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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
 
Clio09: What exactly is an array of 48 optically controlled resistors and why can't my LSA have this feature? ;)Clio09

Not too sure Tony, could be 48 1% trimming resistors to go with the quad unmatched ldr's, which in my opinion would detract from just a quad matched set, as the i/o impedances for both channels would not remain constant, and there would be another component in the signal path.
Or it could be 48 x ldr's in some paralleled setup for series and shunt ldr's, again this would detract from a simple matched quad set. Or it could be a 48 position switched volume control instead of continuous rotary pot with a quad matched set, this would then sound the same as my production Lightspeed Attenuator.

Cheers George
dgarretson
2,603 posts
 
Thinking about those DartTZeel and Constellation preamps with LDR volume controls, I just replaced a four-deck Shallco series resistive attenuator in a differentially balanced tube preamp, with a 100K impedance LDR volume control. This is a relatively simple series/shunt arrangement, with fixed series resistors and a single LDR per channel as a variable shunt between phase and anti-phase of the balanced signal. One nice thing is that if limited to use as a shunt, the LDR only needs to operate through a range of 35R-10K ohms or thereabouts(similar to the range used in a passive) to provide a wide range of attenuation down to -70db in a 100K attenuator. It's still breaking in, but so far it's clear that an LDR volume control is superb. Thanks to George for popularizing their use!
clio09
3,360 posts
 
Dgarretson - So you finally were able to get it done. I had recalled earlier you were attempting this. I take it the Shallco is the one used in the MP-1 and that is now replaced by the LDR? If you can provide some more details on how you did this I would appreciate it. I've always been interested in building a balanced LDR preamp, but George has always recommended to proceed with caution on that front and I understand his reasons why.
georgehifi's avatar
georgehifi
10,312 posts
 
Dave is using it the way the first Mk1 Lightspeed Attenuator was designed. This is a series resistor with an ldr for the variable shunt to ground. This is still better than any potentiometer, but when I compared it to the MkII Lightspeed Attenuator which is series LDR and shunt LDR exponentially harder and costlier to make, it was clearly another step up in sound quality and usability, so much so that I recalled all the Mk1's and they were all converted over to MkII status, and all the owners were amazed at the difference. It was more dynamic, punchier in the bottom end and a more transparent sound stage, also the min volume went down closer to zero, and it had more stable i/o impedances at different settings. Dave you should try to make a MKII you will be very pleased with the difference again.

Cheers George
dgarretson
2,603 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.
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