2. amps-preamps
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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
clio09
3,341 posts
 
Bill -

Go to the Arthur Salvatore site and read the instructions for the test (see link). There are test instructions for LP and CDP. Ravel's Bolero is a good track to use because it has low passages at the start that will not cause damage.

You're right though, loud passages or even an unexpected loud passage might cause damage. With no volume control on the CDP you wouldn't be able to mitigate damage unless you were to hit the pause or stop button very quickly.

If you are still using the TRL Samsom amps IIRC they have an input impedance of around 67k ohm. The sensitivity is just under 1V. Any CDP with 2V or greater output should be able to drive it. The question is will there be enough volume and will you like the sound. Also, perhaps not with the test but if using any passive preamp the output impedance of the CDP will come into play. Typically this should be under 200 ohms, but under 100 ohms is even better.

http://www.high-endaudio.com/RC-Linestages.html
grannyring's avatar
grannyring
6,436 posts
 
Thanks Tony,

My CD player is a Cary 306. It puts out 3V - higher then the 2V suggested in this thread.

I am checking on the output impedance from Cary. But my amps are the Samson's as you say and the 67k ohm figure seems low? George says the amp's input impedance should be higher then the preamp's output impedance. Even if the Cary is 100 ohms I may have a problem? I think I just figured it out - the "k"ohms on my amp are much higher then the ohms (not K) out of my CD player?

Sorry, but I am not a big spec guy :-)
clio09
3,341 posts
 
Bill,

While the 3V output of the CDP is quite sufficient, the output impedance is 220 ohms single ended. George may need to weigh in here on how that might affect the matching between CDP and Lightspeed. It should work as typically you want a ratio greater thant 10:1 between the input impedance of the passive preamp and the output impedance of the source. The greater the ratio the better. Same is true for the ratio between the input impedance of the amp to the output impedance of the passive preamp. What complicates matters is that the output impedance of passive preamps vary by volume setting.

The 67k ohm input impedance of the Samson amps is lowish (maybe best to check with Paul on that spec, I'm going by memory here) but workable (under 50k ohm would definitely pose some issues). I prefer at least 100k ohm. However, I did use a passive preamp with my D-225 for quite some time with no problem whatsoever.
pubul57 OP
3,712 posts
 
Anthony, I'm not sure if you would know this, but if a CD player can be switched for output level (The EMM Labs can be switched between 2v and 3.9v)is there any theoretical reason why one should sound better or different, assuming 2v plays loud enough? I imagine based on Ralph K's comments to you on "opening" up an attenuator, you would be better off with the lower voltage - though I'm not sure if Ralph's comments apply to the LS as I think it has the same input and output impedances at all volume levels (I think).
clio09
3,341 posts
 
Paul,

To me, if 2V plays loud enough then go with the lower of the two voltages. Why use a jackhammer to drive a nail. 2V was plenty for the RM-10, Auricle Music Blocs, and S-30, and the S-30 had nearly 3 times the sensitivity rating as the other two amps.

I was under the impression the output impedance did vary on the Lightspeed. If it doesn't, that is another significant benefit versus other passive designs.

George?
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