why turntables have a ground wire
That thin extra wire carries no music, yet leaving it loose can make a quiet turntable hum through both speakers. Here is what it actually does.
A turntable can be wired to the correct input, play at the correct volume and still play a steady hum behind every record; then you notice the random little screw on the side of its red and white plug sockets, and you wonder why you are getting so much background noise. Let's consider: because the cartridge signal is minuscule and the phono stage amplifies everything it receives, including the electrical rubbish you never asked it to give you, you hear the rubbish amplified.
The answer is usually simple. If the turntable provides a separate ground wire, connect it to the proper ground terminal on the phono stage or amplifier receiving the turntable signal. In simple terms, the wire keeps the metal casing of the deck and the sensitive phono circuit at the same electrical reference. It is not a second audio channel, it is not a protective mains earth, and it should never be attached to a random screw in the hope that any piece of metal will earth it.
why a turntable is so sensitive to hum
A phono cartridge's output is a much smaller signal than a CD player, streamer or other line-level source. The phono stage has to add a large amount of gain before your amplifier can use it, it has to reach 'line-level' and the phono stage is scrupulously fair and honest: anything in the signal, whether that be unwanted noise or your beautiful music, gets the same treatment.
UK mains electricity runs at 50Hz, so mains-related interference is often heard as a low, steady hum. A stronger 100Hz buzz can come from power-supply ripple or another fault in the system. Sometimes the nature of the sound does not diagnose the exact cause on its own, but a hum that appears only on the phono input is a useful sign that the turntable path itself deserves your attention first.
The tonearm and other metal parts of many turntables need a reference to the phono stage, and it's the separate ground lead that provides it. Without that connection, a small voltage difference or picked-up interference can find its way into the signal. Therefore, by joining the two intended ground terminals, the unwanted current has a controlled path, so tightening one unassuming wire can make the hum disappear.
where the ground wire goes
Follow the music leads and the answer normally reveals itself. If the turntable's RCA pair plugs directly into an amplifier input marked PHONO, the ground wire belongs on the amplifier's nearby GND or turntable-ground terminal. If the cable pair goes first to a separate phono stage, the ground wire will follow and go there instead (if the phono stage permits grounding). The phono stage's output then carries a line-level signal onwards to AUX, CD or LINE on the amplifier.
To attach the lead, just loosen the little screw, the ground terminal, place the spade connector or bare wire beneath it, and tighten it finger-tight. The connection should hold firmly without crushing the wire. Keep the ground lead close to the turntable interconnect rather than allowing it to interact with any mains cables or power supplies.
However, consider, some turntables will not have a separate grounding wire because the return is arranged through the signal cable or internal electronics. The sound of silence is not a missing accessory. Do not add a homemade wire merely because another deck has one, and do not assume an unused screw on the chassis is the secret terminal the manufacturer forgot to mention.
turntables with a built-in phono stage
A turntable with a PHONO or LINE switch can send two very different signals through the same RCA sockets. In PHONO mode, the deck is sending the cartridge-level output and still needs a phono stage later. In LINE mode, its internal phono stage has already raised and equalised the signal, to bring it to 'line-level' so it belongs in the AUX, CD, LINE or any normal red and white RCA analogue input.
The switch does not create a universal grounding rule. Some manufacturers still ask for the separate lead in LINE mode, while other designs handle the reference internally. Keep or remove the wire according to the turntable instructions rather than treating LINE as permission to disconnect it automatically. The signal level and the grounding arrangement are related parts of the deck, but they are not the same setting.
when connecting the wire does not cure the hum
Connecting the ground wire cures the most classic problem, but not every hum is due to a missing grounding wire. For the initial checks, start with the volume control on the amplifier; if it makes no difference to the hum, you can rule that out. Ensure the equipment is switched off, and all the RCA plugs are tight, secure and properly seated. Ensure any PHONO or LINE switch matches the amplifier input. Consider too that any loose shielding or poor plug connection in your RCA lead can induce a hum just as convincingly as an unattached ground wire.
A hum can be due to your local neighbourhood, so check there also. Move the turntable and its delicate signal leads away from the amplifier's mains transformer, plug-in power supplies, mains blocks, and chargers. Consider, a cartridge can pick up a magnetic field directly, so just moving the turntable one shelf can achieve more than replacing a perfectly good interconnect with something more expensive because it's beautifully braided.
If after trying the aforementioned the hum still remains, it's a good idea to consider reconnecting all the equipment in your hi-fi system; disconnect the source equipment one item at a time, powering down between changes. Consider that a television, aerial connection, computer or another earthed component can create an additional path and complete a ground loop elsewhere in the system, so try and remove those from the area also. Never remove the protective earth from mains-powered equipment as an experiment. A safety earth protects people; it is not a noise-control accessory and no quiet record is worth defeating it.
A hum in just one channel points more readily towards an RCA plug, damaged lead, headshell contact or even tonearm wire. A hum on every amplifier input points beyond the turntable's path. Also, if a fuse or protection circuit operates, a component becomes hot, a cable looks damaged or touching metalwork changes the noise dramatically, stop swapping wires and ask your local dealer. Troubleshooting should narrow a safe fault, not create a new one.
the two-minute checklist
Follow the turntable's RCA leads to the phono stage or PHONO input, then connect the separate wire to the dedicated ground terminal on that same box. Match PHONO or LINE to the input you are using. Keep the cartridge and signal leads away from transformers and power supplies. If the hum remains, check plugs and cartridge leads, then isolate other sources one connection at a time. Never use a random screw and never defeat a protective mains earth.
Once the signal path and ground connection are clear, build the complete plan for your system before buying a supposed cure. The little wire carries no music, but when the design provides one it gives the music the silence it deserves. Happy listening.