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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
dhl93449
455 posts
 
George:

Interesting concept, but what about the distortion of these devices as shown in the Silonex data? .1% THD is not low these days, particularly if you listen to folks like Nelson Pass. I know you will say there are a lot of tube equipment that produces distortion higher than this that still sound great, but I still have a problem with a so called "passive" device having a non-linear distortion characteristic. A pot or switched attenuator will have nearly zero distortion.

Also have a bit of a problem with folks calling this a passive attenuator as it need a power supply to operate.

And the rectification process you describe may be occuring, but I have to ask if microsecond type transients are really audible. You will get the same artifacts by testing many types of solid state amps if you use the same storage scopes and look for micro-second transients on the leading/falling edges of square waves. Does not prove that these artifacts are audible, even if they are present. Still I applaud you for trying to find some explanation for the effect. I also wonder why you might think this is universal to switched resistor types, as these vary widely due to the relay contacts and rotary switches used in the designs. A switch or relay can have a much higher contact pressure than a pot.

Sorry if I have repeated subject matter previously covered, this was a mongo thread.
georgehifi's avatar
georgehifi
10,312 posts
 
At line level, at 2vrms which a cdp hardly ever hits, even with peaks, you get .2% of 2nd harmonic which is pleasing to the ear, and as we know with tube amps this can be 10 x this figure.
All tech info/graphs and circuits can be found at this site http://www.diyaudio.com/forums/analog-line-level/80194-lightspeed-attenuator-new-passive-preamp.html which is more for the diy'er in which I give all info on how to make one yourself, sounds as though you maybe capable, you should try it, and maybe be converted.
Even Nelson Pass has designed a buffer in this diy thread for the Lightspeed so it can drive his low input impedance amps which some are 20k and 10k.
It is a massive thread 100x more than this one with over 4,000 posts with some pretty knowledgeable tech guys giving their input for the diy'ers.
Cheers George
georgehifi's avatar
georgehifi
10,312 posts
 
If you wish to see the measurments at CD playing levels, even better than what I said in the last post, I have posted the Silonex graph and an Audio Precision distortion measurement graph, by the measurers calculations is said to be .0025% only at cd levels.
http://www.diyaudio.com/forums/attachments/analog-line-level/232027d1311212462-lightspeed-attenuator-new-passive-preamp-silonex-ns32sr2s-distortion-measurments.jpg

Cheers George
georgehifi's avatar
georgehifi
10,312 posts
 
An interesting little snippet for those of you who own or have built a Lightspeed Attenuator, this will give you maybe some bragging rights.
Not only does the $28k DarTZeel NHB 18-NS preamp have a form of Lightspeed Attenuator controlling the volume (which they tried to patent).
But now there is a new player on the block Constellation Audio with their $60k fet line stage preamp the Altair which also uses a Lightspeed Attenuator form of volume control. (at least they didn't try to patent it)
http://www.constellationaudio.com/ca/reference-series/altair/altair-features

Cheers George
clio09
3,360 posts
 
Optically controlled resistors for volume control.
Instead of using a potentiometer to perform volume control within the circuit, we use an array of 48 optically controlled resistors. These resistors are controlled through a temperature-compensated tracking circuit that maintains ideal signal balance in all conditions. Because they are completely electrically isolated from the control circuits, the optically controlled resistors are as sonically benign as an ordinary resistor.

See George, if you use fancy language like this you can get a lot more money for your Lightspeed.

What exactly is an array of 48 optically controlled resistors and why can't my LSA have this feature? ;)
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