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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
 
The (diode thingy) "diode effect" is bought around by very fast music transients from the source, CDP or phono, which can be in the order of 100's of volts per micro seconds (volts per uS) these happen as the name implies in micro seconds.
While I have no reason to doubt the audibility of diode effects that may occur with many volume controls, I would respectfully point out that this explanation strikes me as fundamentally flawed.

For line-level amplitudes, swings occurring at rates of 100's of volts per microsecond would represent spectral components (frequencies) at several tens of MHz. No source material and no source component will provide signal frequencies remotely approaching those rates. Not to mention that the amplifier, the speakers, and our ears would not respond to them even if they were somehow present.

Regards,
-- Al
pubul57 OP
3,712 posts
 
Al, but we do know that contact quality is important[?], so would "no contact" have to be better? for whatever reason or theory?
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almarg
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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....

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