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It is currently 20 Apr 2019, 08:19

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 Post subject: Re: Turntable hum
PostPosted: 22 Mar 2019, 20:33 
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Joined: 09 Oct 2012, 19:43
Posts: 334
Location: Vancouver Canada
The pic is just confusing the issue.
You have 4 wires coming out of the needle. The wires are fed down the arm and into the chassis. They then connect to heavier
shielded cables to make connection with the amp. The chassis ground connection however, contrary to the pic, should not be
connected to the audio cables at all. Chassis gnd should be brought threw a third wire from turntable gnd to stereo amp gnd.
The 4 wires feeding to the cartridge should NOT be connected to chassis gnd at all, anywhere, from the cartridge to the phono
input on the stereo amp. Make sure the 4th wire shown in the pic is not connected to the chassis at the solder terminal where the wires are soldered. Use this terminal for gnd and isolate the cartridge connection from that terminal. Try it. Some turntables are not so fussy but some are.

A second thought is, have you made sure the 2 wires for left channel are connected at the cartridge's left chnl pins. Then same for right channel. Mixing these connections could/will have adverse effects like hum.


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 Post subject: Re: Turntable hum
PostPosted: 22 Mar 2019, 21:29 
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Joined: 04 Jun 2008, 20:59
Posts: 3953
Location: Arizona, USA
Hi, I have seen the arrangement in the drawing and it is indeed as the company supplied them. Even now some cartridges have their shield (and thus the metal parts of the phono) connected to the ground terminal of one of the channels. IMO it is not a smart thing to do and will cause problems....but then who am I to know ;) . Anyhow the modification they propose to fix the hum is fine. It separates the signal ground from the chassis ground.

Now back to the hum...if it only occurs when the needle is on the record and it only occurs then when the platter is moving it means it is related to mechanical noise from the platter. The only way something like hum can get through is from the motor or the platter bearings. I believe this particular turntable is a belt drive (*) and so the motor should not be transmitting hum (noise from the motor spinning at some frequency related to the AC mains frequency) to the platter. The platter is normally attached to the plinth with a sturdy bearing assembly and can pick up any vibrations that are introduced to the plinth. If the motor mounts (rubber bushings) are hardened from age they will do this. A motor with loose internal bearings can also vibrate at a multiple of the AC mains frequency. The only other way I can envision hum getting into the system is if the platter is not metalic and the cartridge is picking it up directly from the motor. In that case it would vary depending on where the arm actually is at the time. Further in would typically be worse than at the outside edge of the LP. The cartridge you indicated you have is not one noted for direct hum pick up so I don't think this is the case. I have several ones made by Grado that are sensitive in this manner and they really do not work well when the motor is not well shielded and the platter is not metal.

*If it is an idler wheel drive then the drive wheel can cause noise, but typically it is rumble and not anything like hum.

Good listening
Bruce

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 Post subject: Re: Turntable hum
PostPosted: 25 Mar 2019, 09:38 
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Joined: 19 Oct 2018, 15:30
Posts: 90
Location: Montréal, Québec
The thorens TD165 is belt driven. The motor is bolted to the chassis top plate, but the plater spindle and bearing are on a separate suspended chassis that is held to the top plate with 3 springs. The tone arm is also fixed to the suspended chassis, so mechanical vibrations to the platter should be very minimal. With the belt of, the platter turns litterally for minutes with a hand spin. The motor shaft and pulley wasn’t loose the last time I checked.

I might still have a ground issue though. If the sheild of the cartridge is internally connected to one of the 4 terminals, I am creating a ground loop by adding a separate ground wire to the preamp. Because the tone arm’s tube is grounded to the chassis, Maybe I am creating two ground paths if there is continuity between one of the channels and the tube. I will open it and make shure that the 4 wires are actually separated from the chassis.

I came back home from the Montreal audio show yesterday with some new vinyls, and quickly connected the turntable to my system to give them a try. I totally forgot to connect the ground wire to my preamp and only realized that this morning. The hum didn’t seemed as bad as the last time I tried the truntable, so it makes me think that the added ground wire is creating a second ground path. There was still a hum, but lighter than it was before.

I’ll check everything out and keep you posted with the results. Thanks !


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