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LDR attenuators - Better balance with DC bias
LDR attenuators - Better balance with DC bias
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Old 28th March 2012, 02:29 AM   #11
BFNY is offline BFNY  United States
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Originally Posted by gootee View Post
It could be done with a micro, and memory for the measured LDR curves. But this way it could be done with no memory, or processor, which could also be accomplished by using pilot tones that were outside of the audio range.
Yep, but on the other hand, is it really a real time problem? It would be also possible to set volume, hit a "cal" button, and have the out of band tone do it's thing for 1-2 secs, then turn off. No one could really complain about that, because it's normally off.
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Old 28th March 2012, 03:23 AM   #12
kudamakri is offline kudamakri
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We could ask for Nelson Pass' comments, he had used DC to match the LDR resistances in some commercial guitar pre-amp set up some time ago
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Old 28th March 2012, 12:44 PM   #13
wapo54001 is offline wapo54001  United States
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LDR attenuators - Better balance with DC bias
The devil is in the details. Have you played with LDRs? Some points to consider:

1. At higher resistances, it takes quite a while for resistance to completely stabilize. At high resistances, it can take a minute to completely stabilize and then it wanders around a central value +/- a few hundred ohms.

2. Whatever control mechanism you use, it must be capable of accurate control over a range of maybe 15ma at 40 ohms to .005ma at 10K. There is a wide range of sensitivities, I'll attach a list I compiled using my present current control arrangement. At the high end, a change of .001ma can change the resistance by 1K ohms down, or 10K ohms up (a range of 9K~20K). With some of these devices at 10K ohms, a slight change in current will make the unit act like it fell off the edge of the world, resistance-wise.

I believe that I've read that udailey has limited the upper end of his Lighter Note to 8k ohms to avoid the drop-off at 10K.

3. What measurable DC voltage will you use in a device whose resistance will vary across a range of 40 ohms to 10K ohms, always keeping in mind that power is limited to 50mW?
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Last edited by wapo54001; 28th March 2012 at 12:46 PM.
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Old 28th March 2012, 03:43 PM   #14
AndrewT is offline AndrewT  Scotland
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I would expect that the monitoring circuit would compare the DC output voltage from the two sides. Make an adjustment and then compare again. Alternatively, a VERY SLOW time constant could be used to compare the two DC outputs.
One could put "cal" on a timer, 2seconds or 2minutes or whatever. Then it holds and disconnects itself.
regards Andrew T.
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Old 28th March 2012, 05:51 PM   #15
DF96 is offline DF96  England
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This all sounds like a lot of work to avoid a simple pot. The LDR resistance wandering mentioned above could generate more LF/subsonic noise than a track-wiper contact.
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Old 29th March 2012, 04:19 AM   #16
gootee is offline gootee  United States
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It's not just to avoid a pot, is it?

1. The LDRs wouldn't need to be matched.
2. LDRs can't really be matched, anyway. They don't just differ by an offset. Their R vs LED Current curves all have different shapes.

Some of the possibilities:

1. Use digital memory and auto-calibrate once, or at every startup, or whenever the Cal button is pressed. Auto-cal would result in some type of lookup table for each LDR/LED device, so that the two channels could be controlled to give accurate resistances for each of the four devices, for every setting of the attenuation control.

2. Don't use digital memory and use an out-of-band signal (DC or HF) and a automatic feedback control system to continuously either a) set all four resistances for correct and matching attenuation, or, b) cause the slave channel's two LDR/LED devices to track the resistances of the master channel's devices, for whatever resistances happened to result in the master channel's devices, from the attenuator pot setting.

3. Similar to 2, above, except that the out-of-band signal would only be engaged when the attenuation control was changed. Then the resulting settings would need to be remembered, somehow, maybe with digital memory or digital pots, or sample-and-holds.

It might have been nice if the music signals could have been used, to constantly auto-cal, by using current- and voltage-sensing instrumentation amplifiers, RMS-to-DC convertors, and analog dividers, to calculate a voltage corresponding to each resistance, continuously, which would have been fed to differential integrators that would have also used setpoint voltages derived from a Lightspeed-style pot's output voltages and would have then provided control voltages to voltage-controlled current sources for the LEDs.

That might have worked, somewhat, if the time-constant was extremely large/slow (except when there wasn't any music!). But it seems like it would be far better, anyway, to hold the resistances completely constant, except when the attenuation is being changed by the user, which probably rules-out 2a, above. But, the same sort of system might work as part of the auto-calibrator implementation for number 3, above. Maybe only one such measurement-and-control system would actually be needed, with switching to let it look at each of the four LDR/LED devices sequentially during auto-cal.

But I have to say that it's looking like wapo54001's approach (see his thread), which is basically like number 1, above, I think, is looking better and better, although 3b, above (like 2b) might be simple-enough to make it reasonable to attempt.

Last edited by gootee; 29th March 2012 at 04:28 AM.
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Old 31st March 2012, 07:00 AM   #17
godfrey is offline godfrey  South Africa
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Originally Posted by wapo54001 View Post
The devil is in the details.
No kidding. The more I look at these things, the more I don't like them.

This chart sums it up nicely; you've basically got a choice of slow or unpredictable.
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Old 31st March 2012, 07:10 AM   #18
nezbleu is offline nezbleu  Canada
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LDR attenuators - Better balance with DC bias
Doesn't all this just raise the same old question: "What problem are LDR volume controls supposed to solve?" (followed by "And do they?")
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Old 31st March 2012, 08:20 AM   #19
godfrey is offline godfrey  South Africa
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They seem to just replace one set of problems with another (worse) set.

I wonder if one couldn't get better results using JFETS instead of LDRs as the variable resistance elements. Here's links to a couple of relevant documents:

It should be easy to get just as much distortion as with the LDRs, for those that like that sort of thing. Ahem, let me rephrase that.... Those that like the sound of LDRs despite their distortion may also like the JFET version despite it's distortion.

Sample circuits below.
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Old 31st March 2012, 12:23 PM   #20
godfrey is offline godfrey  South Africa
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Oboy! I just realized you can make the distortion cancellation variable and have a dial-a-distortion knob on the front panel next to the volume control, for recordings that could use a little extra warmth.
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