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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
 
Paul, I'm not certain but I suspect that the reason for the Atma-Sphere S30's high sensitivity number may be that it reflects the voltage DIFFERENCE between the + and - signals at its balanced input, which would be twice the amplitude of either signal. Not sure how its rca input is configured, or if the 3V sensitivity spec is meant to apply to that input as well as to its balanced input.

Also, although I realize you were referring to a 92db speaker just in answer to Jult52's question about "why does it matter," it's worth noting that the MMG's sensitivity is spec'd at 86db/2.83 volts/1 meter, which for its 4 ohm nominal impedance is only 83db/1 watt/1 meter. Although as a planar speaker its acoustic output will fall off relatively slowly as listening distance increases.

Jult52, the only ways that occur to me for stepping up the dac's output voltage would be:

1)Putting an active gain stage somewhere in the path between the dac and the power amp, but of course that would defeat your purpose in using a passive preamp.

2)Modifying the dac internally -- lots of potential issues there.

3)Using some sort of audio step-up transformer -- lots of issues there as well.

Best regards,
-- Al
pubul57 OP
3,712 posts
 
Almarg, I suspect you are right, Ralph's spec sheets don't specify single-ended sensitivity specs. So, given a 3db loss, or a 50% reduction in wattage, assuming a worse case scenario, it will seems Jult52 more than sufficient power to drive his speakers VERY loud in most rooms, I assume Maggies are not for headbangers. So, yes there is a slight loss in max power, but do you think it would be relevant? But then again I come from drive my Merlin VSMs with 27-60 watts amps.
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almarg
9,670 posts
 
So, yes there is a slight loss in max power, but do you think it would be relevant?
My guess is that it would not be a problem, especially considering this statement by Jult52 in the other thread:
My power amp has a continuous rated output of over 500w which I never fully max out being a law-abiding middle-aged person :) I like the power for the instantaneous peaks, especially at the onset of sounds and for the feeling of headroom.
BTW, my rough estimate of 3db as the amount of the amp's power capability that would become unusable was based on the following:
1)1.3db due to the disparity between dac output voltage and amp input sensitivity.
2)0.5 to 1.0db due to tolerances or possible inaccuracies in those specs.
3)0.5 to 1.0db estimated margin in the recording, between the highest musical peaks and digital full scale. That's what I've actually seen on a number of commercial cd's, that I've analyzed with an audio editing program called Sound Forge.

Best regards,
-- Al
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georgehifi
10,312 posts
 
All this is not an issue, most amps/speakers are fine.
If you have an amp that is .5 or 1v input for full output sensitivity and 110db speakers, then you my have a problem with having to use the Lightspeed way down on the volume and never being able to go past 8 o'clock on the volume. And an active pre would be even worse in this situation. But this is very rare as most amps are 1.5 to 2v sensitive and most speakers are 93db below.

Cheers George
clio09
3,360 posts
 
I think you'll be fine. Your CD has a good amount of output voltage and your amp isn't too insensitive. You should definitely be able to swing the volume knob past 12 o'clock which is ideal.

I do use my LSA with the S-30 single ended inputs. I've also removed two of the driver tubes and shorted the tube sockets, effectively lowering the gain of the amp 12db. In each case the LSA worked fine. My speakers are rated 92db, but that is conservative. 94 or 95db is probably more accurate. Had they been under 90db I doubt the LSA would have worked well with the S-30.
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