LM317 experiments and measurements

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I assume that the injection of the sine wave onto the rail, is to simulate PS modulation by the load. I'm not 100% convinced as yet of the significance of this test. I would have thought that feeding the actual device (which uses the PS) a full magnitude sinewave and then doing the fft on the rails would be more telling...

Yes a full-magnitude sinewave would provide a 'spray' of harmonics on the rails rather than a single tone. But wouldn't something like pink noise be even closer to what happens with real music?

I also note in the link above to jackinnj's other post that the fundamental on the super-reg is lower than it is on the LM317... It is not clear if this is because the LM317 is amplifying the fundamental, as well as producing harmonics, or whether the fundamental supplied to the LM317 was higher than the one supplied to the super-reg...

From what jackinnj is saying - applying a sinewave from the AP's generator - the differing levels most likely reflect the differing output impedances of the two regs. The Jung reg being significantly lower impedance at 1kHz than the LM317.
 
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Yes pink noise would be an interesting test, but you may need to watch real time fft to get a sense of what is going on... From what jackinnj said about percussion, some test tones with sharp transients would probably also be interesting to observe.

Tony.
 
YES! I remember, ok, I'm a bit of an idiot here for forgetting...

Because I'm the guy who read your Linear Audio article on regulators and sent Jan an email saying "hey, why don't you measure THD on the output of your regulators?" and then he told you and you did measure, and the results were damn interesting.

FYI I tested plenty of LDOs and they all have more or less the same behaviour, ie, output impedance decreases with increasing load current, which means high THD on the output.

This is logical, as pass device Gm increases with current. Loop gain is enough to control output impedance, but they're not designed for low THD.

If it supplies anything which draws a distorted current anyway, we don't care. But if it's a class-A stage which draws a current which is an exact replica of the signal, we might as well not have THD on the rails.

Bonus question:

- Load current is a sine
- Regulator output voltage is a sine with harmonics due to regulator THD
- Since this voltage appears across the output capacitor, it means the harmonics create a current which flow into the output cap
- This current also flows into GND and adds harmonics to GND
- Level is probably harmless... should check this.

I didn't want to mention your name specifically (which Jan had passed along to me), but i have been giving credit in the form of "a correspondent to LinearAudio remarked..."

It was a great insight...
 
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