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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
georgehifi's avatar
georgehifi
10,312 posts
 
To give some measured examples of interconnects
1: With IC's at 100pf per foot a 1mt = approx 300pf
this with the Lightpeed at it's highest output impedance has a HF rollof of -3db at 76khz

2: Same with 200pf per foot 1mt = approx 600pf
this with the same gives HF rolloff of -3db at 38khz

3: At 300pf per foot same gives a HF rolloff of -3db at 25khz.

Good interconnects are usually below 200pf per foot. As not to create low pass filters with passives and quite a few tube preamps.

Cheers George
fiddler
581 posts
 
Teajay,

I may indeed craft a scintillating response if you have anything of significance to add here. However, at the moment the motivation is absent.
pubul57 OP
3,712 posts
 
Ralph is a very sharp mind and I am sure that he is not confusing Flecher-Munson for something else. It does seem however that in the real world with low capacitance IC (my total capacitance with 2 meters is 72pf) that the impedance impact is true, theoretically correct, but inaudible under these circumstances, especially since my hearing is proabaly down 20db at 15khz:) But, obviously with very long cable runs it might very well be the case that buffering is called for. What I still don't understand and would like to hear from Ralph is why gain is of any value except in the obvious cases where the source output is very low and perhaps with some sources and his amps which are 2.83v sensitive. One of the reasons I think his OTLs are among the finest sounding amplifiers of any kind is that he eliminates the complexity and distortion inherent in output transformers, so the tube is directly coupled to the driver in the speaker. A philosophy of less is more, less complexity (in parts and wires)more purity in sound. In some sense the very same thing that is appealing about a passive, bufferless linestage. In the same way that a transformer leads to distortion and bandwidth limitations, it seems to me that the electronics neede for gain in a linestage has some of the very same "problems" as a transformer has in a tube amp. But I do realize I know close to nothing about electronics, and this may be a false metaphor, but I don't understand why.
georgehifi's avatar
georgehifi
10,312 posts
 
Saying all that now if you have a series output cap on your source that is too small it can rolloff the bass with the combination of the Lightspeed and poweramp input impedances, to give an example for this.

1: Source series output cap of 10uf into the combined 7k Lightspeed & 47k poweramp = 6k the cap is seeing which gives a LF rolloff of -3 at 2.6hz

2: 5uf series source output cap will be double that at -3db at 5.2hz

And so on. 2.5ufwill equal -3db at 10.4hz

I like direct coulpled source outputs as this then is not an issue, as well as the best sounding cap is NO cap.
Cheers George
almarg's avatar
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almarg
9,670 posts
 
01-13-11: Georgelofi
To give some measured examples of interconnects
1: With IC's at 100pf per foot a 1mt = approx 300pf
this with the Lightpeed at it's highest output impedance has a HF rollof of -3db at 76khz
2: Same with 200pf per foot 1mt = approx 600pf
this with the same gives HF rolloff of -3db at 38khz
3: At 300pf per foot same gives a HF rolloff of -3db at 25khz.
Good interconnects are usually below 200pf per foot.
George, from those numbers I infer based on bandwidth = 1/2piRC that the maximum output impedance of the Lightspeed is around 7K (which I see is confirmed in your post immediately above). Other passive preamps may be considerably higher, however, at worst case settings. And the issue concerned compatibility with long interconnects, not 1m interconnects. 15K ohms, for instance, in combination with 15 feet of 50pf/ft cable results in a -3db bandwidth of only 14kHz.

Ralph & George, thank you for the explanations concerning bass effects. Just to clarify, though, it should be noted that those effects are essentially unrelated to cable capacitance or cable length.

Best regards,
-- Al
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