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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
 
Paul one of the tube guru's on diyaudio said the solid state part is at the input, and it's even stated that the 12AU7 in cascode mode is directly attached to the rca outputs in the manufacturers blurbs on a website.
I asked what other preamps used this cascode output stage, and he said Rouge Audio used to on their preamps but their poweramps are very high, the M120 monoblocks I had here were 1meg! input impedance. And also he said some early Lamm preamps were also cascode output.
It is ok to have this high output impedance if you have over 100k or more on the poweramps, but you do get the problem of having to use low capacitance and short interconnects 1.5mt or less, just like with the Lightspeed.
Cheers George
clio09
3,360 posts
 
33k ohm output impedance is rather high, especially given that the Concert Fidelity amps have an input impedance of 47k ohms. Is that 33k ohm constant or at a specific frequency or perhaps a range? Now I know Masa Tsuda personally and have been working the Concert Fidelity rooms at the last few CES/RMAF shows. I'm here to tell you Masa in my opinion is a great designer and the system of components he brings to the shows has incredible synergy and sound. I'm having a hard time getting my hands around this so I'm going to go straight to the source and find out more information.
georgehifi's avatar
georgehifi
10,312 posts
 
Hi Tony,this is what a cascode 12AU7 will be judging by the tube gurus. 33k is not bad so long as the poweramp is 100k or more. What was the input of the power amp used at the CES/RMAF.

Cheers George
georgehifi's avatar
georgehifi
10,312 posts
 
This is the output description "The 12AU7 tubes are on the back panel virtually bonded to the output connectors."

This is where the solid state part is used "It uses a solid-state analogue switch for input switching and an analogue solid-state volume control selected for its sonic quality and tracking accuracy."

All this can be read here, half way down.
http://www.positive-feedback.com/Issue41/concert_fidelity.htm

Cheers George
clio09
3,360 posts
 
George, yes I saw that review and was aware of the other information. As I stated in my previous post, amps input impedance is 47k ohms. What I am having a difficult time with is why the designer would build a preamp circuit with 33k ohm output impedance (assuming this is accurate) to mate with amps that have not much more relative to input impedance. It makes little sense given I have spent approximately 150 hours or so listening to this system and it in no way sounds like a bad match to me.
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