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Nakamichi PA-7 Hum

Hi,
Hope everyone has a great holiday season.
I am the original owner of a PA-7. If my house was ever robbed the first place I would look to know if my life, as I have known it, was over is my audio rack. I will stay with the PA-7 until it dies or I do.
My problem is that any new equipment that I try to run through the preamp (Nakamichi CA-5)that is not CD related i.e. cable TV audio lines, dvd players - anything new makes the amp hum. It is loud enough to be annoying but is drowned out when the volume is loud enough. Needless to say I have never left anything connected to it that caused it to hum for fear of doing damage.
Any ideas why it does that and if there is some type of line filter that I should have and don't know about? I would love to play DVD's, cable, etc. through the PA-7 but as long as I have had it only CD's play through without a problem.
Thanks for any help.
edmallia
weiserb's avatar
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weiserb
473 posts
 
You many have introduced a ground loop into the system by connecting the audio system to a Home Theater system, Cable TV box, and or powered subwoofer. The cable is ususally guilty followed by the Sub which usually gets pluged into a different outlet. I installed two isolation transformers from Jensen transformers (not cheap about $ 150+ each )My background is black as night now.

Unplug the above items to see if they are at fault when you plug them back in
hifigy
59 posts
 

I had the same problem with my PA-7 & CA-7. The hum was intoduced by the audio cables coming from anything connected to my cable tv system. This was the quick and cheap way to create an isolation transformer. Take 2 75ohm/300ohm transformers that come with vcrs or tvs. Hopefully you'll have one with pigtails (300ohm) and one with screw teminals. Connect the pigtails to the teminals. You'll now have two "f" connectors at either end. Connect one end to your cable feed and one to your video system. This was essentially what the old "Magic" box was from a few years back but, instead of $99, you've spent maybe $5.

Happy holidays
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swampwalker
5,138 posts
 
Yep, Hifigy is right. Its an easy way to check to see if that is the problem, since most of us have dozens of those little buggers lying around in a drawer somewhere. I then went and got the $10 RS isolation tranny which of course looks a lot neater.
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marakanetz
5,473 posts
 
With time the internal power supply of the amp may become noisy due to the lack of DC filtering. If this is the case you'll need to inspect PCB for the loose soldering joints and replace power supply electrolytic capacitors.
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almarg
9,670 posts
 
With time the internal power supply of the amp may become noisy due to the lack of DC filtering. If this is the case you'll need to inspect PCB for the loose soldering joints and replace power supply electrolytic capacitors.

Very true, but in this case he said that the problem does not exist when he selects a cd player with his preamp, while it does exist when he selects other line-level inputs.

Which tells me the problem is unlikely to be internal to his power amp, or to his preamp for that matter. I think it's more likely to be an open return path (shield connection) in an interconnect, or one of the other things I mentioned above.

Or, as others have said, a ground loop with the catv system IF the problem only occurs when there is a connection to something within that system (which I suspect is not the case based on the wording of his original post).

Regards,
-- Al
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