strange hum

**bass dan** · #1 03-19-2009, 10:06 AM

i have been using a fishman pro plat pre/di with really great results until last night. it had really annoying hum when run into the pa head. at first i thought it was a bad cord. i checked all the cords and found when i touched the 1/4 cord at the input the hum went away.

the humming noise was much louder with a xlr cord from the pre to the pa head than a 1/4 cord, but still was humming. i figuredit was a grounding problem, but the ground lift made no change to the hum. the only way it would go quiet was if i was contacting the cord with a part of my body. interesting enough when i toggled the pre/post eq switch the hum went away in the pre position but was right back in the post position.

anyhow i did the gig with a 1/4 out to the pa head which was still humming but at much less volume than the xlr cord. the hum was not so loud with the 1/4 cord that i couldnt use the pre, but still it sucked. we have played this venue many times without this issue. the only variable was we were plugged into a different outlet than usual. after the gig i decided to plug the pa head into the outlet we usually use at this venue and had the same problem.

i played 2 gigs the night before at 2 different venues with no issue. and today i plugged the preamp into my acoustic image contra and it was dead quiet. anyone have any idea what may have caused this annoying hum? i'm guessing it was the venues power supply, but like i said i've played there multiple times without this issue. the guitar player was using a lr baggs preamp and he had no hum noise at all from his. i have a gig tomorrow night and now i'm starting to worry i'm going to have this issue again. any ideas?

robgrow · #2 03-19-2009, 12:11 PM

I'm a little confused by your description. Are you using an amplifier with the Fishman Pro-EQ/DI as well as sending a signal to the P.A.? Assuming you don't have any signal cable issues and that you've tried the ground lift switch on the Fishman EQ, the problem may be related to the venue's electrical power. If at all possible, plug your amp and the P.A. into the same outlet. This should help eliminate some possibilities such as different electrical phases, neutral problems, etc. Since Fishman doesn't specify that the Pro-EQ's XLR direct out is transformer balanced, I'm going to assume that it's not. In some cases, only a transformer-type direct box will provide adequate isolation to eliminate hum problems. I always carry a direct box equipped with a Jensen transformer just in case.

**bass dan** · #3 03-19-2009, 12:22 PM

thanks robglow. i am not running a amp, just straight to the pa. sorry for the confusing discription. i usually run a xlr out of the pre to power the unit with phantom power. using a 1/4 cord instead of the xlr reduced the hum but did not eliminate it. the only reason i tried the 1/4 cord instead of the xlr was just a desperate attempt to get rid of the hum. could i run the fishman with the jensen di? if so it may be a good thing to have for venues with funky power.

robgrow · #4 03-19-2009, 12:41 PM

Quote:

Originally Posted by bass dan

thanks robglow. i am not running a amp, just straight to the pa. sorry for the confusing discription. i usually run a xlr out of the pre to power the unit with phantom power. using a 1/4 cord instead of the xlr reduced the hum but did not eliminate it. the only reason i tried the 1/4 cord instead of the xlr was just a desperate attempt to get rid of the hum. could i run the fishman with the jensen di? if so it may be a good thing to have for venues with funky power.

Have you tried the ground lift switch on the Fishman Pro-EQ in both positions? Since you're not using an amp, the switch should be in the "gnd" position. Also have you tried swapping the cable between your bass and the Fishman and the mic cable between the Fishman and the P.A. mixer with other cables that are known to be in good working order? It's easy to overlook something simple.

The transformer-isolated direct box would probably only help if you were using an amp.

**bass dan** · #5 03-19-2009, 02:41 PM

yes i tried the ground switch in both directions and cables that i know to be good. i dont have the pa at my house so i had to run it into my contra amp. it is dead quiet into my amp at home. i also tried using a different channel in the pa last night but it still hummed. i guess i'll find out tomorrow at the gig if its the fishman or if it was a problem with the power at that venue. i'll bring my contra as a back up.

fdeck · #6 03-19-2009, 08:54 PM

I experienced something like this once with my GK combo, where the ground lift switch did not help. I think if there is a large enough common mode signal, it can bleed sound back into the amp if it is an "active balanced" direct output as opposed to a transformer. In this case it was an outdoor gig and there was probably some massive AC wiring problem.

**bass dan** · #7 03-21-2009, 07:38 AM

must have been the venue. worked perfectly last night. would a power conditioner solve a problem with a venue having screwy power?

fdeck · #8 03-21-2009, 11:56 AM

I doubt a power conditioner would help. Most are glorified power strips, with some EMI and spike protection. Even the regulated ones don't have isolated outputs, AFAIK. If you get to a sufficient level of isolation, then isolating the audio signal via a transformer would probably end up being the more portable and cost-effective solution.

A couple of additional measures are worth noting. Plugging everything into a single outlet is likely to result in fewer grounding issues. And an outlet tester is good to have along if you suspect problems. There's a chance that the problem at the "bad" venue might be associated with some wiring fault that also makes it dangerous.