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Author Topic: New Compressor and hum/feedback in system  (Read 1172 times)

Jeremy George

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New Compressor and hum/feedback in system
« on: January 13, 2007, 01:28:17 pm »

Hi, my church just got a dbx 1046 four channel compressor. I connected it to our Soundcraft LX7ii mixer using the inserts for groups 1 and 2 and used the following settings:

Threshold: -25 dBu

Ratio: 2:1

Output Gain: +1

Limiter: Off

OverEasy: On

I tested it with our pulpit mic and we immediately got what sounded like a mid-low frequency feedback but it never built upon itself so it would stay at a steady level. It was almost like a hum but louder and had more of a feedback characteristic. When I used the built in bypass switch on the compressor it did nothing to the hum but when I routed the mic directly to the mix bypassing groups 1 and 2 which had the compression the hum immediately stopped.

What would be causing this? Looking at the settings again I probably had the threshold set too low but I’m not sure what else would cause this.

Thanks,
Jeremy
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Ivan Beaver

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Re: New Compressor and hum/feedback in system
« Reply #1 on: January 13, 2007, 03:40:13 pm »

Is it doing this on all channels or just one?
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Can I have some more talent in the monitors--PLEASE?

Ivan Beaver
dB Audio & Video Inc.

Bob Watkins

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Re: New Compressor and hum/feedback in system
« Reply #2 on: January 13, 2007, 07:26:26 pm »

Make sure of a couple things.
1. Are you using balanced cables?.
2. Make sure the compressor is on the same leg as the rest of the sound system (for AC power)

It sounds like ground noise.

Can you try to put it in a channel instead of a group insert and see if it has teh same problem?

Bob W
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Ivan Beaver

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Re: New Compressor and hum/feedback in system
« Reply #3 on: January 13, 2007, 07:44:39 pm »

The inserts on the LX7 are unbalanced, so that option is out.
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Can I have some more talent in the monitors--PLEASE?

Ivan Beaver
dB Audio & Video Inc.

John Ward

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Re: New Compressor and hum/feedback in system
« Reply #4 on: January 13, 2007, 08:05:10 pm »

Bad cable(s)?

Try cleaning the send / return jacks on both.....

What else may have been assigned to the subs? Any open mics?

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John Ward

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Re: New Compressor and hum/feedback in system
« Reply #5 on: January 13, 2007, 08:08:15 pm »

Or any DI'd instruments / amps that may have been on?
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John Ward

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Re: New Compressor and hum/feedback in system
« Reply #6 on: January 13, 2007, 08:16:37 pm »

Hopefully someone received my reply(s).....
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Jeremy George

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Re: New Compressor and hum/feedback in system
« Reply #7 on: January 13, 2007, 10:34:40 pm »

Thanks for all your replies. I went back this afternoon and turned down the gain which seems to help but I couldn’t fully test it since I was the only one there. None of the other channels are affected but I’m still not sure what is going on.

If I turn the threshold to -40 dBu the hum almost completely stops so it seems more like a sort of feedback. Another reason why I think it could just be feedback that is being compressed is that if I turn up the gain without using the compressor I get the same exact tone of true fast building feedback.

I’ll test it tomorrow before the service and if I am still having problems I can always bypass the comp.

Thanks again,
Jeremy
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Brad Weber

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Re: New Compressor and hum/feedback in system
« Reply #8 on: January 14, 2007, 12:14:45 pm »

There is a wiring detail for the inserts in the User Manual (page 24 in the one I'm looking at), verify that you have the correct wiring.  Or describe more details of how it is wired.

For one thing, make sure the +4dBu/-10dBv switches on the rear panel are set correctly.  If they are in the -10dBv position they apparently boost the input level and then attenuate the output level which could affect how the compressor operates.

Have you verified gain structure?  You should then be able to use the metering on the dbx to look at the input and output levels.  Bypassing the 1046 apparently just direct connects the input to output so I'm not sure if the gain settings, etc. are engaged.  You might want to set it to a 1:1 ratios, very high threshold and turn off the OverEasy, this should let you see basically just the device gain and levels with no processing.

Keep in mind that compressing does let you increase the overall average level so you may be compressing on one thing and then turning up the overall level thus increasing the frequency content causing the feedback.  For example, you may be compressing based on some very dynamic baritones, letting you bring up the overall mix including the alto frequencies that cause feedback.
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Brad Weber
muse Audio Video
www.museav.com

ProSoundWeb Community

Re: New Compressor and hum/feedback in system
« Reply #8 on: January 14, 2007, 12:14:45 pm »


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