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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
atmasphere's avatar
atmasphere
11,793 posts
 
Clio09 a low output impedance will mean that the preamp can drive a load of less than 1000 ohms without loss of bandwidth, voltage or increase in distortion. You don't need negative feedback to do that. In the old days, tube circuits did that with an output transformer. You can imagine, being an OTL manufacturer, that we use a different technique (which is patented).

If your DAC uses coupling caps, they would be fairly close to the output of the unit, but there is no set rule on that. You might ask with the manufacturer.

Except still I do not believe anyone has yet built the transparent active preamp, as they all sound so different, more so I believe than the sound of different interconnects.

George and I feel very much the same way in this matter, which is to say that many line stages do seem to fall well short of the ideal. But at the same time it is apparent that George has not heard *all* the line stages out there.
pubul57 OP
3,712 posts
 
I think Ralph also makes a high quality volume control as an option on some [?] all of his amps. I suspect this is the very best way for an amplifier, but for the fact that audiophiles tend to like to mix and match gear and this has the sort of perceived limitation that you have with an integrated; in that sense. I would love to have an LDR volume control on my M60s, and or a Pass B1 with an LDR, as I think the LDR is this best method for building an attenuator (without mechanical contacts, wear and tear). I suspect if the Built-in volume control was available with the M60s when I bought them, that is the path I would have take, but Ralph's preamp undoubtedly works brilliantly with his amps.
georgehifi's avatar
georgehifi
10,312 posts
 
01-14-11: Banquo363
Differently, remaining with this type of amp (with volume control), if we use it in conjunction with the LSA:
1. could we use the amp's volume control to mitigate any impedance mismatches between the two? I understand that these are two different variables, but they are physically related.
2. and, what adverse effects can we expect to remaining faithful to the source? 01-14-11: Banquo363

To your top question, yes if the volume control was 50kohm or more and there were no resistors to ground after it before the first active input stage. But saying that it will still be better if the pot was removed completely and a say 100kohm input resistor was inserted to ground.

Q1: No, not with the Lightspeed

Q2: If just using the pot on the input (no Lightspeed)it all comes down to the quality of the pot, as I say even the top ones Alps, Bournes, Penny @ Giles ect, all sound different because of light wiper pressure, materials used eg: wiper being metal on carbon or plastic conductive track, it is lightweight contact that mimics in a small ways a diode, (a proper diode in the signal path would try to rectify and AC music signal into DC), very bad. And it is this that a soldered in resistor gets rid of, the lightweight contact.

Cheers George
pubul57 OP
3,712 posts
 
George, I know you have worked on the LSA for a long-time, so I imagine it is very well "sorted", is there conceptual room for improvement (sound side, not aesthetics)with the LSA that you have in mind or is a MKII not likely, other than possibly offering more inputs/outputs/balanced if you chose to expand the product line? Frankly, I think if there is a patent involved, licensing would make a lot of sense, more time surfing:)
georgehifi's avatar
georgehifi
10,312 posts
 
The best possible way of getting the best transfer from source to poweramp is to have a cdp with a digital domain volume control, and have no preamp Lightspeed or whatever. This would be the ultimate, however a digital domain volume control in the cdp usually at less than 75% of full output starts to what's called "bit strip" and instead of getting full 16 bit resolution you start to loose resolution to 14 bit and so on the more you attenuate. I have yet to see one that doesn't "bit strip" at 75% or less level.
Even a question I emailed DCS (on behalf of a Lightspeed customer who believed his Puccini and Elgar did not "bit strip") about these very expensive units, and I asked "do these units bit strip at half volume?"
David J Stevens (of DCS) reply was.
"Hi George
Thanks for your interest in dCS and your questions.
As for the digital volume control, provided that you perform the volume adjustment at the correct point in the processing chain and you do it with sufficient precision then there is no reason to lose any signal.
Best Regards
David

Now this reply says to me that they do "bit strip"

Cheers George
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