Hi there,
I am tackling some massive hum I'm developing on my custom-restored Thorens TD-150.
A particular feature of my restoration was that I included the step-up transformers inside the TT cabinet, to have the weak signal travel as short as possible. I am using a Denon DL103 LOMC cartridge and a pair of Sowter 9570 step-up transformers. That's a combination that I'm not going to change any time soon.
As mentioned, the system generates a lot of 60Hz hum. I tried several arrangements, mostly trying to separate the signal ground from the metal body and motor, without much success. I have an earth ground in my mains and a binding post that I'm connecting to my preamp. the SUT cans are screwed to a copper sheet bonded to the chassis.
I am mostly confused by how I should route the grounds from the SUT cans, the SUT ground wire proper, the pickup ground wire, and the negative signal wire.
The latest arrangement I'm trying is illustrated here:
Note that I am not sure if the cart GND is really bonded to the tonearm hardware. I just realized after I attached this image...
What am I getting wrong?
Thanks,
gm
I am tackling some massive hum I'm developing on my custom-restored Thorens TD-150.
A particular feature of my restoration was that I included the step-up transformers inside the TT cabinet, to have the weak signal travel as short as possible. I am using a Denon DL103 LOMC cartridge and a pair of Sowter 9570 step-up transformers. That's a combination that I'm not going to change any time soon.
As mentioned, the system generates a lot of 60Hz hum. I tried several arrangements, mostly trying to separate the signal ground from the metal body and motor, without much success. I have an earth ground in my mains and a binding post that I'm connecting to my preamp. the SUT cans are screwed to a copper sheet bonded to the chassis.
I am mostly confused by how I should route the grounds from the SUT cans, the SUT ground wire proper, the pickup ground wire, and the negative signal wire.
The latest arrangement I'm trying is illustrated here:
Note that I am not sure if the cart GND is really bonded to the tonearm hardware. I just realized after I attached this image...
What am I getting wrong?
Thanks,
gm
I'm not familiar with the TD-150, but I prefer to keep signal and chassis grounds separate. So my suggestion would be to disconnect the SUT ground from chassis earth. The tonearm and TT can be routed together but separate from the SUT. The cartridge coils themselves are not grounded and don't need to be.
It's worth mentioning that putting the SUT inside the TT in close proximity to the motor is probably more prone to hum pickup than locating it elsewhere, far away from the motor. I wouldn't rule this out as the source of hum even if the motor is switched off.
It's worth mentioning that putting the SUT inside the TT in close proximity to the motor is probably more prone to hum pickup than locating it elsewhere, far away from the motor. I wouldn't rule this out as the source of hum even if the motor is switched off.
Thanks. I rewired according to your suggestion and I'll test later.
Could I also merge all the SUT & tonearm grounds to the negative pole of one of the output jacks? Or will that create a ground loop?
The transformers and signal wiring are placed about 11" (28cm) away from the motor, I suppose it's far enough to avoid interference. I could also build a copper casing around the SUT if I wanted to be extra safe. Some signal wires are quite long and I'll trim them once I settle on a good solution.
Could I also merge all the SUT & tonearm grounds to the negative pole of one of the output jacks? Or will that create a ground loop?
The transformers and signal wiring are placed about 11" (28cm) away from the motor, I suppose it's far enough to avoid interference. I could also build a copper casing around the SUT if I wanted to be extra safe. Some signal wires are quite long and I'll trim them once I settle on a good solution.
You could, but to iterate, I prefer to keep audio and chassis grounds separate. The only place they come together is at the preamp. This avoids any possibility of a loop.
I'm not sure how well a copper shield will help (steel or mu-metal is better), but keeping audio leads short and twisting them tightly together is essential.
I'm not sure how well a copper shield will help (steel or mu-metal is better), but keeping audio leads short and twisting them tightly together is essential.
The chassis and motor grounds would be routed separately to earth. What I was suggesting is to join the signal grounds at the output jack ground instead of the binding post:
I built a pair of Step-Up Transformers in a separate metal box, with internal mumetal shield. It is far from the turntable or any magnetic field source. Even so I had to play with the orientation. Eventually I found that the mains wiring in the wall has a strong stray field, and my transformers pick it up. I'd never place them close to the turntable motor.
You could, but only if the arm ground is not connected to chassis ground. It's not elegant, but it might be ok.
OK, since I still was getting a lot of hum, I connected the individual grounds to alligator clips to do point-to-point tests. At a certain point, the noise was so bad I was picking up radio frequency and I could hear a Christian radio playing in my amp.
Maybe, then, help came from Jesus in the form of a web page: http://jelabsarch.blogspot.com/search/label/MC step up transformers
That page shows a very clear and simple grounding scheme. What worked for me: 1) connect input and output signal grounds; 2) connect the SUT cans (which are mumetal shields) to the chassis; 3) connect the SUT grounds (which are not connected to the cans) and the tonearm ground together and to a binding post the goes to the pre ground (straight to the main star). The hum is still there but greatly reduced. I have an all-DIY 3-piece stack (TT, pre + phono, power amp) and I keep finding grounding flaws, so I'll focus on the other stages now. I can tell that the TT stage is quiet because the hum doesn't increase when I raise the preamp volume.
Thanks for all the help!
Maybe, then, help came from Jesus in the form of a web page: http://jelabsarch.blogspot.com/search/label/MC step up transformers
That page shows a very clear and simple grounding scheme. What worked for me: 1) connect input and output signal grounds; 2) connect the SUT cans (which are mumetal shields) to the chassis; 3) connect the SUT grounds (which are not connected to the cans) and the tonearm ground together and to a binding post the goes to the pre ground (straight to the main star). The hum is still there but greatly reduced. I have an all-DIY 3-piece stack (TT, pre + phono, power amp) and I keep finding grounding flaws, so I'll focus on the other stages now. I can tell that the TT stage is quiet because the hum doesn't increase when I raise the preamp volume.
Thanks for all the help!
Where do you find mumetal sheet or foil? In the US I can only find it on wholesale distributors' sites.
I might wanna build a mumetal box around my tonearm connections to further shield them from noise.
It was years ago, I ordered it on eBay from an Ukrainen vendor. It was a strip of about 1 meter x 20 cm. Mumetal or permalloy, I am not sure.
Step-up transformer should be as close as possible to ampliifer because they don't like shunt capacitance across their secondary. If lead from turntable to step-up transformer is twisted pair in screen it won't pick-up hum.