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6th July 2012, 01:26 AM  #1 
diyAudio Member
Join Date: Sep 2009
Location: Santa Fe, Argentina

Cathode Follower, where is the 3d H gone?
Hi gents
I was playin on LTSpice around the idea of a tube line amplifier, capable of driving a solid state amp trough some long cables, While working at it, I found that this one, not optimally biased, exhibits an FFT behaviour that seems to me, somewhat curious, It is a simulation error, or such a circuit can perform that way in the real world? 
6th July 2012, 04:03 AM  #2 
diyAudio Member
Join Date: Aug 2009
Location: Johnson City, TN

I've seen harmonic cancellation in multistage amps before. They simulate and measure nearly the same.
Harmonic content varies with bias point. If you get two stages biased at the optimal (but possibly different) points they may do considerable harmonic cancellation. However, it usually is very configuration specific. 
6th July 2012, 05:55 AM  #3 
diyAudio Member
Join Date: May 2007

Hey M,
Error due to to short time! .tran 0 100m 60m 1u This is what you should expect when doing a proper FFT: Last edited by revintage; 6th July 2012 at 05:58 AM. 
6th July 2012, 06:49 AM  #4 
diyAudio Member
Join Date: Apr 2008

indeed. do not absolutely trust the LT FFT analysis. it's a great tool, but toying with timesteps, simulation time, compression and other options can give wildly different graphs for the same circuit. Always measure realworld performance and if there should be a 3rd harmonic, there will be.

6th July 2012, 11:48 AM  #5 
diyAudio Member
Join Date: May 2007

A 'negative' amount of 3rd should have warned you that the FFT was in trouble. FFT does lots of calculations on samples of your waveform. Small numbers coming out of the FFT are the result of lots of cancellations of big numbers, so more likely to be be meaningless than bigger numbers (e.g. fundamental) if you have insufficient data, wrong windowing etc.

6th July 2012, 01:23 PM  #6 
diyAudio Member
Join Date: May 2007

Hey,
Donīt complicate this to much, its only a matter of knowing how to use a program. Mosquito didnīt know how to use the LTSpice FFT, thats all. With the .tran commando I gave, it will work like intended. 
6th July 2012, 01:37 PM  #7 
diyAudio Member
Join Date: May 2007

Simply following your advice may make it work, but he still won't know why it makes it work. I generally find understanding is a useful thing. I deliberately kept my comments very simple  did I mention orthonormal basis sets for innerproduct spaces?

6th July 2012, 02:26 PM  #8 
diyAudio Member
Join Date: Aug 2010
Location: London,UK

in infinite dimensions?

6th July 2012, 02:35 PM  #9 
diyAudio Member
Join Date: May 2007

Finite dimensions for the FFT. Infinite for a real Fourier analysis.

6th July 2012, 03:35 PM  #10 
diyAudio Member
Join Date: Sep 2009
Location: Santa Fe, Argentina

Hi again
Many thanks You all for the sharp comments. In fact looking at the first image , the small ammount of negative 3dH must be warned me about somethig gone wrong. Lesson learned. LTspice , and particulary the FFT analysis, are powerfull tools, but if the user isn't completely aware about what he is doing, it's somewhat like give a razor blade to a monkey... :)) Again thanks Lars, DF96 and the others for the useful info and tips. I'm here just to learn. Last edited by Mosquito; 6th July 2012 at 03:37 PM. 
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