output LR filter - diyAudio
Go Back   Home > Forums > Amplifiers > Solid State

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
Reply
 
Thread Tools Search this Thread
Old 8th February 2002, 08:28 AM   #1
Helix is offline Helix  United Kingdom
diyAudio Member
 
Helix's Avatar
 
Join Date: Jul 2001
Location: UK
Default output LR filter

Most of the class AB amps i have built seem to have a parallel L R output filter. Say a inductor of 10 turns and a 1R resistor. i was wondering if any one could tell me :
a) Why is it there?
b) Can one recalculate the turnover freq, and make the inductor smaller?
  Reply With Quote
Old 8th February 2002, 08:58 AM   #2
Geoff is offline Geoff  United Kingdom
diyAudio Member
 
Join Date: Apr 2001
Location: UK
Helix

The output inductor is fitted to ensure that amplifier stability is maintained into capacitive loads. Common values are 2 to 6mH (10 to 20mohm dcr) with a 1 to 10ohm damping resistor in parallel to reduce ringing. Air cored inductors are preferred.

If you are using normal speakers, and the speaker cable is not one of the high capacitance types, it should be possible to remove the inductor/resistor completely. Alternatively, the inductor/resistror could be replaced with a suitably rated 0R1 resistor which will ensure stability but will reduce the maximum power a little, particularly into a 4ohm load.

Geoff
  Reply With Quote
Old 8th February 2002, 09:34 AM   #3
Helix is offline Helix  United Kingdom
diyAudio Member
 
Helix's Avatar
 
Join Date: Jul 2001
Location: UK
my speaker cable isn't one of the strange types, but the speakers are 3 way and may introduce a bit of capacitance from the crossover networks, i think?

the amp in question has a
"inductor of 2mH and you make it by winding 15 turns of 18 SWG enamelled wire on a 3W wire wound resistor"
where the resistor is 1R.

i was wondering if you could not eleminate it (just incase) but just reduce the values a bit.
  Reply With Quote
Old 8th February 2002, 10:05 AM   #4
Geoff is offline Geoff  United Kingdom
diyAudio Member
 
Join Date: Apr 2001
Location: UK
Helix

The smallest inductance value I can remember seeing was about 1mH (10 to 12 turns of 22# round a 2W 10ohm resistor). There is no way that I know of for easily calculating the required inductance. The normal method of determining it would be to measure the amp into various capacitive loads and to observe the overshoot and ringing on a square wave.

Assuming that you do not have the test equipment to do this, I see no reason why you shouldn't reduce your inductor to say 10 turns and listen to the results. There may or may not be an audible difference between the two inductance values. My guess is that there won't be.

Geoff
  Reply With Quote
Old 8th February 2002, 11:36 AM   #5
Helix is offline Helix  United Kingdom
diyAudio Member
 
Helix's Avatar
 
Join Date: Jul 2001
Location: UK
ok, i will just leave it be then, i just thought that a 1R resistor in to 4 ohm load would loose a little power at 100 or so watts.

isn't L for a air core solenoid coil :
L=(u*N^2*A)/l
where:
u = permeabilty of free space
N=number of turns
A=cross section area
l = inductor length
  Reply With Quote
Old 8th February 2002, 11:39 AM   #6
Helix is offline Helix  United Kingdom
diyAudio Member
 
Helix's Avatar
 
Join Date: Jul 2001
Location: UK
having said that, the inductor resistance is far lower and hence the resistor value is meaningless
  Reply With Quote
Old 8th February 2002, 12:06 PM   #7
Geoff is offline Geoff  United Kingdom
diyAudio Member
 
Join Date: Apr 2001
Location: UK
Helix

Sorry, I obviously hadn't woken up properly when I made the previous posts. All references to mH should have read uH.

At audio frequencies the resistor has virtually no effect since the low impedance of the inductor effectively bypasses it. The resistor comes in to play at higher frequencies when the impedance of the inductor begins to rise towards a similar value to that of the resistor. In your case, with a 1ohm resistor and 2uH inductor (I assume your 2mH was in error as well), this will be somewhere above 200kHz.

Geoff
  Reply With Quote
Old 8th February 2002, 12:08 PM   #8
hifi is offline hifi  Sweden
diyAudio Member
 
Join Date: Mar 2001
Location: sweden
BTW: i once read an article on magnetic coupling between output coils in an amplifier....but i dont remember the conclusions or where i read it, Electronics World? ....

anybody?

/micke
  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
Output filter Skorpio Class D 5 8th August 2008 06:44 PM
What output filter to choose Groundloops Digital Source 2 9th November 2007 01:06 PM
Output filter m-tech Class D 11 22nd September 2007 07:04 AM
BTL output filter ted12 Class D 16 26th April 2005 01:55 PM
What matrial for output filter? lumanauw Class D 62 16th March 2005 05:15 AM


New To Site? Need Help?

All times are GMT. The time now is 12:01 PM.


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