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Old 1st July 2013, 09:45 PM   #591
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If that is the case, then you have unwittingly sabotaged your simulation in order to match it to real life.

You don't need to use the Hann window to see this. Set the sine start delay to 0 and make all DC blocking or bootstrapping caps 1F. The DC drift will no longer obscure the harmonics.

EDIT: Now I see you exclude the sine start in your FFT. I found this was extra effort when I could do basically the same thing in the .tran statement. Still, the noise floor is caused by drift.

Last edited by keantoken; 1st July 2013 at 09:49 PM.
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Old 1st July 2013, 09:52 PM   #592
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Quote:
Originally Posted by keantoken View Post
If that is the case, then you have unwittingly sabotaged your simulation in order to match it to real life.

You don't need to use the Hann window to see this. Set the sine start delay to 0 and make all DC blocking or bootstrapping caps 1F. The DC drift will no longer obscure the harmonics.
As I do not want to add 1F caps and other magics to fake the measurements I think it is better to skip one cycle ...
"Hann" was your Idea ...
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Old 1st July 2013, 09:59 PM   #593
astx is offline astx  Austria
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Quote:
Originally Posted by greierasul View Post
Your FFT is a little bit strange... thare are no high frecqvency energy from crossover distortion ? Can you put the open loop gain bode plot ?
Believe me - there is no energy from crossover distortion but about 10 to 15 ppm driving THD21k@200W@8R in real life ...
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Old 1st July 2013, 10:31 PM   #594
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Default Michael's criteria for goodness?

Quote:
Originally Posted by michaelkiwanuka View Post
Your naiveté and ignorance of elementary analogue electronics is simply shocking.
Michael, thank you for your erudite opinion.

If you would enlighten us unwashed masses ... I'm at a loss to understand your point of view. I note
  • you spurn all 'real life' evidence
  • your SPICE examples all show poor stability & terrible THD. If I've missed one that doesn't, please point it out.
  • every other person's circuit you 'modify' immediately takes on these characteristics that most of us try to avoid
Are there some esoteric characteristics that you feel are more important than these common measures?

Surely you are not just posting to point out that everyone else is cerebrally challenged?
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Old 1st July 2013, 11:22 PM   #595
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Quote:
Originally Posted by astx View Post
As I do not want to add 1F caps and other magics to fake the measurements I think it is better to skip one cycle ...
"Hann" was your Idea ...
Yes, Hann was my idea. I never said it was the problem.

The 1F caps don't add any magic. They won't change the harmonics. They will do absolutely nothing unless your DC operating point was solved very badly.
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Old 2nd July 2013, 02:16 AM   #596
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Quote:
Originally Posted by astx View Post
... and here the result with added 10R as explained ...
Stability betters significant but is not enough for an amplifier to go into production! There is still tendency for oscillation. (See attached picture of 20kHz square wave test using long ribbon cable. Short ribbon cable shows a bit more oscillation).

Distortion level compared to TPC/TMC not so bad but not better as predicted.

long ribbon cable:
THD21k@200W@8R
0.00186% (bw 80 kHz)
0.00229% (no bw limit)
short ribbon cable:
THD21k@200W@8R
0.00197% (bw 80 kHz)
0.00239% (no bw limit)
==> THD betters with longer cable!
Thanks for this Toni.

I can't go any further until I can sim the level of oscillation you show. I only get some of this behaviour WITH the inductance of the ribbon cable and Clean earth link to Star earth. The small measures you've tested for me show I'm on the right track but not nearly there.

My sims have both ribbon inductance AND capacitance. They show MORE oscillation and no THD change with longer ribbon so they are missing some major factor(s).

Quote:
Originally Posted by astx
Using asc file from post #278 and merging with current input/vas stage development shows:
THD20k@200W@8R with TMC 68p/470p/470R
~ 0.0006% ( only 1-2 ppm worse compared to current output stage)
Toni, are these are sim results?

Regarding the Cherry version .. as my sim results show only 2dB improvement over TMC and the measured THD for all versions is 10dB worse, there is really no point pursuing Cherry until the sim shows the wriggles on slew limiting. In theory, the main advantage of Cherry over TMC is less xover and your measurement techniques don't show any. Something else is the dominant THD mechanism.

I'm certain the issues are related and have a feeling I'm on the verge of a breakthrough in understanding big Power Amps with IPS & VAS on a separate PCB.

Again thanks for all your hard 'real life' work Toni.

PS I've made a bid for that broken PA amp so if successful, this beach bum may take you up on your spare PCBs.
_____________

For those simulating THD, I'll repeat my recommendation for Eugene Dvoskin’s Total Harmonic Analyzer from the LTspice Yahoo Group. Bear in mind the various caveats I've mentioned in previous posts. It takes out all the guesswork and fudging to get a good 'noise' floor. Close study of the code will reveal what is necessary to get sensible results eg the correct delay etc. The amp must have little LF roll-off but this is easy to arrange without mucking up the sim.

You can extend it from the default 10 harmonics to as many as you want. The harmonic plots I've shown are very useful to pinpoint the dominant THD mechanisms. And so is the residual.
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Old 2nd July 2013, 04:13 AM   #597
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Quote:
Originally Posted by Dave Zan View Post
I don't see how unconditional stability is the same as "conditional" "stability", even with quotes.
I didn't say 'unconditional' is the same as 'conditional'.

You'll have to read my 200 pg explanation I did have an Idiot's Guide[1] on this website but can't seem to find it.

Quote:
But he found a solution that was important, non-obvious and unique. Any maths book will show the importance of uniqueness as a mathematical property. ....

.... There is only a maximum feedback function. The particular network to implement that will depend on circuit values, transistor capacitances, trace inductance and capacitance and so on. If you are lucky these may result in a fairly decent feedback function with just a "Cherry" capacitor as the only added discrete component. But that would be exceptional. The extra resistor and capacitor of TMC are just a couple of extra variables to tweak the feedback function closer to optimum.
Are you referring to his continuous 9dB/8ve 45 degree Loop Gain function? You gonna do this on your amp?

Impossible to get even close to mythical Bode with 'pure Cherry' or plain Miller.

However, TPC & TMC have enough flexibility for a good approximation. I've done this with TPC in da old days[2].

Never tried evil TMC for real.
__________________

[1] Of course, 'Idiot' refers to the author and NOT to Guru Zan

[2]Also a version which replaced the Cherry cap with a 2-pole network. The TPC version worked well compared to plain Miller. But the 'pure Cherry' amps already reached the limits of my THD gear at that time so couldn't see if 'TP Cherry' was an improvement.

Might do some more work if I can find a collaborator for the 'real life' stuff.

'TP Cherry' is double differentiating (2nd order) making Enhanced VAS + EF2 (4th order) into a 2nd order system which re-enters the realms analysed by Prof Cherry. Should be more stable if done right and also help IPS THD
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Old 2nd July 2013, 09:47 AM   #598
astx is offline astx  Austria
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Quote:
Originally Posted by keantoken View Post
Yes, Hann was my idea. I never said it was the problem.

The 1F caps don't add any magic. They won't change the harmonics. They will do absolutely nothing unless your DC operating point was solved very badly.
Got it. Attached a fft picture of the second half of a 1ms sample to get DC stabilized. Now we can see a more detailed fft.
Thanks again - there is a lot to learn!
Attached Images
File Type: png fft_20k_200w_8r.png (109.6 KB, 225 views)
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Old 2nd July 2013, 09:52 AM   #599
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Quote:
Originally Posted by kgrlee View Post
...
Toni, are these are sim results?
Yes. My discrete darlington version hasn't seen the real life - maybe this summer there will be a breadboard version for testing ...

Quote:
Again thanks for all your hard 'real life' work Toni.
you are welcome!

Quote:
PS I've made a bid for that broken PA amp so if successful, this beach bum may take you up on your spare PCBs.
Simply send me your address by PM and you will get some spare PCB's for testing.
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Old 2nd July 2013, 12:19 PM   #600
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Quote:
Originally Posted by kgrlee View Post
Are you referring to his continuous 9dB/8ve 45 degree Loop Gain function?
Bode's actual solution is a bit more complex and lot more subtle than that.
9dB/8ve is just an over-simplification that has been repeated until the misconception has become accepted.
I do plan to do as close as I can to the real deal in my amp.

Quote:
Impossible to get even close to mythical Bode with 'pure Cherry' or plain Miller.

However, TPC & TMC have enough flexibility for a good approximation...
Exactly my point

Best wishes
David

Last edited by Dave Zan; 2nd July 2013 at 12:39 PM.
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