Noise from pedals in a patchbay when not in signal path

[James Perrett]

If your iMac is providing the ground then it is likely to be via a fairly tortuous path which could easily cause problems. You ought to try a much more direct path to ground.

[Sam Spoons]

I don't know if iMacs have the same characteristic as MBPs but it's probably worth checking that the earth path between the earth pin in the mac and the earth of the chassis or USB lead is low resistance. MBP's PSUs have a resistance to earth of around 1kΩ, (but only if you use the 'optional' extension lead, if you use the included plug adapter there is no connection to earth).

[Echos]

If your iMac is providing the ground then it is likely to be via a fairly tortuous path which could easily cause problems. You ought to try a much more direct path to ground.

I can lift one of the speaker grounds, but then I have two paths to ground, one via the speaker and one via the iMac which is breaking the single path to ground rule isn't it? Unless I buy a USB isolator which seem to be incredibly expensive for a decent one. At least that way I would be able to provide a single path to ground.

Do you have any other suggestions?

[Echos]

A lot is going to come down to specifics of exactly how you've got your patchbay wired and the path the cables are taking, which is very hard to describe. I don't know if you could draw it out for us at all and post a photo (you have to host elsewhere and link to it).

Here you go: I've drawn up the connections. (Sorry. Adding images inline appears to be broken).

[Echos]

I don't know if iMacs have the same characteristic as MBPs but it's probably worth checking that the earth path between the earth pin in the mac and the earth of the chassis or USB lead is low resistance. MBP's PSUs have a resistance to earth of around 1kΩ, (but only if you use the 'optional' extension lead, if you use the included plug adapter there is no connection to earth).

Thanks for the suggestion, but I've checked the Mac's plug and it's definitely got a metal earth pin. Can't believe they don't include an earth on the laptop power supply. That's terrible.

[Hugh Robjohns]

I can lift one of the speaker grounds, but then I have two paths to ground...

There's nothing wrong with having 100 ground paths, provided any circulating ground currents don't get into the audio ground reference anywhere.

So it's really all about the detail of the engineering of the specific kit and how it's connected. These things can't usually be fixed with a simplistic approach.

It's late here now so I'll take a look at you diagram tomorrow to see if anything obvious is apparent.

[James Perrett]

If your iMac is providing the ground then it is likely to be via a fairly tortuous path which could easily cause problems. You ought to try a much more direct path to ground.

I can lift one of the speaker grounds, but then I have two paths to ground, one via the speaker and one via the iMac which is breaking the single path to ground rule isn't it?

There is no hard and fast single path to ground rule. It is a convenient suggestion when people encounter ground loops but in a well designed system you can have as many ground connections as you need.

The problems arise when you include a poorly designed piece of kit that relies on there being no currents flowing in the ground lines of the signal cables. This kit will be susceptible to ground loops. If everything is properly grounded through its power supply then there should be fewer problems because any ground currents should flow through the mains connectors rather than through the signal cables.

The ground connection on the Mac is likely to go through various components which may not affect the DC resistance but may increase the impedance at audio frequencies. So the ground on the Mac may actually make things worse. (NB I don't own a Mac these days so I'm making this assumption based on my experience with other computers.)

Hang on - I've just seen your diagram and noticed a power conditioner. Take it out - they're not normally needed and can cause more problems than they purport to fix. Take out the Radial box too - it isn't needed and is only going to confuse things.

[Sam Spoons]

I don't know if iMacs have the same characteristic as MBPs but it's probably worth checking that the earth path between the earth pin in the mac and the earth of the chassis or USB lead is low resistance. MBP's PSUs have a resistance to earth of around 1kΩ, (but only if you use the 'optional' extension lead, if you use the included plug adapter there is no connection to earth).

Thanks for the suggestion, but I've checked the Mac's plug and it's definitely got a metal earth pin. Can't believe they don't include an earth on the laptop power supply. That's terrible.

Lots of laptops don't have an earth connection*, the MBP does but only if you use the 'extension cable' and even then that is a high resistance path. Both the adapter and the extension lead have a metal earth pin so it's existence won't tell you if the it is actually connected or whether it is low resistance, you'll need to get the multimeter out I'm afraid.

* Laptops invariably require a low voltage DC supply and, as such, don't require a safety earth.The PSU is usually a class II device (double insulated) so does not require a safety earth either, if they are class I they will have a three pin connector including a safety earth but may or may not connect that through to the laptop.

[ef37a]

I would tie all the pedal jack sleeves together but not return that link to another ground. That way every pedal is grounded whether on or off.
I assume the 'all off' condition is not required?

Dave.

[Echos]

If your iMac is providing the ground then it is likely to be via a fairly tortuous path which could easily cause problems. You ought to try a much more direct path to ground.

I can lift one of the speaker grounds, but then I have two paths to ground, one via the speaker and one via the iMac which is breaking the single path to ground rule isn't it?

There is no hard and fast single path to ground rule. It is a convenient suggestion when people encounter ground loops but in a well designed system you can have as many ground connections as you need.

The problems arise when you include a poorly designed piece of kit that relies on there being no currents flowing in the ground lines of the signal cables. This kit will be susceptible to ground loops. If everything is properly grounded through its power supply then there should be fewer problems because any ground currents should flow through the mains connectors rather than through the signal cables.

The ground connection on the Mac is likely to go through various components which may not affect the DC resistance but may increase the impedance at audio frequencies. So the ground on the Mac may actually make things worse. (NB I don't own a Mac these days so I'm making this assumption based on my experience with other computers.)

Hang on - I've just seen your diagram and noticed a power conditioner. Take it out - they're not normally needed and can cause more problems than they purport to fix. Take out the Radial box too - it isn't needed and is only going to confuse things.

Thanks. The problem with pedals is they all seem to share ground between audio and power cables. I picked five of my pedals, a Boss, a Strymon, an EHX, a Wampler and an Eventide. I took the pedal and plugged a power cable in (with the other end of the power cable connected to nothing), then plugged a jack into either the in or out jack and again connected it to nothing, I then did a continuity test with a multimeter and in all cases there was a clear signal path between the power ground and the jack ground. This bring the case it's hard to see any way of eliminating ground loops because as soon as there is a connection between two pedals via a TR cable, there is a ground loop running from the power supply to the first pedal via the power cable, along the TR cable between the two pedals and back down the other pedal's power cable. This is surely multiplied for every pedal added to the signal chain. It would be a different matter if the signal path was isolated from the power supply, but if there is an inherent ground route via all power supply cables and via the audio cables that run between them I can't see any way of eliminating it.

The reason I have the Radial before the speakers is to ensure there aren't multiple paths to ground. If I remove it there will be three - via each speaker and via the iMac. Although given what I've said above about the pedals, there are already many paths to ground via each pedal's power cable. All connected by the signal path.

[James Perrett]

I think you are far too fixated on eliminating multiple paths to ground when you may find that the more paths to ground there are, the quieter the system.

The main thing to avoid is to have half of your setup plugged into one wall socket and the other half plugged into a wall socket on the other side of the room. Keeping everything plugged into the one wall socket should be fine with no extra precautions.

[Sam Spoons]

Have you tried it though? Hardly ant wall wart PSUs have a safety earth...