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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
atmasphere's avatar
atmasphere
11,575 posts
 
Yes, Al, George seems to have missed my point entirely.

If you think about what happens when you install a resistance after a capacitor, then you have a start at what is happening with any passive system. The cap has a variable impedance vs frequency, which changes depending on the time constants in the system that it is part of. At some low frequency the impedance will be seen to increase. This is how capacitors are used to roll off low frequencies in equalizers and how the low frequency poles in active electronics are set up.

When you add to that impedance, you are changing the timing constant. This is simple math. In effect you are increasing the output impedance of the source. When that happens, you get a bass rolloff when that resulting composite source interacts with the input impedance of the amp.

The thing to note here is that none of this has anything to do with the *quality* of the passive control. It can be the best out there (and the Lightspeed is certainly on the short list in that regard) and this will still happen because these effects arise out of simple physical laws **not the quality of the control**.
georgehifi's avatar
georgehifi
10,312 posts
 
And here is the Flecher-Munson 1933 study done, for percieved frequency loudness at different db levels. As you can see at low levels one needs to boost bass quite a bit to remain flat to the ear. Half way down the page is the graph everyone can understand.
http://www.jimprice.com/prosound/db.htm

Cheers George
tradeontheweb
43 posts
 
Georgelofi "So now you'll only find them in car stereos and midfi home equiptment"

And certain speaker manufacturers (who shall obviously remain nameless here), that like to boost those frequencies for the "startle factor" in demos .... seems to work too, as people are still paying the big $$'s for their products ?
georgehifi's avatar
georgehifi
10,312 posts
 
Almarg Ralph is referring to the increase in output impedance of a passive preamp that occurs as the volume control POSITION is reduced from maximum,Almarg

This higher output impedance will only roll off the highs and won't touch the bass if and I state "if" the interconnects are extremmly high in capacitance, above 300pf per foot, and if they are that high they should not be called high-end, they need to go into the bin.
I belive what Ralph is hearing is the Flecher-Munson effect, in the F&M graph I have posted the link to.

Cheers George
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georgehifi's avatar
georgehifi
10,312 posts
 
To give some measured examples of interconnects
1: With IC's at 100pf per foot a 1mt = approx 300pf
this with the Lightpeed at it's highest output impedance has a HF rollof of -3db at 76khz

2: Same with 200pf per foot 1mt = approx 600pf
this with the same gives HF rolloff of -3db at 38khz

3: At 300pf per foot same gives a HF rolloff of -3db at 25khz.

Good interconnects are usually below 200pf per foot. As not to create low pass filters with passives and quite a few tube preamps.

Cheers George
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