Phase Linear 700 Series II - 60Hz Hum

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craigstrat

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Joined 2004
I am running a pair of JBL 4343's, powered by a pair Phase Linear 700's bi-amped. One is a 700B (low amp), one is a fully complementary 700 Series II (mid's and hi's amp).

I like to cut the mid-hi amp gain (Series II) back about 3dB. The more I cut the gain, the more a 60Hz hum gets louder. It's loudest point is when the knobs are at about 12:00, then it decreases all the way down to full off.

Is my problem caps that need to be replaced? (electrolytics?? filter caps??)

Could the internal crossovers' caps be going due to age and heavy usage??

Or could it possibly be a ground problem with the active crossover that I'm using... Which by the way is based on the MOX design for the 4343's, with the help of Ian MacKenzie.

Any help is greatly appreciated.

Thanks,
Craig
 
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d3imlay

Member
Joined 2004
If you disconnect the input jacks is there still any hum? The 700's are quiet with no input attached. The setting of the pots shouldn't make any difference. Whenever I work on any P/L amp I always change the RCA's out as they get loose and intermittent.

The PS caps should have less than a few hundred millivolts of ripple at idle. At full load it will be slightly over 1 volt.
 
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d3imlay

Member
Joined 2004
burnedfingers said:
Power supply caps are a problem with this amp especially considering its age. Replace them.
Click to expand...

Easier said than done. The originals are 9800uf @100V. They run right around 100V. With caps you buy today you don't get the headroom like you did 20 years ago. I'd want to go with 125V. Problem is that even at 100V they're hard to find and must be the same size or smaller.

If anyone can point me to some please do.
 
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