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Old 24th March 2012, 02:23 AM   #1121
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Hello, just woke up.

That looks like a legitimate class AB amp distortion profile. To get rid of that slant, you can measure the voltage across caps in the simulation which show large LF drift, and replace them with voltage sources. Another way is to make the coupling caps 1GF; I prefer this when I can remember to do it. But before all this you should use the Hann windowing. It's not that the Hann window is less accurate than Rectangular (LTSpice default), it's that the Rectangular window is horribly inaccurate for audio. So anyone still reluctant to window their FFT, take note.

The Hann window should clear up the noise floor, but if the drift is large enough the slant will still overcome the harmonics. In this case replace coupling caps and any caps that can cause signal drift with 1GF values.

Hope this helps.
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Old 24th March 2012, 10:05 AM   #1122
tvrgeek is offline tvrgeek  United States
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What I found last night was I needed to do a lot more periods to get the resolution I am expecting. At least 1000 cycles. That helped a lot. I compared several windows. I am most familiar with Hamming. I looked at Hann, and Blackman as well. They all changed the slope but the actual harmonics stayed within half a dB, so the measurements were not effected much. In SoundEasy measurement system, no window is preferable, in TrueRta, Hamming is preferable. This suggests it is quite dependent of the code. I prefer the resolution on square window, it was the slope that I know is not real that had me worried. Straight lines in a log plot are a warning! You are right in the Hann giving the most believable looking trace.

It took a bit to get all the config screens matched up so your suggested parameters worked. Changing the screen is a lot quicker than changing the config boxes. Neat trick, plus it documents on the drawing how it was tested.

This particular circuit has no coupling caps. One Miller comp and one in the feedback to set the LF. (not the original Erno design). I'll see if the feedback cap is the problem source.
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Old 24th March 2012, 10:18 AM   #1123
tvrgeek is offline tvrgeek  United States
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sho-nuff
Changing the LF cutoff in the feedback removed the slope. 100uF to 100F. I would never have guessed that. I can cut the cycles down to about 100 and still get enough points in the low side to be believable, or at least what I expect from using old analog analyzers back in the day.

My biggest impediment now is lack of models for modern transistors. Some things I guess are only possible on the bench.
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Old 24th March 2012, 10:22 AM   #1124
tvrgeek is offline tvrgeek  United States
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results
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Old 24th March 2012, 10:30 AM   #1125
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It is almost always the signal DC coupling caps, as they always have the longest time constant in a power amp.

You need 1000 cycles because that is how long it takes for the DC drift to mature into a recognizable signal by the FFT.

Are you using my Ampsim2.asc file? Just copy the directives and circuit nodes (Vin, Vout) to the right places in your schematic, and there will be no need for filling in dialogues, it just works out of the box.

Spice simulation

Furthemore, this post makes it pretty clear what FFT windows you have to choose from, and which will be most accurate. The Rectangular window gives good resolution, but with reasonable simulator settings it's not an improvement, you don't need to see in between the harmonics.

LTSpice FFT simulation settings and inconsistent results.

Attached is the FFT for my current simulation. This one has more DC drift than usual. Must fix that somehow.
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Old 25th March 2012, 05:23 PM   #1126
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Default output impedance

a few years back i was doing some simulations and some physical experiments with amp output impedance. i found out some very interesting things. i'm posting the paper i wrote in the process...

oh well, too big for here.... i'll link to it... http://www.electro-tech-online.com/a...ing-factor.pdf
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Old 25th March 2012, 05:34 PM   #1127
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Aww, it doesn't work.
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Old 25th March 2012, 05:42 PM   #1128
AndrewT is online now AndrewT  Scotland
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I can't see the PDF either.
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Old 25th March 2012, 10:55 PM   #1129
gootee is offline gootee  United States
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I right-clicked the link and selected "Open in New Window". After it failed, I logged in at Electro-Tech-Online and then the PDF file came up.
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Old 26th March 2012, 04:04 PM   #1130
TzeYang is offline TzeYang  Malaysia
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does anyone have PMBFJ620 JFET spice model?

Thanks in advance.
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