2. amps-preamps
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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
 
I have just received some information regarding the output impedance of the Concert Fidelity CF-080 that should put the discussion to rest. According to the designer, the 33k ohm output impedance is an incorrect assumption. In addition to the tubes, the circuit design uses a capacitor and a resistor at the output to prevent harmful DC signal to be transmitted to the power amp.

So the actual measured output impedance, which depends on the frequency of the CF-080, is between 5.2k ohms and 900 ohms as measured between various points within a frequency range of 20Hz to 20kHz.

From this I would say that the CF-080 will match up just fine with most power amps out there. This includes the Concert Fidelity amps with 47k ohms input impedance. Not that this was ever in doubt from my perspective having heard the system multiple times and regarding it as one of the best I have heard.


I fully trust this information as it came from the designer himself. As such I don't feel there should be further debate on the subject.
pubul57 OP
3,712 posts
 
And I assume Terry is using the XLR connection which is 30kohms on his amps input so it so match ok, assuming not too much time is spent at 5,2kohms. But this is theory and Terry is a pretty experienced listener, I have to assume the CF works very well in his system. Well, I guess that is enough on the CF sojourn....
clio09
3,360 posts
 
CF-080 is single ended only. The designer, in addition to well informed opinions on impedance matching, implements single ended circuits to take advantage of short signal paths and to keep things simpler.
georgehifi's avatar
georgehifi
10,312 posts
 
I take what you've found out onboard Tony. I can safely say a cap on the output will not change the output impedance of the tube in the audio band for the better (lower). And a resistor would also not change it for a lower impedance, only for higher impedance.
So there is something else we don't know, if what you were told is "5.2k ohms and 900 ohms" As I find it very disconcerting that the output impedance has not been included in the specs or design on the website.
As just one of a few of the tube gurus I queried about cascode output said. And they all say basically the same thing.
Quote" If you want 6 or 12dB gain, you don't use a cascode.
If you want lowish output impedance, you don't use a cascode. (NB. in a preamp you will want low output impedance, unless you are incompetent).
If you are serious about valves, you don't hang them out horizontally at the back of the chassis, even with fancy roll-bars. This is asking for poor contact as the valve slowly drops out of the socket.Quote"

Cheers George
teajay
1,702 posts
 
Well Tony,

You just got a taste of what I keep referring to as George's arrogant know it all attitude, this time towards a gentlemen we both respect, Mr. Masa Tsuda. The quote's from George's "tube gurus" particlarly the crack about, " you don't hang them horizontally at the back of the chassis" shows how ignorant he is about why Mr. Tsuda has them in that position to begin with.

And this is the guy you would want to hang out with and enjoy his company? Well, let's see what your going to do now. Come up with another excuse or rationalization for George's put down of someone you know is a brilliant designer, builder and your friend or finally stand up to this know it all who's full of himself.
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