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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
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almarg
9,670 posts
 
Hi Paul,

Yes, there are significant differences.

An active buffer stage will provide a gain very close to 1, while providing a high input impedance (to the stage itself) and a low output impedance.

A transformer or autoformer will transform impedances in proportion to the square of the turns ratio that is selected, while at the same time causing a voltage reduction (or gain) proportional to that turns ratio. Example:

If a TVC is set to provide 12db of attenuation, that corresponds to providing an output voltage that is 1/4 of the input voltage. So the turns ratio for that setting would be 0.25. The output impedance would be ((0.25)squared) or 1/16th of the output impedance of the source component. The load impedance seen by the source component would be 16 times the input impedance of the destination component (presumably the amplifier).

If the TVC were set for unity gain (no attenuation), there would be no impedance transformation, the output impedance would equal that of the source component, and the input impedance would equal that of the destination component. But as the amount of attenuation is increased, the output impedance will decrease rapidly, since it is proportional to the turns ratio squared.

Best regards,
-- Al
pubul57 OP
3,712 posts
 
thank you.
lstack
2 posts
 
Just wanted to weigh in on using a Lightspeed in very unfavorable impedance conditions. I have a DAC with an output Z of 10k and an amp with an input Z of 15K . All logic tells you that inserting the Lightspeed between those two devices would spell certain sonic disaster. I had resigned myself to using an active preamp until I could solve the impedance mismatch either by using a buffer or replacing equipment. My active tube Preamp is an Airtight with an input Z of 100K and an output of 600 Ohms easily fulfilling the "10 to 1" rule.

Having already pulled the Lightspeed out of the system knowing it could never work anywhere near as optimally as my expensive tube preamp , I suddenly decided to compare the Lightspeed to the Airtight. With only one input it was a simple swap and within the first 30 seconds I realized that the Lightspeed even in a terrible sub optimal impedance match was wiping the floor with my active preamp ANYWAY. Just thought I would give more than a sliver of hope to those who think the Lightspeed won't kick your expensive active preamp to the curb even in a bad impedance mismatch. Yes George warns over and over that the Amp "should be over 50K input Z" and the source "should be 200 ohms".

The last thing I ever expected was to hear the Lightspeed be more dynamic , more extended, more spacious and 3D and to basically make a very very fine and expensive preamp sound absolutely woolly by comparison and all this in what was supposed to be a very impedance unfriendly situation. Now all I want is to do is stop slapping Ohms Law in the Face and try to get the impedance matches where they belong so I can hear the Lightspeed in its full singing glory. I can't even imagine how much better it could be , but I will sure try to get the matches right and let the Lightspeed cut loose.

SO , for a quick recap, Just because your impedences don't fall in the 10 to 1 rule or better, don't think the Lightspeed won't easily dethrone your expensive active preamp anyway and make what you thought was the best Preamp you ever heard sound broken by comparison.
clio09
3,355 posts
 
Funny, I've been considering trying an Airtight of late. Guess I'll cross that off my list.

I have used the LSA as well with unfavorable impedance matching. Into am amp with 21k input impedance. It did sound good, although you will definitely hear the difference when using it with optimal impedance.

The "10 to 1" rule is overrated IMO.
pubul57 OP
3,712 posts
 
Another one bites the dust:)
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