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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
 
To all who are not very technical, try to bear with me here.

The (diode thingy) "diode effect" is bought around by very fast music transients from the source, CDP or phono, which can be in the order of 100's of volts per micro seconds (volts per uS) these happen as the name implies in micro seconds.
When we view a static (constant) 1k-10k square waves on an ocilloscope all looks fairly fine going through a good pot (Alps Bournes ect).
But it's when you pulse the waves at micro seconds (transients) an then store it on a very good super fast digital storage ocilloscope play the micro second pulses back frozen in time, and magnify the corners of the waves it's then you see the ringing/occilations effect of the light wiper contact on the conductive track of the pot, (I have named it for want of a better word, "dynamic contact bounce")It's the wiper being ever so slightly bounced on the track.
When pressure is applied to this wiper (with in my case a wooden skewer) so the wiper presses down hard down on the conductive track, the ringing occilations stop when doing the same test. When a soldered resistor is in place of the pot it's not there either, when a Lightspeed Attenuator is in place of the pot it also is not there either.
All pots no matter how good bounce to a certain degree this is why they all sound different, they are all fundamentially flawed.
Cheers George
georgehifi's avatar
georgehifi
10,312 posts
 
Sorry I should clarify a bit better the first paragraph above, the figure 100's of volts per micro second, is in reference to "rise time" in volts per micro second of musical transient figures, not what a cdp or dac can put out of it's output in volts, but rather the speed at which they can happen.
Like when very fast opamp specifcations of some of the better opamps, they can have rise times of 1000's of volts per micro second, but they only have 18v rails, these are very good for the i/v (current to voltage) converters after digital to analogue chip before the ouput stage in cdp's
Cheers George
atmasphere's avatar
atmasphere
11,793 posts
 
Clio09, I had the opportunity to compare the Lightspeed against one of our own preamps and I have to say it was the most neutral passive I have heard.

Like any passive I have heard, it had less bass impact than the preamp, which as I have already mentioned is mathematically unavoidable with a passive if you have a coupling capacitor at the output of the source (in this case a DAC). I contest the idea that *all* switches will have issues relative to a light-activated device (my concern would be the linearity of the light activated device...); obviously in practice both can be quite good.

FWIW the Shallco uses gold contacts, with a double-spring-loaded wiper.

However the preamp in this comparison had not only the Shallco switch (custom-built) but a stage of gain, a set of coupling caps and then a direct-coupled vacuum-tube buffer. It was also driving 24 feet of cable, where the passive was driving 3 feet. The two were gain-matched to avoid Fletcher-Munson errors.

The big difference was in the bass as I mentioned. If you go with the idea that the preamp was hampered by its active circuits, then the idea that its volume control is audibly inferior falls apart.

Its my contention that one of the biggest failings of many tube line stages is the coupling cap found at their outputs, so we found a simple way to get rid of it in out designs. Apparently, that is a bigger deal than I had thought.
georgehifi's avatar
georgehifi
10,312 posts
 
Capacitors on output stages of dacs and cdp's are evil necessity with tube output stages, and yes they will not be a suitable match for passives, not only that, the tube output stages on dacs & cdp's are usually too high in output impedance (more than 200ohm) for passives, being sometimes 2kohm (2000ohm).

As for really good multi (24 or 36) position switched resistor "double leaf contact" volume controls that Ralph uses. These are way ahead of quality high end pots (Alps Bournes Penny ect)
But with the single leaf switched resistor, I did also confirm contact bounce, as you can imagine there is no second leaf on the other side of the wiper to clamp it, like squeezing your fore finger and thumb together with the wiper between them like Ralph's double leaf does.

Cheers George
atmasphere's avatar
atmasphere
11,793 posts
 
Having done some audition in this regard (we 'switched', if you will pardon the pun, to our custom Shallco part about 15 years ago as a result of some of these auditions), the difference in contacts that George is describing above is clearly audible.

Its my opinion that the volume control is what shoots many preamps down (especially preamps with remote control) before they can even get off the runway. One of the consequences is that the majority of line stage technologies are in a deplorable state- I don't fault anyone for thinking that a passive might be better.
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