Hum on MM cartridges

[davidro]

G'day all

I'm currently using Sherwood PM-9805. I use 2 MM cartridges being a stock AT cartridge and an M97xE. Both produce noticeable hum from say 9-10'o clock volume.

The hum decreases to about 60% if I touched the headshell, tonearm, phono cable, amp (whether it's connected to an inbuilt phono stage or a separate phono pre) or even the CD player connected to the amp.

I changed headshells too. Same result.

BUT if I used an HOMC cartridge, it's all quiet. Zilch hum. Just like a CD player.

Wonder what's causing this?

Cheers

 

[Doug G.]

If your headshells are metal, I know the M97xE must be isolated from the shell because its metal mounting and body touching the shell and the little gound strap coming out of the cartridge on the left channel ground pin will cause a ground loop.

This is because of the more-than-one ground paths and if they aren't all exactly at the same voltage potential, an AC voltage will be developed between them and amplified.

You will need to look at the AT and see if the same thing is true there.

Cartridges with plastic or other insulating material bodies will not exhibit this characteristic.

Plastic screws and spacers or other insulators can be used to isolate the cartridge. Just so the metal on the cartridge doesn't touch the metal headshell.

The alternative is to remove the little ground strap on the cartridge but I don't like to do that.

Doug

 

[Hackster!]

Are the two "hum" carts wired the same way in the headshells as the "no hum" cart? If you're using a headshell that's a different brand than the turntable, wiring "red to red" etc. doesn't always work; the pinout in the tonearm's bayonet receiver will vary from one manufacturer to the other. That can lead to hot and ground getting reversed, etc. Just a thought.

 

[catman]

G'day all, these sorts of hum issues can be 'complex'. :yes: My Shure M97xE's are in metal headshells and the earthing strap's have not been removed, yet I have no hum issues at all.

However I also have a rather nice Empire cartridge where it was necessary to remove its cartridge earth stap to stop hum/ buzz. Every situation tends to be different. :yes: :scratch2: Regards, Felix aka catman.

 

[davidro]

If your headshells are metal, I know the M97xE must be isolated from the shell because its metal mounting and body touching the shell and the little gound strap coming out of the cartridge on the left channel ground pin will cause a ground loop.

This is because of the more-than-one ground paths and if they aren't all exactly at the same voltage potential, an AC voltage will be developed between them and amplified.

You will need to look at the AT and see if the same thing is true there.

Cartridges with plastic or other insulating material bodies will not exhibit this characteristic.

Plastic screws and spacers or other insulators can be used to isolate the cartridge. Just so the metal on the cartridge doesn't touch the metal headshell.

The alternative is to remove the little ground strap on the cartridge but I don't like to do that.

Doug

I was aware of that headshell issue. So I tried the M97 without having it bolted to the headshell in order to bypass the metal issue. Still the same hum.

Are the two "hum" carts wired the same way in the headshells as the "no hum" cart? If you're using a headshell that's a different brand than the turntable, wiring "red to red" etc. doesn't always work; the pinout in the tonearm's bayonet receiver will vary from one manufacturer to the other. That can lead to hot and ground getting reversed, etc. Just a thought.

That's a nice thought! I might try swapping the wiring. But the non-problematic HOMC is wired in the same way.

G'day all, these sorts of hum issues can be 'complex'. :yes: My Shure M97xE's are in metal headshells and the earthing strap's have not been removed, yet I have no hum issues at all.

However I also have a rather nice Empire cartridge where it was necessary to remove its cartridge earth stap to stop hum/ buzz. Every situation tends to be different. :yes: :scratch2: Regards, Felix aka catman.

Yeah the hum issue can be very tricky.

 

[davidro]

Swapping wires was no success
On the other hand, I found out unplugging the CD player helps.

 

[lini]

David: Well, the basic reason why it happens with the MMs, but doesn't with the MCHO, is that the latter has a much lower coil impedance, which makes it less sensitive. That's also the reason why it's possible to nudify MCs, while it wouldn't be a good idea to free an MM from its shielding shell around the coil & polepiece arrangement.

Did you already check for possible hum inducers like big transformers near the table and experiement with placement?

Greetings from Munich!

Manfred / lini

 

[guiller]

Does the TT allow for changing the interconnect cables?

 

[davidro]

David: Well, the basic reason why it happens with the MMs, but doesn't with the MCHO, is that the latter has a much lower coil impedance, which makes it less sensitive. That's also the reason why it's possible to nudify MCs, while it wouldn't be a good idea to free an MM from its shielding shell around the coil & polepiece arrangement.

Did you already check for possible hum inducers like big transformers near the table and experiement with placement?

Greetings from Munich!

Manfred / lini

I can't really think of any big transformers. But I'll try to see if I can get rid of anything on the way. Thanks for the tip!

Does the TT allow for changing the interconnect cables?

It does and I've tried that too. There was no difference.

 

[cgutz]

Don't know if this could cause it, but is the polarity on the AC cord of the amp proper? If the case is hot instead of neutral, possible AC feedback to the TT? The fix would be flip the AC plug for the amplifier over so the spades are reversed (assuming you are in USA with 110/120 AC).

I once measured the case to a ground and found my amplifier was plugged in wrong. Being vintage, it didn't have a polarized plug, and the case had voltage. Flipped the plug over, and marked the proper orientation with a sharpie.

Just a thought, someone else correct me if I'm wrong and this could not cause hum. Some people claim it will make the amp sound better...

 

[davidro]

Don't know if this could cause it, but is the polarity on the AC cord of the amp proper? If the case is hot instead of neutral, possible AC feedback to the TT? The fix would be flip the AC plug for the amplifier over so the spades are reversed (assuming you are in USA with 110/120 AC).

I once measured the case to a ground and found my amplifier was plugged in wrong. Being vintage, it didn't have a polarized plug, and the case had voltage. Flipped the plug over, and marked the proper orientation with a sharpie.

Just a thought, someone else correct me if I'm wrong and this could not cause hum. Some people claim it will make the amp sound better...

I'm in Australia so spades can't be reversed. But it may be they wrongly assembled the plug to begin with. My amp was bought new a couple of yrs ago so wouldn't make a vintage unit...

 

[Hackster!]

David, I swear, it sounds to me like the polarity may be reversed in your cart wiring, especially since touching them reduces the hum.

Two questions- has it been this way since you started using these headshell rigs with your table, or has it just started suddenly, on its own? And are you absolutely sure that the pins on the carts are wired to the correct pins in the headshell?

Not trying to be tiresome, just want to help...

 

[davidro]

David, I swear, it sounds to me like the polarity may be reversed in your cart wiring, especially since touching them reduces the hum.

Two questions- has it been this way since you started using these headshell rigs with your table, or has it just started suddenly, on its own? And are you absolutely sure that the pins on the carts are wired to the correct pins in the headshell?

Not trying to be tiresome, just want to help...

John no way I'd find you tiresome! On the contrary thank you very much for trying to help. Unfortunately I do not recall precisely whether it started all of sudden or not. I suspect it was always like this. But I'm absolutely sure they're wired correctly, because I've tried swapping them in various ways the other day, which in some case resulted in louder hum.