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1970s Design Indulgence

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Graham Slee View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 22 Jan 2022 at 7:46am
Rod tells us the switch on charge is

Q = CV

then goes on to explain about the average area under a sine curve as opposed to the rms value of the curve: 0.637 as opposed to 0.707.

I've been running my experiments at 8W rms, not his 8W peak, so my charge at 1kHz must be root2 bigger

Q = I x t = 1.41428A x 0.637 x 0.5ms = 0.0004504 (coulombs)

It seems to me like this is going to cause some loss of negative output swing, or is it positive?

On the way to the speaker it must lose something in topping up the capacitor - perhaps it loses voltage? We can rearrange Q = CV to

V = Q/C

and then plug in the charge and divide it by the capacitor value (ours is 2200uF)

0.0004504/0.0022 = 0.205V

All I'm doing here is looking for patterns because I have 2nd harmonic distortion, which although isn't much, it isn't that nice valvey distortion, so I'd like to bury the 2nd harmonic in the noise floor.

As we've been dealing with peaks, could 0.205V be related to the peak voltage for 8W rms?

Being asymmetrical, perhaps it relates to the peak to peak swing?

As it's 22.6V p-p. Dividing 0.205V by 22.6V we obtain 0.0091.

To turn it into a percentage we multiply by 100, and then we must divide it by the loop-gain of the amplifier at 1kHz, and we find that is also 100x, so the lazy mathematician can simply leave it as 0.0091%.

But the loop gain is simulated not measured and may be higher, and also the distortion meter sums the harmonics, but reads something like 0.0075%. It is not too far from the calculated 0.0091%.

What I'm trying to get at is if the distortion is related to the output capacitor size, because if it is, then double the capacitance halves the error voltage. I guess the only way is to try it for real, and if it does make any difference, then it "proves" the hypothesis, and if not, it eliminates this particular approach.

Not simple enough for Google-Bot to understand...
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