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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
 
The Rothwell attenuators work just like the Lightspeed Attenuator. That is, they have a soldered fixed series resistor with another shunt fixed resistor to ground, this forms a voltage divider just like the Lightspeed Attenuator.
Except the in the Lightspeed Attenuator these series and shunt resistors have variable resistance with the amount of light that shines on them. So what you have is a variable voltage divider in the Lightspeed Attenuator.
The only problem with the Rothwell's is that they change the input impedance of the device they are attached to, so this needs to be taken into consideration when using them, with passives and tube preamps.

Cheers George
devilboy
1,176 posts
 
Thanks, George. The input impedance of my amp is 470K. I can't imagine that being an issue with the Rothwell's.
georgehifi's avatar
georgehifi
10,312 posts
 
10-11-11: Devilboy
Thanks, George. The input impedance of my amp is 470K. I can't imagine that being an issue with the Rothwell's.Devilboy

No that is not a problem, what is the problem if you go passive or tube preamp, is that while the 470k input of your amp is fine for these preamps, when you plug in the Rothwells then this changes that 470k for something much smaller. What I don't know as I have never measured the shunt to ground resistor of the Rothwell, It maybe say "10k?" then this is seen by the passive or tube preamp not the 470k, and then it will not be a good impedance match for them.
As Clio09 found out when he used them, it wasn't the Rothwells them selves that he didn't like it would have been the input impedance change of the poweramp he heard that then was not a good match with whatever pre he was using at the time.
There is no free lunch, you have to do your homework, regarding impedance matching. Like I said before it goes on right through the whole system from source through to speakers, up to about 6 to 10 times, you can't do anything within the components themselves unless you are a good tech, hopefully the designers of individual components have done the right matching inside their units, but you can look at the output and inputs of each component to make sure you have a good match and give everything a fighting chance to sound it's best.

Cheers George
almarg's avatar
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almarg
9,670 posts
 
Devilboy, I have a pair of the 10db Rothwell's, and as measured with my not particularly accurate analog multimeter the series resistor has a value of about 21K, and the shunt resistor has a value of about 9.5K. As you realize and as George has confirmed, the 470K input impedance of your amplifier certainly presents no issues. (As you no doubt realize, the attenuators should be placed directly at the input connectors of the amp, so that there will be no interaction between their output impedance and cable capacitance).

With the attenuators so located, and driven by the LSA via a short cable, the LSA would see a load impedance of about 30K. I suspect that is not an issue either, based on the assumption that the impedance "looking back" into the LSA is essentially resistive and does not vary significantly as a function of frequency, given that you will be driving it with a solid state DAC having low output impedance. Hopefully George will confirm that assumption.

You will not be able to drive your amp to full power, though, even with the LSA turned up all the way, as 10db attenuation will reduce the DAC's 2V maximum output to about 0.63V.

For most volume control settings of the LSA, btw, the attenuation resulting from insertion of the "10db" Rothwell's will be more like 11 or 12db, because the impedance "looking back" into the LSA will sum together with the Rothwell's series resistor.

Concerning the sonic effects the Rothwell's may have under properly impedance matched conditions, I'm not using them with the VAC Renaissance 70/70 MkIII amplifier I currently have, but I noticed no adverse effects when I previously used them with a lesser quality EL34-based Paxthon amplifier.

Regards,
-- Al
devilboy
1,176 posts
 
Jeez, if 470K isn't enough, what the hell is?
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