Class d - Clipping - Help - diyAudio
Go Back   Home > Forums > Amplifiers > Class D

Class D Switching Power Amplifiers and Power D/A conversion

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 2nd March 2010, 05:25 PM   #1
diyAudio Member
 
Join Date: Nov 2004
Location: Bucharest
Default Class d - Clipping - Help

Hello.

This schematic is a car amplifier, and i wish to change with your help!
Rail +/- 116v, and clipping start at ~73Vrms of output (without loud)
I suppose that clipping is introduced by preamplifier section.
Please help me with some ideas , what minor changes I need to do to eliminate that clipping!
Attached Images
File Type: jpg CLASS D.JPG (196.1 KB, 205 views)
  Reply With Quote
Old 2nd March 2010, 07:33 PM   #2
diyAudio Member
 
runebivrin's Avatar
 
Join Date: Jul 2003
Location: Huddinge, Sweden
Send a message via MSN to runebivrin
If it's clipping at 73V RMS, that means it has peaks that are 73*SQRT(2) = 103 volts. With a class D amp there's only so far you can get the duty cycle, so it's not really that far off.

However, looking at your schematic and measurements has me thinking there's something wrong with your setup. There's 4.68 Vrms on the input, but 5.33 Vrms on the non-inverting leg of the op-amp. Considering the signal is at around 50Hz and looking quite dirty, I'm wondering if what you're measuring isn't bad grounding of the oscilloscope probe.
__________________
Do wizards use spell checkers?
  Reply With Quote
Old 2nd March 2010, 07:40 PM   #3
81bas is offline 81bas  Germany
diyAudio Member
 
Join Date: Jan 2006
Location: Kiel
Quote:
Originally Posted by mgm2000ro View Post
Hello.

This schematic is a car amplifier, and i wish to change with your help!
Rail +/- 116v, and clipping start at ~73Vrms of output (without loud)
I suppose that clipping is introduced by preamplifier section.
Please help me with some ideas , what minor changes I need to do to eliminate that clipping!
I suppose you use the input signal of about 10-15 KHZ, right? and if your output filter has too much inductance and too high capacitance, then it's cutoff frquency will be at this frequency already and you will newer get full rail to rail output swing at this frequency... Check it at 1KHZ instead, or at 100HZ at all...

P.S.: oh sorry, I see now, it is 50HZ... then I have no idea

Last edited by 81bas; 2nd March 2010 at 07:43 PM.
  Reply With Quote
Old 2nd March 2010, 08:28 PM   #4
diyAudio Member
 
Join Date: Nov 2004
Location: Bucharest
I change R57 + R37 with a smaller resistor.
I removed R57 (short circuited it) and the clipping threshold moved to 75V.
Is modifying the values of the resistors a good solution ?
  Reply With Quote
Old 2nd March 2010, 11:11 PM   #5
Eva is offline Eva  Spain
diyAudio Member
 
Eva's Avatar
 
Join Date: Oct 2003
Location: Near the sea
Send a message via MSN to Eva
You may not like the result if the output stage of the amplifier is allowed to clip. Clicks, sticking to the rails, and oscillations may be the result. In many class D circuits the modulator is intentionally made to clip before the output stage, or the signal is pre-clipped according to instantaneous supply voltage.
__________________
I use to feel like the small child in The Emperor's New Clothes tale
  Reply With Quote
Old 3rd March 2010, 11:09 AM   #6
81bas is offline 81bas  Germany
diyAudio Member
 
Join Date: Jan 2006
Location: Kiel
Quote:
Originally Posted by mgm2000ro View Post
I change R57 + R37 with a smaller resistor.
I removed R57 (short circuited it) and the clipping threshold moved to 75V.
Is modifying the values of the resistors a good solution ?
You have got the answer in second post in this topic already from user runebivrin: with +/-116 rail voltage you newer get 116 RMS voltage at output! With +/-116V rail voltage theoretical limit is 116/sqrt(2) ~= 116/1.414 ~= 82Vrms. And practically class D amp has a limit in minimal pulse width and has Rdson resistance of output mosfets, which limits the maximal output swing. To understand the things better, switch your oscilloscope to show you the Vpp (peak to peak) instead of Vrms, and connect it to the OUTPUT of your amp
I would not modify any resistors in this amp to get another additional 8-9 volts at output, otherwise probably you will end with die amp.

Last edited by 81bas; 3rd March 2010 at 11:12 AM.
  Reply With Quote
Old 3rd March 2010, 11:12 AM   #7
ssanmor is offline ssanmor  Spain
diyAudio Member
 
Join Date: Jan 2002
Location: Madrid
You also don't know what are the actual rails voltage at the moment of clipping. You should monitor the output and at least one rail at the same moment with a dual channel oscilloscope, and check how far are the output from the rail when clipping.
  Reply With Quote
Old 3rd March 2010, 03:29 PM   #8
diyAudio Member
 
Join Date: Nov 2004
Location: Bucharest
Thank you very much, I will check what happens when it is under loads.
One probe on rail and and other to output!

Best regards,
for all!
  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
Recovery from clipping, soft clipping hahfran Solid State 49 23rd February 2010 01:03 AM
Clipping richie00boy Class D 2 16th January 2008 05:36 AM
class-d behavior at clipping raintalk Class D 7 13th September 2007 12:34 PM
Clipping Class D ? Joules Class D 31 19th January 2006 07:27 AM


New To Site? Need Help?

All times are GMT. The time now is 02:56 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