PRS MT15 effects loop hum

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mesaboogieE34L

mesaboogieE34L

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I've had problems with effects loop hum from my PRS-MT15 ever since I bought it, nothing plugged in the loop and it was dead quiet. The amp is completely useless when using the loop though, gates don't help either. Tried swapping 12at7 in V5 and it didn't do anything.

Finally did some more research and PRS recommended to use the shortest cables for utilizing the loop, I have some two foot guitar cables that I'm using and that did the trick, dead silence with the Zuul+ in the loop.
IMG_20250219_181139830~2.jpg
 
mesaboogieE34L said:
I've had problems with effects loop hum from my PRS-MT15 ever since I bought it, nothing plugged in the loop and it was dead quiet. The amp is completely useless when using the loop though, gates don't help either. Tried swapping 12at7 in V5 and it didn't do anything.

Finally did some more research and PRS recommended to use the shortest cables for utilizing the loop, I have some two foot guitar cables that I'm using and that did the trick, dead silence with the Zuul+ in the loop.View attachment 390032
Click to expand...
Tbh, that is bs to have to use that short of cables. But i am glad it is working for you!
 
I'm going to try some 5 or 6ft cables next, I was using 20ft.
 
I had a MT15 when they first came out. Killer amp for sure but the loop was sensitive to a hum. It really all came down to placement of my cables which were 15 footers. I had to make sure they went around one side of the amp and didn't get close to the power cable.
 
This is an issue with pretty much all PRS high gain amps. First gen USA archons have notoriously noisy loops. Never got mine fixed but they have a fix for it. The MT100 can be pretty noisy too
 
The wires connecting the effects loop board to the main board need to be shielded. I replaced mine with a single 2 conductor plus shield coax cable. I wish I did it years ago. Same with replacing the 500k lead master linear taper pot with a log/audio taper pot. Another useful mod is to replace R66 with a 1M pot, audio taper preferred, to get a depth/resonance control.

Also, note that the standby switch is not a traditional design. It mutes (grounds) the signal after the effects return. I took mine out and put the depth pot there.
 
PiggySmallz said:
This is an issue with pretty much all PRS high gain amps. First gen USA archons have notoriously noisy loops. Never got mine fixed but they have a fix for it. The MT100 can be pretty noisy too
Click to expand...
The loop on my Archon has some noise. I notice it when I first plug into the loop. I get used to it pretty quick, not an issue, imo. I felt the same way about the Bogner 3534. Didn't bother me, but some folks couldn't tolerate it.
 
I had the same problem with my 2022 MT-15. I tried all kinds of tube swaps. All kinds of cables. The best is when I put a 12Au7 in the V5. It didn't get rid of all the noise but, it made it almost inaudible. (The bad sound).
 
My USA Archon loop was pretty garbage even with the FX loop fix applied (removing C2 and swapping R6 for a 47k 1W), so I'm not surprised the MT15 also has an issue.
 
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