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Old 24th June 2012, 04:21 AM   #6331
a.wayne is offline a.wayne  United States
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Quote:
Originally Posted by SY View Post
Others did- but joules per watt is just as nebulous. What's the rail voltage? What's the load? At what frequency? And all the other bits I mentioned.

Someone talked about using enough capacitance to keep the amp running for 5 seconds after turnoff. I'd rather have an amp that collapses faster than that so that in the event of a power outage, my speakers don't suffer a full power BANG from the preamp.

Rail sag? Get a bigger transformer- the caps shouldn't be a band-aid.

Seriously Sy , power cuts, Then have the pre stay up for 10 ...

The caps are not for transformer rail sag , Isn't for maintaining voltage during diode switching, storage and controlling ripple.. I Guess best to keep adding caps until no detectable sonic advantages or measurable ripple ...
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Old 24th June 2012, 06:33 AM   #6332
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Originally Posted by jacco vermeulen View Post
Which is a different ball game, because it's not just twice the capacitance, divided in half per amp side, but also ripple suppression at half the impedance.
Joule per watt, or C per A(V), in both cases it all depends on which load impedance is targetted.


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Old 24th June 2012, 06:40 AM   #6333
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Frankly, I don't understand what the whole argument is about. It's fairly clear that there is no, nor can there be any universal answer to how much capacitance you need, as it varies as per several variables.

However, if you compare John's answer for a nominal 100 WPC/8 Ohms amp with the little formula proposed by Motorola, I think you'll find a very big percentage of agreement.

I think assuming 1.5 Joules of energy per every dissipated 10W of power is nice, safe bet. If you know the load might be an evil one, go for 2 Joules per dissipated 10W. Just don't forget to clearly define what you expect from the amp into low loads, say 2 Ohms, or you might run out of steam. Work it out for both 1.5 and 2 Joules, and find your own niche somewhere in between.

I could be wrong, but I don't think it can get any more specific than that.
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Old 24th June 2012, 02:26 PM   #6334
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And that's basically what I was looking for.

For some reason when you ask for a rule of thumb or general practice, people fly into a fit and want to hit you over the head about "no Magic Answers" or "You can't generalize" or some such. Sheesh..... All I wanted to know is if anyone had noticed a relation between energy stored/output power and sound quality. (Sound Quality vs Measurements). It appears that no one posting here thinks in those terms, fair enough. Tho we can see that someone at Motorola did.

FWIW, many deluxe amps have much more than 1.5 joules per 10W. Is it just overkill?
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Old 24th June 2012, 02:53 PM   #6335
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You worked with Hiraga , what was his magic number/formula...?
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Old 24th June 2012, 02:53 PM   #6336
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I just had to do this with a valve amp . 330 uF was not enough and was humming ( 450 V 140 mA ) . So for the loss of 5 V I used a capacitance multiplier ( Darlington and 10 uF , 4 K , 430K ) . It was bits from my useful box and worked a treat ( 10 to 1 reduction and nicer looking ripple ) . It needed 10 nF from collector to base . It even sounded nice ! With transistor amps my feeling is ripple current tells you all you need to know . Then add caps as you fancy . My goodness this simple thing has dragged on .
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Old 24th June 2012, 02:55 PM   #6337
a.wayne is offline a.wayne  United States
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It's not simple , most get it wrong , hence the questions...
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Old 24th June 2012, 03:10 PM   #6338
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I beg to differ a little . Say whatever maximum current will be . Times it by 1.5 for safety . Job done . The manufacturer will give the ripple ratings . Then add caps as your ears require . I would admit that the currents that might be required baffle most of us . Would anyone like to say ? Lets talk daft speakers and stupidly high wattage's ( 1 ohms 600 W ) . One problem is that the big amps draw current at the crest of the sine wave . Sometimes the caps hardly get a look in . My friends in the PA world use 3 phase power as it helps replenish the caps more often . They find there comes a time when no matter how big the caps are 3 phase does it better .
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Old 24th June 2012, 03:36 PM   #6339
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Quote:
Originally Posted by a.wayne View Post
You worked with Hiraga , what was his magic number/formula...?
Bigger is better. See "le Monstre", so named because of its power supply. An exercise in "How far can you go?". On average Hiraga's power supplies were bigger than normal, it was an important part of the design. Of course, the school of massive power supplies has come into vogue recently, so the average seems to be creeping up.

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Originally Posted by nigel pearson View Post
The manufacturer will give the ripple ratings. Then add caps as your ears require.
OK, as the ears require. That's a tough one. What I was hoping to find is that there may be an approximate range that many designers have used over the years that seems to work well. I'm sure the range will be fairly broad, but what is it? Just because most folks have never thought of it in those terms does not make it an invalid metric. And of course just because I thought of it does not make it valid, either. I'm curious.

Perhaps stored energy doesn't relate well to any sonic qualities. Maybe PSU impedance is a better metric?
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Old 24th June 2012, 03:39 PM   #6340
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Originally Posted by Pano
Tho we can see that someone at Motorola did.
Not necessarily. Motorola were mainly hard-headed engineers, so it is very likely that their rule of thumb was derived in the way I explained i.e. the criterion was not 'sound quality' but x% droop in 10 ms (or 8.33 ms in US). This could be regarded as a ripple calculation, but ripple calcs usually assume full power sine-wave; I assumed full power square wave.

If your amp has good PSRR (including low power supply IM) then it may be the case that there is nothing more to be said: low enough ripple guarantees good enough sound. When people try to explain anything further they usually quickly fall back on myths and legends rather than genuine explanations. Now it may be that we don't know how much hum is low enough, or how much ripple IM is low enough, given that we can't have zero of either unless we use battery power. 'Too low to measure' is meaningless, as you can always measure lower if you try a bit harder.
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