Lightspeed Attenuator a new passive preamp - Page 27 - diyAudio
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Old 17th January 2007, 03:47 AM   #261
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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.
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Old 17th January 2007, 08:23 AM   #262
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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
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Old 17th January 2007, 08:37 PM   #263
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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.
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Old 18th January 2007, 09:00 PM   #264
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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.
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Old 18th January 2007, 09:20 PM   #265
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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.
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Old 18th January 2007, 10:30 PM   #266
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I tried to explain this contact phenomena before about 200 posts ago in this thread, Iíll 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 cornerís 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 100ís 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, Iíve 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 youíve done a clean up of your RCAís.
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
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Old 18th January 2007, 10:58 PM   #267
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Sounds reasonable to me, George.
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Old 18th January 2007, 11:14 PM   #268
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The contact bounce idea reminds me of Frank de Grove's (RI-DIY-P) unswerving preference for mercury relays. His observations regarding them perhaps pointed to the same phenomenon.
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Old 18th January 2007, 11:55 PM   #269
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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
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Old 19th January 2007, 05:15 AM   #270
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Having tried different relay types for switched attenuators, I can also support Frank's preference for mercury wetted relays.


Patrick
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