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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
grannyring's avatar
grannyring
6,530 posts
 
When I take temps in my preamp (tube) I have never read temps any higher than 90 - 102 degrees F. If the room temp is 75 degrees, then I assume no issue?

For cost and hassle reasons the thought of an LDR is very interesting. Shallco attenuators are very expensive and a lot of work. They sound fantastic and I must admit doubt that a low cost LDR would sound as good.

But, I have never directly compared in an active preamp and am open to trying. If it sounded as good, then it would be a nice alternative for sure.

Not sure if anyone has done this type of comparison in an active preamp. Hard to do without having two of the same preamp for comparison. I suppose one could solder in and out the two attenuators, but going back and forth would be a HUGE hassle.
clio09
3,360 posts
 
I believe Audiogon member Dgarretson put an LDR volume control in his Atma-Sphere preamp. In fact he may have mentioned it in this thread somewhere. You might try pinging him.
grannyring's avatar
grannyring
6,530 posts
 
Thanks Clio09. I will check with him and his results.
grannyring's avatar
grannyring
6,530 posts
 
Clio09,

As an FYI. I now own a Music Reference RM9 MKII that I modified with Duelund CAST caps. Based on comments from you and others I thought I would try it. I love this amp! Very musical and engaging.
dgarretson
2,603 posts
 
Clio09, Thanks for remembering. I replaced a balanced Shallco attenuator with a 100K balanced LDR attenuator in my AtmaSphere MP-1 phono preamp with great results. It is a relatively simple project, provided that use of LDR is limited to the shunt position in conjunction with a high-quality fixed series resistor such as TX2575. One of the many nice things about this application is that the useful range of attentuation across the full arc of the dual control pot can be varied to your desired gain structure by varying the value of the control pot. To obtain a range of attenuation between around -70db and -30db I used a 500K control pot with dual 50K trim pots for L/R balance. Depending on ambient and chassis temperature one must occasionally rise from the couch to trim channel balance. The improvement in sonic performance is well worth the effort.
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