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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
agear
1,362 posts
 
Hmmm. An explosion of words. I will add to it with my own verbal diarrhea.

I have a few questions:

1. Pubul57, you started this thread (a good one a might add), and you are in a good position to speak to the attributes of passive and active pre-amps. You appear to own both (LSA and Atma-sphere). Any observations on differences between the two particularly in light of what Bill has described?

2. Tvad, you stated the following:

"Grannyring, your further explanation further reinforces the idea of the LSA preamp being less colored than the TRL preamp."

I am unable to follow your logic here. Bill's description does not do that at all.

"If one accepts the definition of a passive preamp as a device that passes the source’s signal unaltered, except for attenuation of gain, to the amplifier; and assuming a proper impedance and gain match between source, LSA preamp, amplifier, speakers and cabling, then one also accepts that the recording is being reproduced as if the passive preamp were not in the system at all."

That is a definition only. The LSA uses components which can "influence sound." If anyone takes the time to review the DIY threads as I did, you will discover a few things about the LSA in this regard. The LDRs, which appear to be the heart of this unit, functions as a variable resistor, and according to some, have their own "sound" which contrasts with traditional metal resistors, etc. Furthermore, based on one gentleman's measurements, the impedance of the LSA varies with degree of attenuation: "Similarly, the Lightspeed's simulated output impedance varies from about 37 Ohms to about 14.6 kOhms, as the attenuation level is varied from maximum to minimum." derived from http://www.diyaudio.com/forums/analog-line-level/159163-
ldr-photoresistor-attenuator-preamp-general-2.html

So Tvad, you cannot make those simple, blanket statements in regards to active pre-amps. Both influence things from an engineering standpoint and thus potentially influence sound.

3. Tvad, you made the following statement in Tony's review thread discussing the LSA and Truth passive:

"Those who are considering the $18,000 Concert Fidelity CF-080 might also consider the US made SMc Audio VRE-1, which is also a "passive with balls" ($14,950)."

A passive with balls? Why does a passive need balls? It would be very interesting to get your impressions of the LSA vs your VRE-1....
clio09
3,360 posts
 
Great post Dave.

Regarding the allegation of "forwardness," assuming that we are not talking about strident aggressiveness, the quality of forwardness in a top component is often a good thing in the sense that the piece sounds more alive, faster, dynamic and resolving. The listener is literally closer to the music, in fact the stage may extend both forward of and to the rear of the speakers. In this scenario depth-of-field cues are delivered through high resolution. Instruments appear layered in depth more by virtue of low-level cues than by soundstaging per se.

I have a recording that illustrates this very well. The soundstage is very deep, yet the piano is quite forward in it. The vibes float effortlessly in the middle with amazing lateral movement. On some tracks the soundstage does in fact exceed the speakers. The CD is The Wonderful World of Ron Carter and IMO is well engineered. Anyone who enjoys jazz trios with excellent bass playing should look into it. The music will certainly give your system a lower frequency work out if nothing else.
clio09
3,360 posts
 
I don't know of one resistor based passive that doesn't vary the output impedance based on the attenuation level. I think it is just the nature of the beast and as Andrew mentioned the LDR design is not immune to this (Side Note: The Truth Preamp does not vary the output impedance but that is because it uses active buffers on the outputs). It is one of the reasons why it is important to get the right impedance matching and in some cases to pay attention to cabling to attain the best sound from such a design. TVC passives have the same issue (see here, scroll down the page http://www.stevens-billington.co.uk/page102.htm) but are more forgiving. It was one reason I preferred TVCs earlier on before encountering the LSA.

Also, Ralph Karsten mentioned a number of times that the best sound from a passive will occur closest to the maximum point on the volume control (least attenuation). That is because the further around the dial you go the lower the output impedance. I'm never at much lower than 12 o'clock on my LSA and with my Atma-Sphere S-30 amps where I can vary the input sensitivity and gain, I'm never at less than 3 o'clock. So it's safe to say I'm operating at lower output impedance running into high input impedance amps (minimum 100k ohm, maximum 250k ohm).
grannyring's avatar
grannyring
6,517 posts
 
Tvad, you understand me correctly. Sorry I should have been more clear perhaps. Further, so we can get on more common ground around this topic, I feel the software/recording used to judge accuracy should be one that is very close to live sound - a great recording. This gives a good base in which to judge the system. If I have heard a given brand/type piano in an intimate setting on many occasions I have a good sense of how its sound. When I listen to good recording of said piano on a system, I am in a good position to judge that system's accuracy.

If it sounds thin or lacks body, then I know that system is not getting it quite right in that particular area. I now understand this system will behave the same way on any recording regardless of the recording's quality.

A recording may be heavy laden with warmth and body, but this system will play it with less of those attributes and therefore not be true to the recording.

I have good recordings of piano and Van the Man and have seen both live on many occasions. Based on this I prefer one system over another.
agear
1,362 posts
 
George, it does not make sense to state that system synergy manifests as 2D or 3D. Please explain.

"What you should do Grannyring to see if the Dude is artificially giving depth, is what I preach all along, put your CDP straight into your poweramp (Bolero Test) no preamp. Put on a quite cd so you can then ascertain a good level of cd to play, then swap in the Lightspeed then your Dude and see which is closer to no preamp. The one that is, is the one that is truer to the source."

I have done that test repeatedly. Digital done direct (Bolero Test) always sounds flat, hard, and 2D. Not like real life at all. Not sure what that specific test adds.
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