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Old 25th January 2004, 12:19 PM   #21
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Quote:
Originally posted by pjacobi

SPICE trivia:
Building upon advice from the LTSpice mailing list, I used following Parameters and was able to resolve down to -160dB with less than 2 minutes simulation run time.

V3 N011 0 SINE(0 5 9k765625) AC 1
.tran 0 5.12m 1.024m 0.015625u
.fourier 9k765625 V(out)

The parameters for the graphical FFT were
262144 points
Hann window
5 points binomial smoothing
Yes, I know one can fine-tune the parameters to improve things,
but I don't understand FFT well enough to figure out how and
haven't bothered. Of course, if one can get such big savings in
run time, it is definitely worth it.

However, I tried your suggested settings above and get nowhere
close to the the performance you indicate, except in run time.
I get approx. -60dB 3rd order distorsion on the source signal!!!
The other odd harmonics also stick out and the general noise
floor is around -80dB.

BTW, be careful with notation like 9k765625 above. It seems that
LTSpice take it, but it is not Spice standard AFAIK. Normally I think
everything after the k is discarded, but maybe that is only for
letters? I might be wrong, though, and it is only letters that are
discarded, not digits.
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Old 25th January 2004, 12:41 PM   #22
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Christer, Peter, have you come to any conclusion? Maybe you should start making a prototype One good thing has come out of this and this is making narrow peaks in the fft chart. This I must test.

Note also guys that you have noise in real life.... -160 dB...!
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Old 25th January 2004, 01:39 PM   #23
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Per-Anders,

I don't have the equipemnt to measure distorsion on these levels,
so building a prototype for the purpose of measuring is a bit
pointless for me, I think. Bulding for listening is another thing, though.

Don't bother too much about the very low distorsion figures. We
don't know how much they say about the distorsion in a real
circuit. However, I do think that comparative simulations like these
may say something about the relative behaviour of the circuits.
If circuit A gives a clearly lower distorsion than circuit B in a simulation,
I think it has a good chance of doing that in reality too. A lot of
other factors will enter, not the least noise as you mentioned.
However, I think the main reason for distorsion is still the intrinsic
exponential characteristic of BJTs, so I think/hope/assume/... that
simulations with idealized BJT models can tell something about the
relative merits of topologies. Note however, that my simulations
are only about distorsion so far. There are other important factors
such as phase and frequency response to consider for real
circuits, as you very well know. That is not unrelated, but brings
in more parameters so it should be kept as a separate issue,
I think.

As for conlcusions, I think that is too early. More simulations would
be useful and I am also trying to do some theoretical thinking
about it, with pen and paper, or lying on my back on the sofa.
I would also guess there are already ccomparative studies
between CFPs and followers in the literature that may already
have (some of) the answers. However, to quote Nobel prize
winner Anthony Leggett, "that is no reason not to do it yourself
because you always learn something from doing it".
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Old 25th January 2004, 01:47 PM   #24
pjacobi is offline pjacobi  Germany
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Hi Per-Anders,

Quote:
Originally posted by peranders
Christer, Peter, have you come to any conclusion? Maybe you should start making a prototype One good thing has come out of this and this is making narrow peaks in the fft chart. This I must test.

Note also guys that you have noise in real life.... -160 dB...!
Yup, noise and everything will ruin the best theoretical numbers, but it doesn't hurt to get THD below 0.0003% (110dB) for a starting point.

The problem with actually building these suckers are

a) all semiconductors quickly run away and hide when I switch on the soldering iron (and right they are)

b) I'm lacking 90% of the necessary measuring equipment to let the prototypes tell me anything (but I may join forces with a more practically oriented friend of mine, real soon now)

Regards,
Peter Jacobi
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Old 25th January 2004, 01:56 PM   #25
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Quote:
Originally posted by pjacobi

a) all semiconductors quickly run away and hide when I switch on the soldering iron (and right they are)
Are you sure you can tell the difference between a cockroach and
an op amp?
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Old 25th January 2004, 02:08 PM   #26
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Quote:
Originally posted by pjacobi
b) I'm lacking 90% of the necessary measuring equipment to let the prototypes tell me anything (but I may join forces with a more practically oriented friend of mine, real soon now)
.... you could also settle with using your ears

You could use a sound card and Rightmark FFT, then you can get a picture of the performance better or worse than the soundcard.

http://audio.rightmark.org
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Old 25th January 2004, 02:18 PM   #27
pjacobi is offline pjacobi  Germany
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Default SPICE trivia

Hi Christer,

Thanks for the warning about the non-standard notation, I assume you've changed it before your test run.

Quote:
Originally posted by Christer
[...]
However, I tried your suggested settings above and get nowhere
close to the the performance you indicate, except in run time.
I get approx. -60dB 3rd order distorsion on the source signal!!!
The other odd harmonics also stick out and the general noise
floor is around -80dB.[...]
What flavour of SPICE are you running? Do you have the standard .fourier in addition to the graphical output you've shown? Any difference between these two?

As the set of parameter worked on .fourier, which is handled by the SPICE core itself, I did assume it will work for every SPICE which allows .fourier.

Perhaps some parameter tweaking (switchoff data compression, keep more digits etc) is necessary.

I didn't see THD on the input at all:
http://www.linearaudio.de/scratch/CFP.txt
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Old 25th January 2004, 03:00 PM   #28
PMA is offline PMA  Europe
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I have been running the power amp output stage based on similar topology shown in Christer's post #3 for more than year and a half. Made enough measurements and comparative listenning tests. This circuit enables to reach very low THD and IMD in real life and is sonically superb.
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Old 25th January 2004, 04:49 PM   #29
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Nice work Christer and pjacobi! A very interesting thread.

Quote:
Originally posted by ojg
(...)In the FFT window try switching between "Using extent of simulation data" and "Specify time range". In the text-box change the 4.999998ms to 5.0ms (numbers for example only, I think you see what I mean).
LTSpice seems to automatically choose the wrong stop time in the FFT dialog most, but not all of the time. I've seen very big improvements in the FFT residual from making the seemingly small change mentioned above by ojg. One strange quirk - if you choose Gear integration instead of trapezoid (just as a test, not generally recommended), LTSpice seems to get the stop time right every time. Weird. Another weird thing LTSpice does is automatically enable compression, which needs to be off to minimize the FFT residual. So I find that I'll get everything working right with a low residual, then quit LTSpice and start it again later. Then I always forget to go back into the control panel and disable compression again. I'll get a poor residual until I disable it.

Christer, I think the change in residual you're seeing with different topologies may be due to the time step issue. Based on info from the LTSpice mailing list, a good approach seems to be taking the number of cycles you're doing the FFT over, multiplying by the period, dividing by the number of FFT points and specifying this value as the minimum time step. If this number ends up being smaller than what SPICE would compute automatically for the time step, then the actual time steps used will be the same as what's needed for the FFT. If it's not, LTSpice needs to interpolate to get the FFT time points. This can be one reason for the poor residual.
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Old 25th January 2004, 08:09 PM   #30
andy_c is offline andy_c  United States
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Quote:
Originally posted by andy_c

LTSpice seems to automatically choose the wrong stop time in the FFT dialog most, but not all of the time.
Well, updating to the latest version of LTSpice (2.08m) seems to have fixed the problem with the incorrect stop time in the FFT dialog. I also tried the .FOURIER directive, and it worked great on Christer's circuit. Nice to have it calculate THD without having to do this manually! So I tried the .FOURIER command with my power amp simulation, and the results were 30-40 dB worse than my spectrum plots. I tried changing the time steps but wasn't able to improve this. It looks like care is required with .FOURIER to make sure it agrees with the spectral plots.

With the transient simulation set up as follows:

SINE(0 5 10k 0 0 0 12)
(12 cycles)
.tran 0 1.2e-3 0 3.051757813e-8
FFT start time = 7e-4
FFT stop time = 1.2e-3
Number of FFT points = 16384
(The time step is 5 cycles/16384)

I get a residual that's about 180 dB below the fundamental with a simulation time of about 3 seconds in Christer's circuit, using a 900 MHz Athlon. For the circuit without the CFP, I get
3rd harmonic = -79 dBc
5th harmonic = -105 dBc
7th harmonic = -131 dBc

same as Christer got.
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