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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
clio09
3,355 posts
 
The Atlas monoblock is 3.1V to full power so I'm assuming Bill has the stereo version which is 2.3V. That should pose no issues for the LSA IMO.

The gain stage in the amp is tube driven. Output stage would appear to be solid state. The amp does double its power output from 8 to 4 ohms and it has an output impedance that is incredibly low and somewhat of a norm for solid state. I am curious how Jim White gets such a low output impedance from a zero feedback design.

While the LSA does match up well with the Atmas-Sphere S-30 in my system, I do feel it matches up better with the Music Reference amps I have. Driving the S-30 direct from my Resolution Audio Cantata (5.5V buffered output) seems to offer some benefits as well, but it is not as pure a signal transfer. In fact nothing has been as pure as the LSA in my system.

If you go to Devilboy's system page he comments on the fact that he did not do a head to head comparison between the Tap-X and LSA. In my direct comparisons with my Slagle DIY unit and the LSA they are pretty close, but I give a slight edge to the LSA, again it is more pure where there is some coloration from the autoformers.
georgehifi's avatar
georgehifi
10,312 posts
 
I believe I stand to be corrected, I see that it's the Atlas "monoblocks" that are 3.1v sensitivity, these are not suited for unity gain active or passives preamps, they will work but fine with maybe a source headroom disadvantage.
However I now been told the stereo version of the Atlas is better at 2.3v sensitivity, much better for unity gain actives or passives, this is why you like it, still not as good as 2v or less sensitivity amps, which most are.

Cheers George
georgehifi's avatar
georgehifi
10,312 posts
 
Clio09 I am curious how Jim White gets such a low output impedance from a zero feedback design.Clio09

Could be that it has no "global" negative feedback, there are such things as "local" feedbacks, and even feedforward circuits. It all in the wording.

Cheers George
grannyring's avatar
grannyring
6,479 posts
 
Devilboy, it is great when you find something like the LS in your system that really works perfectly and exceeds all your expectations. Very excited for you!

I tried the LS in another system and it simply did not work. But reading everything here I was tempted to try it again with my new amp and speakers. I am always open minded and just had to give the LS another try.
I purchased one a few weeks ago and inserted it into my system. My new amp has an input impedance of 475k ohms and I actually measured it at 570k ohms! My new speakers are also relatively efficient so I felt I could give the LS a real chance to sing!

Well, I put the little LS on top of my 70 pound reference preamp and gave it a second whirl. I took notice immediately as my music sounded wonderful .
Dead quite background with nice tone and no hint of grain or any type of nasties. Nicely detailed and certainly earned my respect. For $500 and in the right system, the LS is a bargain of large proportion.

After several weeks of longer term listening I have come to respect this little passive. I would go as far as saying it kept pace and even bested some big dollar active preamps I have heard. Actives that sell for several thousand new. However, in the end I sold it as my tube preamp just surpassed it in too many ways. While my preamp costs $4000 new, compared to the wonderful LS it is not overpriced considering the improvement it brings to my music collection.

So I would not say all high end gear is overpriced as good values do reside in other places. I do agree that we must be careful as it is indeed easy to overpay in this hobby.
pubul57 OP
3,712 posts
 
Never a truer word has been spoken:)
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