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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
3,345 posts
 
Interesting comment on the buffered approach Dave. I believe when John Chapman re-introduced the Tap-X with the Slagle autoformers the remote allowed you to choose from buffered or unbuffered (only on the multi-input models though). In his testing he said he couldn't distinguish a difference between the two, but his goal was obviously to allow a user to address impedance mismatches. The Truth preamp I wrote about in another thread uses photo cells and a buffered output. Output impedance is extremely low, something like 2 ohms. It supposedly can drive cables up to 30 ft. in length. It works nicely with my Atma-Sphere S-30, but as Pubul57 mentioned if you don't need the buffer, and my other amps don't, why add anything to the mix.

In this scenario the comparison of LSA to other preamps becomes more of a contest between volume controls-- which is a critical and oft-neglected determinant of a preamp's performance.

I agree we often forget the impact of the volume control in preamp designs. The concept of removing the volume control from the equation was what got me interested in the LSA to begin with. The LSA doesn't need a high quality volume control since the design eliminates the impact on its performance.
dgarretson
2,592 posts
 
Clio09, Of course in LSA the series/shunt photoresistor IS the volume control. The knob that you twist merely controls the control. I'm suggesting that by adding a companion active buffer, the LDR control could be directly compared to a conventional volume control(e.g. potentiometer, stepped attenuator, etc.) independent of systems matching. Chances are good that an LSA buffered in this manner would come out ahead of most active preamps. And by switching off the buffer function you would know immediately whether the problem is a mismatch.
clio09
3,345 posts
 
Wasn't one of the active buffer designs posted on the DIY forum a tube version? I recall seeing the one from Nelson Pass as well.
georgehifi's avatar
georgehifi
10,312 posts
 
Yes, Nelson Pass very kindly did get involved, and designed a nice direct coupled discrete (no yuk opamps) DIY buffer for the Lightspeed Attenuator, because some of his amps were 12k and 20k input impedance, and this made Lightspeed Attenuator match those low impedance amps correctly.

Cheers George
clio09
3,345 posts
 
George, aren't the Burson Audio buffers an FET design that does not use ICs? Their RCA version includes a volume control too. Curious as to why the balanced XLR version doesn't. Might have liked to the balanced XLR version give a spin with my Atma-Sphere amp.
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