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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
georgehifi's avatar
georgehifi
10,312 posts
 
Channel to channel matching using quad matched sets as I do, gives the obvious left/right channel balance (centralized sound image & stagging), and more a more stable i/o impedance.
All the other copiers of the Lightspeed Attenuator on the market, do 2x matched pairs only, which is still very hard to do, some are not even doing matched pairs but series resistance and only one matched pair. Doing matched quads is exponentially more difficult than doing 2x match pairs, but the difference in sound quality and sound stagging can be heard, when A/B'd against each other.
I go for plus/minus 1-2 db and then calibrate for the majority of the usable volume range. Then all matched quads are potted in hard wax to keep them as temperature stable as each other, so drift is keep to an absolute minimum. Then a final calibration done again before a 24hr paying time, then checked once again before being shipping out.

Cheers George
pubul57 OP
3,712 posts
 
George, is it correct that as the volume is louder (less attenuation) the output impedance goes up? If so, given that I can switch my CD output from 2v to 4v, does it make sense to use the higher gain so I can use the volume control at lower settings and keep the output impedance lower?
georgehifi's avatar
georgehifi
10,312 posts
 
The ratio impedance between the series ldr to the shunt ldr is what causes the level difference, this is why it's called a voltage divider, but one that can be varied in both planes. EG: If they are at the same impedance you have half volume coming through from the source.
As for the 2v or 4v choice, it depends on the poweramps gain and speakers efficiency, go with the one that gives you good reasonably loud listening level at mid position, but it doesn't really matter too much. With one of my very low gain amps I listen around at 3 o'clock with the others around 10 o'clock and you'll never guess which one has more dynamic headroom, yes the one that's at 3 o'clock the low gain amp, not because of the Lightspeed but because the amp has itself has less restrictive (compressive) circuitry.
It does not matter where the volume is for listening, so long as all the i/o impedances are right, and you can reach your loudest level you want to hear.
There is no such thing as dynamic headroom compression with the Lightspeed Attenuator volume control as it can pass anything the source gives out, unhindered. The only thing that will sound artifically dynamically better is a DBX unit (dynamic range enhancer) but we all (old school audiophiles ) know how disgusting they make the music sound.

Cheers George
vicks7
182 posts
 
Hi All,

Can anyone help me in determining why, on some CDs, even with Lightspeed at full turn, I am not getting enough volume.

System is Audiopax Model 88 amplifiers, Acoustic System International Tango R Speakers and an APL modified Esoteric X01 CD/SACD Player with Zu and ASI Liveline interconnects.

Is there anything I can do to "help" the Lightspeed?

Thanks

James
pubul57 OP
3,712 posts
 
I think the Audiopax is a fairly low gain amplifier (17db and less?)- it might need some additonal gain from an active preamp, or possibly a TVC passive with 6db gain. No expert on the combo you have, but a guess.
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