Amp humming when i power pedals in front of the amp with a power supply

presvis20

Member
Messages
17
I have a question for you. . When i power my pedals in the effects loop and in front of the amp my amp is humming. When i use batteries instead of a power supply for the pedals in front of the amp the hum dissapearsl and when i try to power them with any power supply i do get the hum again. Note that that happens with the pedals i have going in front of the amp.
. Note that i do use mxr brick and a t rex classic and i already checked my ma draw and its fine. I get this with my jvm205h and with my 6505+. What can i do? Thanks
 

dbun

Member
Messages
844
You have a ground loop problem

You need to use an isolated power supply for what you are trying to do. The MXR Brick is not isolated.
 

cap47

Member
Messages
2,273
I use a Dunlop DC Brick with no issues. Used a Radio Shack power supply once that made mine hum and got rid of it.
 

Kyle B

Silver Supporting Member
Messages
5,099
You have a ground loop problem

You need to use an isolated power supply for what you are trying to do. The MXR Brick is not isolated.
Maybe - But it could also very easily be a supply that's not putting out pure DC. How to tell --- Unplug one or more pedals (so the load on the supply is less). If the hum also decreases, then the supply has too much ripple. If it remains constant, then it's ground-loop
 

presvis20

Member
Messages
17
Guys the thing is that i tried different supplies with the pedals on the front of the amp and it is still humming. I dont use the same power supply to power the pedals in front of the amp, i used different ones and i still get the hum... Any opinions??
 

Kyle B

Silver Supporting Member
Messages
5,099
How many pedals? Do you get hum if you run just ONE pedal with any one of those supplies??? What are the ratings on the supplies??? What is the polarity of the plug?????
 

presvis20

Member
Messages
17
Even if i put one pedal it still does that. I am using the right power supplies with adequate amperage. Even a single wah wah with so little ma draw when i power it with power supply i still get the hum. But its not only the wah. Any pedal i power infront of the amp produces hum except if i use batteries.
 

Kyle B

Silver Supporting Member
Messages
5,099
Are your power supplies designed for guitar pedals, or did you buy 'em from WalMart?? Guitar pedals are unusual -- Most have a (+) shell and (-) tip. This is backwards from 99% of the other power supplies in the world.

There is more to the supplies than amperage and voltage ratings. The DC output, if not filtered well, would not look "DC" at all.

Graphically, the input & output look something like this:


AC input is RED. Unfiltered DC output is BLUE. Filtered is GREEN.

If your power supplies are unfiltered and putting out the BLUE waveform, this is bad and will cause major humming. Technically this is "DC" and the transformer manufacturer isn't lieing, but it's not what you want for an audio application. With adequate filtering the power should look like the green lines.

Unfortunately, without an oscilloscope, you'd have no way of knowing whether this is the issue or not (but I strongly suspect it is).

You could easily fix said issue by adding filtering. In this case, the filter is simple as a big-a$$ capacitor --- Something rated like 1000uF @ 25V (or higher) should fix it.



If you have an ohmmeter, you can check for a ground loop. There should be infinite resistance between the AC input (The pins that plug into the wall) and the output plug. You need to check all 4 possible connections (each wall pin to each part of the power plug) If there's continuity among any of them, you're setting up ground loops. Solution would be to get an electrically isolated supply.
 
Last edited:

TimmyP

Member
Messages
2,488
Are you plugging the power supply into the same AC socket as the amp? If not, try that.

Does your amp have a death cap in it?
 

donnyjaguar

Member
Messages
4,194
KyleB has got it. I built myself a regulated 9Vdc supply with 6 plugs on it. Internally I used a 7909 regulator with built in thermal shutdown. Its good for 1A which is plenty. I don't know who makes these commercially but someone should if they don't. No hum here.
 

Mark Robinson

Platinum Supporting Member
Messages
8,737
Plain as day it is a ground loop. Many guitar stomp boxes connect one leg of the D.C. to the signal ground. IF your power supply also supplies ground and is not isolated, you get a loop. Batteries are intrinsically isolated. Get a Voodoo Labs Pedal Power II. It will cure this problem, unless you have more than 8 effects. Then you have to start lifting D.C. grounds. Also, lift the ground on whatever pedal power supply you have. The alternative is to make a little terminal strip, and lift individual D.C. grounds which are redundant to signal grounds.

The pedal Power II solved all my hums. The Dunlop D.C. brick was a brick. I could not take it back to Sam Ash quick enough. I would think the Dunlop folks would know better?? They've got some smart people there. Too bad.
 

Shiny_Beast

Gold Supporting Member
Messages
10,604
Yuo could also try gong the other route, power them all off the same adaptor, loop and floor. I think making the ground references the same might at least keep the ground loop hum to a minimum.
 

Kyle B

Silver Supporting Member
Messages
5,099
Plain as day it is a ground loop. Many guitar stomp boxes connect one leg of the D.C. to the signal ground.
IF
* the transformer is galvanically isolated (i.e. there is no direct metal connection between the primary & secondary)

AND
* Neither the "+" nor the "-" of the transformer (hate to use the word "ground"!) has no direct metal connection to 3rd pin earth ground

THEN
*there can be no ground loop.

Yes? No?

Discuss :)
 

donnyjaguar

Member
Messages
4,194
I have noticed that some pedals look for an AC input. This includes some of the older Boss ones I have. Most will run on DC though, provided they are used with a "normal" DC model. I suppose there's a mechanism here for noise to get in.
 

CIOKS support

Member
Messages
177
Hi,

Older BOSS pedals with a ACA label aside the adapter input socket are not intended for AC but a unregulated 9V DC supply. I recommend to use a 12V DC regulated voltage for these, since they are intended to be powered of a higher voltage than the standard 9V. Optimally 12-13V, which is the voltage an ACA adapter puts out at lower loads, corresponding to what an analogue ACA pedal draws. Also you can daisy chain an ACA pedal with another PSA pedal (or use our Split Flex to power two) and then you can use regulated 9V, since the limiting resistor and a protection diode are then by-passed.

Best,
Poul Ciok,
CIOKS
 




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