20 year old A-75 buzzing - diyAudio
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Old 12th January 2012, 05:58 PM   #1
daly41k is offline daly41k  United States
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Default 20 year old A-75 buzzing

I have an A-75 Pass/Tharagard amp that I built in 1992 and have used a lot ever since. Recently I have retired and moved and I had an extra room in the house that I could use for a library and I put together a system using the A-75 amp. I am after all these years getting buzzing in the tweeters and hum in the midranges. It must be 120hz since the woofers are quiet. I hate ground loops and I am wondering if anyone is still using this amp and has run into this problem. This amp has followed me all over the world and it could be that the caps in the PS are getting old. The front end regulators are working fine and I have the all the grounds star grounded at the caps with a line to the chassis ground. The amp still uses the original grounding scheme.

What is funny is that when I run single ended inputs to the amp and switch it to balanced mode the noise goes down by half. I am assuming this is due to common mode rejection in the front end mosfets. The amp still sounds great when playing music and all DC offset measurements are less than 80mv. I hate to think of tearing this thing down to look for cold solder joints. I tightened all the wire connections with no luck. Yes it runs very warm per design and I have not had any other problems. All other measurements are normal per the original article. I haven't made any changes or upgrades since I built this amp. The tranny is dead silent. I get 1-2 mv when I check the chassis ground vs other signal grounds on the board and connectors. I can't tell if this is the Radio shack digital meter error or something else. I haven't put my scope on it since I can hear the problem. If anyone has got an easy idea I am all for it. With music playing at a comfortable level from my listening position 8-10ft away I don't notice it. With no music I can hear the noise at my chair. You know that us Pass labs philes have to have everything perfect or we will be thrown out of the club. I saw Nelson's new amps on his web site but alas I don't own a vineyard and can't afford them.
I am glad for this website. By the way I have to get my XA100's working again. I had to partially disassemble them when I moved. I have never got the absolute offset DC to ground below 6volts even thought I got matched Jfets when tested but they don't act that way in the circuit. The dc offset between the +/- outputs is near 20mv. thanks for any suggestions--dave
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Old 12th January 2012, 08:28 PM   #2
Botte is offline Botte  United States
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The power supply caps in the voltage doubler for the front end is my guess.
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Old 12th January 2012, 09:00 PM   #3
daly41k is offline daly41k  United States
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Ok I will check those out. By the way I uncovered a 20 year old mistake. I never connected the center tap of the tranny to the chassis ground. I put an alligator lead between these points and the buzz stopped. I will have to solder in a lead. I have some residual hum but it has been greatly reduced. I will check or just replace the caps next. Botte-thanks for your ideas.

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Old 12th January 2012, 09:00 PM   #4
eLarson is offline eLarson  United States
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Any halogen lamps (usually have a dimmer in them) or dimmer switches for the lights in the room?
Would a woodchuck bother to chuck MDF?
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Old 13th January 2012, 08:40 PM   #5
daly41k is offline daly41k  United States
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Location: Phillipsburg, New Jersey
elarson-thanks and a good point. The area I have the amp is an abandoned theater area.

I am just getting into fixing it up and there are all kinds of wires, lights etc. I will have to experiment. Who knows what the grounding was? dave
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