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Sound Reinforcement - Forums for Live Sound Professionals - Your Displayed Name Must Be Your Real Full Name To Post In The Live Sound Forums => LAB Lounge => Topic started by: Miguel Dahl on January 15, 2019, 05:11:22 pm

Title: Hum from Guitar amp with WL: Guitar -> Beltpack -> Reciever -> Guitar Amp
Post by: Miguel Dahl on January 15, 2019, 05:11:22 pm
I have a problem I can't solve.

We're running the chain described in the topic. The sennheisers are powered by the ASA1-antenna splitter which sits in the rack. There's 4 receivers in one rack. The rack has laid out all the connections to the back panel. XLRs, BNCs and power. This is the only time I've gotten noise from this setup.

From the receiver we use xlr - jack adapter cable. I've narrowed it down to the antenna cables. If I disconnect all antenna cables, the amp goes silent, but if i just touch an antenna cable to the bnc the hum is there. I've disconnected the ASP212 splitters which we use to feed 4x racks and just ran the antenna cable straight from an antenna (A2003, passive) to one of the bnc connectors on the back panel, either one of the three (antenna A, Antenna B, Link output), and the guitar amp hums.

This is as far as I've gotten. No matter what I try, it's an antenna cable touching a BNC connector which causes the hum. Anyone?

Edit: And we've tried with a standalone "some other" sennheiser reciever, and it was dead quiet, it was powered by a wall wort.
Title: Re: Hum from Guitar amp with WL: Guitar -> Beltpack -> Reciever -> Guitar Amp
Post by: Stephen Kirby on January 15, 2019, 07:34:01 pm
Sounds like a basic ground loop to me.  Does it happen if the amp and receiver rack are plugged into the same outlet box?  Don't use the cheap white power strips, which have terrible ground contacts.  The more such connections the ground from the amp to the rack goes though, the more likely there will be hum.
Read the power section of the forum and do a search on Poor Man's Distro to get a better understanding of grounding issues.

DO NOT, under any circumstances put a ground adapter on anything to cut the safety ground.
Title: Re: Hum from Guitar amp with WL: Guitar -> Beltpack -> Reciever -> Guitar Amp
Post by: Miguel Dahl on January 15, 2019, 07:36:41 pm
Sounds like a basic ground loop to me.  Does it happen if the amp and receiver rack are plugged into the same outlet box?  Don't use the cheap white power strips, which have terrible ground contacts.  The more such connections the ground from the amp to the rack goes though, the more likely there will be hum.
Read the power section of the forum and do a search on Poor Man's Distro to get a better understanding of grounding issues.

DO NOT, under any circumstances put a ground adapter on anything to cut the safety ground.

I don't know why it should hum due to a ground loop since the antenna cables are not tied into a different ground potential..they just go to the antennas.. Racks and guitar amp are on the same ground, power from the same distro.

And btw. This is not just happening to one amp. I'ts happening to acoustic instruments which goes through a preamp, with the same ground as the racks, and another amp as well. So there's not just one guitar amp in question.
Title: Re: Hum from Guitar amp with WL: Guitar -> Beltpack -> Reciever -> Guitar Amp
Post by: John Sulek on January 15, 2019, 08:16:14 pm
I don't know why it should hum due to a ground loop since the antenna cables are not tied into a different ground potential..they just go to the antennas.. Racks and guitar amp are on the same ground, power from the same distro.

And btw. This is not just happening to one amp. I'ts happening to acoustic instruments which goes through a preamp, with the same ground as the racks, and another amp as well. So there's not just one guitar amp in question.

What happens if you plug the antenna bnc directly into the ASA splitter, bypassing the connector panel which appears to be tying the wireless and amp grounds together?
Title: Re: Hum from Guitar amp with WL: Guitar -> Beltpack -> Reciever -> Guitar Amp
Post by: Mike Caldwell on January 15, 2019, 08:21:30 pm
What's the other end of the coax cable/feed through barrel connected to?

How is the the XLR to 1/4 cable wired?

Try using an everyday 1/4 or 1/2 whip antenna as a test.
Title: Re: Hum from Guitar amp with WL: Guitar -> Beltpack -> Reciever -> Guitar Amp
Post by: Miguel Dahl on January 16, 2019, 09:43:09 am
We moved the rack a few meters so now it's standalone without the powercon/coax/network links between racks and now it's silent.
Title: Re: Hum from Guitar amp with WL: Guitar -> Beltpack -> Reciever -> Guitar Amp
Post by: Keith Broughton on January 16, 2019, 10:18:48 am
I suggest trying a transformer, with ground lift, to isolate the audio output  from the wireless receiver to the associated amplifier/preamp inputs.
This would go between the output of the receiver and any adapter cables going to each of the amps.
Title: Re: Hum from Guitar amp with WL: Guitar -> Beltpack -> Reciever -> Guitar Amp
Post by: Miguel Dahl on January 16, 2019, 12:01:00 pm
False alarm,the noise is still there after we moved it.

I got hold of a trafo. Noise way less with ground connection. Ground lift engaged = noise all gone.
Title: Re: Hum from Guitar amp with WL: Guitar -> Beltpack -> Reciever -> Guitar Amp
Post by: Keith Broughton on January 16, 2019, 02:44:16 pm
False alarm,the noise is still there after we moved it.

I got hold of a trafo. Noise way less with ground connection. Ground lift engaged = noise all gone.
It's the ground connection that is the problem so you could try adapter cables with a ground lift.
I presume you are adapting from 3 pin (XLR) to 1/4 mono plug.
Try pin 2 of the XLR to tip of 1/4" plug and pin 3 of the XLR to the sleeve of the 1/4" plug.
Do not connect pin 1 of the XLR to anything.
See if that works.
 If not, you can use the TX ground lifted option for each of the amps/preamps.
Title: Re: Hum from Guitar amp with WL: Guitar -> Beltpack -> Reciever -> Guitar Amp
Post by: Miguel Dahl on January 16, 2019, 05:43:34 pm
It's the ground connection that is the problem so you could try adapter cables with a ground lift.
I presume you are adapting from 3 pin (XLR) to 1/4 mono plug.
Try pin 2 of the XLR to tip of 1/4" plug and pin 3 of the XLR to the sleeve of the 1/4" plug.
Do not connect pin 1 of the XLR to anything.
See if that works.
 If not, you can use the TX ground lifted option for each of the amps/preamps.

Thanks, I'll try this after the production is over, as it works now, although the guitar sounds a bit skinny, which I guess is because of a cheapo transformer.

I tried inserting an XLR ground lift cable with pin 1 not connected at one end, but the hum got worse, but I don't know how the premade XLR -> 1/4" are wired.