ADA Preamps > Original MP-1

ground loop issues

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ang3lus:
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.

Rusty:

--- Quote from: ang3lus on March 04, 2020, 01:27:19 PM ---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.

--- End quote ---


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.

rnolan:
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.)

ang3lus:
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.

Rusty:
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.

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