Output filter
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m-tech
diyAudio Member

Join Date: Feb 2006
Output filter

I built my first class D amp. It's a full bridge, single supply, based on LT1162 bridge driver. It sounds great, but I have some questions about output filter. Currently, my output coils are wounded on separate cores ( core EF25, 1mm air gap ), but I saw that they can be wound on same core.
1. one core is advantage ( smaller PCB, costs... ), but are there some disadvantages ?
2. how to calculate C3, C4, C5 ?

thanks.
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 7th September 2007, 04:11 PM #2 m-tech   diyAudio Member   Join Date: Feb 2006 Location: Belgrade Somebody ?
 7th September 2007, 04:27 PM #3 Keruskerfuerst   diyAudio Member     Join Date: May 2007 Location: Germany C3 = 1/2 C2 (C2 from the left picture C4=C5= 2 * C3 (C3 from the right picture)
 7th September 2007, 05:10 PM #4 m-tech   diyAudio Member   Join Date: Feb 2006 Location: Belgrade Are you sure ( C4=C5=2*C3=C2 ) ? I saw a few schematics and always is C3>C4. What about 1st question ?
 7th September 2007, 05:23 PM #5 Baldin   diyAudio Member     Join Date: Jan 2005 Location: Copenhagen First, when the coils are wound on the same core you only need half the windings for each coil, compared to winding on two separate cores. Second, you don't need C4 and C5. C3 should be half that of C1. Thirdly, remember that when calculating L1/C1, you should set Rload to half that of the seaker, in your example 4 ohm due to the ballanced mode. __________________ Visit www.sensibleaudio.dk or read latest news on Baldin's Blog
 7th September 2007, 06:26 PM #6 theAnonymous1   diyAudio Member     Join Date: Feb 2004 Location: Anonymityville Maybe this will help...... http://www.murata.com/emc/knowhow/pd...-1/26to28e.pdf
m-tech
diyAudio Member

Join Date: Feb 2006
Thanks for all.

Quote:
 Originally posted by Baldin First, when the coils are wound on the same core you only need half the windings for each coil, compared to winding on two separate cores.
But, this is full bridge amp ( not two halfbridge connected in bridge ). L1 and L2 are connected in series with speaker and calculations told me that I need 60uH ( L1=L2=30uH ). So, if they wounded on same core, L1 and L2 should be 15uH each ? Why ? ( I'm confused now )

Quote:
 Originally posted by Baldin Second, you don't need C4 and C5. C3 should be half that of C1.
What is purpose of C4 and C5 ? As I sad earlier, a saw them on few schematics.

 10th September 2007, 09:05 AM #8 Ouroboros   diyAudio Member     Join Date: Dec 2003 Location: Nottingham UK If you only have a cap across the speaker (and not also a cap from each side of the speaker to ground), then any common-mode noise produced by the amp will radiate and give you big EMC problems. It is ALWAYS wise to fit C4, C5 to help filter out common-mode noise. (I've got caught by this myself before). Believe me, there will be C-M noise at RF produced by the switching!
 10th September 2007, 07:04 PM #9 Baldin   diyAudio Member     Join Date: Jan 2005 Location: Copenhagen This is of course true. I use these my self in my bridged design. These only have to be like 10% of C3 in value. As you say Ouroboros they are ther to reduce commonmode noise, but remember that they will add to the filtering effect. So if you have C3 = 1uf and C4=C5=100nF then the resulting "C3" will be 1,05uF (stil pretty close to 1uF though ) __________________ Visit www.sensibleaudio.dk or read latest news on Baldin's Blog
 22nd September 2007, 07:04 AM #10 Cyguy84   diyAudio Member   Join Date: Sep 2007 Oh, as far as C3 is concerned-- I was curious. From what I have seen, a good reason to include C3 is because it basically doubles its effect in the filter. What other reasons are there to include it? (By doubling, I mean connected as such, there is a virtual ground in the middle of it, and so the capacitor can be modeled as 2*C3 on both sides, going to ground, just like the first schematic. This is all basic to most of you, but its something I at least know )

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