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Old 14th June 2013, 12:33 PM   #9441
fas42 is offline fas42  Australia
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But that means that each output device is dissipating just under 7W at idle, and for a three pair amp, that's 42 Watts. This clearly implies much more efficient and robust heat sinks than are found in dinky little commercial amps. This in turn means more internal real estate, more weight and (AAAAARGH!!!) more money. You can't get away with heat sinks rated at say 1.8 or 1.5, you need to get down to 0.8 or 0.7 - I use two 0.6 heat sinks and believe me, they do get nicely hot.

Payback is that the amp works to about 3 Watts/8 Ohms in pure class A before it crosses over to class B. Given that statistically I spend 98% of my listening time with a constant dissipation below 1W, it turns out that this way I spend like 99% of my time in pure class A. Class B serves me only to do occasional hard transients, or when the feeling takes me to be a headbanger every now and then. But that's 15 lbs of heat sinking alone.
My old Perreaux, which I still have, is a pretty good cooker - the heatsinks ran at about 60degC. In raw form, it did a very nice job while it stayed in class A, but the limitations of the power supply crippled it quickly once a bit of class AB was called for - I spent a lot of time redoing, and redoing, the power supply so that it had a decent reserve of energy - a technician would have a heart attack if he saw the insides ...
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Old 14th June 2013, 12:58 PM   #9442
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@Nigel

Nige, you know this, I told you before - I regard global NFB as icing on an aleady great chocolate cake.

It's a bit like ironing out a drip-dry shirt; you don't have to do it, but it does work that little bit better if you do do it.
I just did that !

My girlfriend buys washing powder ( Daz , Dash in France ) . She is as poor as a church mouse and fiercely independent . When asking why not buy cheaper products in liquid form she said " because powder grinds out the dirt " . I laughed , she laughed back and said it showed how little I knew . This morning reading this thread I thought of this and said how do I know she is wrong ?

She is a brand snob . If I bought her the worst possible Champagne she will jump with joy . If I buy the best Cava she is not so happy . If I bought her Riesling she might poor it down the toilet . She has many bits of paper so is no fool . I challenged her to come on a blind sparkling wine tasting . She looked very worried . What if she likes the Riesling ! As for these !!!!!

This vineyard is near me . I won't be buying Colleen any I guess .

http://www.youtube.com/watch?v=mfMfHFoOhH4

Last edited by nigel pearson; 14th June 2013 at 01:08 PM.
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Old 14th June 2013, 01:10 PM   #9443
fas42 is offline fas42  Australia
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'Performance' is an objective word, in the context of electronics. I assume you have careful measurements to confirm this?

Could you offer an explanation of how 'lousy connections' can audibly affect domestic audio yet a similar effect seems not to be seen in radio or instrumentation, where low level IM would be much more damaging, unless seriously bad connections are used?
Only by ear. Let's say, you have a recording of solo piano: I have heard many times the typical standard of reproduction of that instrument. We have a real piano in the room, I have numerous times experienced close proximity to other, nominally quite superior pianos, I have a strong idea in my head what the subjective impression should be, especially when played vigorously. If a system can't convey that impact and tonality then I would judge the 'performance' to be inadequate ...

With regard to connections, the problem seems to be the sensitivity of the ear, its ability to adapt extremely rapidly to changing levels, to pick out meaning amongst conflicting input. Instrumentation quite often effectively uses time averaging, and only needs to deal with accuracies of the order of 60dB within a range - once you start asking for 80 or even 100dB accuracies things start to become much, much harder - the slightest perturbation can dramatically affect the reading.
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Old 14th June 2013, 01:18 PM   #9444
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Strange. Unless your amp is unstable open loop (very unusual), NFB is like to degrade stability. The true purpose of NFB is well-described in all the standard textbooks: lower distortion, lower output impedance, wider frequency response, behaviour set by passive components rather than active components etc.
Unusual for amps to be unstable without global NFB? I think the opposite is more likely to be true, unless you have an open loop full power bandwidth below say 1 kHz. In my approach, that is WAY greater.

Also, you AIM for 20 dB of global NFB, but exactly how much you end up using has to be confirmed by live samples, no math can assure a safe amount except by fluke, in which case you never needed the math anyway. But in general, I do get it right to 20 dB +/- 2 dB.

As for "it says so in the books", well, it was said in books that the Earth was flat for much longer that it has been recognized that it ain't necessarily so, to quote Bronsky Beat.

You know, every time I learnt the answer to a question which bothered me, that opened up just more questions and made me realize how much more I do not know yet, and probably never will. That never stopped me from searching anew, though. Heck, that's what I do it for, to learn something new.

While you mention what is indeed standard book fare, it does not mean that it's all unconditionally true. I see it as the quick'n'dirty way of doing things, even if some amps built around that doctrine did sound wonderful. Exactly the same could be said of my approach: it makes better than average sound more probable than otherwise, but does not guarantee it.
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Old 14th June 2013, 01:19 PM   #9445
DF96 is offline DF96  England
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Originally Posted by fas42
With regard to connections, the problem seems to be the sensitivity of the ear, its ability to adapt extremely rapidly to changing levels, to pick out meaning amongst conflicting input. Instrumentation quite often effectively uses time averaging, and only needs to deal with accuracies of the order of 60dB within a range - once you start asking for 80 or even 100dB accuracies things start to become much, much harder - the slightest perturbation can dramatically affect the reading.
Communications receivers and active antennas routinely cope with dynamic ranges of 90 or 100dB, with no time averaging.

The comparison seems to be between your sighted subjective impressions and measurable objective criteria. Which should we place more faith in? If your ears really are as good as you believe, then you ought to make your money by hiring them out to people developing sonar etc. They could then scrap all their expensive test equipment and expensive computer programmers etc. and just rely on your opinion to develop and debug the next generation.

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Originally Posted by dvv
Unusual for amps to be unstable without global NFB? I think the opposite is more likely to be true, unless you have an open loop full power bandwidth below say 1 kHz. In my approach, that is WAY greater.
For most SS amps the open loop bandwidth may be much less than 1kHz. In any case, are you confusing bandwidth with stability? Sometimes the output stage may be unstable with certain loads and global NFB might tamr this, but this is quite unlikely because the frequency of instability is often way too high for the loop to do much about. An amp like this could have very strange clipping behaviour. No, most SS amps are perfectly stable open loop but adding GNFB can reduce stability so compensation has to be added.

Now you may design unusual amps which are unstable open loop but that is not the norm.

Quote:
As for "it says so in the books", well, it was said in books that the Earth was flat for much longer that it has been recognized that it ain't necessarily so, to quote Bronsky Beat.
Actually, the Earth has been known to be spherical for rather a long time. Like servo/feedback theory, this is open to scientific investigation. In every field there are always a few people who think the basic facts are wrong, or don't apply to them, but they or their customers usually find that Nature cannot be fooled.

Last edited by DF96; 14th June 2013 at 01:29 PM. Reason: extend to answer dvv
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Old 14th June 2013, 01:21 PM   #9446
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I will in the meantime have to continue in "blithe disregard"...
Not unexpected. Handwaving is easier than actually doing real experiments.
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Old 14th June 2013, 01:28 PM   #9447
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And doing experiments is sometimes a good way to dodge admitting that you hear no difference where others do.

The real truth has a nasty way of somehow always managing to take some middle ground between the "objectivists" and the "subjectivists".
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Old 14th June 2013, 01:47 PM   #9448
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Everyone who has no trouble deciding, when they hear music playing and don't see what it comes from, whether it's the 'real thing' or merely reproduction is up to the mark. The little clues are easily picked up, register with your memory bank of past experiences, and decision made with barely the slightest twitch of the conscious mind.

Experiments could easily degenerate into a bizarre parody of instinctive listening, like forcing a person to fully engage the rational, thinking mind to walk -- okay, switch this muscle on for 0.2 secs, wait for that nerve trigger to fire, OK, twist this bone 5 degrees to the right, and apply 2.4 Newtons force to the balancing sinew ... oh dear, oh dear, I'm falling over ... why did that happen ...?!!
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Old 14th June 2013, 02:06 PM   #9449
DF96 is offline DF96  England
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Walking is a good analogy to listening. Can a person actually get from here to there while remaining upright and unaided? If they can only 'walk' from here to there while the tester has his eyes and ears shut then there will always be the suspicion that they actually crawled or used motorised transport, while vehemently maintaining that they walked it. Being asked to walk a short distance does not create stress in adults, except for those who actually can't walk!

Similarly, those who claim to hear things should be able to demonstrate this in the absence of other clues. If not we are justified in expressing doubts about their hearing, however sincerely they believe it themselves.

As it is, amps with poor circuits and poor performance but high price tags are often found to 'sound better' than other amps with good circuits and low price tags, but only when the extra information is available to the 'listener'. It doesn't take a genius to work out what is probably happening, especially as this phenomenon is so well-established that it even has a name.
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Old 14th June 2013, 02:13 PM   #9450
rdf is offline rdf  Canada
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Could you offer an explanation of how 'lousy connections' can audibly affect domestic audio yet a similar effect seems not to be seen in radio...
Apples and oranges. The harmonics of a radio carrier are far outside the operating bandwidth of most devices. A radio station at 96.9 MHz occupies a max +- 0.1 MHz. The first rectification harmonic would be 193.8 MHz, invisible to receivers. Many broadcast transmitters as well have very dirty modulation schemes but the 'distortion', falling in a complete different radio band, is easily filtered by the output section/broadcast antenna system.
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