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30th November 2014, 06:36 PM  #1 
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Join Date: Feb 2003
Location: Jakarta

Comparing FFT of Two Amplifiers
First, we cannot judge an amp from FFT alone (in fact, I don't want to tell detailed differences from other criteria). But still, I want to hear what you can see in an FFT alone (i.e. which one do you prefer: A or B).
So no more information other than the image showing FFT of two amplifiers. Each with 1W, 5W and 40W FFT. I haven't built the two amplifiers, but is in the soldering process for amp B and drawing and transistor matching process for amp A. 
30th November 2014, 09:06 PM  #2 
diyAudio Member
Join Date: Mar 2007

FFT spectrum should be presented with linear frequency scale due to linear sampling in frequency domain. Regardless, the harmonics are better visible in linear scale. When you show FFT spectrum, the parameters of analysis must be specified (number of samples in record, window function, etc.). The data outside dynamic range of analysis aren't valid.
Regards, f. 
30th November 2014, 09:29 PM  #3 
diyAudio Moderator

You do understand that "FFT" is a mathematical operation, not a measurement technique? I think what you mean here is "single frequency distortion spectrum."
You have vastly too little information to make any sort of intelligent judgment. Frequency response, distortion, noise, source impedance, stability. That's your basic measurement set, and yes, you can use FT techniques to measure all of them, once you actually have some real amps to compare.
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30th November 2014, 09:57 PM  #4 
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I agree with SY  build your amps and measure them equally, then present the FFT for viewing. Until then, it's just pretty pictures.
Also, if you haven't built the amps, where did you get the FFT that you posted?
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30th November 2014, 10:46 PM  #5  
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Quote:
Type 7770  FFT Analysis  Type 7770  Brüel & Kjær f. 

30th November 2014, 10:47 PM  #6 
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A fast Fourier transform (FFT) is an algorithm to compute the discrete Fourier transform (DFT) and its inverse. Fourier analysis converts time (or space) to frequency and vice versa; an FFT rapidly computes such transformations by factorizing the DFT matrix into a product of sparse (mostly zero) factors.[1] As a result, fast Fourier transforms are widely used for many applications in engineering, science, and mathematics. The basic ideas were popularized in 1965, but some FFTs had been previously known as early as 1805. Fast Fourier transforms have been described as "the most important numerical algorithm [s] of our lifetime".[2]
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30th November 2014, 11:31 PM  #7 
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Yes, exactly. You are conflating different concepts an FT is a mathematical transformation of data gathered in a measurement no matter what the measurement. FFT is a particular algorithm used for the calculation.
@Jay: Build two amps, generate frequency response, distortion spectra at various frequencies and power levels, source impedance versus frequency (at different levels if you're dealing with a poor design), noise, and stability. Use FT techniques on the data if you like. Try all this with a few different loads, including speakers. Then you can start talking about making valid comparisons.
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30th November 2014, 11:34 PM  #8  
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Quote:
Of course, I know you are right that technically the information is insufficient and that there's a necessity to build both amps (which I will). It's like I'm sure both will work (which it might not ) and I'm guessing their quality from FFT alone to choose which one to build (I regard other criteria, even tho different for both amps, are equally acceptable). Quote:
Quote:
Quote:
Assuming that parts and power supply will be cost no object (the amps were simulated without the PS), I believe ampB will have more details. But look at second order harmonic domination (and absence of higher order harmonics) in ampA for example. In my experience, such simulated performance is always telling the truth. 

30th November 2014, 11:42 PM  #9  
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Quote:
Simulation, although useful, ain't the real thing.
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30th November 2014, 11:53 PM  #10 
diyAudio Member

Hi Jay, the "A" spectrum looks more like the one for VFA, the "B" spectrum looks like CFA one. In general, the less harmonic components are visible  the better. Also, having 3rd harmonic significantly lower than the 2nd one, and then just a little bit of 4th and that's it  a very good profile. The "B" spectrum looks much more "nervous", but it doesn't automatically mean the amp "B" is bad. It may have other benefits  better slew rate, square wave and phase responses...
Cheers, Valery
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