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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
grannyring's avatar
grannyring
6,479 posts
 
Actives also need to be well matched with the amp. My cuurent tube active was a poor match to the Sunfire Signature amp which has an input impedance of 24k ohms.

The bass was muted and highs were etched. The Sunfire amp does not sound this way at all nor do my speakers or pre. I inserted a modest SS amp with an 100K ohm input impedance and all was much better.My system is magic again. Impedance matching is also important for actives. I think most, if not all, of our opinions of actives vs passives are based on the synergy of the system they are placed in. It's the total system stupid:-)
georgehifi's avatar
georgehifi
10,312 posts
 
09-17-11: Grannyring Actives also need to be well matched with the amp. My cuurent tube active was a poor match to the Sunfire Signature amp which has an input impedance of 24k ohms.Grannyring

Yes Grannyring, tube output active preamps are just as susceptible to poweramp impedances as passives are.
I looked at what Teajay is using. The output stage of his Concert Fidelity CF-080 is a 12AU7 in cascode configuration direct to the SE output rca's, I thought this is a high output impedance. So I checked with our tube gurus, they confirmed to me for this tube to be use in this configuration in it's best linear way, it's output impedance will be around 33K! And at best it will only give 10mA maximum output current.
This is fine if it sees a poweramp with 100k or higher input impedance, but he is using it into a Pass Labs XA100.5 which has 20k SE input impedance??? Bad Match this one!!

Cheers George
teajay
1,691 posts
 
Sorry George,

You have two mistaken assumptions regarding my system. First, I use Pass Labs XA-60.5's and not the 100.5's. Secondly, and I think more importantly, since I biamp my system any preamp I use active or passive the signal is first directed to a Bryston X-over then to my two pairs of monoblocks to drive my MG-20's. Therefore, your premise regrading a bad impedance match is invalid in my situation.

I have no idea regarding what the impedance rating is of this active X-over, just that your LSA did not play music in my system like CF-080 does regardless of the reasons why.

georgehifi's avatar
georgehifi
10,312 posts
 
Sorry correction, XA60.5, it is the same 20k for the single ended input as the XA100.5
georgehifi's avatar
georgehifi
10,312 posts
 
Also if it is feeding any of the Bryston Xovers this is the same bad match 15k or 20k input impedance depending on model, one needs to do their homework before spending the big bucks.
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