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Old 19th April 2012, 05:13 PM   #4481
bcarso is offline bcarso  United States
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dV/dt for a capacitor charging/discharging with a constant current is i/C. Hence 10mA and 10pF yields 1000V/usec, 2mA and 50pF gives 40V/us.
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Old 19th April 2012, 05:17 PM   #4482
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Originally Posted by Bazukaz View Post
But why very high slew rate is needed?

One example is Quad ESL.
Doe to very capacitive loading on amplifier full power bandwidth is just few kHz depending on amplifier current rating. That would translate to max. slew rate really low compared to today's standards. And, if you try to exceed it, amplifier's short circuit protection probably kicks in. Should be horrible?

Still some people think Quad esl are one of best sounding speakers.

So why bother with square waves ? IMO it is just useful to test stability of amp/preamps.
"Some people" will always think all sorts of things.

Also, using just one speaker, or even one not too popular TYPE of speakers, to prove a point is hardly convincing. Although I agree that slew rates as such, isolated on their own, don't mean much really.

By the same token, I could now call my own speakers (local manufacture) the standard, even if they are an exceptionally easy load to drive, nominal impedance 8 Ohms, minimum 6.5 Ohms, worst case phase shift 20...20,000 Hz -25 degrees at around 200 Hz, efficency 92 dB/2.83V/1m. They would in fact level the playing field to unrealistic comfort levels, enabling tha humblest of amplifiers to achieve its best. But this is unrealistic because in the other room, I have a pair of AR 94 speakers, which are a bitch to drive and make quite a few amps huff and puff for breath.

Square waves are an invaluable tool in designing, as they represent a multitude of even simpler sine waves, and no matter what anyone says, they do let us see at least approximately what happens with our amp as it is subjected to various drive regimes. Not ideal, but certainly very useful.
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Old 19th April 2012, 05:51 PM   #4483
a.wayne is offline a.wayne  United States
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Agree on the square wave deal ...

I was reading an article last week , about a test done by one of the B&K designers , according to him , this setup have the ability to test amplifiers from .0001 watt to 1 watt and by his evaluation, amplifiers that show .00001% thd @ 100 watts and sound subjectively average, exhibit 1 % thd or higher @ .01 watt . The belief is this new testing procedure , has the best correlation between measurements and good sound ..
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Old 19th April 2012, 06:41 PM   #4484
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Quote:
Originally Posted by bcarso View Post
dV/dt for a capacitor charging/discharging with a constant current is i/C. Hence 10mA and 10pF yields 1000V/usec, 2mA and 50pF gives 40V/us.
Yes, I lost a zero somewhere...
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Old 19th April 2012, 07:44 PM   #4485
dvv is offline dvv  Serbia
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Originally Posted by a.wayne View Post
Agree on the square wave deal ...

I was reading an article last week , about a test done by one of the B&K designers , according to him , this setup have the ability to test amplifiers from .0001 watt to 1 watt and by his evaluation, amplifiers that show .00001% thd @ 100 watts and sound subjectively average, exhibit 1 % thd or higher @ .01 watt . The belief is this new testing procedure , has the best correlation between measurements and good sound ..
Well, not a moment too soon. It was about time somebody did something really worthwhile in that arena, even if it is no longer so "in" as it used to be.
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Old 19th April 2012, 08:10 PM   #4486
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On the subject of voltage slew rate, overall I feel it's too exaggerated. In my view, as long as your amp can slew 1V/usec per every peak volt of output, I believe you're home and dry regarding SID, TIM and such like. Using FET inputs, this is very likely to be more to much more anyway, without any special effort.

Looking back, it seems to me that the problems encountered many years ago were in fact the result of irresponsible compensation - the easiest thing to do is to throw in a compensation cap until it stops oscillating or exhibiting signs of instability, certainly far easier and cheaper than further researching and refining your circuit. But, as ever, after the battle, everybody's a general.

In my view, and I am totally with Thorsten on this, the trick is to judge a proper balance between local and global NFB, and this will vary from case to case, from one topology to another, from one set of requirements to another. Like T., I also happen to think it's better to clean up house locally, leaving the global NFB to be more or less like icing on a cake.

Where we differ (I think?) is in how much should be left over for global. T. mentioned that 12 dB was ideal; I don't know how he got that particular number, but I am not convinced that's always ideal. Yes, my own HK 680 integrated uses exactly that much, but overall, I tend more towards 20 dB as being ideal, 12 dB still has reminants of that "not quite completed" design I dislike. But at 20 dB, of the several amps I have heard, not one had even a hint of it.

Since amps using that much (or little) global NFB don't exactly grow on trees, my sample was small, and all of it was hand built by enthusiasts. Not a terribly reliable sample, I'm afraid.

BTW, my simulator says my voltage slew rate is around 390 V/usec (1.3 mA and 3.3 pF). Thus, even in the very unlikely case of having to double the value of the compensation caps, I still have more than I could ever possibly use. Open loop full power bandwidth is just over 80 kHz at -3 dB point and stable (according to the sim) - into a 4 Ohm load.

It's a habit I picked up from the Germans decades ago. Make it work well into 4 Ohms and it'll work even better into 8 Ohms.

Another habit I picked up also from the Germans, specifically Studer/Revox (although a nominally Swiss company, their actual home audio manufacturing was just across the border, in Loeffingen, Germany) is to get the amp to work to say 300 kHz, and then put in a first order filter at the input, limiting this to say 200 kHz. Seems prudent to me.

Jan will probably know better, but if memory serves, Philips used that gig as well.
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Old 19th April 2012, 08:22 PM   #4487
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Originally Posted by dvv View Post
[snip]Jan will probably know better, but if memory serves, Philips used that gig as well.
Don't know about Philips in detail, but they were very conservative, more like "it measures great therefor it MUST sound great". They were at the time an engineers' outfit, not very good at marketing. Their video casette system was arguably better but lost from VHS marketing power.

A few years ago I spoke with the guy who invented the moving magnet cartridge, but Philips thought that piezo cartridges were all that would ever be needed. This guy went to work for Bauer at Shure and the rest, as they say, is history.
Sorry for rambling on like that...

jan
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Old 19th April 2012, 09:33 PM   #4488
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Originally Posted by janneman View Post
Don't know about Philips in detail, but they were very conservative, more like "it measures great therefor it MUST sound great". They were at the time an engineers' outfit, not very good at marketing. Their video casette system was arguably better but lost from VHS marketing power.

A few years ago I spoke with the guy who invented the moving magnet cartridge, but Philips thought that piezo cartridges were all that would ever be needed. This guy went to work for Bauer at Shure and the rest, as they say, is history.
Sorry for rambling on like that...

jan
I couldn't agree more with you.

I used to say that somebody should do Philips a favor and send shock troop into the marketing department, with orders to use extreme prejudice - shoot anything that moves.

I had their fantastic N4520 open reel deck, all 28 kilos of it - it was a truly outstanding product, which never really made it, like their Black Tulip range, because I was sure they had like the worst marketing in the world.

Having worked with Philips on their PC products for five years, I know that their technical and commercial staff was one of the best in the business - but again, their marketing slowed things down like I couldn't believe. Their SE European HQ was in Vienna, relatively near, so I saw a lot of them. To this day, I think very fondly of them.
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Old 19th April 2012, 09:53 PM   #4489
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Originally Posted by dvv View Post
I am glad we agree, W., but this begs the logical question: why is this not recognized widely, instead of promoting only the first half of the whole?
Why?
Because they want to give mo better business opportunities.
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Old 19th April 2012, 10:04 PM   #4490
tvrgeek is offline tvrgeek  United States
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Originally Posted by a.wayne View Post
Agree on the square wave deal ...

I was reading an article last week , about a test done by one of the B&K designers , according to him , this setup have the ability to test amplifiers from .0001 watt to 1 watt and by his evaluation, amplifiers that show .00001% thd @ 100 watts and sound subjectively average, exhibit 1 % thd or higher @ .01 watt . The belief is this new testing procedure , has the best correlation between measurements and good sound ..
.0001W. Yea right. In the noise. Is .01 watt valid? ONE watt would be. Going for a standard 10X resolution, start at .1W. I think it was mentioned a few thousand posts ago of an Italian mag that tested at many powers across the spectrum and produced a nice 3-d waterfall plot. That would be insightful information. It still leaves out the difference in amps in proportion of high to low order harmonics and if that changes across the spectrum or with power. We need at least a four dimensional plot. Frequency, power, distortion, ratio high to low. We need to be able to slice the plot on any axis. If you have the data, Matlab can do the plot.
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