
Home  Forums  Rules  Articles  diyAudio Store  Blogs  Gallery  Wiki  Register  Donations  FAQ  Calendar  Search  Today's Posts  Mark Forums Read  Search 
Solid State Talk all about solid state amplification. 

Please consider donating to help us continue to serve you.
Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving 

Thread Tools  Search this Thread 
30th November 2014, 05:36 PM  #1 
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. 
30th November 2014, 08:06 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, 08:29 PM  #3 
On Hiatus

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 
diyAudio Moderator

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  BA3 Amp  BA3 Preamp  F6 AmpCampAmp 
30th November 2014, 09:46 PM  #5  
diyAudio Member
Join Date: Mar 2007

Quote:
Type 7770  FFT Analysis  Type 7770  Brüel & Kjær f. 

30th November 2014, 09:47 PM  #6 
diyAudio Moderator

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  BA3 Amp  BA3 Preamp  F6 AmpCampAmp 
30th November 2014, 10:31 PM  #7 
On Hiatus

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." 
30th November 2014, 10:34 PM  #8  
Banned
Join Date: Feb 2003
Location: Jakarta

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, 10:42 PM  #9  
diyAudio Moderator

Quote:
Simulation, although useful, ain't the real thing.
__________________
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  BA3 Amp  BA3 Preamp  F6 AmpCampAmp 

30th November 2014, 10:53 PM  #10 
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 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
__________________
If you can't explain it simply, you don't understand it well enough (c) Albert Einstein VIRTUAL ZERO distortion AUDIO 
Thread Tools  Search this Thread 


Similar Threads  
Thread  Thread Starter  Forum  Replies  Last Post 
comparing various Alpair's  Bob Brines  Full Range  49  15th August 2017 04:35 AM 
Comparing drivers  ziocalepino  Full Range  13  17th March 2011 08:17 AM 
Comparing  Bose(o)  Chip Amps  5  29th July 2005 09:19 PM 
Comparing sensitivities  Grahamt  MultiWay  2  20th September 2004 06:42 PM 
Comparing GC's  yuval  Chip Amps  20  10th June 2004 05:26 PM 
New To Site?  Need Help? 