2. amps-preamps
  3. 1276356977

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
2,601 posts
 
George, given the non-linearity of LDRs at high impedances, I don't think that it is practical to adapt your Mk II approach to a high-impedance volume control for an active preamp, whether balanced or single-ended. The Mk I approach with fixed series resistor and variable shunt LDR, may be the best that can be done in this regard. If the system is sensitive enough that in normal use the volume control is operated in the region between say -25db and full attenuation, then the Mk I approach will present a stable & benign input impedance similar to a traditional switched ladder.

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. If one is committed to an active preamp, it's nice that a few hundred dollars and some relatively simple DIY gets the job done with LDRs.

Clio09, my delay was due to the nagging fear that the failure of an LDR shunt LED will take the preamp to full volume, with disasterous conseqences downstream. A simple insurance bet is to parallel the LDR shunt with a fixed resistor, whose value is chosen to put a brake on max volume.

To control the LDR LED segments, I use a 500K dual log pot as a master volume control for coarse adjustment, and a 50K log pot on each channel to trim balance. The LEDs are powered 5V by a Twisted Pear Placid current-shunt regulator kit. This set to pass 10ma to each LED.
c1ferrari
464 posts
 

Dave,

MKII LSA in a true differential balance circuit topology...
Balanced LDR preamp -- great suggestion, Clio09 :-)

Vbr,
Sam
clio09
3,360 posts
 
Well it might be a great suggestion but in the MkII version it appears to be somewhat impractical according to George. IIRC the single ended version of the LSA uses 4 matched LDR modules. In balanced mode it would be 8. It's difficult enough to find a lot of 4 tightly matched, to find 8 would be much more difficult. In addition George feels the reliability suffers. I have the schematic George provided for the balanced version. Just don't think it would be worth building. Perhaps Dave's approach using an existing balanced active circuit and substituting the LDR attenuator into it is the closest we can get.

I'm curious though, Burson makes a balanced active buffer, but sans a volume control (unlike their single ended version). Wondering what possibilities exist to take Dave's approach and apply it to the Burson. Could be interesting if it can be done.
dgarretson
2,601 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?
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