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RE: Heathkit AA-121 * PT hum * Coupling cap question

Posted by Jim McShane (D) on January 14, 2015 at 17:36:50

In Reply to: RE: Heathkit AA-121 * PT hum * Coupling cap question posted by lektrik on January 14, 2015 at 16:36:10:

"I think part of the problem might be with the tiny yellow IC film caps since the roll-off with a .047uf cap is where you say it is."

The IC MPW caps are good caps, the bass cutoff is an issue of cap value. So I don't think that's the issue. I use the IC MPW polypropylenes in some locations where I'm replacing mylar/polyester caps to retain some "snap" in the voicing.

"It shouldn't be reaching up into the mid-bass frequencies as I am hearing."

Keep in mind the low frequency rolloff begins at a higher frequency than the cutoff. It can be affecting the bass to some extent an octave or two above the -3db point.

"I never use the yellow caps, but I left them in to find out if the amp worked. They might take longer to burn-in and mellow out than I'm used to with the K-40Y PIOs."

What you are hearing is not just the caps, it's a fairly high frequency bass rolloff. Don't judge those caps by what you hear. They'd sound much better if they were .1 or .15 uf.

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Kimber Kable

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