Go Back   Home > Forums > >
Home Forums Rules Articles diyAudio Store Blogs Gallery Wiki Register Donations FAQ Calendar Search Today's Posts Mark Forums Read

Solid State Talk all about solid state amplification.

Intermodulation Distortion Values below THD Values by SS Amps - is this possible ??
Intermodulation Distortion Values below THD Values by SS Amps - is this possible ??
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
Reply
 
Thread Tools Search this Thread
Old 19th May 2011, 07:56 AM   #1
tiefbassuebertr is offline tiefbassuebertr  Germany
diyAudio Member
 
tiefbassuebertr's Avatar
 
Join Date: Jul 2005
Location: D-55629 Schwarzerden
Default 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.
  Reply With Quote
Old 19th May 2011, 09:03 AM   #2
Elvee is offline Elvee  Belgium
diyAudio Member
 
Elvee's Avatar
 
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 non-existent.

At least the first order IMD, which is nearly always what is specified.
  Reply With Quote
Old 19th May 2011, 10:00 AM   #3
DF96 is offline DF96  England
diyAudio Member
 
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 second-order distortion products at low frequencies can feed back in to produce third-order IM (even if third-order distortion is otherwise low). So IM can be greater than harmonics.
  Reply With Quote
Old 19th May 2011, 12:26 PM   #4
WuYit is offline WuYit  Sweden
Banned
 
Join Date: Oct 2010
Hi,
the IMD level is several times higher than the harmonic distortion, in particular at high frequencies.
  Reply With Quote
Old 19th May 2011, 12:48 PM   #5
DF96 is offline DF96  England
diyAudio Member
 
Join Date: May 2007
No. Take an amplifier with a flat frequency response, and some second-order 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 f1-f2 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. f1-f2 corresponds to the DC shift, and f1+f2 corresponds to the second harmonic.
  Reply With Quote
Old 19th May 2011, 04:26 PM   #6
tiefbassuebertr is offline tiefbassuebertr  Germany
diyAudio Member
 
tiefbassuebertr's Avatar
 
Join Date: Jul 2005
Location: D-55629 Schwarzerden
Quote:
Originally Posted by Elvee View Post
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 non-existent.
At least the first order IMD, which is nearly always what is specified.
Thank you for this advice. This means for me, that to a certainly THD spectrum there is an associated intermodulation distortion spectrum.
  Reply With Quote
Old 19th May 2011, 04:52 PM   #7
WuYit is offline WuYit  Sweden
Banned
 
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.
  Reply With Quote
Old 19th May 2011, 05:10 PM   #8
Elvee is offline Elvee  Belgium
diyAudio Member
 
Elvee's Avatar
 
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 non-linearity.

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.
Attached Images
File Type: gif AmpTHD.gif (48.1 KB, 251 views)
File Type: gif AmpIMD.gif (52.1 KB, 244 views)
  Reply With Quote
Old 19th May 2011, 06:22 PM   #9
DF96 is offline DF96  England
diyAudio Member
 
Join Date: May 2007
Quote:
Originally Posted by WuYit
the number of intermodulation products vastly exceeds the number of harmonic distortion products
Only when the input signal includes lots of separate signals. This is just a matter of combinatorics. Your original claim was that the level of IM was greater. Are you now revising that to the number of IM?
  Reply With Quote
Old 19th May 2011, 07:08 PM   #10
WuYit is offline WuYit  Sweden
Banned
 
Join Date: Oct 2010
No revising. I could have used my preferred term: energy of distortion product.
  Reply With Quote

Reply


Intermodulation Distortion Values below THD Values by SS Amps - is this possible ??Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump

Similar Threads
Thread Thread Starter Forum Replies Last Post
Aliasing Intermodulation Distortion and filterless DACs halcyon Digital Line Level 70 2nd September 2017 09:03 PM
Help with RC values davidlzimmer Solid State 3 6th November 2006 08:01 PM
values taloyd Chip Amps 2 30th May 2004 12:47 AM
Help with R values Burnedfingers Tubes / Valves 0 24th February 2003 11:03 AM
THD, Intermodulation, S/N Diogo Solid State 13 4th September 2002 12:29 PM


New To Site? Need Help?

All times are GMT. The time now is 01:39 PM.


Search Engine Optimisation provided by DragonByte SEO (Pro) - vBulletin Mods & Addons Copyright © 2018 DragonByte Technologies Ltd.
Resources saved on this page: MySQL 14.29%
vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2018 DragonByte Technologies Ltd.
Copyright ©1999-2018 diyAudio
Wiki