2. amps-preamps
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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
dgarretson
2,598 posts
 
Indeed adding a buffer stage to an active preamp at input to the volume control allows the LDR to operate within its more linear range below 10K impedance. However in a balanced mode there is no escaping imperfect common-mode noise rejection if using LDRs in the Mk II switched ladder arrangement. However carefully the LDRs are matched, they always deviate from each other by at least 5%-- not close enough for optimal balanced operation. On the other hand, the simpler Mk I configuration ensures perfect CMRR-- provided that the shunt LDR is located between phase and anti-phase and not between each signal phase and ground.

Any active tube preamp has at least several fixed resistors in signal path in addition to the volume control. Provided that the preamp can tolerate a shunt volume control, I can't imagine that one LDR more or less in place of one of many fixed resistors in signal path will matter much. Moreover, adding an input buffer just adds more components in the signal path and may thus introduce as many problems as it solves.

I think this is an interesting development in the discussion. Combining the LDR technology with active balanced circuits offers relief from the usual SE vs. balanced, passive vs. active debate. Some proponents of Lightspeed tend to dismiss balanced circuits altogether. Personally I am a balanced guy from phono coil all through system to amplifier output, and am not ready to give this up. Moveover, adapting the Lightspeed to a high impedance phono stage source, presents further challenges a passive. IMO it's preferable to be able to combine these all these design constructs with minimal compromise, rather than to insist upon a purist approach that mandates exclusion of a particular construct.

pubul57 OP
3,712 posts
 
"A respected designer of $15K active preamps recently opined to me that approaching SOTA, the quality of the volume control accounts for 85% of the performance of an active preamp."

Very telling.

Does "gain" have anything to do with the claim some make that passive are not as dynamic? or, if true to the experience in some systems, is that attributable to something other than gain?
clio09
3,355 posts
 
I can see the point regarding complications of adding an input buffer to the circuit of an active preamp. It certainly moves away from my simpler is better philosophy. However, I was more curious about adding this type of volume control, or more accurately, attenuator, to the Burson AB160 XLR buffer itself.

I run an Atma-Sphere S-30 in one of my systems and currently am using a Resolution Audio Cantata CD/USB source and running it direct into the S-30 via it's own buffered balanced outputs and using its analog volume control. I also run my Otari MX 5050 with volume control direct into the S-30, just swapping cables when I want to swap sources. I can certainly hear the benefit of a balanced set-up in this case.

I do feel though that I would like to extend this system to add something in the middle that might provide the opportunity to take advantage of a balanced phono stage. One option I'm considering is the Atma-Sphere MP-3, which can be configured as a buffer. However, I'm open to other suggestions as well, including a modification to the MP-3 taking advantage of an LDR volume control.
c1ferrari
464 posts
 
Perhaps Dave's approach using an existing balanced active circuit and substituting the LDR attenuator into it is the closest we can get.
Combining the LDR technology with active balanced circuits offers relief from the usual SE vs. balanced, passive vs. active debate. Some proponents of Lightspeed tend to dismiss balanced circuits altogether. Personally I am a balanced guy from phono coil all through system to amplifier output, and am not ready to give this up.

I like what I'm hearing :-) / reading!

-Sam
pubul57 OP
3,712 posts
 
Does lack of "gain" have anything to do with the claim some make that passive are not as dynamic as actives (lifeless, dull)? That us not my experience, but I assume it is true for some in their systems; so, is that attributable to gain or something that would account for this fairly frequent criticism of passives?
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