th August 2010, 02:48 PM   #33
von Ah is offline von Ah  United States
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I am seriously intrigued by the perkin elmer audio applications document. That is some cool stuff, although mechanically delicate to work with (soldering, mounting leads, etc.).
Though I could never build something like this, I love reading innovative audio science.
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Old 13th August 2010, 03:14 PM   #34
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Originally Posted by von.ah View Post
I am seriously intrigued by the perkin elmer audio applications document. That is some cool stuff, although mechanically delicate to work with (soldering, mounting leads, etc.).
Though I could never build something like this, I love reading innovative audio science.
Why could you not build using an LDR? The wiring is delicate, but that is easily managed if you put the LDRs on a DIP header. I myself use a 24 pin header to hold four devices very comfortably.

If you want something now, kits are available in the Lightspeed thread. My system is somewhat more complex and will not be finished for a while.
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Old 13th August 2010, 04:43 PM   #35
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Originally Posted by von.ah View Post
I am seriously intrigued by the perkin elmer audio applications document. That is some cool stuff, although mechanically delicate to work with (soldering, mounting leads, etc.).
Though I could never build something like this, I love reading innovative audio science.
BTW, using the same document you're talking about, consider using the Silonex device that is sold by udailey which has lower minimum resistance than the PerkinElmer device and is therefore more appropriate to this application.
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Old 14th August 2010, 09:06 PM   #36
gootee is offline gootee  United States
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Actually, Perkin Elmer has a large family of LDR device types that, the last time I checked, out-range the limited product line of Silonex in every way. But once you get down to tens of Ohms, why does it matter, for this application? They also make dual-LDR devices that look interesting.
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Old 15th August 2010, 02:36 AM   #37
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Actually, Perkin Elmer has a large family of LDR device types that, the last time I checked, out-range the limited product line of Silonex in every way. But once you get down to tens of Ohms, why does it matter, for this application? They also make dual-LDR devices that look interesting.
Well, the minimum resistance value is important in the max attenuation range. With 40 ohms, with a 5K Zo, the math says I only get 42dB attenuation maximum. At a lower Zo, a slightly greater attenuation is possible.

Is there a PerkinElmer device that reaches, or almost reaches, 40 ohms? I did not find one when I went looking . . .

At the minimum attenuation end, you only need a much greater number, so it's not a problem.

With reagard to the dual-LDR devices, the dual LDR shares a common wire (3-wire device) and that kinda limits its usefulness.
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Old 15th August 2010, 05:00 AM   #38
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Originally Posted by wapo54001 View Post
Is there a PerkinElmer device that reaches, or almost reaches, 40 ohms? I did not find one when I went looking . . .

.
If you look back through the Lightspeed Attenuator thread from what one our members found when he emailed them, you'll see this was a misprint by Perkin Elmer they forgot to add a zero and should have read 400ohms. They don't make anything that is suitable with low enough resistance. And the dual one would be nice but it's also way to high resistance.
Cheers George
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Old 15th August 2010, 11:12 AM   #39
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Originally Posted by georgehifi View Post
If you look back through the Lightspeed Attenuator thread from what one our members found when he emailed them, you'll see this was a misprint by Perkin Elmer they forgot to add a zero and should have read 400ohms. They don't make anything that is suitable with low enough resistance. And the dual one would be nice but it's also way to high resistance.
Cheers George
That could have been me. I found their documentation had a typo in it, and you had to look at the graph and compare it to the text list of parameters. And it was the "K" that was left off -- so the actual bottom was about 1.2K ohms rather than 1.2 ohms as published.
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Old 15th August 2010, 01:54 PM   #40
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Quote:
Originally Posted by georgehifi View Post
If you look back through the Lightspeed Attenuator thread from what one our members found when he emailed them, you'll see this was a misprint by Perkin Elmer they forgot to add a zero and should have read 400ohms. They don't make anything that is suitable with low enough resistance. And the dual one would be nice but it's also way to high resistance.
Cheers George
You mean the company whose IR spectrometer I used for qualitative organic chemistry made a typo?

VTL5C3 1.5 ohms at 40mA.
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