Ground Loops 101
Let’s start our discussion of ground loops with two pieces of chassis grounded equipment. Class II equipment can exhibit hum and buzz too, but the mechanism is a little different, but we’ll talk about that in another installment. We’ll also start with an “unbalanced” signal connection since it is more prevalent in commercial equipment (RCA-type connections and stereo mini-plug connections are examples of unbalance signal connections).
In the first example below a ground loop is created when the two pieces of equipment are plugged in and connected. DUT 1 might be a CD player and DUT 2 might be a Receiver. The signal from DUT 1 is passed through a signal cable to DUT 2. The “signal ground” connection is made between points “C” and “D”. Generally points “C” and “D” are also connected to the metal chassis. The “chassis ground” connection is made where the “earth ground” wire attached to the chassis. This happens at points “A” and “B”. The bare wire connection at points “H” and “J” on the AC outlets are earth grounded by the wire running back to the breaker panel.
The ground loop is formed, starting at point “H” to “A” to “C” to “D” to “B” to “J” and back to “H”. For simplicity we’ll just say “HACDBJ”. Any magnetic flux that passes through the loop will cause an electric current to flow in the loop. Any time current flows in a wire there will be a voltage impressed on that wire. Any voltage on the signal ground connection “CD” will become part of the audio signal and will be amplified. By virtue of the fact that your whole house is wired with AC wiring with currents flowing every which way, the ground loop will be bathed in magnetic flux. (Cable shielding is only effective for electric fields.)
Also any current on the earth ground between points “H” and “J”, regardless of where it comes from (like a light dimmer on the same circuit) will create a voltage in proportion to the current on “HJ”. Since the connection “HACDBJ” is in parallel with “HJ” part of the current will flow in “HACDBJ” and part of the current will flow in “HJ”. In electronics terms a parallel path is known as a “current divider”. The current flowing in “CD” will have an associated voltage, and this will be added to the signal from DUT 1.
The second example above is essentially the same as the first. The difference is that the two pieces of audio equipment are powered by two different circuits on the breaker panel. DUT 3 sends a signal to DUT 4 along an unbalanced cable with a signal ground “CD”. The chassis are grounded at “A” and “B” and the outlets are grounded at “H” and “J” just like before. The big difference here is that now the ground loop goes all the way back to the breaker panel. The loop is now a very large loop and can have even greater potential to pick up hum from magnetic fields. Also, now you have ground and neutral currents on the neutral bus bar coming into play. Ground current and neutral current from all the other circuits in the house can now put voltage on your signal ground “CD”.
So what can be done?
1. Make a block drawing of the components in your system, showing the signal connections (a single connection is fine for stereo connections) and the power cables. Also show in your sketch which power cables have an earth ground. Identify the house circuits that you are using, and show any other equipment that is sharing the same circuit (light dimmers, motors – like appliances, things with switch mode power supplies – computers and such, and things that might draw a lot of current or be injecting noise on the ground).
2. Study the diagram to see where your trouble might be coming from. Then try some trouble shooting and make notes of what happens. Only make one change at a time; do and undo the change so that you can have an A/B comparison.
3. Make loops smaller. Smaller loops have less magnetic flux inside them. Try running the AC cables right next to each other; use twist ties to hold them together temporarily. Shorten signal cables and twist tie them up.
4. Does your AC cable or signal cable hang in a “half turn” around a power transformer. (If this needs a sketch please let me know.) The cable will become part of the transformer. If your equipment cables hang in loops; tie them up. Reroute cables to eliminate or shorten loops.
5. Move or reroute cables away from cables that carry large currents or are known to have currents with high harmonic distortion (almost every appliance/piece of equipment that is not resistive will have varying levels of high harmonics on the AC power line/cord). Distance is your friend in these cases. Try a different cable orientation or a stack your components in a different order. The sensitive part of a PCB in one component may be right on top of a noisy transformer or rectifier in another. Maybe it sounds crazy but a sheet of steel between components can help, and in extreme cases it may be necessary to wrap a steel sheet around the offending transformer (a magnetic “short” circuit). I have measured hum reduction on the order of 25-30dB by doing this. But it’s better and easier just to move the noisy transformer away from the sensitive equipment.
6. Reduce the ground voltage between components. Plug all your components into the same AC outlet. Use a power strip if you have to. If there is a noisy light dimmer on the same circuit and you plug the system into a single outlet you can reduce the ground voltage difference (the voltage on “HJ”) to zero or nearly zero. Make sure your power strip is rated for 15-20 Amps.
7. Plug the offending equipment into a different circuit outlet, so that the noisy ground will not be on the same line as your audio equipment. Or if there is a permanent fixture/appliance (e.g. a ceiling light on a dimmer), plug your audio equipment into a different circuit.
8. If you are building a house, it can’t hurt to run a dedicated AC line to your equipment. This will generally cost $200-$300. If you can talk them into it, run a 20A circuit with 12AWG with a corresponding breaker and outlet instead of a 15A circuit. Keep the run as short as possible and don’t run it parallel with any other runs. And for a truly esoteric touch (I’m not claiming this will help, but it might) run the line in flexible steel conduit. If you are in an existing construction, it is not impossible to run a dedicated line, and sometimes there are creative solutions to getting an AC line where you want it, using an existing line that is more noise free.
9. Transformer coupling. Sometimes there is no other way… perhaps that can be its own installment.
10. The “pin 1 problem” (this is in reference to pro or “prosumer” products using balanced connections). This may require internal modifications... the manufacturer may already know about if if they have a product with this issue... but that's for balanced connections, and that's another discussion for another time.