Distortion - diyAudio
Go Back   Home > Forums > General Interest > Everything Else

Everything Else Anything related to audio / video / electronics etc) BUT remember- we have many new forums where your thread may now fit! .... Parts, Equipment & Tools, Construction Tips, Software Tools......

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 10th March 2011, 12:12 AM   #1
diyAudio Member
 
Join Date: Nov 2008
Default Distortion

I can not get my head around this:

If you have 2 identical amps fed the same signal, each produces 10% of THD (never mind any other artifacts). What is the (theoretical) THD if these amps are in series?

E
  Reply With Quote
Old 10th March 2011, 12:30 AM   #2
diyAudio Member
 
abraxalito's Avatar
 
Join Date: Sep 2007
Location: Hangzhou - Marco Polo's 'most beautiful city'. 700yrs is a long time though...
Blog Entries: 109
Send a message via MSN to abraxalito Send a message via Yahoo to abraxalito Send a message via Skype™ to abraxalito
Haven't done the theory so I'll take a wild guess - 14%?
__________________
Seek not the favour of the multitude...rather the testimony of few. And number not voices, but weigh them. - Kant
The capacity for impartial observation is commonly called 'cynicism' by those who lack it.
  Reply With Quote
Old 10th March 2011, 12:33 AM   #3
diyAudio Member
 
Join Date: Sep 2004
Location: Napier, Hawkes Bay
THD will be 21%

(1.1)^2 = 1.21
  Reply With Quote
Old 10th March 2011, 09:14 AM   #4
Elvee is offline Elvee  Belgium
diyAudio Member
 
Elvee's Avatar
 
Join Date: Sep 2006
I don't think the problem is as simple and straightforward as it looks.
Let us take some examples:

The amplifiers have a gain of -1, and the distortion is small enough (let's say 1%) to render higher intermodulation products negligible.
In this case, the second amplifier will add harmonics in antiphase with that of the first, and the final result will be ~0%.

Another example: the amplifiers have a gain of 1, and the distortion consist purely of clipping.
In this case, once the signal has been clipped, it will pass unchanged through the other stage.

Etc etc

I doubt it is possible to find a correct generalization.
  Reply With Quote
Old 10th March 2011, 10:57 AM   #5
knutn is offline knutn  Norway
diyAudio Member
 
Join Date: Mar 2006
Location: Oslo
It is very complex. However you can calculate the total distortion mathematically, if you have time enough....

The simplest example is an amplifier with purely second harmonic distortion. This is input to the next amplifier (also with purely second harmonic distortion). The output is the wanted squared gain (first harmonic), a second harmonic (also a function of the gain in the amplifier), and a third and fourth harmonic (both gain dependent, in different ways). In other words: it is a nonlinear function, and you cannot predict the distortion in a general manner without knowing the spectral properties of the distortion.
  Reply With Quote
Old 10th March 2011, 11:56 AM   #6
DF96 is offline DF96  England
diyAudio Member
 
Join Date: May 2007
THD alone does not give you enough information. However, you could make an estimate, given certain vaguely reasonable assumptions.

Assume the THD is pure second harmonic. Then the two together would give either 20% or 0% second harmonic, depending on whether the amplifer inverted or not. You would also get 1% third harmonic and 0.1% fourth harmonic in either case.

This shows the limitations of distortion cancellation. It always introduces higher order distortion, so better to design for low distortion in each stage.

Last edited by DF96; 10th March 2011 at 11:57 AM. Reason: cancellation limits
  Reply With Quote
Old 10th March 2011, 12:55 PM   #7
knutn is offline knutn  Norway
diyAudio Member
 
Join Date: Mar 2006
Location: Oslo
As an example: Here is a 2SK170 FET amplifier fed with 5mV (peak). The output is scaled down to feed a new 2SK170 FET amplifier with about the same amplitude. As you can see, the distortion of the last amplifier is much lower due to cancellation. But remember that in reality, the second amplifier normally would have the output signal of the first amplifier as an input signal....
Attached Files
File Type: txt thd.txt (2.0 KB, 10 views)
  Reply With Quote

Reply


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
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
How much distortion is too much? dead rat Class D 4 3rd December 2008 08:42 AM
Distortion. bigwill Tubes / Valves 8 14th July 2007 11:47 PM
Non Linear Distortion testing - Harmonic Distortion JMB Multi-Way 0 20th July 2004 03:54 PM
What causes distortion? matthias Parts 1 5th June 2004 05:47 AM
The look of distortion. Joel Tubes / Valves 3 26th June 2003 11:51 PM


New To Site? Need Help?

All times are GMT. The time now is 08:53 AM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2