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Old 31st March 2012, 06:11 PM   #21
gootee is offline gootee  United States
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I tend to agree that using LDRs almost seems like it would be more trouble than it's worth.

The main problem they are supposed to solve is simply the same one that all stereo attenuation schemes are supposed to solve: A stereo attenuator is needed. A couple of other possible problems that they might be able to mitigate are mentioned below.

One thing that LDRs should be able to do better than other passive attenuators is automatically adjust (or automatically be calibrated) for nearly-perfect matching of the two channels, at every attenuation level. But I don't know if that is a big-enough problem with other passive attenuators to warrant using LDRs, with the added complexity of auto-cal or auto-adjust.

And the same thing (automatic matching or auto-cal'd matching) could probably be accomplished a little more-easily with a dual pot and one LDR. Or, with more parts but less design effort, a stepped attenuator could be carefully matched.

One thing that LDR-based attenuators have, that none of the other types of passive attenuators have, is a complete lack of mechanical contacts in the signal path. But I don't know how important that is to sound quality, or how much better it might be in terms of long-term reliability and performance.
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Old 31st March 2012, 06:41 PM   #22
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Quote:
Originally Posted by gootee View Post

One thing that LDR-based attenuators have, that none of the other types of passive attenuators have, is a complete lack of mechanical contacts in the signal path. But I don't know how important that is to sound quality, or how much better it might be in terms of long-term reliability and performance.
I think that's not the issue. When I first started reading about using LDRs in attenuators, I asked the following of the Lightspeed thread:

"May I ask for you opinion about the "sound" of LDRs -- how it affected your system when you switched to LDR input switching? Did you hear a difference? If so, what was that difference?"

The one-word answer I got was "transparent" and no one disagreed with that assessment.

So, it seems (I've never heard a set so I don't know), it is the sound quality that is the potential, and we are willing to sacrifice time and energy to solve the peripheral issues that the LDR presents. The important piece that complex drive circuitry can (potentially) provide that the Lightspeed cannot is close tracking between channels which obviates constant fiddling with the balance control. This lack seems to be the main problem with the Lightspeed. That, and very loose impedance control which can also affect sound quality.

So in my project I am assuming that the LDR's sound is worthy. My goal is to have the sound quality but also add to it

1) excellent tracking between channels
2) relatively tight impedance control
3) low current through the LDRs and the pot at any volume level to assure pot and LDR longevity
4) convenient remote control capability with readily available motorized control pots and circuits.

I think that the question of sound quality has been asked and answered. That's not the issue any more.
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Old 31st March 2012, 07:10 PM   #23
gootee is offline gootee  United States
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I hear what you're saying about sound quality, wapo. But I still have lingering questions and doubts. For example, I wonder about the magnitude of the improvement in SQ, which would help determine whether or not it justifies the effort.

Also, I generally like to discount listening test results such as "sounds better". I want the most-faithful reproduction, even if it "sounds worse".

And I haven't done enough research but keep seeing claims that the LDRs add distortion. But, if there is added distortion, there's not enough good data to know if they're still better overall.

Despite all of that, I do still strongly tend to "believe" that they could be made to give more accurate reproduction than other passive attenuator types, and could have fewer long-term problems or changes in accuracy.

I do have a big pile of VTL5C2 devices, here. Maybe someday I'll make the time to hack something together and do some measurements.
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Old 31st March 2012, 07:39 PM   #24
DF96 is offline DF96  England
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Quote:
Originally Posted by wapo54001
The one-word answer I got was "transparent" and no one disagreed with that assessment
I thought that some measurable distortion at higher attenuations is a known issue. And of course poor channel balance will not help the sound, especially if the system does not have a balance control.
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Old 31st March 2012, 09:07 PM   #25
gootee is offline gootee  United States
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I too wonder about the reported distortion. But wapo's design, and most of the ones being discussed in this thread, should provide nearly-perfect channel balance at all attenuation settings. Maybe you were thinking of the typical uncalibrated/uncontrolled types of LDR attenuators.
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Old 31st March 2012, 09:37 PM   #26
DF96 is offline DF96  England
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Yes, of course. I forgot that this is the 'good balance' LDR thread!
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Old 1st April 2012, 11:06 AM   #27
godfrey is offline godfrey  South Africa
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Quote:
Originally Posted by gootee View Post
One thing that LDR-based attenuators have, that none of the other types of passive attenuators have, is a complete lack of mechanical contacts in the signal path.
The JFET option mentioned a page or so back offers that too. Depending how strict your definition of "passive", there's also the attenuator chips that use electronically switched resistor networks.

Moving away from passive solutions, I'm almost tempted to suggest analog multipliers (e.g. Gilbert cells), but I suspect noise would be a problem.

Quote:
Originally Posted by gootee View Post
Despite all of that, I do still strongly tend to "believe" that they could be made to give more accurate reproduction than other passive attenuator types, and could have fewer long-term problems or changes in accuracy.
I'm not so sure about that. Things like the "light history" effect tend to put a spanner in the works. It means that identical LED currents will give different attenuation depending on whether, for example you just switched the unit on a few minutes ago or it's been left on overnight. Similarly, Turning the volume up to a certain level will give a different result to turning the volume down to the same level, all of which makes calibration a bit awkward.

In terms of things like history effects and response times, JFETs are far better than LDRs as variable resistance elements for attenuators. Temperature dependence is likely to go the other way though.
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Old 1st April 2012, 11:18 AM   #28
godfrey is offline godfrey  South Africa
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Changing the subject entirely (again)....

It seems the two main problems with channel balance in LDR attenuators are:
A) Matching and aging of the LEDs.
B) Matching and aging of the photocells.

The first of those can be neatly sidestepped by using one LED to illuminate two photocells. While that severely limits your options for trimming, it allows the interesting possibility of having a purely mechanical balance control, with no added electronic components at all (think moving shadow ).
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Old 1st April 2012, 01:28 PM   #29
gootee is offline gootee  United States
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If you really want to get around (or not worry about) all of the device-physics-related calibration problems then I think that the only certain way would be with LDR resistance measurement and feedback control system, which could probably be all analog if desired.

That would require out-of-band pilot tones, or DC, either continuously or possibly only when the knob position was changed. If done only when the knob position was changed, then some sort of memory (at least four bytes, or four digital pots) would probably be required.
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Old 1st April 2012, 01:40 PM   #30
godfrey is offline godfrey  South Africa
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I like the pilot tone idea, but it would probably require a filter in the audio path at the output. It may as well be continuous, though. The idea of "on demand" recalibration when the the knob is moved is tempting, but I don't see a real advantage unless the filter is switched in and out of circuit during and after the adjustment.
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