2. amps-preamps
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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
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banquo363
371 posts
 
I've been having issues with the sound out of my system and finally had some time to diagnose.

I've narrowed it down to the LSA or my tube amp (music reference rm10). The symptoms are the following. I'm getting diminished power/volume and muffled sound quality. Every other track or so the volume and sound quality will briefly return (the volume will suddenly shoot up) and then go back down after a few seconds or so. It's not isolated to one speaker or the other, so I'm ruling out an interconnect or wire issue. I've changed all the tubes on the amp and the problem persists. So I'm suspecting the LSA. I'm using a wall wart power supply and not battery pack. But what in the LSA could cause the symptoms I'm hearing? More importantly, how can it be fixed? Preferably without having to send back to Australia?

Help. Thanks.
clio09
3,360 posts
 
Are you able to try another preamp in the chain to see if the problem persists. Although it may not be tube related, there could be something else going on in the amp that could cause this issue. Trying another preamp will help you further isolate the problem. If you don't have another preamp let me know as I have a set of EVS attenuators I can loan you. I'll be in Newport Beach next week so you can pick them up in room 417 where I will be exhibiting.
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banquo363
371 posts
 
Thanks, Clio09. I think I figured it out. I was premature in blaming the LSA. You are right. I opened up the rm10 and found that one of the wires to the fuse box is disconnected. Now if only I knew how to solder :).

That's cool that you'll be exhibiting. I'll be sure to swing by.
clio09
3,360 posts
 
I can bring my soldering iron to the show if you want to haul the RM-10 over. Also, Sal Zambrano from Audio Summa should be at the show too and he works with Roger, maybe he can help.
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banquo363
371 posts
 
Well, I spent the day learning how to solder and fixing the detached wire. While it was fun, it didn't help. The very first track I played after all that revealed the same problem.

So, I dragged out a beater preamp (thanks for the suggestion, clio09) I had stored in the garage and hooked it up and listened for 45 minutes: no issues. Afterwards, I immediately plugged in the LSA and replayed the same cd: problem returns.

So, it's not the amp after all. It must be the LSA. I tried a different wall wart and the problem persists. I tried a different outlet and the problem persists. As I noted above, the problem is not channel specific: imaging is centered as usual. Perhaps there's a power regulation problem in my unit?

I should note that one of the output rca jacks is loose, but that's been that way ever since I've had the unit. By loose I mean I can rotate it a quarter turn back and forth with little resistance. If I try to go past the quarter turn, there's resistance.

Anyone else experience these symptoms?

Clio09: do you or your tech know the insides?; I'm loathe to send it to Australia but it'll be easy to carry it to Newport Beach :).
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