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19th May 2011, 07:56 AM  #1 
diyAudio Member
Join Date: Jul 2005
Location: D55629 Schwarzerden

Intermodulation Distortion Values below THD Values by SS Amps  is this possible ??
In most cases the mentioned IM values are above the THD values. But there are amps, where this is reversed, e. g. by the Altec 9444B, go to
Altec Lansing Professional Amplifier 9444B Specification List page 6 Is this possible in general, and if yes, what means this for the audible perception by listening tests? So far I have never noticed this detail. 
19th May 2011, 09:03 AM  #2 
diyAudio Member
Join Date: Sep 2006

It is not only possible, but even very likely.
Some examples are discussed here: http://www.diyaudio.com/forums/equip...ml#post2559836 And you can see that for a severe case of crossover distortion, the IMD is practically nonexistent. At least the first order IMD, which is nearly always what is specified. 
19th May 2011, 10:00 AM  #3 
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Join Date: May 2007

Intermodulation and harmonics are both created by nonlinearity, so if other things are equal they will vary together. Other things are not always equal. By definition, harmonics are at a higher frequency than the input signal so may be affected differently by feedback, as in SS amps this reduces at higher frequencies. So there is a mechanism for harmonics to be greater than IM.
On the other hand, problems with local decoupling can mean that secondorder distortion products at low frequencies can feed back in to produce thirdorder IM (even if thirdorder distortion is otherwise low). So IM can be greater than harmonics. 
19th May 2011, 12:26 PM  #4 
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Hi,
the IMD level is several times higher than the harmonic distortion, in particular at high frequencies. 
19th May 2011, 12:48 PM  #5 
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Join Date: May 2007

No. Take an amplifier with a flat frequency response, and some secondorder nonlinearity. Apply a signal at frequency f1  you get some DC shift and the second harmonic at 2f1. Now apply a signal at the same level but frequency f2  again you get DC and 2f2. Now apply both signals together. You get twice the DC shift, both harmonics at the same level as before, and intermodulation at f1f2 and f1+f2 at the same level as the harmonics. Why is it the same level? Because it is created by exactly the same process: multiplication of trigonometric functions. f1f2 corresponds to the DC shift, and f1+f2 corresponds to the second harmonic.

19th May 2011, 04:26 PM  #6  
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Location: D55629 Schwarzerden

Quote:


19th May 2011, 04:52 PM  #7 
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Join Date: Oct 2010

DF96,
unfortunately yes; the issue is a bit more complicated than in your presentation. A (more advanced) mathematical approach would certainly be interesting to show how the number of intermodulation products vastly exceeds the number of harmonic distortion products, the disproportionality rapidly growing with nonlinearity. 
19th May 2011, 05:10 PM  #8 
diyAudio Member
Join Date: Sep 2006

To go further, here is another example: an underbiased class B amplifier.
Nothing special, nothing exotic, just a very average, very common type of nonlinearity. You can see that the third harmonic is 42dB down (THD is 1.32%). Now, the IMD test: the first order IM product between the 10KHz and 11KHz is 55dB down. Of course, if you take into account higher order products, the comparison is more balanced, but when an IMD figure is given, it applies to one product, not the entire spectrum. The THD figure, as its name implies, applies to the whole spectrum. 
19th May 2011, 06:22 PM  #9  
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Quote:


19th May 2011, 07:08 PM  #10 
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Join Date: Oct 2010

No revising. I could have used my preferred term: energy of distortion product.

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