At least a 1 farad cap. I would actually advise 2 fd. My amp is running 600 watts w/o dimming with a 1 farad cap. Actually, I didn't get much dimming before the cap was installed.
Dave
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99 SVT
Sony ES CDX-C910
Sony Digital EQ/X-over
Sony Changer
McIntosh MC440M
Adire Audio Shiva
MB Quart/Vifa seperates