Hum in Jim Williams mod Adcom GFA 545
- Thread starter jdurango
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radardoug
Well-known member
You should hear an Eventide 949 on 50 Hz power if you think thats a problem. And what about if you connect a speaker to it? Thats when you worry about hum. An amp that hums physically at that level is not uncommon.
Rob Flinn
Well-known member
EmRR said:
I thought he was no longer with us ........
ruairioflaherty
Well-known member
Rob Flinn said:
The real Jim Williams of Linear Tech is no longer with us - http://www.computerhistory.org/atchm/an-analog-life-remembering-jim-williams/
The other Jim Williams still reigns over at Gearslutz, thankfully correcting the seemingly endless mistakes made by "lesser" engineers and designers. One piece of gear at a time he's ensuring that every mic pre can also pass light, that all caps are black gate and that every opamp used has a slew rate of 3000V per uS.
Rob Flinn
Well-known member
ruairioflaherty said:
Aha. Well that's a good reason to keep avoiding Gearslutz ....
Jim (the one still alive) can be a little extreme but generally does more good than bad with his equipment modifications. He uses empirical objective measurements from his real test equipment and often makes gear better than needed, not a sin.Rob Flinn said:
JR
PS He charges for his mods, so why give them away? That said if one of his modded pieces is misbehaving there is an implied responsibility to help determine that his mod is not at fault. I suspect posting your query in geekslutz forum of gearslutz might get a response from him.
Rob Flinn
Well-known member
JohnRoberts said:
Fair play, I appreciate that. I don't need him as a reason to not visit Gearslutz.
moamps
Well-known member
jdurango said:
This amplifier is simple in topology and very friendly for testing and servicing, so if you have a DMM and basic knowledge of electronic, you can check it very easily.
The schematic you can find here
https://www.audioservicemanuals.com/a/Adcom/Adcom-GFA/70846-adcom-gfa-545-service-manual
Open it, power it on and check out DC voltages on PS capacitors and output transistors bias (6mV). If all readings are correct, check it finally with a cheap speakers. There is a lot pictures of the interior of the amp on web so, I believe, you will easily notice what Jim or anyone else changed. Does it hum equally in horizontal position?
Usually this type of hum is sign of not optimally designed power transformer, or of a high level of the power grid distortion (DC offset etc).
radardoug
Well-known member
Um, guys, he is talking about the acoustic hum of the amplifier with nothing plugged into it or out of it.
Correct, acoustic hum with nothing plugged in is the issue here. It seems to be something misbehaving, especially with the seemingly erratic behavior. I'll try checking with JW of AU himself, but as some of you have pointed out, he can be a bit "temperamental" and "seemingly erratic" as well. That said, you can't say the guy doesn't take his craft seriously or maintain VERY high standards.
Acoustic hum in a transformer can be caused by lousy mains power. Have you metered your mains voltage? Any weird loads.jdurango said:
Who is JW of AU himself?
JR
Winetree
Well-known member
JW of AU
Jim Williams of Audio Upgrades
http://audioupgrades.com/upgrades/upgrades.htm
Jim Williams of Audio Upgrades
http://audioupgrades.com/upgrades/upgrades.htm
Is the power transformer a torroid? If so, check that the mounting bolt is tight and/or add a neoprene damping pad. If it is an old style laminated transformer, be sure all four bolts are tight. I had an old Acoustic brand bass amp in recently where all four were loose and it hummed like that guy who can't remember the lyrics...
