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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
agear
1,362 posts
 
George, I did end up reviewing the DIY forums and had a few thoughts and questions.

1. Are Silonex NSL-32SR2 LDRs used in the LSA? From my limited understanding, this entity consists of a Cadmium Sulphide photocell coupled to an LED within a light-tight enclosure. This is used in lieu of a traditional metal resistor and may account for (as you insinuated) for the LSAs sound. What is the typical lifespan on a LDR?

2. Does the input and output impedances of LDR-based attenuators vary with attenuation level?

3. It seems that the LSAs performance is contingent on system conditions. What are the "ideal" conditions for the LA? Sensitive speakers, short ICs, and amps with an high input impedance? One blog on the LSA I found on DMS Audio stated the following: "In a system with sensitive speakers it works amazingly well. However with difficult loads such as a power amp needing an active pre-amp with voltage gain like my Nelson Pass F5 or the F3, or using speakers under 90+ db then it is not as great. It still has the same wide sound, deep bass, detail and naturalness, but it sounds 2D with no projection. It just lacks that edge and bite of an active stage."

Your comments on this?
clio09
3,360 posts
 
From elsewhere in this thread are some of George's comments:

Regarding using the LSA with low sensitive speakers, a question/answer:

Grannyring: I have a question for you owners of this exciting volume pot. Will it work well on speakers that are 87 db efficient like Soundlab speakers and other ESL's? I say this assuming the amps are very powerful and the spec criteria mentioned by the poster are meet by the source unit and amps.

George: My Martin Logan Monoliths ESL's are around 86db, I am never more than 11 o'clock for very very loud listening. I make it a point of asking customers with low efficiency speaker where the position of the volume control is for good loud level listening, and the worst was 82db speakers and he was still only at 1 to 2 o'clock for loud listening sessions.

Regarding system requirements George states:

If your source output impedance is less than 200ohm and your poweramp input impedance is more than 47Kohm then the Lightspeed Attenuator is a shoe in, no buffer will sound better.

I have read where short interconnects are best suited to the LSA, less than 2m.

Grannyring has my LSA right now and is using it with his Sound Lab speakers and Atma-Sphere amps. He's given me positive preliminary thoughts on the sound so perhaps he'll weigh in on this later.

I personally have tried the LSA with scenarios that are not as ideal as previously mentioned. For example, an amp with less than 47k ohm input and a digital source with more than 200 ohm output impedance. One thing that never happened in my usage was a change to a 2D sound stage, However, I will add that in my system imaging and sound stage are recording dependent.

I also now use battery power for the LSA. There is a difference versus using the stock wall wart, but I can't identify it. However, I will say I prefer the battery power method. Just something about it that sits better with me.
agear
1,362 posts
 
Hmmm. From an engineering standpoint, the DMS opinion on system requirements makes sense to me and lines up with the logic as applied to passives in general. Your experience contradicts his and another TRL owner how has heard it. System/ear etiology I presume....
grannyring's avatar
grannyring
6,517 posts
 
Tony was very kind to let me play with his Lightspeed unit. I really appeciate his kindness. I will post comments once I have heard more kinds of music etc...
agear
1,362 posts
 
Keep us posted Bill. That should be interesting. I am still interested in hearing it at some point just for the sake of keeping an open mind.

Strong work Tony on your review of the LSA and Truth passives. I saw that Stereophile gave the LSA a commendation for value and performance recently. Congrats to George for the long years of hard work.

Tony, which config do you have with your LSA in the stream? VAC>>Audiokinesis or are you using that other amp listed in your system?
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