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Old 9th August 2012, 08:36 PM   #471
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ah is that the reason. Also not keen with soap? Let me not talk about that but you understand I hope.
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Old 9th August 2012, 08:40 PM   #472
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If you have an MSc and are a teacher at a university I am an astronaut OK ?
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Old 9th August 2012, 09:08 PM   #473
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Tom/Frank please do not be discouraged by our illustrious Master of Science since his contributions is rather nebulous and unfounded.

Please continue with the excellent work as I believe we are moving closer to the rule of thumb that may solve many power amplifier problems.
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Old 9th August 2012, 09:15 PM   #474
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Originally Posted by Nico Ras View Post
Tom/Frank please do not be discouraged by our illustrious Master of Science since his contributions is rather nebulous and unfounded.

Please continue with the excellent work as I believe we are moving closer to the rule of thumb that may solve many power amplifier problems.
Capacitance multiplier....
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Old 9th August 2012, 09:29 PM   #475
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Capacitance multiplier....
Okay liching1952, show us what you want us to try out.
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Old 9th August 2012, 09:33 PM   #476
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try to analise that C multiplier and find out yourself that it is a nonsence circuit without any advantage.
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Old 9th August 2012, 10:33 PM   #477
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liching1952,

In your country there are at least two manufacturers of audio power amplifier, Goldmund and FM Acoustics. Do you have any comments/experience on how these power supplys are designed?
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Old 9th August 2012, 10:48 PM   #478
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liching1952,

In your country there are at least two manufacturers of audio power amplifier, Goldmund and FM Acoustics. Do you have any comments/experience on how these power supplys are designed?
I have not saw the schematic so I cant say anythink about it. If you give the scheme, I can give comments. However I think its just straight, bridge rectifier and huge Capacitor of 100.000uF or so. Thats all.
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Old 10th August 2012, 12:08 AM   #479
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No problems, Nico, I've still at the job! Tom's transformer model is something I've been dissecting, it's somewhat different to what I've seen before, and I'd like to get a good sense of how it works.

In the meantime Terry has been offering up excellent material in regards to the transformer, there are all sorts of "tricks" out there to squeeze the last bit of performance out of how these parts do their job. But, ultimately there is always a physical limit: a simple way of looking at the first stage of a linear PS is that the transformer is a converter of energy from one form to another, the form in this case being voltage level. So you can never get more energy out than goes in, no perpetual motion machines here! And the VA rating of the transformer, the nominal maximum energy delivered in a period of time is the key parameter here. That rating is not an absolute maximum, if you have a 300VA transformer you could use tricks to force it to deliver 500VA, say, but you would be pretty stupid to do so, because the transformer would be tremendously stressed, it would overheat enormously, hopefully blow its internal fuse, or cook itself to a cinder, or at worse set your house on fire!

At a very simplistic level, a 300VA transformer if perfectly incorporated into a supply could allow the amp to deliver somewhere near 300W continuously, the voltage sag would be engineered away by being super clever in the circuit design. But is it worth it? Probably easier all round just to buy a transformer well beyond what's apparently required, rules of thumb come in nicely now.

As Terry stated, the leakage inductance and winding resistances of the transformer are what makes the voltage sag badly when running at high power. In Tom's transformer the leakage inductance is 1.4mH and secondary winding resistance is 0.29R; if replaced with an ideal transformer the voltage droop disappears. Throwing vast quantities of C at the "problem" doesn't help the continuous power drain issue, the transformer can't refill the caps fast enough, the amp will always start clipping.

But, lots of intelligent C will help ride through the musical peak demands of normal music with minimal twitching, modulation. The trick, as Nico says, is to work out just the right combination to get the job done as well as possible -- good ROI.

Frank
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Old 10th August 2012, 01:01 AM   #480
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Nicely put Frank! and thanks for extracting the numbers (silly question - how many watts is the nominal load? I know its audio, but from an analytic perspective Pout is much more useful).

1.4mH = 0.440/0.528 Ohms at 50/60Hz, which is 1.5/1.8 times larger than the resistance.

Skin & Proximity Effect:
if the wire diameter is < 5mm we can completely ignore skin & proximity effect, regardless of the number of layers in the xfmr at 50/60Hz - this is because for phi/delta < 0.5 Fr = Rac/Rdc = 1 for any no. of layers (phi = effective conductor thickness = equivalent rectangular conductor with no gaps).

problem is we dont draw sinusoidal current - not by a long shot! skin depth is ~9mm at 50/60Hz, but at the (predominantly odd) harmonics it drops to:

3rd ~ 5mm, 5th ~ 4mm, 7th ~ 3.4mm, 9th ~ 3mm

so as long as the wire diameter is less than say 2mm you can safely assume constant R up to the 9th harmonic. whereas for a really big xfmr with say 4mm wire thats only true up to the 3rd harmonic - above that the resistance will increase - but not proportional to sqrt(Fharmonic) - it will rise faster than that due to proximity effect (the no. of layers in the winding).

I thinks its fairly safe to ignore this completely (unless you're planning on driving say 2R speakers so have a stupidly low DC bus) and assume R is constant.


The voltage dropped across the xfmr resistance & inductance is the vector sum of all the harmonics. the resistive portion is just Ipeak*R, and the inductive portion will look similar but shifted in time (its the derivative, but the current is sinusoidal-ish so the derivative will be much the same shape).

with Xl = 1.5...1.8xR the vector sum is dominated by the inductive drop (the R supplies 20...15% of the total) so a first-order analysis can simply ignore the resistance.

with a 15% conduction duty cycle (1.5ms conduction time @ 50Hz) the crest factor (IEEE def. CF = Ipk/Irms) is 3.53. But the ratio of peak to 1st-harmonic-peak is 5.0. So the voltage drop across the leakage inductance is FIVE TIMES larger than that you would expect given the load WATTAGE.

So an excellent 1000W transformer with 2% leakage will drop 2% volts when running at 1000W, but it will drop 10% when running at 1000VA with D = 0.15. Hmm, I'll see if I cant jack up a pretty curve....

Last edited by Terry Given; 10th August 2012 at 01:12 AM.
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