A few weeks ago, I got a radio station turntable. It’s a Technics SP-10 Mk II, with an SME tonearm and Stanton cartridge. Both the cartridge and the turntable chassis are grounded to the chassis of my stereo amp. (The AC wall outlet is properly grounded.) But there is still a little bit of hum, 'way in the background. I have to turn the stereo volume up to about 2 o’clock before you can hear it (much too loud for music listening), but 60 Hz hum is still present.
The instructions that come with the cartridge say that there is a ground strap on the left ground pin. Would removing it eliminate the hum? I can’t think of where else it might be coming from. Only the phono input on the amp hums, and it didn’t hum at all until I plugged in the new tonearm.
Perhaps trying a new cable from the phono to the amp would help. Also recheck that your phono is properly grounded to the amp and maybe check that the wall outlet is grounded correctly. As a last resort power down whatever else in the house you can, maybe there’s some feedback off a computer/fridge/washing machine etc…
Some cartridges are also more susceptible to hum than others. It might be worth experimenting by swiching some headshell + cartridge combos around if you have others.
Thanks for the replies. The tonearm is connected to the amp by a heavy-gauge cable with gold-plated RCA connectors. It doesn’t run near any AC cords. I can’t try any other headshell with this tonearm. It has a one-piece unit that’s the headshell and the shaft, that plugs into the rear section with a proprietary pin-socket connection. All I could do would be to install another cartridge, but this hum would most likely still be present.
Trying to eliminate 60 Hz hum can be a real PITA. As an example, I operate an electrodynamic shaker here at work (which is similar to a one-channel stereo system). The shaker table has always had a 60 Hz component. Last year I made some modifications to the grounding arrangement between the controller and amplifier, and the 60 Hz component only decreased by about 50%.
One of the main causes of 60 Hz hum is something called a “ground loop.” In most low to midrange stereo systems, all low-level signals are “single-ended,” which means the signal is always referenced to earth ground. This arrangement works O.K. if the impedance between any two grounds is very low. But if the impedance is not low, and if there’s a small voltage difference between two grounds, a small current will flow. This current is usually 60 Hz, and it finds it’s way into the signal path.
To eliminate a ground loop problem, you must use what’s called a “single point ground,” which (obviously) means that all ground connections must be made at one point. Practically speaking, this can be very difficult to do in most systems. In lieu of a single point ground, you can try tying grounds together using conductors with very low impedance (e.g. copper braid or copper ground plane).
It should also be mentioned that high-end stereo systems get around this problem by using a differential arrangement.
A quickie way to make an attempt at single-point grounding is to run the entire system off a good quality plugstrip. And I mean everything - turntable, amp, and all other sources such as tape decks and CD players.
The vast majority of hi-fi separates (excluding amplifiers) are double-insulated anyway, and have no earth wire to the mains to form a ground (gnd) loop. And yes, the LH gnd wire on the cartridge does indeed go to the thin grounding wire that’s connected to the earth terminal on the amplifier. The cartridge LH gnd also goes to the LH gnd of the phono cable.
Looking at the RIAA equalization curve, you can see that at 50Hz the preamp gain is +20dB. That’s a lot of gain at 60Hz still, with low signal levels from a relatively high impedance source with single-ended outputs, and it’s not surprising fishbicycle’s phono amp is picking up hum at high volume levels. It’s only to be expected, and the noise levels described sound about right. Some of the hum might even be coming from the amp - disconnect the phono leads at the amp end, and see how much hum remains.
Some old hi-fis used to incorporate 50/60Hz notch filters to frig away some of the hum. Took a chunk out of the music too, of course. They also sometimes had a high pass “rumble” filter at about 30Hz or so to attenuate the pickup of mechanical vibration from the turntables moving parts.