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Lightspeed Attenuator - Best Preamp Ever?

The question is a bit rhetorical. No preamp is the best ever, and much depends on system context. I am starting this thread beacuase there is a lot of info on this preamp in a Music First Audio Passive...thread, an Slagle AVC Modules...thread and wanted to be sure that information on this amazing product did not get lost in those threads.

I suspect that many folks may give this preamp a try at $450, direct from Australia, so I thought it would be good for current owners and future owners to have a place to describe their experience with this preamp.

It is a passive preamp that uses light LEDs, rather than mechanical contacts, to alter resistance and thereby attenuation of the source signal. It has been extremely hot in the DIY community, since the maker of this preamp provided gernerously provided information on how to make one. The trick is that while there are few parts, getting it done right, the matching of the parts is time consuming and tricky, and to boot, most of use would solder our fingers together if we tried. At $450, don't bother. It is cased in a small chassis that is fully shielded alloy, it gets it's RF sink earth via the interconnects. Vibration doesn't come into it as there is nothing to get vibrated as it's passive, even the active led's are immune as they are gas element, no filaments. The feet I attach are soft silicon/sorbethane compound anyway just in case.

This is not audio jewelry with bling, but solidly made and there is little room (if any) for audionervosa or tweaking.

So is this the best preamp ever? It might be if you have a single source (though you could use a switch box), your source is 2v or higher, your IC from pre-amp to amp is less than 2m to keep capaitance low, your amp is 5kohm input or higher (most any tube amp), and your amp is relatively sensitive (1v input sensitivity or lower v would be just right). In other words, within a passive friendly system (you do have to give this some thought), this is the finest passive preamp I have ever heard, and I have has many ranging form resistor-based to TVCs and AVCs.

In my system, with my equipment, I think it is the best I have heard passive or active, but I lean towards prefering preamp neutrality and transparency, without loosing musicality, dynamics, or the handling of low bass and highs.

If you own one, what are your impressions versus anything you have heard?

Is it the best ever? I suspect for some it may be, and to say that for a $450 product makes it stupidgood.
pubul57
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almarg
9,670 posts
 
Hi Paul,

Yes, I certainly don't question that, assuming there are no significant tradeoffs in the "no contact" approach. As I indicated, what I was questioning was simply the explanation that I quoted.

Best regards,
-- Al
pubul57 OP
3,712 posts
 
Not sure if diode effect theory drove George's design or the design was in search of a theory to explain its performance. It does seem he is doing something right, as others like Dartzeel deploy this approach in their active linestages where it serves as an attenuator in place of an ALPs, DACT, TKD, etc.
agear
1,362 posts
 
Thanks Al. Your elucidation of the "diode effect" was helpful and confirmed some of my suspicions.

"All active circuits produce distortion/colouration whether they have no gain or unity gain." Can you be more specific? And what about amps? Do I need to revert back to the straight wire with gain?

I have talked to two other designers of pre-amps who disagree with the fundamental premise behind the superiority of light-based volume controls. That does not make them right, but the gospel according to George is just that. Gospel to some and apocrypha to others. We need more detailed feedback from actual engineers who make this stuff. Things are much more complex than we want to admit. I know that is not going to happen for reasons stated above.

If you were to rank system components in terms of importance, where would the volume control be?

If this is a paradigm shift, it appears to be away from tubed pre + SS amps to passive pre + tube amps. The "distortion" remains. The villain has just changed clothes....

pubul57 OP
3,712 posts
 
09-21-11: Teajay
Hi Unsound,

One of the oldest forms of knavery in a debate is to share very nasty implied information by saying that it was proposed by someone else, not the speaker, to get the dig in and than avoid the heat from the audience.

If a desinger is not using LDR, I would be surprised that they would argue for the superiority of LDR. Which does not mean the an LDR is better, but they would have little incentive to suggest that it is, even if it were.

I'm glad Al joined in, I always learn something from him.
clio09
3,355 posts
 
I have talked to two other designers of pre-amps who disagree with the fundamental premise behind the superiority of light-based volume controls. That does not make them right, but the gospel according to George is just that. Gospel to some and apocrypha to others.

I've talked to some notable designers as well, including Roger Modjeski, whose word I will take as Gospel on many topics, but not this one. Not that he was not correct on a generic level about issues with LDRs. These are things that have been said before by others more knowledgeable than most around here and can be referenced on the DIY site. So nothing new there. I think what many miss is what George does to eliminate the main criticism of LDRs which is the issue of drifting.

It is said that when ones work is copied it should be considered a form of flattery. Obviously some serious preamp manufacturers have found enough in George's work to copy it. I trust he is flattered.

Regarding the paradigm shift. I for one have tried both and prefer having tube amps to tube preamps (although I will be digressing a bit by the end of the year). In my conversations with some designers the indication was that active preamps are responsible for a larger source of the noise in ones system than amps (assumption is we are talking similar designed equipment).

Case in point at least in my system and others that have allowed me to do this simple test. Using shorting plugs on the inputs of my amps I cannot hear any noise (with my ear to the drivers) coming from my Music Reference amps (both sets). With my Atma-Sphere amps I get a minute level of noise from the tweeter. Adding any passive preamp I own (a TVC and the LSA) to the chain and then shorting the inputs on the preamp the noise level does not change (regardless of volume level). I can't say that for active tubed preamps (mine and other's systems), especially those that use tubes. There have been cases as well with noisy amps when I have done these tests outside my system, so that just adds to the noise floor in most cases.

Also, from the perspective of the preamp in the chain, I agree with Ralph and his opinions of the volume control. It is nearly always the weakest link in a preamp design and will be responsible for sound deterioration. The LSA and some other very expensive switches will eliminate or minimize this, but even in some expensive preamps you get nothing more than an Alps pot.
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