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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
 
I read that as well and find #4 interesting. Wasn't the whole idea of using LDRs and optocouplers in George's design to take the switch out of the equation. In other words a $5 switch would work just as well as a $200 switch.

Regarding #1, this is true in my system, but not sure what the point is. It doesn't degrade the sound. As for #3 I guess by virtue of the fact that the Warpspeed uses a SLA battery, the designer must feel battery power is better. While I liked using a battery power supply with my LSA, I can't say it was better, just a little different, but I could never really pin point why. I now use the battery PS on one of my LSA's and the linear PS on the other.

So for me clarification on #2 and #5 would be helpful.
pubul57 OP
3,712 posts
 
Agree that #1 is completely irrelevant to me in my system and not worth "fixing" - the nature of the beast. I have the feeling that way for the LSA to be better lies in ergonomic issues, and I don't think can easily be addressed and remain being a true LSA "neat".
clio09
3,360 posts
 
True, I can't see what more can be done to the circuit. I think George already addressed that going from MkI to MkII. I think we can forget about any upgrade in ergonomics for the time being.

Reading about the Warpspeed I find the designer has this thing he calls a Squid in the circuit, but I think it is commonly known as the VCCS. IIRC an idea he got from Nelson Pass. I'm a little fuzzy on that though. Maybe George can clarify.
georgehifi's avatar
georgehifi
10,312 posts
 
2: Never had any problems with the ldr side, as my prototype is still as it was when new and that's 7 years now powered 24/7. When I say in the instructions to leave the Lightspeed Attenuator at 1/2 volume when leaving it powered up and not listening, it's the led's I'm thinking of, even though when at full or min volume they are never at their max mA rating, it is in my opinion to share the use evenly so I say leave it at half way if not in use (listening).

5: The best is as it is now, it's simplicity in it's purest form, as the music signal only goes through one soldered resistor (ldr) from input to output, anything else would need to go through more devices. As for the power to drive the led's, it is known that battery is the purest form of dc power, and so many times we have A/B'ed between the wall wart and a Lithium Ion battery and nobody could detect the difference reliably some say they hear a difference but can't tell what it is. Even golden eared Sam Tellig, need to see with his own eyes if he has the wall wart or battery attached to his Lightspeed when he's listening.

As for the Warpspeed not interested in it, too much other crap being put into the signal path.
As for doing any other things to the Lightspeed, I would like to have more inputs, but the only way I would do them is to use Mercury Wetted relays for the input switching (which I believe are being made illegal) and that would be still not as good as only one input with no switching. So use one of these, and then you can still go direct when your really want to hear your system at it's best. http://www.decware.com/newsite/rotary.htm

The only thing I may do in the future if I find a reliable set is a remote control volume and I would make sure it's retrofittable to all older Lightspeeds. But I have not yet seen a system I'm happy with that has long term reliable supply and quality. For a remote volume you need, a quality remote, receiver circuit, motorized pot and power supply to run it and the receiver

Cheers George
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banquo363
371 posts
 
I don't know what it is (thanks Almarg for the explanation above), but I've been switching my short (1.5 ft) and long (3ft) interconnects as described above and I always get better sound when the short IC is between LSA and rm-10. Perhaps it's as Almarg noted: that there's an attenuation of bass due to variations in the impedance of my tubed phono stage. At any rate, I now get more dynamic and better bass (articulate and rich). I get 15-20db dynamic swings (as measured by my radio shack meter) on symphonic music, from the average sound level to peaks. Not bad at alI, I think. What counts as a good dynamic swing?

Joe Morello's drum whacks can now sound like a shot in the dark.

I've made a myriad of changes to my system of late and I'm continually impressed by how the LSA allows me to note them all, good and bad. I guess that it's the LSA's neutrality 'at work'.

Thanks, Clio09 for the shorting plugs idea above. I made some and went through the procedure. My rm-10 is not dead quiet, close but not quite. Transformer hum, I believe. But the LSA with shorting plugs adds no additionally noise. My phono stage however...There's a dreaded ground loop I think. I bought a cheater plug but that didn't help. Mercifully it's not too loud. I need to have the LSA at 3 o'clock to be bothered by it at my seat.

On a side note for those using the rm-10: I've been using NOS GE El-84's and they're fantastic. Superior to the stock tubes and to the Genelex reissues.
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