LDR photoresistor attenuator preamp, in general

[romana]

Courtesy of Stereophile, I'm another newcomer to this thread. However, I've managed to read through quite a bit over the past few days, as well as some other related threads. I don't recall seeing this question raised, but apologies if it's in there somewhere and I missed it:

Let's say that one prefers dual-mono (separate L+R) volume controls, which would also give you a de facto balance control. Is it practical to keep the channels separate and replace the dual-layer pot with a pair of single deck pots, running L to one and R to the other. It might also eliminate the need for the 1K trimmers, since the separate controls would effectively take their place. Likewise, it could make matching the LDRs less critical. Comments, anyone?

Also, from somewhere in the thread, I gather that the primary impedance of the control pot doesn't have to be 100K or 500K. For example, my existing phono preamp uses 10K attenuators. If I were to make a direct replacement, is there any reason not to use 10K log pots with the LDR attenuators, as well?

And, if my count is correct, there are actually three different sets of DIY PC boards available:

1. dvb-projekt (Oliver's) basic implementation of George's circuit,
2. Udailey (Uriah's) updated boards, and
3. Maximus (Paul's) VCCS IR remote controlled version.

I've also found at least four finished or finished for drop-in projects on the market (not counting Paul's stuffed and tested VCCS):

1. George's Lightspeed,
2. Yeo's (DIYParadise) Eva (1 & 2),
3. the SKA OptiVol, and the
4. P&S Opto-Potentiometer.

Are there any others to add to either list (omitting complete components like those from Melos or DarTZeel)? For nostalgia, we could add the now discontinued Halcyon.

Finally, I'd love to see some form of alpha-numeric LED read-out or other visual reference implemented for the DIY schemes. If someone can add specific instructions or point to a solution that even a novice like myself can work with, I'm sure others would appreciate it, too.

Thanks!
Romana

[anubisgrau]

there's more finished stuff - actually these seem to be more elaborated in significant areas:

myth preamplifier
www.myth.rs

hornshoppe truth
Audiophiletalk - The Truth Pre Amp !

plus a PSM LS that was discussed here - as a DIY thing

[romana]

Thanks, Hornperfect, for mentioning those.

So, to update the lists, there are four different sets of DIY PC boards available:

1. dvb-projekt (Oliver's) basic implementation of George's circuit,
2. Udailey (Uriah's) updated and redesigned boards,
3. Maximus (Paul's) VCCS IR remote controlled version, and
4. ZenMod's PSM LS Controller.

The four finished or finished for drop-in projects on the market (not counting Paul's stuffed and tested VCCS or the discontinued Halcyon) are:

1. George's Lightspeed,
2. DIYParadise (Yeo's) Eva (1 & 2)--apparently developed by his associate VTC4
3. the SKA OptiVol, and
4. the P&S Opto-Potentiometer.

Finally, the more mainstream commercial preamps that incorporate LDR technology as a feature are:

1. The Myth,
2. the darTZeel NHB-18NS, and the
3. Melos SHA Gold.

According to the ongoing thread over at Audiophiletalk, "The Truth Pre Amp !" does not use LDRs, though the exact technology is not revealed. See page10 where the developer, Ed, says, "The Truth does not use optocouplers and is an active device." He's somewhat coy about the details, but seems to infer that the heart of the system may be a photovoltaic cell.

"Film at 11,"
~R

[amt]

Originally Posted by romana

Finally, the more mainstream commercial preamps that incorporate LDR technology as a feature are:

1. The Myth,
2. the darTZeel NHB-18NS, and the
3. Melos SHA Gold.

You might include the Hafler Iris too.

amt

[romana]

Originally Posted by amt

You might include the Hafler Iris too.

You're right about that. Apparently, it was developed by Jim Strickland (formerly of Acoustat), after David Hafler sold the company.

John Atkinson's review in the June, 1989, Stereophile discusses the Iris LDR attenuator in some detail.

His description of it as based on "a Cadmium Sulfide (CdS) photosensitive resistor, as used in countless photographic lightmeters," suggests that Ed, of "The Truth" preamp, may be playing a semantic game with his technology (though it's not worth getting too twisted about it).

[romana]

Originally Posted by tinitus

If you like to devellop a more advanced diy optocoupler version, it might be better and more interesting to have this in a seperate thread...

Granted, this thread revolves around George's LS LDR circuit, but here are some Big Picture Questions:

1. Is the LDR technology inherently superior in and of itself, such that sonically, the associated circuit is not really critical to the end result or do different LDR circuit designs clearly sound different from each other?

2. A related question is, will different brands of caps or resistors in the same LDR circuit change the sound, as happens in most audio designs?

3. Has anyone here built or used two or more of the DIY or drop-in circuits and compared them from a sonic point of view?

4. Or has anyone built-up two or more of the same boards, using different passive components, then compared the sound? Have any of the DIY developers done this or have the component choices been based on availability, price, or fit, rather than direct comparison? Let's assume that practical measures, like staying with the Silonex LDRs, are a given, regardless of the design.

In part, I posted the summary lists in order to sort out the different approaches. If one can end up with better sonics through an alternate approach, then starting a new thread with this in mind, may make sense. However, if the end result is just a more complex route to the same destination, then it's only an exercise with no practical benefit, other than experimenting for the sake of experimenting.

I know this sounds somewhat obsessive-compulsive, but little things often make a difference.

Ciao for now,
~R

[tinitus]

Hey, its ok, or you wouldnt have been moved to a new thread

Its an effort to keep things/threads organised, nothing else

If you want to have the thread title changed, we can manage that too

[anubisgrau]

i think that the thread name "LDR passive preamp/attenuator" would be more appropriate than the given one.

[jmudrick]

Originally Posted by romana

You're right about that. Apparently, it was developed by Jim Strickland (formerly of Acoustat), after David Hafler sold the company.

John Atkinson's review in the June, 1989, Stereophile discusses the Iris LDR attenuator in some detail.

His description of it as based on "a Cadmium Sulfide (CdS) photosensitive resistor, as used in countless photographic lightmeters," suggests that Ed, of "The Truth" preamp, may be playing a semantic game with his technology (though it's not worth getting too twisted about it).

Interesting. I don't know that Ed is being that mysterious, but as his is a commercial enterprise (of sorts, if you know Ed), I wouldn't expect him to run out and post a circuit diagram straight away. Ed says of his Truth Preamp:

"It has an input impedance too high to measure, output impedance is a couple ohms, bandwidth is to 60Mhz, slew rate is a couple hundred V/microsecond.

There are no capacitors or resistors in the signal path. There is no potentiometer in the signal path. It uses photo cells to control the volume. It does not use optocouplers. It is an "active" device and suffers none of the "problems" that "passive volume control/pre amps" have. It can drive long (30 feet) cables with ease."

After considering the Lightspeed and Eva for quite a while I've been sufficiently persuaded by those who have heard Ed's unit to buy one myself. By incorporating an active buffer with the LDR volume control it seems that Ed is on to something. I haven't heard the Lightspeed but am happy to post my impressions of the Truth vs. my Endler attenuators once I've got the unit back home and have had some time with it.

[anubisgrau]

Originally Posted by hornperfect

i think that the thread name "LDR passive preamp/attenuator" would be more appropriate than the given one.

may i repeat my plead once again - the current title of this thread is really not appropriate and it just doesn't do a justice.