Comparing FFT of Two Amplifiers
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Jay
Banned

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.
Attached Images
 CompareNewAmps.png (40.2 KB, 267 views)

 30th November 2014, 08:06 PM #2 fonon   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, 08:29 PM #3 SY   On Hiatus     Join Date: Oct 2002 Location: Chicagoland 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. __________________ "You tell me whar a man gits his corn pone, en I'll tell you what his 'pinions is."
 30th November 2014, 08:57 PM #4 6L6   diyAudio Moderator     Join Date: Oct 2010 Location: Denver, Colorado 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? __________________ Illustrated Build Guides - Pass F4 - Pass B1 - Pass F5 - Pearl 2 - O2 HPA - Hagerman Bugle 2 - Pass Aleph J - Me at BA '13 -Pass F5Turbo - BA-3 Amp - BA-3 Preamp - F6 AmpCampAmp
fonon
diyAudio Member

Join Date: Mar 2007
Quote:
 Originally Posted by SY You do understand that "FFT" is a mathematical operation, not a measurement technique?
Not exactly. FFT is mathematical operation used as measurement technique. It's a kind of transformational methods of spectral analysis (in technical classification).
Type 7770 - FFT Analysis - Type 7770 - Brüel & Kjær
f.

 30th November 2014, 09:47 PM #6 6L6   diyAudio Moderator     Join Date: Oct 2010 Location: Denver, Colorado 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] __________________ Illustrated Build Guides - Pass F4 - Pass B1 - Pass F5 - Pearl 2 - O2 HPA - Hagerman Bugle 2 - Pass Aleph J - Me at BA '13 -Pass F5Turbo - BA-3 Amp - BA-3 Preamp - F6 AmpCampAmp
SY
On Hiatus

Join Date: Oct 2002
Location: Chicagoland
Quote:
 Originally Posted by fonon Not exactly.
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.
__________________
"You tell me whar a man gits his corn pone, en I'll tell you what his 'pinions is."

Jay
Banned

Join Date: Feb 2003
Location: Jakarta
Quote:
 Originally Posted by SY 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.
I guess the more information doesn't mean the more informed decision? For example if there were 10 criteria, amp-A excels in 4 criteria and amp-B excels in 6 criteria, doesn't mean that amp-B is better.

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:
 Originally Posted by 6L6 I agree with SY - build your amps and measure them equally, then present the FFT for viewing.
If I have built the amps, I will have no questions as I know how they both sound.

Quote:
 Originally Posted by 6L6 Also, if you haven't built the amps, where did you get the FFT that you posted?
Simulation of course?

Quote:
 Originally Posted by fonon 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.
Is it not visible (the harmonics) in this windowing? The "exact numbers" are not valid, I know, but subjective comparison is I believe valid.

Assuming that parts and power supply will be cost no object (the amps were simulated without the PS), I believe amp-B will have more details. But look at second order harmonic domination (and absence of higher order harmonics) in amp-A for example. In my experience, such simulated performance is always telling the truth.

6L6
diyAudio Moderator

Join Date: Oct 2010
Quote:
 Originally Posted by Jay Simulation of course? ... In my experience, such simulated performance is always telling the truth.
I am a Professional Learjet Captian by trade. I can tell you all about simulation. (And the fact that the sims cost usually 3X the price of the airplane...)

Simulation, although useful, ain't the real thing.

 30th November 2014, 10:53 PM #10 vzaichenko   diyAudio Member     Join Date: Jan 2010 Location: Moscow, Russia 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 3-rd harmonic significantly lower than the 2-nd one, and then just a little bit of 4-th 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 __________________ If you can't explain it simply, you don't understand it well enough (c) Albert Einstein VIRTUAL ZERO distortion AUDIO

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