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

Showing 9 responses by jult52

Thank you for the responses and information, gentlemen. It's very much appreciated. To clarify, I have modded Maggies with a sensitivity of about 89 or 90dbs.
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?
Pubul - I see. So you are saying that I would indeed lose some of the amps output potential due to the mismatch between the voltage output/input gap. Is there a solution to the mismatch other then changing to a different amp or DAC? Can DAC output voltage be increased? (I know it can be decreased by installing a resistor.)
Devilboy - I note that you have a Bent Audio Tap (as well as the Metrum Octave). Could you do a quick comparison of the LSA and the Bent? That I believe would be of interest to everyone on this thread.
I compared the LSA with a switch-mode ps and the linear TeraDak 9v supply discussed earlier in this thread. I am going to report a null result: I could not reliably differentiate between the sound of the two. The TeraDak carried no benefits in my system.
Hi everyone, I have a question about the TeraDaK U9VA power supply, which I recently bought. The ac input is too large for the 2.1mm connector used by the Lightspeed. I asked the TeraDak manufacturers how to convert the plugs but the language barrier is preventing me from getting an answer.

What adapter have the folks using the TeraDak with the Lightspeed been using? If you have an online link, that would be very helpful. Thanks.
I pushed the TeraDak ac input plug into the LSA and, though it isn't the right size, it powers the unit. Problem solved. Thanks for the advice, Banquo & Lacee.

I'll try and do some careful listening and come back with a report on how the TeraDak compares with a SMPS wallwart.
I want to add my very positive initial experience with the Lightspeed here. After receiving some good advice on amp input voltage here, I purchased and plugged in the LSA into my system about a month ago and am very impressed with it in terms of:

1) detail - every recording I listen to now has added lines, inflections and colors;

2) color - the diversity of sound colors I now hear is maybe the biggest single change I've experienced. I listened to Philippe Herreweghe's sonically-excellent recording of the Mozart c-minor serenade for 8 wind instruments soon after getting the LSA. It was like listening to a kaleidoscope of timbres, with each of the instruments differentiated by register, type of attack and dynamic. Amazing.

3) musicality - this is maybe the most surprising effect, for which I was totally unprepared. The performers actually seem to be more "human". There's also a "spaciousness in time", in that the melodic lines "breathe" more (boy, is writing about music hard). My working hypothesis is that this is a result of the added detail: one can now hear more of the inflections given to the notes, especially during the initial attack, and this decrease in blurring at the transient edges of sound adds ease and expressivity to the music.

So I want to thank George, Pubul and Clio for turning me onto a component that is the end-state for my system (off the merry-go-round!). A few technical comments:

1) With my 1.8v DAC feeding my 2.1v input amp, I find most of my listening to be in the 12-2 o'clock range.
2) I swear there was some break-in in the bass with the LSA, lasting maybe 5 hours.
3) I am a newbie compared to the very experienced audiophiles here. I think suggesting a DAC/passive preamp/high impedance amp combo is a must whenever recommending a path forward for a newbie or someone with a less expensive system. I have a combined $2.5k sunk into my components between transport and speakers (ex-cables) and I think I'm done. Beat that price/performance ratio!

System: Transport -> Nixon TD-2 DAC with added power supply -> LSA -> W4S ST-500 -> framed Maggie MMGs.
The LSA replaced a modded Forte F-40 ss preamp which I think is very musical but which I now believes is grainy (not its rep).