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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
almarg's avatar
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almarg
9,670 posts
 
Gentlemen,

In a situation where the LSA is being driven by a component whose output impedance is low to the point of insignificance at all frequencies (which appears to be the case here, with the Metrum Octave DAC being solid state and having a nominal output impedance of 85 ohms), and where the LSA is driving a load impedance that is purely resistive (i.e., the Rothwells), and *IF* the LSA truly acts at any given volume setting like a pair of resistors, I don't see why it shouldn't be able to drive 30K without issues.

IIRC the LSA's output impedance may be as high as about 7K when driven by a low impedance source. Obviously 30K/7K falls far short of meeting the 10x guideline, but if all of the impedances that are involved are essentially resistive, and therefore constant as a function of frequency, why would there be a problem (other than the inability to drive the amplifier to full power, as I had pointed out)?

Best regards,
-- Al
georgehifi's avatar
georgehifi
10,312 posts
 
We did a comprehensive listen test with the Lightspeed Attenuator with 20 odd "golden ears" from our audio society.
The amp we had was specially modded on the input that we could changed the input impedance in steps on the run while listening from 200k down to 10k. The source was 10ohm output impedance.
What we found that everyone agreed on was there was no change from 200k down to 47k only when we got down to 33k it was felt by some that a slight compression was noticed, then when we went down to 20k all noticed slight compression.
This is why I state in the Lightspeed Attenuator info/instruction sheet the poweramp should be 47k or more, to get 100% transparency and dynamic transfer, 33k is still ok and safe to use, but you may notice a slight compression of the dynamics.
pubul57 OP
3,712 posts
 
George and AL, Ralph Karsten always makes the point of "controlling the ICs" as the reason for needing and active line stage, and I have to believe Ralph is right about many things regarding equipment, though I also recognize he has an interest in selling active line stages...... to what degree do you find his observation true and under what circumstances - valid with 1 meter or less of ICs, true but not to observable in practice?
almarg's avatar
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almarg
9,670 posts
 
George, thanks for providing that background.

Paul, I've always interpreted the phrase "controlling the interconnect cables" to mean simply minimizing or eliminating their effects.

As you know, Ralph has made the point that in the case of a balanced cable driven by a balanced output circuit having very low output impedance, that is capable of driving 600 ohms without issues, and with a 600 ohm termination applied at the destination end, cable effects and cable differences will be completely eliminated.

I see no reason to doubt that, and I too have great respect for Ralph's opinions. But obviously most available equipment is not designed to those standards, which is one reason among a great many why assembling a system inevitably involves compromises and tradeoffs. The bottom line on those compromises and tradeoffs, though, seems to simply be the common sense notion that there are many paths that can lead to excellent results, including both passives and actives.

All technically explainable cable effects that I can envision are either directly proportional to or highly correlated with length, so I would not consider controlling the effects of a 1 meter or shorter cable to be a major factor in those tradeoffs.

IMO.

Best regards,
-- Al
pubul57 OP
3,712 posts
 
Thank you. I do love my Atma. Pre/Amp combo too, nice to use a $20 balanced IC and get SOTA performance.
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