Low Pass filters for class-D audio testing?? - 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 9th December 2012, 05:08 PM   #1
diyAudio Member
 
Zero Cool's Avatar
 
Join Date: Sep 2004
Location: MN
Default Low Pass filters for class-D audio testing??

I need to build a better set of low pass filters for my test setup. I need to build a set of high power filters for my scope and distortion analyzer inputs that will allow me to test class-D amps up to 500 watts from 20-20khz.

What I have now is 12db oct at 20khz and that allows me to test up to 5khz at full power, if I go over 5khz at more than a few watts i burn them up which has happened more than a few times.

SO. Suggestions? maybe 24db at 100khz? with an 8/4 ohm resistive load?

Can I use standard speaker crossover calc's to do the math??

where Eva?

Zc
__________________
It's not about what you can BUY...It's about what you can BUILD!

Last edited by Zero Cool; 9th December 2012 at 05:23 PM.
  Reply With Quote
Old 9th December 2012, 05:20 PM   #2
KSTR is offline KSTR  Germany
diyAudio Member
 
KSTR's Avatar
 
Join Date: Jul 2007
Location: Central Berlin, Germany
Check specs of AP's AUX-0025 Filter Unit (attached).

Note that you DON'T attach the Load to the filter output, rather the other way round.

A speaker Software would certainly do it (LSPcad demo version), or LTspice.
Attached Files
File Type: pdf AUX-0025_Specifications.pdf (88.9 KB, 91 views)
  Reply With Quote
Old 9th December 2012, 05:26 PM   #3
diyAudio Member
 
Zero Cool's Avatar
 
Join Date: Sep 2004
Location: MN
I will check it out. I am not using a software based test set up. and right my filters connect to the input of my scope and my dist an. but my filters are made up of a series resistor / inductor and a capacitor to ground and as you start getting above 5khz there is sufficient energy to burn up the low wattage resistor and inductors as you are getting into the tail of the 12db down points of the 20khz filter
__________________
It's not about what you can BUY...It's about what you can BUILD!
  Reply With Quote
Old 9th December 2012, 05:30 PM   #4
diyAudio Member
 
Zero Cool's Avatar
 
Join Date: Sep 2004
Location: MN
Ok well it's great that AP makes that product but i am quite sure I can't afford ANYTHING with the AP name on it. I need to BUILD one!
__________________
It's not about what you can BUY...It's about what you can BUILD!
  Reply With Quote
Old 9th December 2012, 05:49 PM   #5
KSTR is offline KSTR  Germany
diyAudio Member
 
KSTR's Avatar
 
Join Date: Jul 2007
Location: Central Berlin, Germany
Well, the AP filter is on the order of 800EUR and quite unlikely to be found used. I have one at work but it's still having the screws sealed so no way to peek inside...
  Reply With Quote
Old 9th December 2012, 06:28 PM   #6
diyAudio Member
 
Join Date: Dec 2003
Location: Munich
I am typically using passive RC filters in the range of 3rd...5th order. Similar to the 4th order filter in the attached picture.

Chose a 5W type for the 100 Ohms and 1 W for 330 Ohms, all the rest 1/4W resistors. Caps should be MKP types with 100V rating (lower quality will still work....).
Attached Images
File Type: jpeg CarrierFilter.jpeg (30.6 KB, 152 views)
File Type: jpeg CarrierFilterTransferFkt.jpeg (75.0 KB, 154 views)
  Reply With Quote
Old 9th December 2012, 07:35 PM   #7
diyAudio Member
 
Zero Cool's Avatar
 
Join Date: Sep 2004
Location: MN
I need to get out to 20khz flat before roll off. part of my testing involves full power frequency response so i need to make sure the filter is not causing problems at 20khz
__________________
It's not about what you can BUY...It's about what you can BUILD!
  Reply With Quote
Old 9th December 2012, 08:19 PM   #8
KSTR is offline KSTR  Germany
diyAudio Member
 
KSTR's Avatar
 
Join Date: Jul 2007
Location: Central Berlin, Germany
I would think with a well-known roll-off you could account for that easily in the measurements. Flat response (how flat, <0.1dB?) and sharp roll-off complicates filter design and construction quite a bit.
  Reply With Quote
Old 9th December 2012, 08:26 PM   #9
diyAudio Member
 
Join Date: Dec 2003
Location: Munich
Of course, you can adjust the transferfunction according your taste.
The shown values were just anticipated from your first posting to fit -3db at 20kHz. Feel free to make the filter faster unless your analyzer gets irritated because the switching residuals are not suppressed sufficiently anymore.
Or simply measure the full power frequency response with a function generator and the scope directly, you do not need any filter for that at all.
  Reply With Quote
Old 10th December 2012, 03:52 AM   #10
diyAudio Member
 
Zero Cool's Avatar
 
Join Date: Sep 2004
Location: MN
Quote:
Originally Posted by KSTR View Post
I would think with a well-known roll-off you could account for that easily in the measurements. Flat response (how flat, <0.1dB?) and sharp roll-off complicates filter design and construction quite a bit.
Yeah within .1 to .2db accurate to 20khz. most of the switching frequencies seem to be 300-400khz or so so I figure if i have a 100khz low pass filter that should be good enough to keep the junk out of the analyzer yet give accurate results.
__________________
It's not about what you can BUY...It's about what you can BUILD!
  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
Low pass filters for removing ultrasonic noise? billybudapest Analog Line Level 25 17th July 2014 01:17 PM
Very Simple Low and High Pass Pre-Amp Filters Help houndazs Solid State 7 11th July 2011 05:46 PM
DIY Low Pass and Mid/High Pass (Bass Blocking) Filters runslikealpaca Everything Else 1 19th June 2009 10:59 PM
Need formulas for multi-poles RC low pass passive filters gaetan8888 Digital Source 9 13th March 2008 03:59 AM
A question on low pass filters Vaughan Subwoofers 15 10th October 2007 04:23 AM


New To Site? Need Help?

All times are GMT. The time now is 10:06 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