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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
ts0711
91 posts
 
I for one would love to see this thread die already. I've followed it on and off from the sidelines for the last 15 months and for the most part it's little more than a three way conversation among like minded audiophiles... At one point I even suggested to George that he start his own forum, but as he said it would distract him from surfing. cool.

Here's my take, if anyone cares. Even if the LSA is the best, I'll never buy it. It's just to restricting in the real word. I need the flexibility offered by full function preamps, both tube and passives.

I've lived for years with a John Chapman, Bent Audio NOH. It's a passive similar to the Music First and based on the same S&B transformers. (Mine are copper.). It has 4 sets of RCA inputs and 2 xlr inputs, it outputs 2 sets of either single ended or balanced feeds, sports 6db of gain, has flooring grounds, etc.... I run 20 foot interconnects to mono blocks and a sub. Short interconnects and one input - no way.

I've also lived with a great dual mono tube preamp that offers similar flexibility.

I guess since this tread won't go away I will.
pubul57 OP
3,712 posts
 
That is precisely what I do when I am not interested in a thread, ignore it. BENT was good, LSA was better in my system. But it is true, the BENT offers much more flexibility, but that was not a priority for me with a single source and short IC to amp, if it were the BENT would be at the top of my list for a passive, especially the version with Slagle auto former, even better than the S&B, at least to my ears. At this point, while I love the LSA, what is interesting is the new approaches taken, especially as the DIY tinkering community pursues new forms of passive forms of attenuation. Though for ergonomics, ease of use, and input/output flexibility, the BENT was heck of an excellent passive preamp.
georgehifi's avatar
georgehifi
10,312 posts
 
Ok some normal hifi speak for a change, I never been a big fan of balanced as 90% of the time to make them balanced a lot of the equipment manufacturers just bundle in more opamps into the single path to make it balanced input or output, very few are true balanced and discrete all the way through, you need to see the circuit diagram to find this out. Even the advantage of it does nothing for me, it's just so I'd be able to run 20mt interconnects with good noise figures. In the end most hi end poweramps speaker outputs are + and ground, single ended again.

I have tried doing balanced Lightspeeds quite a few times and even when double quad ldr's are matched perfectly they go out of calibration after just a few hours sometimes days because they are exponentially more critical to temp drift. And it is exponentially harder to match double mathed quads for balanced as it is single matched quads for single ended.
It can be done with sensors that would measure a test tone before each listening session and then do an auto calibrate circuit, but that then takes them away from being a matched set with the same i/o impedances for each channel, they would get wildly different readings and therefore each channel will sound slightly different to the other at different levels. Or you could do a feedback type arrangement then your asking for more crap in the signal path and making them active and creating distortions /colourations.
After one and a half year of developing Wapo seams to have hit a brick wall with his on diy, http://www.diyaudio.com/forums/analog-line-level/170381-precision-led-ldr-based-attenuator-15.html#post2687081
what I presented to them is the way it is, with development going back to 1974 in all sorts of formats, but they love to tinker, and good on them, this is how man reached the moon, but it's sad to see them as he said "This project has hit what appears to be an insurmountable brick wall."

Cheers George
pubul57 OP
3,712 posts
 
Love the DIY community, unfortuantely, I can only be an observer:)

What I found interesting at the start of the Warpspeed thread was the identification of a few "problems" with the LSA and attempt to improve upon the LSA:

-the inability to adjust to complete silence
-on low/high volume level settings, power levels on the LEDs endanger/shorten/toast the life out of the optocouplers
-the need to improve the power delivery to the optocouplers
-the need to improve on quality of the volume adjustment pot
-the Lightspeed, simple as it is, still has a number of variables/design factors that affect performance

How many of these are valid but not too relevant or important, valid and relevant but insurmontable due to a brick wall and sonic price to pay for addressing the issue, invalid (are the LEDs really in danger)?
almarg's avatar
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almarg
9,670 posts
 
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.
While I have no reason to doubt the audibility of diode effects that may occur with many volume controls, I would respectfully point out that this explanation strikes me as fundamentally flawed.

For line-level amplitudes, swings occurring at rates of 100's of volts per microsecond would represent spectral components (frequencies) at several tens of MHz. No source material and no source component will provide signal frequencies remotely approaching those rates. Not to mention that the amplifier, the speakers, and our ears would not respond to them even if they were somehow present.

Regards,
-- Al
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