Lightspeed Attenuator a new passive preamp

I bucked up and bought a Lightspeed Attenuator from George. I've played music through it for a handful of hours, so I have but a preliminary perspective on its sonics.

I think George has a winner on his hands. First impressions are: the unit is quiet as the night is dark, it excels in coherence and resolution, and high frequencies are to die for---holy cow, no grate or edge, and what delicacy. The HFs alone are worth the price of admission.

Comparison preamps are an Emm Labs active pre stuffed to the gills with teflon bypass capacitors, a DIY Paradiso TVC and an Audio Consulting Silver Rock TVC. The latter is my reference; it beat out a Placette I once owned, and comes close to the LA, but mucks up the HFs in comparison.

Thanks for a great product, George. More listening will tell, but the LA seems an appreciable step forward.
 
L.A. ? think I like it. Thanks for the great preliminary appraisal Tom, hope you can add some more after listening for a day or so, maybe it will give the hundreds of wanabee diy'ers the kick up the you know what to build it for themselves, I know because of the countless PM emails I've recieved.
I think maybe the hassle of matching the 4 x LDR's has turn them off doing it, but really it's not so bad $40aus will get more than enough to match up 4 and build your own Lightspeed Attenuator, and matching them takes no more than a hour or so.
The end result is a passive preamp to which the sound of makes you think seriously about what we've been listening to all these years in the way of preamps (active or passive)

L.A. umm kinda Californian, jurys still out

Cheers George
 
Wow, what a preamp! Shockingly good! I can find no fault with its sonics. That, I suppose, should be expected because it so elegantly solves so many problems associated with preamp design.

I've had several, mostly involuntary thoughts listening to it, like, "geezus, that actually sounded like a cymbal." I won't belabour the others because they all practically come to the same point.

Best pre I've heard by a considerable margin. George deserves a fricken award.
 
Another day of listening and I have yet to hear anything I dislike about this unit. Everything is three notches more realistic through it.

Why does the Lightspeed perform so well? I'm familiar with the difference changing a cheapo resistor for a Vishay makes. That difference is considerably smaller than changing my previous reference pre, the AC TVC, for the Lightspeed, and the AC beat out a Placette. Personally, I'd place my money on George's contact theory.
 
By the way, George's contact theory is consistent with Herve Deletraz's perspective on contacts and switches, whose audio approach I rather appreciate. I personally can't see how a switch wouldn't become a diode to some degree if just due to oxidation of the contacts. The contact bounce theory also makes sense to me.
 
I tried to explain this contact phenomena before about 200 posts ago in this thread, I’ll try this time to explain what I found a bit more clearly.
First it had nothing to do with corrosion of the contacts in potentiometers and switches, it was the fact that all had a very small pin point size contact area between the surfaces. In potentiometers it is even worse because the pressure is so light as not to wear out the carbon or plastic resistance track, Switch resistors are almost as bad especially if the rotary switch is of the single leaf variety, dual leaf is better one each side of the common, but still I saw a problem even with these.
The way the problem was found, I managed to borrow for a day a very trick digital 10 gigahertz for memory storage oscilloscope from the CSIRO a massive research company here in Australia, god knows what it was worth 50k or so it made my high end Tektronix look like a toy. With this scope I was able to store and magnify the corner’s of a cd level 1 to 2 volt 1k square wave, and what I saw was an extremely high oscillation embedded in the original this oscillation disappeared once I put extra pressure on the wipers of the pots and switches with a non conductive toothpick, all I can say is that the rise time is 100’s of volts per micro second and it causes the contacts to become poorly conductive at these levels and creates a diode effect and tries to rectify the AC music signal into DC, I’ve for want of a better word called it contact bounce, you guys call it what you will but the proof was there and disappeared once more pressure was applied, in the case of RCA connectors the area is 10 to 20 times larger and the pressure much higher, so the phenomena does not seam to happen, but I think maybe it does, ever wonder why your system sounds cleaner after you’ve done a clean up of your RCA’s.
And please all you non believers, don't say anything till you've seen it for yourself, on a scope like I had the good fortune of borrowing. Not on a toy 400mhz Tektronix like mine, you won't see it.

Cheers George
 
Cannot go there

The single source linestage does not appeal to me. My more complex and most likely less effective solution is using a five position, 4 pole switch for a stereo 3 position. Ground positions 2 and 4 so mute is one click away. And doubling up the wafers to parallel the contacts. If the theory about polarity switching is true, this should cancel out.
I can see where getting the contacts out of the signal path can be a big improvement. And glad you purchased one from George. He is a true gent for sharing the details. I am a believer too after building one.

George
 
Sorry to have to post this up, but I find it necessary with the amount of emails that I'm getting bombarded with.

I will not sell the separate parts, if I do it for one I have to do it for all, and that's just impossible for me to do

I've given all the info on how where and why to DIY the Lightspeed Attenuator yourself in these threads, for a little of your own time you can Do It Yourself, I only sell the completed product.

I hope all understand and don't think that I'm a big nasty Ausie Oger who won't help them.
Thanks for your understanding, hopefully.

Cheers George
 
As a relatively long term user of the Lightspeed (L.A. ?) I can offer a couple of insights as to what I have found with this little marvel of Georges.

I purchased a completed model from George, single input/output and have used it with over 10 different pairs of speakers, about the same number amps, both Valve and SS.

A wise old (sorry Pete) audiophile once said to me that it helps to have one truly blameless component in your system from which to build from and to evaluate other components. For me, that's been the Lightspeed. Although it has come and gone out of my system over the last 12 months, it has always been my preamp of choice when I change anything else in the system. It has allowed me to evaluate and judge speakers, amps and sources very accurately, as it just gets out of the way, adding no colouration or sonic signature of its own. To me, that's its greatest strength - the ability to do it's job and allow whatever is playing to come through cleanly and clearly. I might not always like what I hear with the Lightspeed inline, but at least I know I'm hearing the truth !

This can be a double edged sword though, as components and tracks that were previous favourites are now ruthlessly revealed in all their glory (or not as the case may be !) However, I firmly believe that my system is better for having had the Lightspeed involved in it's growth.

I'm not one for getting into the technical aspects of circuit design etc and only have a rudimentary knowledge at best of how the Lightspeed works - but I can vouch for the fact that it has beat almost everything else I've tried. Occassionally I have wanted to add a certain sound to my system and that's the only time I have swapped it out for a Valve pre or something else. Otherwise, it's usually in there.

For such an innexpensive component, comparatively, the Lightspeeds' a true audiphile addition to any system - something proven by the number of big dollar preamps I know are now languishing unused in systems around Australia.

I hope that this helps those that are still pondering the wisdom of inserting something this "low key" into their mega-dollar rigs, but at the price and performance of this unit, you'd be mad not to give it a try.

Current System.

Teac Esoteric P500/D500 Transport and Dac Modified.
Lighstpeed
Peter Gay 50wpc Class A SS Power Amp
Altec Barcelonas with Piezzo Twin Diaphram Supper Tweeters

Alternate Amps

Sonic Impact T-Amp
Beam Echo/Avantic SPA21
UCD180 Digital Monblocks
Melody 2A3 Integrated
ME850
ME55
Melody KT88 Monoblocks
 
All the pot does is vary the voltage to the LED's of all 4 LDR's, it does not work in stereo mode.
One side of the pot takes the voltage up to both channels of the series LDR's and the other side of the pot takes the voltage down to both channels of the shunt LDR's.
If you can get your head around this, you will then realize that the quality of the pot has no bearing on the sound quality of the LDR's.
It's one of those weird things that you have to do a double take on to understand whats happening. The pot has no bearing on the channel balance of the output and is not connected in any way to the input signal or output signal.

Cheers George
 
Hi chaps, been lurking for some time now whilst building my new amp and wondered if I could integrate one of these puppies?
From my schematic below you can see that I love to keep it simple (KIS as George puts it)!:D
I have put everything in one box to reduce the parts count and keep the amount of joints/connections to a minimum. That’s why I love the thought of using this idea.
Looking at the schematic of the ‘Lightspeed’ it appears to work in shunt mode, which will affect the gate resistor on my FET whenever the dial is turned. So, my question is… can it be used as a regular volume pot with a fixed grounding resistor and only the LDR’s on the signal line?
I don’t want another Cap in there before the attenuator as I’m just multiplying the number of parts and caps certainly have a sound. I can’t put the cap before the attenuator because the T-amp has 2.5V bias on the input.
I’m only a beginner here so if I’m talking complete hogwash then please correct me!:D
 

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Do not bother

Using just a fixed shunt and variable series element will not give much range in attenuation. You need to vary both to keep impedance fairly constant.
A better place for the attenuator is in front of the jfet buffer. And use it close to the design GeorgeHIFI has kindly posted. This works, and works real well.
Took mine to a Bottlehead meet last weekend. Never got to do any preamp swaps to compare, but hooked to my old My_Ref everyone was complimentary of how it sounded. I am biased, but thought the pair sounded better than any of the other amps listened to that day. There were some kits, a couple commercial, and a tweaked out vintage tube unit to compare.
How does the chip amp handle the 4.7 ufd input cap? I found them to pop on power up when the input cap was 2.2 ufd and larger. Bigger cap gave bigger pops.


George
 
Not knowing this chip amp, but looking at your circuit I would just sub the 50k pot (which is series shunt anyway) for the Lightspeed as I've designed it, the only thing is how much dc offset is present on the output of your buffer? if over say 10mv then i would have another 4.7uf before the lightspeed, with it's input of around 7k this then will give a -3db point of 4.8hz which is fine, use a good polypropylyne or Blackgate NX though. same for your existing 4.7uf after the Lightspeed.

Cheers George
 
georgehifi said:
Not knowing this chip amp, but looking at your circuit I would just sub the 50k pot (which is series shunt anyway) for the Lightspeed as I've designed it, the only thing is how much dc offset is present on the output of your buffer? if over say 10mv then i would have another 4.7uf before the lightspeed, with it's input of around 7k this then will give a -3db point of 4.8hz which is fine, use a good polypropylyne or Blackgate NX though. same for your existing 4.7uf after the Lightspeed.

Cheers George

Thanks for the reply George, the LDR's don't like DC then?

No issues on the caps, I have some very nice polys;)