Phase Linear 700 Series II - 60Hz Hum

craigstrat · 19th September 2008, 01:38 AM

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

d3imlay · 21st September 2008, 10:36 AM

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.

craigstrat · 24th September 2008, 01:31 PM

I disconnected both RCA inputs. The hum is still present. Both amps are running through a Topaz Isolation Transformer too.

What does this mean?

Amp Caps?

Thanks,
Craig

d3imlay · 25th September 2008, 09:56 AM

Did you check the ripple? Why the xfmr?

Original Burnedfingers · 25th September 2008, 10:34 AM

Power supply caps are a problem with this amp especially considering its age. Replace them.

d3imlay · 27th September 2008, 11:14 AM

Quote:

Originally posted by burnedfingers
Power supply caps are a problem with this amp especially considering its age. Replace them.

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.

EchoWars · 28th September 2008, 02:45 AM

I'm not so sure that's true with some of the top-tier manufacturers, such as Vishay/Sprague (36DA & 36DX) and Nippon Chemi-Con (United Chemi-Con)(36DA). The latter of those two makes exceptionally well-built caps.

19th September 2008, 01:38 AM	#1
craigstrat diyAudio Member Join Date: Jul 2004 Location: New York	Phase Linear 700 Series II - 60Hz Hum 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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21st September 2008, 10:36 AM	#2
d3imlay diyAudio Member Join Date: Sep 2004 Location: Ohio	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.

24th September 2008, 01:31 PM	#3
craigstrat diyAudio Member Join Date: Jul 2004 Location: New York	I disconnected both RCA inputs. The hum is still present. Both amps are running through a Topaz Isolation Transformer too. What does this mean? Amp Caps? Thanks, Craig

25th September 2008, 09:56 AM	#4
d3imlay diyAudio Member Join Date: Sep 2004 Location: Ohio	Did you check the ripple? Why the xfmr?

25th September 2008, 10:34 AM	#5
Original Burnedfingers diyAudio Member Join Date: Nov 2002	Power supply caps are a problem with this amp especially considering its age. Replace them.

28th September 2008, 02:45 AM	#7
EchoWars diyAudio Member Join Date: Dec 2002 Location: Left of the Dial	I'm not so sure that's true with some of the top-tier manufacturers, such as Vishay/Sprague (36DA & 36DX) and Nippon Chemi-Con (United Chemi-Con)(36DA). The latter of those two makes exceptionally well-built caps.

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