Grounding in Guitar Rigs
"Having a quiet guitar rig brings the magic. Having a noisy one is a pain in the amp. " (sigh)
Jody "KingSnake" Page (PedalSnake inventor)
The helpful info on this page apply to all guitar rigs, whether you use PedalSnake or not. We include tips on how to avoid hum in PedalSnake rigs too.
Find out how to avoid ground loop hum when using:
Some Common Different-Ground-Points in Guitar Rigs
If different grounds get "crossed" (connected) in your rig, hum can result. The most common way to cross grounds is by using a "power chain" that connects to pedals in different "pedal chains".
PedalSnake defines your main amp's input as the Primary Ground of your rig Other grounds that may cause hum if crossed with Primary Ground are called "different grounds". The good news is, most rigs have only Primary Ground, and contain no different grounds. To make sure, you can always do our Simple Hum Test.
Some common different grounds found in rigs are:
- Tube-Driven FX Loop Fortunately, most tube amps do not have these---their FX Loops are driven by solid state drivers, not tubes. If you have a true tube driven FX Loop (usually found in more expensive tube amps), see "Avoiding Hum in" Power-Chains, Dual G-Lines, and Dual P-Lines.
- 2nd Amp To avoid hum, most folks use an A/B box with isolated outputs. So use 2 Single G-Lines, not a Dual G-Line (the shared ground wire will "un-isolate" the A/B box).
- Footswitch Lines Don't combine these with an audio signal on a Dual G-Line.
While some success can be had with transformerless designs, we have found that the best hum eliminators are based on true isolation transformers. A very good product is made by EbTech.
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A Simple Hum Test for Your Rig
It only takes few minutes to test your rig to see if it has "the potential for" ground loop hum (which means you're testing your rig for different ground points).
You will be comparing the amp-input ground (primary ground) to other connection points in your rig---like an FX Loop, or a 2nd amp, etc.
You will need:
- 4 guitar cords (at least 2 with metal plugs)
- A short piece of wire for "shorting out" between the pedals.. A guitar cord, knife blade, etc., will also work.
- Connect the 1st pedal
- Pedal Input: To the guitar (with its volume off).
- Pedal Output: To the Amp Input (your Primary Ground), using a cord with a metal plug.
- Connect the 2nd pedal
- Pedal Input: To an output of the "other rig point" (FX send on the amp, or output from a rack unit)
- Pedal Output: To an input (or return) of the other rig point (like FX Return, or input of a rack unit), using a cord with a metal plug.
- NOTE: . Do not Power-Chain the 2 pedals. That starts you out with the 2 grounds shorted together, and we're trying to compare the grounds shorted and un-shorted. Use 2 isolated supplies (a battery is essentially an "isolated supply").
- Turn the amp on.
- The pedals are powered, but stomped "off".
- Turn the amp volume up loud enough to begin hearing some noise.
- Note the level of any hum.
- Take the short piece of wire
- Touch it to both metal plugs on the pedal outputs ("short" them together).
- Short and un-short the plugs while listening to the hum-level.
- Is the hum louder when the plugs are shorted together?
- If so, consider the other rig point as a "different ground"
- To avoid hum, follow all the guidelines belowto keep that rig point isolated from the amp input.
- If all else fails, one of the audio signals can be isolated from the other, breaking the ground loop, by using an audio isolation transformer. Ebtech makes a good one for a reasonable price.
- If you have other rigs points to test, move the 2nd pedal to the next one and repeat the test from Step 2.
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Avoid Hum in Power-Chains
Remember, power patch bays on pedalboards are usually just a power-chain from a single supply.
Most of the time it is fine to connect a power-chain across 2 audio-chains of pedals.
However, if you power-chain across audio-chains and you get more hum, a ground loop was created, and the 2 audio-chains need to be isolated.
FX Loop chains and other audio-chains can sometimes be a problem, but the worst case is when the audio-chains connect to 2 different amps (even if they plug into the same outlet). The path of the ground-loop currents is highlighted at the right. The power-chain is the culprit.
The only sure way to avoid hum using a power-chain is to keep the power fanout connections within one audio-chain (at right).
This is one reason why we recommend wall warts over fancier supplies. They are isolated "transformers" (which is why they are big and bulky), and its not expensive to add one more when needed.
Does your rig have the "potential for" ground loops when using power-chains? Use our Simple Hum Test.
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Avoid Hum in Dual G-Lines
An RS2 Dual G-Line can give you 2 G-LInes while using up only one (1) BaseSnake channel. There 2 sure "hum free" ways to use RS2:
Both of these applications gaurantee that both sides are at the same ground (RS2 has one shared ground wire). All other applications of a Dual G-Line should be tested using our Simple Hum Test.
The following should NOT be shared on an RS2 (use 2 RS1 Single G-Lines):
- Sharing a Footswitch Line with an Audio Line (guitar signal)
- Two stereo amps.
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Avoid Hum in Dual P-Lines
Dual P-Lines can deliver 2 pedal voltages using only one BaseSnake channel.
An MM2-I (or MF2-I) Dual Isolated P-Line functions just like 2 Single P-Lines, and:
- Works with Any 2 Supplies AC or DC, whether they are isolated or not.
- Requires a white 5-wire channel On a 3-wire channel, only the red side works.
An MF2 Dual P-Line shares one ground wire between both sides, so both sides must be at the same ground to avoid hum. They are gauranteed to be hum-free when used:
- Within a Single Audio-Chain of Pedals When using isolated power supplies, a single audio-chain will have one ground---which is the ground of the AC-powered device that the chain's last output connects to.
- With Two Isolated DC Supplies AC-output supplies usually don't share well on a Dual.
P-Lines commonly use a power-chain, so see Avoiding Hum in Power Chains above.
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