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Author Topic: ground loop issues  (Read 357 times)

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ang3lus

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ground loop issues
« on: Time Format »

hey all,

i recently fired up my unit for the first time, it fires up fine but when i try to connect output A or B to a mixer or amp reeturn i get horrible hum, the ground loop hum, my unit is 110v and i'm using a step up transformer to 220v.

the hum happens also when the unit is OFF, so it must be a loop, i'm using a normal guitar cable, maybe using non shielded cables will help ? haven't tried it yet.

when pluggin from headphone out to the mixer or amp return i get no such issues, everything is dead quiet.
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Rusty

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Re: ground loop issues
« Reply #1 on: Time Format »

hey all,

i recently fired up my unit for the first time, it fires up fine but when i try to connect output A or B to a mixer or amp reeturn i get horrible hum, the ground loop hum, my unit is 110v and i'm using a step up transformer to 220v.

the hum happens also when the unit is OFF, so it must be a loop, i'm using a normal guitar cable, maybe using non shielded cables will help ? haven't tried it yet.

when pluggin from headphone out to the mixer or amp return i get no such issues, everything is dead quiet.


Try feeding the mixer or amp return from the MP1 send line out jack, use a known good working guitar lead and keep the mixer or amp volume way down and tell us what you got ?

Unshielded cables are no good, they will make the matter worse.

You say that your MP1 unit is a 110V one and your mains is 220V, then your stepping down, just for correction.
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rnolan

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Re: ground loop issues
« Reply #2 on: Time Format »

Sounds like the A and B output jacks have been replaced with normal jacks that earth to the chassis/case rather than jacks with a plastic/insulated sleeve so they don't connect/contact to the chassis.  This was an issue MikeB discovered when he changed the output jacks on his B200s with normal style jacks and fixed when he put in insulated jacks instead.The Fx send always has a signal (volume controlled by the send/return knob), as Rusty suggests, try it, the signal is after the tube/gain stage but before the chorus (where it goes stereo and then feeds the A and B outs).Always use "known good" shielded leads, as Rusty says, unshielded cables will make it much worse  :facepalm: .You can use the headphone out to feed L/R mixer inputs for recording, the output has the treble reduced (like a cab simulator) to feed full range gear (like headphones, full range monitors etc.)
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Studio Rig: MP2/MB1 > Eurorack UB2442FX-Pro > TC M-One XL > Behringer Truths (with Truth Sub) / QUAD ESL 63s
Live Rig: MP2/MB1 > Alesis Quadverb > Alesis Midiverb 4 > Yamaha AM802 > B200s/Carvin TS100/DCM200L > ADA Split Stacks/Mesa P112s

ang3lus

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Re: ground loop issues
« Reply #3 on: Time Format »

interesting, I thought the unit was not messed with, I will add washers, it makes sense actually.

I will test the unit beforehand thru the send connection, gotta do the battery mod anyway and replace the caps maybe, i haven't been inside yet.

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Rusty

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Re: ground loop issues
« Reply #4 on: Time Format »

Yes, I was thinking something along those lines too as rnolan says about the output jack earths grounding scheme. Those jacks need further inspection.

You will know by looking at the solder joints if they have been re-soldered and "maybe" the wrong jack types were accidentally put in there. I'm not a 100% sure on that as of yet.

Take some good pics when you take the lids off and post them up please for us to inspect, keep good info coming and it should become obvious what the problem is.
« Last Edit: Time Format by Rusty »
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"Let us never forget our beloved founder -
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rnolan

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Re: ground loop issues
« Reply #5 on: Time Format »

There's a picture of the output jack on MJMPs site (http://www.marshalljmpmodshop.net/miscellaneous_parts_and_replacement_tubes.htm).  It's a PCB mounting jack and is clearly isolating.  Unfortunately he's out of stock and it's no longer made.
@MJMP, is there a suitable replacement ?
My thinking here is that you said:1. Hum happens with unit on/off (I'm assuming connected to mixer or amp return in both states) so shouldn't be in the output circuit.  However, if the jacks are fine then there must be something else causing the hum
 2. the headphone out works and is dead quiet, this feeds off the output circuit much the same as the A & B outs but has a couple of caps to filter off the tops a bit so you don't have to change the patch eq (which will have more tops to feed guitar speakers but too much for full range (tweeters) speakers).
It seems (from MJMPs site) the same jack is used for a bunch of stuff (Replacement jack for MP-1 rear input jack (V1.38 models) output,headphone,effect send and MQ-1 input,MQ-1 output), none of which earth to the chassis.
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Studio Rig: MP2/MB1 > Eurorack UB2442FX-Pro > TC M-One XL > Behringer Truths (with Truth Sub) / QUAD ESL 63s
Live Rig: MP2/MB1 > Alesis Quadverb > Alesis Midiverb 4 > Yamaha AM802 > B200s/Carvin TS100/DCM200L > ADA Split Stacks/Mesa P112s

ang3lus

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Re: ground loop issues
« Reply #6 on: Time Format »

well i had some time to check and there's no hum from the effects loop out send to the mixer or return to the amp.

i will pop open the unit today and check
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MarshallJMP

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Re: ground loop issues
« Reply #7 on: Time Format »

Hey R, I have the rear jacks, the one you are referring to is the old style input jack.
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ang3lus

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Re: ground loop issues
« Reply #8 on: Time Format »

unit is completely stock, opened it today, the output jacks haven't been changed, is there anything else to check ?
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MarshallJMP

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Re: ground loop issues
« Reply #9 on: Time Format »

Can you take some hi-res pics of the mainboard and some of the midi board top?
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ang3lus

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Re: ground loop issues
« Reply #10 on: Time Format »

1
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ang3lus

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Re: ground loop issues
« Reply #11 on: Time Format »

Two
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MarshallJMP

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Re: ground loop issues
« Reply #12 on: Time Format »

Pic 1 is the digital side of the mp-1, can you take a pic of the analog side (left side). Also can you take a pic of the bottom side of the midi board.
« Last Edit: Time Format by MarshallJMP »
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