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
I run 1 meter from LAS to Music Reference RM10 (hmm, at least 3 folks with that combo - probably because Roger beleives in passives and builds his amps accordingly)- the cable is 12pf or less the 60pf, perhaps the lowest capacitance cable on the market (Carda Golden Reference).
My record for misspellings in a single comment. George, is there any issue other than capacitance related to cable length? That is, would a 1/2 meter of a quality cable almost always sound better than 1 meter even if it were slightly more capacitive; is something going on with length other than aggregate capacitance? Are any of these differences even relevant a 1 meter or less of IC?
In the system I have that features the LSA my speaker cables are 18' long. The brand is OCOS. They are made by Dynaudio and are known for their performance in long runs. I use 1m interconnects.

As for capacitance on the interconnects. Anything under 60pF/ft. should be fine. I'd venture to say you could go higher if you use 1/2m cables. There are a lot of fine cables out there that meet these requirements.

The best method, if it is possible, is to place the passive volume control at the amps input. Ralph offers this option on the Atma-Sphere line for the M-60 series on up. However, given the design of the LSA, I do not believe it would be feasible to place it at an amps input. I am curious though if the LSA can be hardwired from source through amp.
The HF filter that capacitance of an interconnect that is 300pf for it's total length when with the Lightspeed is 76khz -3db
200pf is 114khz -3db
and for 100pf it is 227.5khz -3db.
As you can see even the 300pf is fine as it is still only -3db at 76khz.
This is why I recommend 100pf max per foot and the use of 1mt length with that maximum capacitance.
But most good quality interconnects are well below 100pf per foot which is even better.
This also applies to high output impedance tube preamps with their interconnects, especially when say a 30kohm output impedance preamp is used, if it has an interconnect with 300pf for it's length, it works out to be -3db at 17.5khz which cuts into the audio band quite severely.

Cheers George