Ground loop finally squashed!

[Mike Dubay]

Huuuummmmm.

Fellow forum members, I have been fighting a ground loop

problem in my system for a good part of a year.

When I went to bi-amping for my Cornscalas there was a bit of hum but

not too objectionable. I am using an Ashley 1001 crossover feeding a

passive crossover for my squawker and teeters, so there are several RCA to

balanced XLR cables needed to be fabricated.

I followed the recommendation by Rane for the cable pin

outs, the crux of the recommendations was to connect pins 1 (shield) and 3

(ground) at the XLR connector, with pin 2 (+) of the XLR going to the center conductor

of the RCA connector and pin 3 of the XLR connector gound to the outer conductor. I used acable that has two conductors with a

third shield to XLR pin 1, white wire was pin 2 (+), and blue wire was pin 3 (-).

When I upgraded to my Selenium squawker drivers to drivers

which are about 6 db more sensitive, the hum was objectionable.

I initially went to solutions instead of spending time

trouble shooting. I lifted the ground of

each of my source components and my squawker amplifier. If anything, the hum was slightly louder. I tried to float the ground of each of my

components, to no avail.

So now a more systematic approach was necessary.

The amplifier for the squawker is a hand wired SE amp. It was tried in another person’s system with

Khorns, 106 db efficiency, with the same preamp I was using; and it was dead

quiet. So the amp does have the ability

to be quiet when necessary.

The hum did not change with volume setting.

I then tried the amp with the RCA inputs shorted. Dead quite.

Ah, ha my first piece of trouble shooting information. The amp can be quite in my system. OK, then hooked the amp to the “High” outputs

from the active crossover, hum is back. I

then removed all inputs and outputs from the active crossover, except for the “high”

outputs, hum still there. Turned off

active cross over, the hum was still there.

So I delved back into both the Rane web page and the Ashley

crossover user’s manual. It is not very

clear in Ashley manual if the pins 1 and 3 of the XLR connector should be

connected when using a cable from the balance out to a unbalanced input, the

Rane web page suggested that maybe there should be a 10 ohm resistor between

pins 1 and 3. The Rane web page also

suggests that using non-balanced inputs are going to be prone to hum, and

wishes the user good luck.

So I made a test cable, XLR to RCA, without pins 1 and 3

connected. I was going to put a 10 ohm

resistor between pins 1 and 3, but decided to try the cable first. Hey!

No hum! I fabricated a second

cable, and now the systems hum is not discernable at my listening position,

putting my ear to the squawker I can hear a very slight hiss.

I hope other members can chime in for an explanation. But I guessing that attaching the ground and

negative pins are not necessary for each and every balanced to unbalanced

cable. By doing this to each one, most

likely created a ground loop between the squawker amp and the crossover.

[mustang guy]

That is good information. Thank you for sharing it. Personally, I wish all XLR connections were balanced. It should ba a standard.

[djk]

"So I made a test cable, XLR to RCA,
without pins 1 and 3
connected. "

So you have no signal ground other than the dirty ground in the power cord.

http://www.sweetwater.com/store/detail/HumXLR/

http://www.sweetwater.com/store/detail/HumX/

HumX schematic:

http://www.audioasylum.com/audio/speakers/messages/31/310195.html

[deafbykhorns]

..

[mark1101]

Sorry it took so long for you to figure this out. Another poster in the Home Theater forum has a thread similar to this one with the same solution (titled "QSC PLX 2502").

When connecting Balanced too unbalanced you only use 2 wires of the RCA on the XLR which means (as you found out) you must disconnect the shield and use only the center and ground.

If you go to the Home Theater thread I am speaking of you will see links where you can purchase RCA to XLR adaptors that come with Pins 1 and 3 disconnected puposely.

Good thing you got it figured out.

[Mike Dubay]

"So I made a test cable, XLR to RCA, without pins 1 and 3 connected. "

So you have no signal ground other than the dirty ground in the power cord.

Not exactly,

pin 1 of XLR connected to cable shield

pin 2 of XLR to positive center lug of RCA

pin 3 of LXR to negative of RCA

But no connection between pin 1 and pin 3 of the XLR connector. The Ashley users manual suggests an additional connection at the XLR cable between pin 1 and 3. Thanks for the Humx info.