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
Oh, and congratulations and good luck on the Electra-Fidelity venture ... seems like you will at least have a good time with it, and beyond that, who knows......
I'm using a content management tool for the layout and publishing. The forum module was very easy to work with. Not as robust as some other third-party options, but since this is the only forum that will be on the site and I don't expect heavy traffic it works for me.

Electa-Fidelity is a playground. I don't expect much from it. Just wanted to do something fun and interesting with my spare time, as well as try to help a few friends out. Now I guess on occasion I'm going to have to add disclaimers on my posts here. What a PITA.
I asked this elsewhere but wanted to move this discussion here since it concerns adding an LSA to my system.

I've read a lot here about DAC output impedance and power amp input impedance but less about sensitivity. To give an example, my DAC has an output voltage of 1.75v and my power amp has an input voltage of 2.1v. So that isn't an ideal relationship for passive preamp purposes but my power amp has a continuous rated output of over 500w which I never fully max out. At what point does the difference in the DAC output voltage and power amp input sensitivity become a problem? How does this problem affect the sound? And if you never push your amp to the outerbounds of its volume capabilities, does it really matter?

Interconnects would be pretty short and impedance out (100ohms) and amp impedance in (62k ohms) shouldn't present a problem.

In the other thread, Almarg responded with some good information and estimated that the output/input voltage discrepancy of 0.35v represented a reduction in volume anywhere from 1.3-3.0db, which effectively means reducing my power amp to maybe the 200-400w category, which is undesirable. What are your thoughts?