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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
clio09
3,360 posts
 
4) control the interconnect cable- which is done by having a low output impedance which swamps (makes negligible) the capacitive, inductive, resistive and other aspects of the cable.

What would be a low output impedance (and how would the base measurement be determined, ex. at 1k ohms)? How would one design a preamp to achieve this? I can think of those active preamps and even zero gain designs that use active buffers. Perhaps using negative feedback? Of course if you're designing a true balanced preamp having it support the 600 ohm standard would be the best way to ensure the cable is removed from the equation, assuming the amp on the other end is of the same design.

Also, I don't believe my DAC uses coupling caps, but how would I know how to determine that for sure. What would I be looking for in the circuit?
georgehifi's avatar
georgehifi
10,312 posts
 
01-14-11: Pubul57
George, does that mean that with some sources, Ralphs argument is correct due to the coupling caps? Pubul57

Yes, as I pointed out in my last post, but I will say that the said caps are usually big enough, not create any problem, except colourations, it's the moded one's that have had their caps and output stages modded that have to be sometimes questioned, are they large enough?

And Ralph's answer to your line stage question is answered with diplomacy, looks like he has the same view on coupling caps, (the best cap is no cap), Except still I do not believe anyone has yet built the transparent active preamp, as they all sound so different, more so I believe than the sound of different interconnects.
Cheers George
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banquo363
371 posts
 
Since I haven't been able to understand a word of the recent postings (you all lost me at 'caps'), let me change direction a bit.

IF we assume that a straight line from source (phonostage) to amp is the best/least colored way to get at the information on the source (vinyl), then can we assume that finding an amp that has a volume control built into it would be a good way to go? Since it would allow for direct connection and also allow for volume control. In such a scenario we would bypass the LSA (or any preamp) and any colorations it may add. And also bypass a set of IC's and whatever potential issues it raises.

I take it that the answer depends upon the nature of the volume pot design/quality on the amp?

Differently, remaining with this type of amp (with volume control), if we use it in conjunction with the LSA:
1. could we use the amp's volume control to mitigate any impedance mismatches between the two? I understand that these are two different variables, but they are physically related.
2. and, what adverse effects can we expect to remaining faithful to the source?

I'm wondering about these questions because I see that David Berning and Paul Grzybek have amp designs with volume control as an integral part, and am curious as to what the (dis)advantages are.
atmasphere's avatar
atmasphere
11,928 posts
 
Clio09 a low output impedance will mean that the preamp can drive a load of less than 1000 ohms without loss of bandwidth, voltage or increase in distortion. You don't need negative feedback to do that. In the old days, tube circuits did that with an output transformer. You can imagine, being an OTL manufacturer, that we use a different technique (which is patented).

If your DAC uses coupling caps, they would be fairly close to the output of the unit, but there is no set rule on that. You might ask with the manufacturer.

Except still I do not believe anyone has yet built the transparent active preamp, as they all sound so different, more so I believe than the sound of different interconnects.

George and I feel very much the same way in this matter, which is to say that many line stages do seem to fall well short of the ideal. But at the same time it is apparent that George has not heard *all* the line stages out there.
pubul57 OP
3,712 posts
 
I think Ralph also makes a high quality volume control as an option on some [?] all of his amps. I suspect this is the very best way for an amplifier, but for the fact that audiophiles tend to like to mix and match gear and this has the sort of perceived limitation that you have with an integrated; in that sense. I would love to have an LDR volume control on my M60s, and or a Pass B1 with an LDR, as I think the LDR is this best method for building an attenuator (without mechanical contacts, wear and tear). I suspect if the Built-in volume control was available with the M60s when I bought them, that is the path I would have take, but Ralph's preamp undoubtedly works brilliantly with his amps.
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