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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
 
Banquo363, have you tried the cheater plug on the RM-10? How loud is the hum? If you have to put your ear to the driver it is okay, but if you can hear it standing in front of your speaker or further away then maybe you should talk to Roger about it. In my system I do not get any hum at all from the RM-10 or EM-7 V12. When adding the LSA and shorting the inputs did you check for noise at various volume levels?

The phono stage is more complicated. If a cheater plug did not work try running a ground wire from your phonostage to another ground point, maybe the center screw on the nearest outlet. Any possibility it could also be your phono cable or cartridge wires?
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banquo363
371 posts
 
Clio09: with shorting plugs, I had to put my ears to the speakers to hear the hum AND buzz on the rm10. However, I just tried it again and now I get dead silence. ???

There is a hum emanating from the transformer when I'm near the amp. It varies in its loudness depending on the fullness of the moon :). I thought until just now that that hum was making its way to the speakers--guess not, or not always.

Yes, with shorting plugs in the LSA, I get roughly the same level of hum and buzz at all volume levels (well, up to 3 o'clock--I've never turned the volume past that). And it was the same level as the RM-10 with shorting plugs, i.e. very, very low. However, today, with the amp dead silent, I get the same buzz and hum with the LSA.

Once I introduce the phonostage, then the objectionable noise appears. With IC's into the LSA but the stage off, the hum is loud. After I turn on the stage, the hum diminishes greatly and is overtaken by or competes with the tube noise in my stage. At ambient room levels, it is barely audible at my seat with the LSA at 3 o'clock. I very rarely listen at that level so it doesn't affect my music. But being neurotic, it still bugs me.

My phono cables are grover huffman's latest. They are supposed to be well shielded. As for lead wires, I swapped out the stock wires on my AT LS-12 headshell with copper litz wires. With everything else going on, it's hard to say what effect that had.

I should just sell my stage and get one more compatible with the LSA and quieter. To be honest though, the system sounds great, so I'm hesitant to mess with it. Do you have a K&K stage, Clio? How's that in terms of noise?
clio09
3,360 posts
 
Banquo363, regarding the RM-10, perhaps some DC on your AC line if you can hear the hum at the amp. It doesn't take much. When I lived in a tri-plex in CA if my neighbor used her hair drier or turned on her window unit AC I would get similar symptoms to what you are describing. Unfortunately dedicated lines were not an option for me.

To really isolate the problems after you begin plugging other components into the amp you should get a set of the Jensen plugs. One of the better investments I have made. Also, I won't advise you to do this, but in my system I float all but one of the grounds. Definitely on all my sources, then either ground the preamp, or ground the amp, but not both. The RM-10 manual discusses this too as I'm sure you know.

The K&K phonostage, along with the Hagerman Trumpet were two of the best phonostages I have owned. The K&K was the quieter of the too. In general it is a very quiet phonostage due to the solid state PS and input. The newer versions are supposed to be better and I'm seriously thinking about revisiting it.
georgehifi's avatar
georgehifi
10,312 posts
 
Hi guys, as I have found out input switching is almost as detrimental to the sound quality as a volume potentiometer, this is why I always recommend a separate good quality input switching unit so you can still go direct when you want to bypass all contacts and get the best out of your system or when you want to show it's transparency off. http://www.decware.com/newsite/rotary.htm

Now the obsessed fiddlers on diy audio have found this out also, and to their credit they have now devised their own way of input switching unit using ldr's
http://www.diyaudio.com/forums/analog-line-level/197894-ldr-based-input-selection.html

I still think the one I posted was simpler and it auto shorted the unused inputs.
http://www.diyaudio.com/forums/analog-line-level/80194-lightspeed-attenuator-new-passive-preamp-153.html#post1640716

Cheers George
clio09
3,360 posts
 
Very interesting and thanks for posting George.

I've grown pretty accustomed to swapping interconnects so any type of input switcher has lost it's appeal.
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