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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
 
Funny, I've been considering trying an Airtight of late. Guess I'll cross that off my list.

I have used the LSA as well with unfavorable impedance matching. Into am amp with 21k input impedance. It did sound good, although you will definitely hear the difference when using it with optimal impedance.

The "10 to 1" rule is overrated IMO.
pubul57 OP
3,712 posts
 
Another one bites the dust:)
almarg's avatar
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almarg
9,670 posts
 
As I see it the 10:1 rule will be of no importance if all of the following conditions are satisfied:

1)The output and input impedances that are involved are both essentially resistive, and do not vary significantly as a function of frequency. That condition will usually not be satisfied if the preamp has a coupling capacitor in series with its output, as is the case with most tube preamps and some ss preamps. I'm uncertain as to how often it will or won't be satisfied if no capacitor is present.

2)The component supplying the signal can handle the load impedance without distortion or other signs of stress. I would not expect that to be a problem in most cases unless the load impedance is very low (e.g., 1K or less).

3)The loss in gain that will result is not an issue. For instance, a 1:1 ratio would result in a 6db loss in the maximum volume that can be achieved, in comparison to a high ratio.

Also, cable capacitance must not be too high in relation to output impedance, but that is a separate issue.

BTW, if the Air Tight's 600 ohm output impedance specification does not make any reference to frequency, and if the design includes a coupling capacitor at its output, the output impedance is likely to rise to considerably greater values at very low frequencies (perhaps 3K or 4K at 20Hz). With a 15K load impedance, that would result in a small but very conceivably audible bottom octave rolloff. A nominal (presumably mid-frequency) output impedance number should be multiplied by 50 to 75x, rather than 10x, to be completely safe under all circumstances, IMO. The 10x number would be appropriate to use if the worst case output impedance is known.

Regards,
-- Al
pubul57 OP
3,712 posts
 
I'm sure I asked this earlier, but can't find my question or the answer - can the LSA be built as a balanced output, if so, any serious advantage to that with let's say an Atma-sphere amp?
clio09
3,360 posts
 
Yes you did ask. I've gone down this road with George. Bottom line he gave me the schematic, but warned that overall this is an inferior design due to the difficulty in matching the additional LDRs (versus single ended operation). In addition, if you can get one built there could be a higher probability of reliability issues, again due the higher number of LDRs to achieve balanced operation.

That being said I recall Audiogon member Dave Garrettson trying to match LDRs for balanced operation to use as a volume control internal to his Atma-Sphere MP-3. He gave up on the effort IIRC. There have been a few people on DIYAudio who have tried to build a balanced unit. I didn't sound like it was a lot of fun.

You really should try it single ended into the M-60 amps. You're going to be really surprised. I get the balanced thing, especially with Atma-Sphere components due to their supporting the 600 ohm standard, but the LSA is quite optimal with these amps too.
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