Why? Ortofon MC Quintet Mono and 2M Mono both hum slightly, but...

I have both an Ortofon MC Quintet Mono and a 2M Mono cart. I installed them both (not at the same time) on a Systemdek IIX turntable with a SME 3009 Series II Improved tonearm (fixed head shell). They both hum slightly and the hum increases with the volume control. The turntable is grounded to a McIntosh C28 preamp. The hum is not that bad, but it almost cuts in half when I select Mono on the preamp. Why does the Mono button decrease the volume of the hum?

 

Anyone?

 

Hard to be sure, but I'd guess it's a grounding issue, I'm guessing your arm is 40+ years old and if it's never been rewired there could be an issue with a cable.

 

The arm was rewired last year.

 

Did you have the issue before the rewire? Has it happened ever since the rewire? Have you tried any other carts without the issue?

 

No on three question; it has the hum with the just two mono carts. My questions is really why does the Mono button reduce the hum on a mono cart?

 

The mono button sums the channels so will be rejecting some of the noise, I still suspect a cable issue, especially as the mono switch reduces the problem and both those carts are basically stereo bodies internally strapped so all four wires are hot, the alternative to it being a cable issue is your phono stage, but assuming you have access to another that's easy to eliminate as the cause.

 

That makes perfect sense that the mono button sums the channels and reduces the hum. I am still wondering why only the two mono carts have audible hum. My stereo carts do not hum on this tonearm.