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Old 7th August 2012, 01:39 PM   #381
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by OnAudio View Post
Here we go
http://www.diyaudio.com/forums/attac...ize-simple.png
This is the way I approach it now.
It is a filter.
This filter should not interfere with the action of the NFB.
This filter should not be the dominant roll-off of the bass frequencies. The input filter does that.
I set the pole of the reservoir capacitance at least a full octave lower than the input filter.

And I am back and looking quite brown compared to my usual complexion. Extended = 50days. I got back a little early. I was missing you all!
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Last edited by AndrewT; 7th August 2012 at 01:44 PM.
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Old 7th August 2012, 04:11 PM   #382
AndrewT is offline AndrewT  Scotland
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Starting to catch up with the posts in this thread, missed out quite a few though.

Have you looked at local decoupling with low and high esr and remote reservoir, again with low and high esr? Then add in the esl effect of the cabling/traces?
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Old 7th August 2012, 04:27 PM   #383
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Welcome Back AndrewT, 50 days leave is almost retirement.
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Old 7th August 2012, 04:31 PM   #384
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Tom & Fred, ....
Sorry I meant Frank
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Old 7th August 2012, 04:35 PM   #385
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Originally Posted by gootee View Post
See the post after yours (i.e. post 372).

It shows that it's exactly as I've been saying all along:

The rail CURRENTS _ARE_ the signal, non-inverted, and they come from the capacitors.

Each rail does one polarity. So the currents look like a push-pull version of the signal.

The rail VOLTAGE just sags a little when the transistors open up, to let just the right amount of current out of the caps to create the load voltage.

Edit: It looks like the current for the negative rail cap (fC5) somehow got inverted, in the first image in post 370 (the 1600 uF version of the zoomed drum strike).

Cheers,

Tom
Yes there are some amplifiers that use this technique to drive the load
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Old 7th August 2012, 04:39 PM   #386
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Originally Posted by gootee View Post
(And I still haven't even gotten to the decoupling caps, yet!)
Like a baby waiting for candy
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Old 7th August 2012, 04:46 PM   #387
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Hi T was wondering about you
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Old 7th August 2012, 11:46 PM   #388
fas42 is online now fas42  Australia
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Originally Posted by Nico Ras View Post
Sorry I meant Frank
No worries, Nico, I'm pleased that people are positive about where this is going!

Thanks,
Frank
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Old 8th August 2012, 02:14 AM   #389
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Quote:
Originally Posted by gootee View Post
Frank,

Good insight. That is the trade-space that needs to be quantified.

The voltage goes back up, a few seconds after that sim ends, by the way. But that points out a possible criterion for choosing the reservoir capacitance: They have to be large-enough to have enough charge to supply a sustained "short term" demand, i.e. at least until the next charging pulse, but also need to be small-enough that a) they don't drag the voltage too low for a worst-case burst between charging pulses, and b) they aren't so large that they can blast out a LOT more current than a charging pulse can supply, because then, if they're unlucky with the size and timing of the demand versus the size and timing of the charging pulses, they could just continue to force the rail voltage lower and lower. That's part of the equation we need!

This looks like a probability problem, too. What fun! (groan).

ESR and ESL play a role there, too, both those of the capacitors and those of the interconnects on both sides of them. And the transformer and rectifiers will probably need to be able to supply extra-big charging pulses, sometimes.

Cheers,

Tom (going to bed now)
I applaud the analytical approach, but isn't getting into probability taking things too far? My conclusions based on doing the engineering math, simulation, and listening are, to minimize the influence the power rail can have on the output, you need 3 things:

1) rails 20V higher (my rule of thumb, yours may vary) than the maximum output swing so that the rails can sag under load but still be comfortably above the maximum output voltage;
2) sufficient capacitance to handle the maximum specified output current (your earlier post showed how to calculate that.) Any more is of dubious benefit, and certainly increases cost. So do the math, figure out how many Farads you need, then apply whatever margin you like. My rule of thumb on the margin would be 20%, remember we're talking about peaks.
3) a design with high PSRR.

That gets you 95% of the way there. The next 4.5% is paying attention to low ESR in the capacitance bank and minimizing inductance in the power leads.
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Old 8th August 2012, 03:17 AM   #390
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Originally Posted by mightydub View Post
That gets you 95% of the way there. The next 4.5% is paying attention to low ESR in the capacitance bank and minimizing inductance in the power leads.
I agree with you, mightydub, rules of thumb get you a long way down the path to the good stuff, top notch sound. But, in my experience, it's that last 0.5% where the real battle is, that's the area where everything measures beautifully, but it doesn't turn you on, vs. sound that makes all the question marks in your head go away ...

Frank
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