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3rd March 2012, 04:46 PM  #1111 
diyAudio Member
Join Date: Dec 2009
Location: Md

slow post, thanks.

3rd March 2012, 04:50 PM  #1112  
diyAudio Member
Join Date: Jul 2007
Location: Central Berlin, Germany

Quote:
Consider the cap replaced with a short, for the moment. Then this sim will have distortion levels at 1e15 (300dB), using the .four command, not the FFT in the graph display. This comes from the small time step (10,000 points per period, derationalized with the Pi/3.14thingie which helps when you want to analyse multiple periods) and the double precision option, and most of all from the directive plotwinsize=0, which tells the program to turn OFF waveform compression (which is lossy). Then put the cap back in. Distortion will skyrocket inspite of the perfect oscillator... because the circuit has not settled from the wideband input (down to DC) that a suddenly started sine intrinsicly shows. It gets even worse when you add a deadtime to the generator. One can use generators circuits that fade in with a raised cosine envelope which reduces settling time quite a bit. 

3rd March 2012, 04:52 PM  #1113 
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Join Date: Feb 2003
Location: ..

I just use Blackman, need 4x longer fft length to accomodate the broadened line, but gives very good spectral spreading noise suppression floor
I only use cursor diff values, any phase measures made relative to the source fundamental 
3rd March 2012, 11:05 PM  #1114 
diyAudio Member

Here is a .asc file with every performance tweak I've found so far (well, not quite all of them, but all of the functional ones). I use this param set for every amplifier I simulate. This will let you tweak simulation speed without causing problems with sample aliasing and compression.

3rd March 2012, 11:10 PM  #1115 
diyAudio Member

WRT the FFT window.
If you open up the preview for the window you choose, you'll see there are many different types. The sinelike ones are the most natural and intuitive. It is important that the tail ends of the window end at 0, or your noise floor will be artificially heightened. The window should average to a straight line if it is placed in a series of windows. I believe I'm correct here but I may be wrong. Playing around with the FFT windows in Baudline (only available for Linux unfortunately) gives you a good idea of what the FFT window does to the spectrogram. A narrower FFT window seems to decrease the Hz resolution of the FFT, so perhaps a good audio window would be Kaiser, adjusted so the spikes are exactly 1 octave wide, so they don't overlap. But I haven't studied this in detail so I'm not sure what the sideeffects would be. I know MikeB seemed to prefer the Kaiser window. EDIT: I'm pretty sure the .four parameter, which automatically calculates THD, uses the rectangular window, which is why it will read a ridiculous THD on a simulation where the FFT shows very low THD (provided you used a good window). It would be nice if there were a way to change the FFT window. Maybe someone has come up with a DIY alternate feature that uses the windowed FFT values.
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The Kmultiplier rail filter! = The Simple Kuartlotron Superbuffer! LTSpice wiki with special attention to new users' troubles Last edited by keantoken; 3rd March 2012 at 11:17 PM. 
4th March 2012, 11:04 AM  #1116 
diyAudio Member
Join Date: Dec 2009
Location: Md

SUPER. Much thanks. I also found some notes and a plug in over on AudioPerfection.com Lots to study. All this as a side track when trying to understand why my Rotels sound better than my other amps. Amp design became fascinating. I have a renewed appreciation for analog design. Everything is a catch22 at the levels mere consumer grade equipment works at. .000something. Not long ago measuring in that range was college physics lab domain. Add the craft and the problem of what measures well does not automatically sound good.
I remember a lecture by TI on digital measurement maybe 30 years ago, but needless to say, the details escape me. In SoundEasy, he recommends a rectangular window. My other freeware analyzers all recommend Blackman. Sometimes the results look suspiciously too good. I am used to just being able to set the sample rate and size, where that is a bit confusing in LTSpice. I had not thought of predefining several signal sources. Much easier than retyping it all every time. Rookie. 
8th March 2012, 02:43 PM  #1117 
diyAudio Member

I just posted some stuff about FFT windowing that probably pertains more to this thread. New users are often confused about FFT windowing, so I think it's important to make it clear. Most people will probably be very happy with the Hann window, but like jcx says, for a lower noise floor Blackman can be used with a larger number of samples.
LTSpice FFT simulation settings and inconsistent results. 
23rd March 2012, 09:56 PM  #1118 
diyAudio Member
Join Date: Dec 2009
Location: Md

Very confused on baseline noise. If I just have a voltage source, 1K resistor back to ground. Run 1K, 1000 cycles through it. Look at the FFT. I have a noise floor at about 80dB and quite a bit of distortion. This implies the sine generator is pretty poor. So poor, I can't really tell much about my simulation as it is better than the generator. Am I doing something very wrong?

23rd March 2012, 11:48 PM  #1119 
diyAudio Member
Join Date: Dec 2009
Location: Md

OK, I FINALLY got the configuration keantoken recommended to behave, so I do get a "perfect" sine wave input. Not, this is my output. No window, 3 point adv default.

23rd March 2012, 11:48 PM  #1120 
diyAudio Member
Join Date: Dec 2009
Location: Md

BTW, this is a modified Hafler DH120 power amp model.

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