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
Hi George. Thanks for the suggestion. Somewhere I have the schematic for the 8002. Ill take a look. Have you considered creating potted drop-in attenuators so that folks could upgrade their current preamps if so desired? For instance say I have found excellent synergy with amp/speaker but the amp is not remotely passive friendly...or another scenario: I have more than one source to switch? Im thinking that your volume control would ( if pot resistance compatible) be a marked improvement as a replacement for existing traditional pots. In a similar twist any thougts on designing a tube output buffer with your ldr attenuator?
The best sounding discrete buffer I have made is a tube buffer called the "SLCF" (Super Linear Cathode Follower). It was first use as the output buffer in the top of thie line Tektronics Ocilloscope, it has the lowest ouptut impedance of any tube buffer, under 100ohms. But it still does'nt sound as good as no buffer.
http://www.diyaudio.com/forums/analog-line-level/80194-lightspeed-attenuator-new-passive-preamp-364.html#post2346943
Cheers George
Is the use of a transformer/autoformer or buffer in a passive the same? Are they just different ways of lowering the output impedance of the preamp, or is there some other function that distinguishes xformers from buffers?
Hi Paul,

Yes, there are significant differences.

An active buffer stage will provide a gain very close to 1, while providing a high input impedance (to the stage itself) and a low output impedance.

A transformer or autoformer will transform impedances in proportion to the square of the turns ratio that is selected, while at the same time causing a voltage reduction (or gain) proportional to that turns ratio. Example:

If a TVC is set to provide 12db of attenuation, that corresponds to providing an output voltage that is 1/4 of the input voltage. So the turns ratio for that setting would be 0.25. The output impedance would be ((0.25)squared) or 1/16th of the output impedance of the source component. The load impedance seen by the source component would be 16 times the input impedance of the destination component (presumably the amplifier).

If the TVC were set for unity gain (no attenuation), there would be no impedance transformation, the output impedance would equal that of the source component, and the input impedance would equal that of the destination component. But as the amount of attenuation is increased, the output impedance will decrease rapidly, since it is proportional to the turns ratio squared.

Best regards,
-- Al