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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
 
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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georgehifi
10,312 posts
 
01-13-11: Almarg 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 Almarg

If it's not the Fletcher Munson low level listening curve, that your bass deficiency is stemming from, then I would seriously look at the value of your source series output cap, and make sure it is
1: That is at least a good quality polyprop cap
2: That it is at least 5uf or bigger.
Cheers George
tsciame
88 posts
 

Wow did you guy's ever loss me... But it gives me something to learn about.

Tony
pubul57 OP
3,712 posts
 
ditto...
atmasphere's avatar
atmasphere
11,615 posts
 
George has pointed to exactly what the problem is. In order to produce proper bass, there can be no phase shift above 20Hz. Fletcher-Monson has nothing to do with it. If it were F/M at the heart of this, the same loss of bass phenomena would be heard by lowering the volume on any active line stage, yet that does not seem to happen. The loss of bass is unique to passives.

To eliminate phase shift at 20Hz requires a cutoff frequency of of 2Hz. IOW, the cutoff has to be about 1/10th the frequency to be amplified. So if the cutoff has risen to 5 Hz, effects will be heard at 50Hz- audible on most speakers. This low frequency phase shift is interpreted by the human ear as a loss of bass impact.

Note that the phenomena will not be had if the source has no output coupling cap. In such conditions there will be no low frequency pole and so no loss of bass energy or phase shift. But the vast majority of sources *do* have output coupling caps, and quite frequently, especially in digital gear, the output levels are so high that any amplifier will be driven into clipping by the DAC or CDP. So any passive used in this case will manifest the loss-of-bass problem.
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