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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
11,793 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.
georgehifi's avatar
georgehifi
10,312 posts
 
We are on the same wave length, like I've said on the Lightspeed Attenuator web site. The diode effect with contacts (potentiometers or single leaf switches) is a fact, and I would go further to say, that they are probably the worst component in the signal path.
Even rca's have this problem also, even with their much larger contact areas and tighter fitment, ever wonder why your system sounds better after a good clean up of all your interconnects rca plugs and sockets? Same deal, "diode effect" because of bad contact area contamination.

Cheers George
georgehifi's avatar
georgehifi
10,312 posts
 
Here's an interesting snippet from the latest Stereophile review of the DCS Debussy Dac which has volume control in the digital domain. Yet two stage adjustable analogue level output stage 2v or 6v, this means you can use it straight into a poweramp and only use the top 25% of the digital volume control, this way you should not be "bit striping".

Quote: The differences in sound between the Debussy into a preamplifier and the Debussy directly driving a power amplifier will depend on the preamp's transparency, dynamic capabilities, and overall quality. Sending signals directly to my Musical Fidelity Titan amp from the Debussy or through my reference preamp, a darTZeel NHB-18NS, I heard minimal differences. Given the darTZeel's price of $29,500, that's what you should expect. :Quote

Guess what the Dartzeel uses as a volume control? you guessed it. A Lightspeed setup, they tried to patent when they first released it but I quickly put a stop to it showing proof to the patent offices in Eu and USA that I first did the LDR volume control back in early 1974.

Cheers George
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