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Old 22nd April 2013, 08:43 AM   #1971
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Quote:
Originally Posted by gootee View Post
That too.

But as you said, the capacitance determines how low the rail voltage will dip for a given RMS load current. So for a given rail voltage and speaker impedance, the capacitance determines the maximum output signal amplitude (and thus the max output power rating) that won't make the amplifier clip.

That is very important, if you are wanting to minimize the capacitance.
I think you are right when the capacitance is very low. However, I think once adequate amount of capacitance is present, the capacitance has only a little effect on the amplifier power.

I am attaching four simulation graphs to illustrate. They were simulated using PSU2. Using LTSpice would produce simular result.

The first graph comes from my planned PSU for my new amplifier. I assumed 5A current draw.

In the second graph, I doubled the amount of capacitance. As you can see, the voltage has not changed by much.

The third and fourth graphs are the same except with idle current of 65mA. See doubling the amount of capacitance has not changed much the voltage.

From these 4 graphs we can also see that the doubling the capacitance has not changed much of the voltage sag when large current is drawn.

That was why I suggested that the amount of capacitance has more to do with ripples and THD-N than to amplifier power.

Please correct me if I am wrong.
Attached Images
File Type: jpg gootee1.JPG (150.3 KB, 132 views)
File Type: jpg gootee2.JPG (153.2 KB, 126 views)
File Type: jpg gootee3.JPG (170.9 KB, 122 views)
File Type: jpg gootee4.JPG (159.4 KB, 122 views)
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Old 22nd April 2013, 08:55 AM   #1972
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Originally Posted by Tony View Post
shouldn't you be thankful you got fuses in there? what could have happened other wise?
Tony,

I think I will be extra careful when I test the new amps this time around and not to make mistakes.

AndrewT gave a valid reason. In all of the previous cases in which fuses were burnt, only one fuse in one rail got burnt at a time, not both. The result was that large amount of DC was placed at the output. Perhaps that was because I was poking around with a probe of a scope or multimetre, and perhaps for once I shorted the output terminals. Unless I am convinced that both rail fuses will always get burnt at the same time, I can't see any point having the rail fuses.

Since we can't place too much capacitance after the rail fuse, the increase of power supply impedance would probably cause some measurable hence audible degradation of THD-N figures above 10kHz.

Regards,
Bill
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Old 22nd April 2013, 11:25 AM   #1973
DF96 is offline DF96  England
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Quote:
Originally Posted by HiFiNutNut
That was why I suggested that the amount of capacitance has more to do with ripples and THD-N than to amplifier power.

Please correct me if I am wrong.
Read the thread - especially the posts by me and gootee. If you have poor PSRR in your amplifier circuit then you will need bigger caps. Even with perfect PSRR you still need caps which are sufficiently large to avoid peak voltage limiting, but diminishing returns set in. Once the cap is big enough, you only get small improvements in power output as the cap gets bigger. Once in this region, it will be better to have a bit more secondary voltage from the transformer (or redesign the output stage so it drops less voltage).
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Old 22nd April 2013, 11:50 AM   #1974
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Originally Posted by DF96 View Post
Read the thread - especially the posts by me and gootee. If you have poor PSRR in your amplifier circuit then you will need bigger caps. Even with perfect PSRR you still need caps which are sufficiently large to avoid peak voltage limiting, but diminishing returns set in. Once the cap is big enough, you only get small improvements in power output as the cap gets bigger. Once in this region, it will be better to have a bit more secondary voltage from the transformer (or redesign the output stage so it drops less voltage).
I totally agree with that and have no dispute at all.

What I was trying to say is that the cap "that is big enough" does not need to be very big to reach most of the desired amplifier current and power, after which, any increase of capacitance has only a small impact on the amplifier current and power, but will improve on PSRR. How much capacitance is necessary after that point depends on the PSRR of the amplifier and the target THD-N figure.
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Old 22nd April 2013, 12:13 PM   #1975
AJT is offline AJT  Philippines
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Quote:
Originally Posted by HiFiNutNut View Post
Tony,

I think I will be extra careful when I test the new amps this time around and not to make mistakes.

AndrewT gave a valid reason. In all of the previous cases in which fuses were burnt, only one fuse in one rail got burnt at a time, not both. The result was that large amount of DC was placed at the output. Perhaps that was because I was poking around with a probe of a scope or multimetre, and perhaps for once I shorted the output terminals. Unless I am convinced that both rail fuses will always get burnt at the same time, I can't see any point having the rail fuses.

Since we can't place too much capacitance after the rail fuse, the increase of power supply impedance would probably cause some measurable hence audible degradation of THD-N figures above 10kHz
Regards,
Bill

easier said than done.... Murphy makes a strong presence...

in that amp i posted which is a super leach, main filter caps are 68,000ufd/100volt computer grade types, after the fuses are pcb mounted decoupling caps of about 100ufd per rail....
never had any issues with that arrangement, i employed a soft start circuit using relay, and a speaker protector circuit using relay, the speaker output terminals are not fused...any output dc greater than 1 volt(such can happen when one rail fuse blows) and the relay is turned off preventing damage to the speakers......this worked for me real time..this is my experience...don't know about others...
..
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Old 22nd April 2013, 12:15 PM   #1976
gootee is offline gootee  United States
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HiFiNut,

This thread is about all of that. Neither of us is disagreeing with you but we have already developed equations for your "big enough", "not very big", and "most of", and are intimately familiar with the behaviors you are trying to describe.

Equations relating the PSRR, THD, and C would be a welcome addition, though.

Tom
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Old 22nd April 2013, 12:19 PM   #1977
DF96 is offline DF96  England
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Quote:
Originally Posted by HiFiNutNut
What I was trying to say is that the cap "that is big enough" does not need to be very big to reach most of the desired amplifier current and power, after which, any increase of capacitance has only a small impact on the amplifier current and power, but will improve on PSRR. How much capacitance is necessary after that point depends on the PSRR of the amplifier and the target THD-N figure.
OK; I thought you were disagreeing with the findings of the thread.

How big the cap needs to be depends mainly on the transformer voltage and the claimed/stated power output. You can design a 100W amp, use a smaller cap and then just call it an 80W amp. On the other hand, if you use a truly massive cap you might be able to describe as a 120W amp.
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Old 22nd April 2013, 03:08 PM   #1978
AndrewT is offline AndrewT  Scotland
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Hifi started with +-40mF and went up from there. That in my book is good enough for 4ohms speaker driving, where transient currents can approach 30Apk.
15Apk from +-20mF is usually good enough for (very) good LF performance into 8ohms speaker.

I think Hifi's examples would be much more illustrative had he reduced the supply rail capacitance to <+-20mF maybe even used +-10mF and +-4700uF as seen in some builds.
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Old 24th April 2013, 01:41 PM   #1979
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Originally Posted by AndrewT View Post
Hifi started with +-40mF and went up from there. That in my book is good enough for 4ohms speaker driving, where transient currents can approach 30Apk.
15Apk from +-20mF is usually good enough for (very) good LF performance into 8ohms speaker.

I think Hifi's examples would be much more illustrative had he reduced the supply rail capacitance to <+-20mF maybe even used +-10mF and +-4700uF as seen in some builds.
Good analysis. Agreed.
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Old 24th April 2013, 01:45 PM   #1980
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Originally Posted by gootee View Post
This thread is about all of that. Neither of us is disagreeing with you but we have already developed equations for your "big enough", "not very big", and "most of", and are intimately familiar with the behaviors you are trying to describe.
Thanks for all that. I am yet to use your Spreadsheet as my Excel version is very old at home and at work I cannot download anything from the net.

Quote:
Originally Posted by gootee View Post
Equations relating the PSRR, THD, and C would be a welcome addition, though.
That is exactly my point.
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