PSU filter capacitor value vs. output voltage swing - diyAudio
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Old 9th February 2013, 02:06 PM   #1
johnr66 is offline johnr66  United States
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Default PSU filter capacitor value vs. output voltage swing

I set up a simple experiment to see how changing the supply filter capacitor values influenced the maximum amplifier output voltage before clipping. I had to use the parts I had on hand and this was a 9-0-9v 2a transformer and a TDA2040 audio power amp IC. The supply was setup to be a full wave split design with the cap values shown filtering each rail. The amplifier's output was connected to a non inductive 4 Ohm load. I measured the maximum output RMS voltage just before clipping at 40 and 120Hz (actually slightly off 120Hz so the signal would not be in sync with the 120Hz supply ripple).

Here are the results of the test
Click the image to open in full size.

The non clipped output voltage did not change much until I went smaller than 2,200uf filter caps. After this, the supply ripple was large enough to start affecting the output swing considerably.

Of course, this is only a 8 watt amplifier. Two channels and higher power demand much more current from the supply.

Last edited by johnr66; 9th February 2013 at 02:08 PM.
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Old 22nd February 2013, 06:37 PM   #2
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Quite interesting.

Do you have any data on the ESR of the caps? If you're adding 1000uF caps one at a time for those results, then I'd be interested in seeing those figures.

It looks like you've exchanged caps for:
470uF
1000uF
2200uF
3300uF
4700uF and finally
8000uF
I take it these are singular across the rails? I'm pairing up two 10,000uF samwha for each rail on a PSU design at present, so it looks like I might need some more. Info on whether to use 2 x 4700 or 1 x 10,000 would be interesting

Thanks for this though!
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Old 24th February 2013, 03:17 PM   #3
MarianB is offline MarianB  Romania
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The PSU filter caps cannot influence the amplifier max output swing, that would imply changing the voltage witch is impossible, as long as the total capacitance is enough to get a small voltage ripple then the output swing is about the same no matter how much caps would be add to that minimum needed capacitance, the test comes just to prove that.

PS: On 50/60Hz linear PSU the caps ESR makes no difference, the freq is small enough.
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Old 24th February 2013, 03:43 PM   #4
DF96 is offline DF96  England
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The PSU filter caps are a significant factor in determining the maximum undistorted output swing. The data shows this. Calculations and modelling show this too. Cap ESR has a small effect.

The reason is that the cap value determines the extent of voltage dip just before the next charging pulse. The voltage dip determines the peak output voltage before clipping takes place. This is all gone into in another thread on here.
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Old 24th February 2013, 03:58 PM   #5
MarianB is offline MarianB  Romania
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I completely agree @DF96, and i think that is just what i sayd, maybe not in so many words, but the fact is as long as you have enough capacitance to get a small enough voltage ripple then the amplifier max output swing cannot be influenced just by adding more capacitance. And to make myself better understud, let say just for the sake of argument that for some X current and Y voltage ripple accepted, you need 22mF, if that ripple is small enough then you have made sure the amplifier's swing is at it's max potential, so it would make no difference if adding let say another 22mF of capacitance. The voltage ripple would be in the desired range either way, that is what i wanted to underline.
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Old 24th February 2013, 04:09 PM   #6
DF96 is offline DF96  England
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You seem to agree with me, then disagree, in the same sentence!

Cap size affects voltage swing, but diminishing returns set in once the cap is sufficiently large. Adding more cap makes a difference, but only a small difference.
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Old 24th February 2013, 04:12 PM   #7
MarianB is offline MarianB  Romania
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That is just semantics... yes theoretically it makes some difference but it is too small to take into account, ok now?
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Old 24th February 2013, 04:34 PM   #8
DF96 is offline DF96  England
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The precise meanings of sentences always involves semantics.
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Old 24th February 2013, 04:55 PM   #9
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Very good John. I obsserved an even greater increase in power into a 6 ohm dummy load when prototyping my "Zero Budget" amplifier. I took the power amp board out of a tabletop "hi-fi" "50+50 watts RMS" . I observed 12 watts RMS both channels driven. I increased psu caps from 3300 uF to 20,000 uF, no other changes, and observed 21 watts RMS before clipping. Then I used two transformers instead of one with 10,000 uF psu caps and obserbed 29 watts RMS before clipping. At this point the performance and subjective sound clarity was so good that I slapped it into an enclosure as a power amp for my second TV.

It was so mediocre when I started, and now it sounds better than my buddy's "mid-fi" Sony reciever (although not as powerful). Total cost? Under $30. Most of the stuff came right out of my electronic junkbox.
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Old 24th February 2013, 04:57 PM   #10
MarianB is offline MarianB  Romania
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Maybe so @DF96 but sometimes there is such a thing like overanalising stuff. theoretically yes, you are verry right, but practically the differences are too small, what difference could really make a few hundreds of mV more swing to a couple hundreds W power amplifier?

Anyway, i think this cannot help anyone so do not see any point in continuing this, if you want it so, then it is exactly as you say and i am sorry to have sayd anything.

regards.
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