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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
3,360 posts
 
I also feel the need to share this story, because it offers a bit of a different use case for the Lightspeed.

My speakers are Audiokinesis Jazz Modules. They are a 92db efficient 12 ohm design with a smooth impedance curve. Very easy speakers to drive. I've used a 225 rated TRL D-225 amp (tests out closer to 300 watts as you know Bill) with it, as well as some lower powered amps. The designer, Duke LeJeune, voiced the speakers to an extent with the Atma-Sphere S-30 amp. While the amps input impedance is 100k (single ended) or 200k (balanced) ohms, the input sensitivity is only 2.83V. This figure is not usually what one, including myself, would consider passive friendly. Since I wanted to hear this combo for its synergy I asked Ralph Karsten, the amps designer (who has heard the Lightspeed), if the single ended Lightspeed and another balanced passive unit I'm going to test out would work well with the amp.

Ralph responded that I was making too much out of the volume control issue. My CD players output was more than enough on its own to drive the amp, basically validating a comment George just made (I was able to run my Otari MX-5050 BII reel-to-reel into the S-30 amps using the decks volume control and there was plenty of drive). In fact Ralph feels the further around the volume control you can get on a passive (closest to wide open) and achieve your optimal listening SPL, the better. With my very sensitive amps I get anywhere from 10 o'clock to 12 o'clock. With the S-30, I can push it a bit further, 1 o'clock or so. I can say it sounded wonderful.

Ralph recommends using an active preamp with his amps but it is important to note that his designs support the 600 ohm balanced standard (pro audio). Not all balanced active preamps support this standard. Of course Atma-Sphere preamps do. The benefit of the 600 ohm standard is that it takes the balanced cable out of the equation. Otherwise, based on my experience, when using appropriate cable lengths, there should be no reason why a passive preamp wouldn't work with less sensitive amps, assuming the other spec criteria are met.
clio09
3,360 posts
 
Pubul57, what is the active preamp "conditioning"? Are you referring to the preamp being some type of noise filter?
georgehifi's avatar
georgehifi
10,312 posts
 
No, thinking more in terms of power delivery and power supply fluctuations, the ability of the output stage of the CD player to drive the ICs and the amp.
georgehifi's avatar
georgehifi
10,312 posts
 
06-16-10: Pubul57
No, thinking more in terms of power delivery and power supply fluctuations, the ability of the output stage of the CD player to drive the ICs and the amp

There should never ever be powersupply fluctuations in the output of a cd player, if there was it needs to go to the recycling tip.
As for the power delivery, (which means current delivery) If your source (cdp) has a low enough output impedance (say less than 200ohms) and it sees an input impedance (load) higher than this there is no current delivery problem, it's only if it sees a input impedance (load) lower than 200ohm then you can start to have current limiting.
Output impedance of the source has to be lower than the input impedance of the next stage, this goes from cdp to pre, then from pre to amps, and then from amps to speakers. And don't forget all interconnects and speaker cabling have some resistance this should be added to the input impedance of the load, this is why one should not use hair thin cabling as it has more resistance and adds the load resistance.
There endth the "Ohms Law" lesson for today need to build some more Lightspeed Attenuators, will back later.
Cheers George
clio09
3,360 posts
 
And don't forget all interconnects and speaker cabling have some resistance this should be added to the input impedance of the load, this is why one should not use hair thin cabling as it has more resistance and adds the load resistance.
Georgelofi

What type of cabling should one use (or do you use) in terms of gauge? Is there a preference over stranded versus solid core. Would cabling like Mogami or Belden suffice? These typically have low LCR specs.
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