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Old 21st July 2007, 11:40 AM   #11
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Does assymetrical slew rate really matter? If the slew limits of either side are not approached then I can't see how it does.
With the example VAS shown the cfp configuration will amplify the feedback signal and will lower the output impedanceof the VAS by a factor equal to the total hfe. If the VAS is fed from a standard LTP then the only pre requisite is that the LTP quiescent current is sufficient to sink the VAS feedback current for the designed voltage swing. I don't think this was messed up at all by Doug Self and his explanation of the LTP and VAS was completely satisfactory. Anyway what's wrong with 0.0006% THD. I've heard some amplifiers that employ a single ended VAS with cDom sound exceptionally good with those sorts of THD figures.
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Old 21st July 2007, 12:00 PM   #12
AndrewT is offline AndrewT  Scotland
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There's nothing wrong with setting a 0.0006% target. What is flawed is his total exclusion of other topologies that may miss that target and concentrate on Cdom exclusively.
In my opinion the current mirror on the LTP also falls into this target setting trap and alledgedly blinkered view.

I have another opinion, these two topologies, Mirror loaded LTP and Cdom, may be why the blameless, trimodal and load invariant amplifiers are often cristicised for less than perfect subjective sound quality. Is it a coincidence that all three variants incorporate ONLY these two topologies in the first two stages?
Now compare the Audiolab/Tag power ampliers often described as "sterile" or "clinical" despite being technically accurate. They too use the same two front end stages.
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Old 21st July 2007, 12:50 PM   #13
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Quote:
Originally posted by AndrewT

I have another opinion, these two topologies, Mirror loaded LTP and Cdom, may be why the blameless, trimodal and load invariant amplifiers are often cristicised for less than perfect subjective sound quality. Is it a coincidence that all three variants incorporate ONLY these two topologies in the first two stages?
Now compare the Audiolab/Tag power ampliers often described as "sterile" or "clinical" despite being technically accurate. They too use the same two front end stages.

OK I confess I have built variations of the blamless amp and also found it to be a little on the sterile side. However, when you analyse the residual thd you will find that it's mostly odd order, particularly if you go to some lengths to balance the LTP current mirror. I suspect that the odd order components are the reason rather than some of the almost fanciful explanations I've seen which relate to potential slewing problems. If you degenerate the LTP emitters and reduce Cdom you will hear a noticeable improvement in that regard, but I think this is purely the reduction of HD3 from the LTP. Cdom will reduce the VAS hf distortion to vanishingly low levels so it's only the VAS and output stage that contribute significantly to THD, and that means it's nearly all odd order.

BTW, according to Bruce Candy (Halcro) his DM58/68 use current mirred LTP's and Cdom VAS compensation and are current source fed. His amplifiers have been well received world wide and are regarded as the best by many. So there are obviously some amplifiers that sound very good with this LTP / VAS design. I suspect that the implementation is the key and that starts with device selection, board layout and power supply isolation and regulation etc.
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Old 21st July 2007, 01:14 PM   #14
AndrewT is offline AndrewT  Scotland
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I thought the general view re Halcro was it's near perfect technical performance but almost the opposite regarding musical enjoment.
But never having been near equipment at this price level, never mind the lack of opportunity to hear them, I am too much of an amateur to have my own view.
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Old 22nd July 2007, 06:50 AM   #15
sandyK is offline sandyK  Australia
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Default Compensation capacitor Cdom

I agree with what David Lewis is saying. This "Blameless and sterile" sounding topology can be made to really "sing" by careful matching of the LTP and Current Mirror devices for both HFE and VBE. Both perform audibly better with suitable degeneration
resistors. Some years ago there was a discussion about Current Mirrors, and the overall consensus was that the SQ was noticeably improved by using matched , high HFE devices in the current mirror, with typically 100R emitter resistors.Best results were found with devices having an HFE around 800 (!!)
In my case,Cdom has been reduced to 68pf polystyrene,as well.
Matching the LTP devices for VBE and HFE, and then IDENTICAL current flow gives even better results. You need to simulate identical loading of the VAS , on the unloaded LTP side.
A close approximation in many cases, could be as simple as a
1% 22Megohm resistor from the tied base and collector of the current mirror, to the -VE rail. I am not using this method,but my
modified Silicon Chip 15W/Ch. Amplifier has a HUGE soundstage and pin point location of instruments and voices. The front end, however, uses a very low noise , very low impedance, PSU addon designed by the late John Linsley Hood.

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Old 22nd July 2007, 07:02 AM   #16
awpagan is online now awpagan  Australia
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Old 22nd July 2007, 07:46 AM   #17
AKSA is offline AKSA  Australia
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Hi Andrew,

Quote:
I thought the general view re Halcro was it's near perfect technical performance but almost the opposite regarding musical enjoment.
Some have said this, yes, but there is no question that these amps are extremely clean. Could it be that your notion of 'musical enjoyment' means a little fudging at the edges of the waveform, dare I say it, a little DISTORTION. (There, I've said it!!)

Quote:
But never having been near equipment at this price level, never mind the lack of opportunity to hear them, I am too much of an amateur to have my own view.
I'm not sure that these are the dearest models, but heck, Andrew, you are more than entitled to a POV, even from a strictly subjective stance, and there is no doubt that you understand the electronics to considerable, objective level. Why wouldn't you have an opinion, and a valid one at that?

BTW, you can even have compensation across the output collectors of the LTP, and the gain of the VAS can be weighted down brutishly with a simple capacitor and/or resistor to ground!

Cheers,

Hugh
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Old 22nd July 2007, 10:23 AM   #18
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Quote:
Originally posted by David Lewis
well received
Yeah right, K-K-K-Ken (*),
but what do YOU know about Amsterdam hookers ?

(* ©dom : a smelly fish called Kessler)
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Old 22nd July 2007, 11:46 AM   #19
Bonsai is offline Bonsai  Taiwan
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What the hell was the hot on Ken Kessler all about? I love this guys reviews - he's one of the few around that speaks his mind (e.g. 'I don 't like cables' = I think 2 grand for 2 metres of cables is straight b.s.).
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Old 22nd July 2007, 12:17 PM   #20
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Each to his own.

The "bright" remarks of Jeremy Clarkson are constantly quoted nowadays, and the program has been appointed the "best" automobile tv-show title.
Some people forgot that 10-15 years ago Clarkson was the turd of the Top Gear series, none of the commercial tv-stations were interested in the big guy.
My abfav car program, untill the fat sloppy guy became the No1 honcho when the BBC was forced to produce a follow-up, never watched it since.

Overhere we had Jan Kool, known for his late '80s statement : Wonder Cable or a pile of new CD's ? An easy choice for me !
Highly skilled bloke, also highly biased.
Reading Kessler resembles eyeballing an article in The Sun, the newspaper at least has nice boobies to watch.
(update, a top of the line STC cable is 20 $-grand for 2m)
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