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View Full Version : Hum Problem




ChrisA
Aug 18, 2013, 10:47 PM
I have a "hum problem". It is very specific and only occurs in this signal chain:

1) Audio Technica AT803a microphone (it is an older lavilier powered by an internal 1.5V battery that terminates with an XLR jack on the belt-clip ower pack

2) Hosa XLR to 1/8: TRS adaptor cable, this one is 18" long and has a transformer insode the XLR shell. It has pin 1 soldered to the shell

3) a Canon consumer level camcorder that has a 1/8" microphone jack

The hum is loud until I touch the exposed metal on the XLR connector. I would call it a ground loop except there is only one cable plugged into the camera. It runs on battery power.

This mic and adaptor combo works fine with my "imix" USB audio adaptor. (yes I know the iMic sucks but it was the only other thing I have with 1/8" TRS input.) Other mics work fine with this camera and adaptor cable (SM58 and Tram mics work with no hum even with same Hosa cable.)

It seems the problem is specific to this exact combination of parts

I made a short 20 second video that shows the problem
hum_demo.mp4 (https://dl.dropboxusercontent.com/u/28915695/hum_demo.mp4)

Liten with the volume turned up and you can't miss it.

QUESTION: Why does touching th shell fix the hum? I'd like to understand this. I know I'm providing a ground but why and how does this work.

What is the Apple tie-in? This sound goes into Logic and FCP



50548
Aug 20, 2013, 02:26 PM
Hi ChrisA,

this may sound stupid, but wouldn't it be more an issue of faulty contact than grounding? This seems to be the case considering that the same problem does NOT occur with other cable chains.

I remember having a similar problem with an adapter cable between my older HK Soundsticks and the analog audio cables used in my DAC. Have you tried applying more pressure with your fingers into the rubberized area or spinning it to see if the problem disappears?

ChrisA
Aug 21, 2013, 12:17 PM
Hi ChrisA,

this may sound stupid, but wouldn't it be more an issue of faulty contact than grounding? This seems to be the case considering that the same problem does NOT occur with other cable chains.

I remember having a similar problem with an adapter cable between my older HK Soundsticks and the analog audio cables used in my DAC. Have you tried applying more pressure with your fingers into the rubberized area or spinning it to see if the problem disappears?

Yes. I tried all of that. The connection is very solid. It is rare to have a connection problem with XLR connectors. The two ends lock with a solid "click" and there is zero "play".

I can move the entire assembly to a different recorder and there is no hum only when connected to this camera is there a problem. Other microphones work fine on this camera using the same cable.

Yes it's really hard to understand why.