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audioloud



Joined: 18/08/05
Posts: 42
Loc: London
PA mixer powered output to line level safely
      #1108112 - 16/06/14 07:06 PM
Hi everyone, I was wondering if anybody can help me with this.
I am in the process of purchasing an old Fender 4 channel PA mixer. I am interested in it as it has built in reverb which I would like to use in the studio . Does anybody knows a way of translating the powered output of the mixer to a line level signal so I can safely use the onboard reverb for external processing? I was imagining a little internal modification just to bypass the mixer's amp, or even better a middle box of some kind that can solve this problem.
I would be very thankful if someone could shed a light on this!


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James PerrettModerator



Joined: 10/09/01
Posts: 10652
Loc: The wilds of Hampshire
Re: PA mixer powered output to line level safely new [Re: audioloud]
      #1108162 - 17/06/14 09:02 AM
Let us know the model number. There's a good chance that it will have a slave output or another similar output that can be used.

--------------------
JRP Music - Audio Mastering and Restoration.
http://www.jrpmusic.net


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audioloud



Joined: 18/08/05
Posts: 42
Loc: London
Re: PA mixer powered output to line level safely new [Re: audioloud]
      #1108210 - 17/06/14 01:27 PM
Hi James, thanks for replying!
The model number is CFA 7001 ** .


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James PerrettModerator



Joined: 10/09/01
Posts: 10652
Loc: The wilds of Hampshire
Re: PA mixer powered output to line level safely new [Re: audioloud]
      #1108231 - 17/06/14 03:04 PM
Did a quick search and it looks like there is no slave output or any other kind of output apart from the speaker output. In that case, the best off the shelf solution would be something like a power-soak that guitarists use. This would provide the amp with a suitable load and also a suitable level output for your recording gear.

If you just want a spring reverb, I would probably look at something else with a slave output socket which would be easier to connect to your gear.

--------------------
JRP Music - Audio Mastering and Restoration.
http://www.jrpmusic.net


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audioloud



Joined: 18/08/05
Posts: 42
Loc: London
Re: PA mixer powered output to line level safely new [Re: audioloud]
      #1108385 - 18/06/14 12:37 PM
I didn't think about a power soak... Should defo work.
I have been looking at getting a spring verb for a while and this mixer came out as a cheaper alternative to a spring reverb only device.
Thanks again for your advice.


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James PerrettModerator



Joined: 10/09/01
Posts: 10652
Loc: The wilds of Hampshire
Re: PA mixer powered output to line level safely new [Re: audioloud]
      #1108390 - 18/06/14 01:01 PM
It might be worth saying that it looks like a transistor amp so it may not need to see a load on it but, as it is so old, I wouldn't risk it unless I had a good look at the circuit first.

--------------------
JRP Music - Audio Mastering and Restoration.
http://www.jrpmusic.net


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Exalted Wombat



Joined: 06/02/10
Posts: 5655
Re: PA mixer powered output to line level safely new [Re: audioloud]
      #1108396 - 18/06/14 02:02 PM
Why do you want a rather noisy spring reverb?


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chris661



Joined: 23/05/14
Posts: 12
Re: PA mixer powered output to line level safely new [Re: audioloud]
      #1108681 - 20/06/14 11:23 AM
I have an old McGregor mixer-amp, with a rather nice springline reverb in there.
The slave output is a pair of resistors, forming a potential divider.

The resistances need to be ~10x less than the input impedance of your device (assumed ~10kohm). Sooo...

Get a pair of resistors, preferably 1/2w or 1w each, one 100ohm, one 1kohm. Put both resistors in series across the amplifier output, connecting the 1kohm to the hot, 100ohm to ground. The output is across the 100ohm resistor, so put an XLR or jack socket on there.

You get an approximately 10:1 attenuation, which means when the amplifier's putting out 20v, you're getting 2v out of the attenuator, which is around line level.
Since the resistors attenuate everything coming out of the amplifier, your noise level should be reasonably low.

Note that this is different to a guitar amp power soak: those try to make sure the amplifier is still working hard (plenty of current delivery) but the speaker is still seeing minimal power. The required resistors can be very large (and heatsink-mounted) to ensure they don't burn.

The attenuator outlined above has minimal amplifier current draw, so you can get away with the same low-wattage resistors you see on circuit boards.

HTH
Chris


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audioloud



Joined: 18/08/05
Posts: 42
Loc: London
Re: PA mixer powered output to line level safely new [Re: audioloud]
      #1109302 - 24/06/14 06:20 PM
Thanks Chris


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