[ad9000]

I am experiencing a constant, moderately loud 60-cycle hum, but only when the stylus is in the grooves. Before I go further and mention the equipment I'm using and lay out the steps I've taken to try to eliminate it, why would I be getting hum only when a record is playing? The signal is pretty much dead quiet when the stylus is not engaged.

I've tried various combinations of gear I have on hand. Tables: Pro-Ject RPM 1.3 and Rega 2, phono pre's: Slee Gram Amp II SE and Cambridge Audio 540P, and cartridges: Soundsmith SMMC2, Denon DL160, Sumiko Pearl. I know some of those aren't the best in terms of compatibility, but the idea was to try to isolate any particular link in the chain that might be causing the hum. I've swapped cables, lifted grounds, turned off my amp and other nearby components, and even moved the entire system to another room.

For what it's worth, the downstream components are a Portal Audio amp and Reference 3A Dulcet speakers.

Any help/insight would be greatly appreciated!

[Æ]

So, you've set the stylus in the groove? With the platter spinning and also not spinning? Have you tried setting the stylus in the groove, but with the deck (turntable) unplugged from the wall outlet?

[galyons]

What arm? Same arm in all of the iterations? If so, check the continuity of the tonearm wiring.  Check each lead to ground to see if there is a short to ground.  If the arm has a removable headshell check continuity of each contact/lead.  Your description indicates a wiring issue only when the arm lead circuits are energized by signal.

Tough one!
Geary

[ad9000]

So, you've set the stylus in the groove? With the platter spinning and also not spinning? Have you tried setting the stylus in the groove, but with the deck (turntable) unplugged from the wall outlet?

Yeah, I have tried that with the exception of unplugging. The tt motor has to be running to get the hum.

[ad9000]

What arm? Same arm in all of the iterations? If so, check the continuity of the tonearm wiring.  Check each lead to ground to see if there is a short to ground.  If the arm has a removable headshell check continuity of each contact/lead.  Your description indicates a wiring issue only when the arm lead circuits are energized by signal.

Geary,
RB250 arm on the Rega and the stock arm on the Pro-ject. I didn't suspect a ground issue, but I suppose it is possible. I'll try what you suggest and report back.
I'm thinking somehow the tt motor is a factor in this, as the problem doesn't occur until the motor is running.

[Wayner]

You are not getting hum from a magnetic field, rather hum from a mechanical vibration that is at the 60 cycle frequency, or combination of harmonics of it.

I have encountered this when I have built my own turntables. The motor/power supply is causing the vibration, the belt is transmitting it and the platter/record is receiving it.

You need to isolate the cause of the vibration with the use of damping techniques. The offender is too coupled to the chain.

Wayner

[ad9000]

You need to isolate the cause of the vibration with the use of damping techniques. The offender is too coupled to the chain.

The motor on the Pro-ject is free-standing, so it would be easy enough to try wrapping it. Any type of damping material you'd specifically recommend?

[Wayner]

That is odd. I'd suggest some experimenting first. Perhaps I'd try some cork under the motor assembly and see if that reduces the hum.

Remember that even when a TT motor is off, there is a small amount of current passed thru to it via the switch capacitor. It filters by frequency, but some small amount of current energizes the motor slightly. To prove this to yourself, simply unplug the motor from the wall outlet and see if the hum stops.

Wayner

[ad9000]

Perhaps I'd try some cork under the motor assembly and see if that reduces the hum.
Wayner

Wayner,
Brilliant suggestion, thank you very much! I wedged some small rigid foam pieces under the motor, which raised it about 1/4" off the surface, and hum is completely gone. I'll probably come up with a more elegant solution, but at least I found the source of the problem.
This forum is a great resource, isn't it?

[Wayner]

All of us old turntable farts have been around the block a time or two. Now, you know a trick. This is how we all learn, from others that came before us.

Great to hear it is getting fixed!

Wayner

[Æ]

Yeah, I have tried that with the exception of unplugging. The tt motor has to be running to get the hum.

OK, so the source of the hum was mechanical from the motor.

[ad9000]

OK, so the source of the hum was mechanical from the motor.

Yeah, since I wasn't even thinking in terms of it being from a mechanical (or electromechanical) source, I was essentially on the wrong track. The table and motor assembly are on a 2" maple platform on isoblocks, which was receiving the vibration from the motor and causing it to resonate through the belt, platter and record in succession and be picked up by the stylus. What's interesting to me is that the hum was exactly 60 hz (I checked on a Strobotuner). I'm sure there's some law of physics that would explain why the vibration is the same frequency as a typical ground hum, but I guess I wasn't paying attention in class that day.

[Æ]

Yeah, since I wasn't even thinking in terms of it being from a mechanical (or electromechanical) source, I was essentially on the wrong track. The table and motor assembly are on a 2" maple platform on isoblocks, which was receiving the vibration from the motor and causing it to resonate through the belt, platter and record in succession and be picked up by the stylus. What's interesting to me is that the hum was exactly 60 hz (I checked on a Strobotuner). I'm sure there's some law of physics that would explain why the vibration is the same frequency as a typical ground hum, but I guess I wasn't paying attention in class that day.

Turntable motors are often, usually synchronous. With the 60Hz line frequency.

[Wayner]

Transformers are known to hum. The old style core and coil ones were made from sheets of steel, welded or crimped together. Once in a while, these laminated sheets would come lose and start vibrating. Since the transformer is operating at 60 hz, the sheets of steel would start to vibrate at that exact frequency and mimic 60hz emf. So many people would be busy trying to rig up Faraday cages to stop the hum, when in fact it was all a mechanical noise. Shielding did nothing to stop it. Securing the laminates did.

I think this problem happens lots of times when dealing with turntables. The bad news is, it's hard to determine if it's emf or mechanical by nature, and that is the first path to take...diagnosis. Then the cure comes.

Wayner

[Æ]

Transformers are known to hum. The old style core and coil ones were made from sheets of steel, welded or crimped together. Once in a while, these laminated sheets would come lose and start vibrating. Since the transformer is operating at 60 hz, the sheets of steel would start to vibrate at that exact frequency and mimic 60hz emf. So many people would be busy trying to rig up Faraday cages to stop the hum, when in fact it was all a mechanical noise. Shielding did nothing to stop it. Securing the laminates did.

I think this problem happens lots of times when dealing with turntables. The bad news is, it's hard to determine if it's emf or mechanical by nature, and that is the first path to take...diagnosis. Then the cure comes.

Wayner

I would never have thought it to be mechanical hum, but I'll tuck that useful bit of information away in my noggin.
The whole episode reminds me of a STEREOPHILE article by Corey Greenberg. He had a problem with his REGA turntable and his particular pickup. The cure was, he added shielding over the motor. Following his advice I obtained some samples of Netic and Co-Netic sheet metal from Magnetic Shield Corp.

[Wayner]

And Alan, sometimes the hum problem is a combination of the 2. That is when hair starts to get pulled out. One hum source mimicking the other one makes isolating the problem a can of worms if both are adding to the situation.

Me thinks luck has lots to do with it. That is why I like to use known cartridges that are EMF hum busters, then if you end up with a problem, you know that it is mechanical in nature. But it also could be poor grounding on the tonearm, or suspension, a stepdown transformer giving off EMF or any number of fun campaign contributors.

It's amazing that some of this shit works at all......

 

[ad9000]

That is why I like to use known cartridges that are EMF hum busters, then if you end up with a problem, you know that it is mechanical in nature.

Wayner,
What cartridges have you found to be in that category? It would be useful to know for future reference.

Thanks.

[ad9000]

The cure was, he added shielding over the motor. Following his advice I obtained some samples of Netic and Co-Netic sheet metal from Magnetic Shield Corp.

I wonder if the type of shielding tape that is meant to be used in electric guitar control cavities would work well for this purpose? It would be fairly manageable and could be easily cut to size.

[Wayner]

An AT440MLa is a very shielded cart, void of most hum issues.

Mumetal is the best on earth shielding material.

Wayner

[BaMorin]

I wonder if the type of shielding tape that is meant to be used in electric guitar control cavities would work well for this purpose? It would be fairly manageable and could be easily cut to size.

Copper foil used in guitar cavities are to stop RFI....or having your guitar become a reciever. The copper foil won't stop any low frequency EMI........60hz is low frequency.