Any reason to add CDom?

TomasF · 9th May 2007, 07:17 PM

Hi

my commercial amp was deliverd without Miller compensation capacitor CDom at the VAS transistor (and is stable). I have experimented with lowering the global NFB and the result soo far is promising, the result was decreased (!) distortion at higher freq. The bass however became a bit more diffuse.

My thoughts now are to revert to the original (higher) NFB and to add a small CDom (<100pF). Thereby more of high-freq negative feedback will be locally at the VAS stage. Can this reduce global NFB at high freq. without sacrifying the bass performance.

Are the sonic drawbacks with "CDom feedback" too large, should I drop the idea or try it?

/ BR Tomas F. Stockholm

PS please excuse my language DS

lumanauw · 10th May 2007, 03:13 AM

If you want good quality sound with Cdom, implement it like the Sackinger paper (JCX pointed to this paper several times). It gives much better sonics than ordinary Cdom implementation.

john_ellis · 11th May 2007, 02:55 PM

Hi ThomasF

I would not advise adding a conventional Cdom!

The increased local NFB does not help reduce crossover distortion - it will instead make this worse. (One reason not to use Cdom's!)

See Cherry's proposals for a better implementation.

cheers
John

9th May 2007, 07:17 PM	#1
TomasF diyAudio Member Join Date: Apr 2006 Location: Stockholm	Any reason to add CDom? Hi my commercial amp was deliverd without Miller compensation capacitor CDom at the VAS transistor (and is stable). I have experimented with lowering the global NFB and the result soo far is promising, the result was decreased (!) distortion at higher freq. The bass however became a bit more diffuse. My thoughts now are to revert to the original (higher) NFB and to add a small CDom (<100pF). Thereby more of high-freq negative feedback will be locally at the VAS stage. Can this reduce global NFB at high freq. without sacrifying the bass performance. Are the sonic drawbacks with "CDom feedback" too large, should I drop the idea or try it? / BR Tomas F. Stockholm PS please excuse my language DS

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10th May 2007, 03:13 AM	#2
lumanauw diyAudio Moderator Emeritus Join Date: Oct 2002 Location: Bandung	If you want good quality sound with Cdom, implement it like the Sackinger paper (JCX pointed to this paper several times). It gives much better sonics than ordinary Cdom implementation.

11th May 2007, 02:55 PM	#3
john_ellis diyAudio Member Join Date: Sep 2006	Hi ThomasF I would not advise adding a conventional Cdom! The increased local NFB does not help reduce crossover distortion - it will instead make this worse. (One reason not to use Cdom's!) See Cherry's proposals for a better implementation. cheers John

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