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Old 24th July 2012, 05:14 PM   #261
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How about we create a special chopper loading module to test the buffering capabilities of an amp?

I'd construct it this way: An idle high impedance of say 10K ~ 100K load on the supply output, that, via a control signal can be switched to say 10 Ohm/10W using a MOSFET for a period of say 10mS.

Then, just record the transient response at the supply output closest to the load and see what effect all kinds of bufferings and different caps, inserted wires to get your parasitic L etc, have on the transient response.

The goal is to work towards a way of buffering that alters the voltage at the load node the least. Reduce spiking, reduce sagging. This will be a relatively easy way to find out how to create the best buffer distribution/arrangements.
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Old 24th July 2012, 05:23 PM   #262
PChi is offline PChi  United Kingdom
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The capacitors suggested in post #54 have an ESR specification of 16 mohm typical, 31 mohm maximum at 100 Hz / 20 degrees C. The impedance at 10 kHz = 30 mohm maximum so they are pretty low impedance at audio frequencies. It's going to require a large film capacitor to reach that low an impedance at 10 kHz.
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Old 24th July 2012, 05:33 PM   #263
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Quote:
Originally Posted by PChi View Post
The capacitors suggested in post #54 have an ESR specification of 16 mohm typical, 31 mohm maximum at 100 Hz / 20 degrees C. The impedance at 10 kHz = 30 mohm maximum so they are pretty low impedance at audio frequencies. It's going to require a large film capacitor to reach that low an impedance at 10 kHz.
Well yes, but look at the cap you linked. Is that a power supply main buffer cap, or would that be your local decoupling caps on the board both at board entrance and close to the output devices?

That's what I mean, the local electrolytics won't be kingsize like a main smoothing cap, they are in the region of 100uF ~ 2200uF, and their ESRs are higher than that of an accompanying 100nF MKT.
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Old 24th July 2012, 05:35 PM   #264
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Could someone give me an indication of the self inductance of say an 8 inch length 2.5mm2 wire? 20 years ago since I last calculated such stuff by hand, bare with me
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Old 24th July 2012, 05:40 PM   #265
tsiros is offline tsiros  Greece
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negligible?

200 nH ?
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Old 24th July 2012, 05:41 PM   #266
PChi is offline PChi  United Kingdom
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Originally Posted by MagicBox View Post
Well yes, but look at the cap you linked. Is that a power supply main buffer cap, or would that be your local decoupling caps on the board both at board entrance and close to the output devices?

That's what I mean, the local electrolytics won't be kingsize like a main smoothing cap, they are in the region of 100uF ~ 2200uF, and their ESRs are higher than that of an accompanying 100nF MKT.

I think that it would be the power supply main buffer cap. I would expect local decoupling on the amplifier PCB as well. At 10 kHz the reactance of a 100 nF capacitor is 159 ohms so the audio current is going to come from the power supply main buffer capacitor.
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Old 24th July 2012, 05:42 PM   #267
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negligible?
I don't know if it would be neglible when talking the size of currents in a decent amp. The L could be just high enough to 'choke' the T=0 current and prohibit initial demand. I was about to put a simulation together of a full rectified supply with buffercaps and ESR simulation, along with the chopper circuit to view the transient response. I wanted to model the supply wire in there too.
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Old 24th July 2012, 05:47 PM   #268
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Originally Posted by PChi View Post
I think that it would be the power supply main buffer cap. I would expect local decoupling on the amplifier PCB as well. At 10 kHz the reactance of a 100 nF capacitor is 159 ohms so the audio current is going to come from the power supply main buffer capacitor.
The reactance (X) doesn't matter, it's not in series with the load, you have to view it as an ideal supply with an internal source resistance equalling the ESR paralleled to the main supply. Think 'shortcutting' a charged cap, that's a HUGE T=0 current and has nothing to do with its frequency dependant impedance.

Amps(T0) = Vcap/ESR

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Last edited by MagicBox; 24th July 2012 at 05:56 PM.
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Old 24th July 2012, 06:01 PM   #269
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PChi,
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I think that your waveform distortion on the positive supply is voltage drop caused by the rectifier current.
It's either due to where the Oscilloscope probe is connected when the extra voltage drop is due to the resistance of the wire and connections (What does the waveform look like when the Oscilloscope probe tip and ground are connected directly to the capacitor terminals).
Or the electrolytic capacitor ESR is rather high.
I've done this experience with different caps (I checked them first with a tan-delta meter), transformers, rectifiers and connections of the probes.
Each time, the phenomenon happened for high values of capacitor.
It has been repeatable for me but I do not exclude an error in my procedure.
Maybe somebody could do the same test and show his findings,
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Old 24th July 2012, 06:12 PM   #270
tsiros is offline tsiros  Greece
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Quote:
Originally Posted by MagicBox View Post
I don't know if it would be neglible when talking the size of currents in a decent amp. The L could be just high enough to 'choke' the T=0 current and prohibit initial demand. I was about to put a simulation together of a full rectified supply with buffercaps and ESR simulation, along with the chopper circuit to view the transient response. I wanted to model the supply wire in there too.
i doubt a 200nH inductance would have any significance

current of what size are we talking about here?
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