You are here

PA mixer powered output to line level safely

For everything after the recording stage: hardware/software and how you use it.

PA mixer powered output to line level safely

Postby audioloud » Mon Jun 16, 2014 7:06 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!
audioloud
Poster
Posts: 12
Joined: Wed Aug 17, 2005 11:00 pm

Re: PA mixer powered output to line level safely

Postby James Perrett » Tue Jun 17, 2014 9:02 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.
James Perrett
Moderator
Posts: 5986
Joined: Sun Sep 09, 2001 11:00 pm
Location: The wilds of Hampshire

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


Re: PA mixer powered output to line level safely

Postby audioloud » Tue Jun 17, 2014 1:27 pm

Hi James, thanks for replying!
The model number is CFA 7001 ** .
audioloud
Poster
Posts: 12
Joined: Wed Aug 17, 2005 11:00 pm

Re: PA mixer powered output to line level safely

Postby James Perrett » Tue Jun 17, 2014 3:04 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.
James Perrett
Moderator
Posts: 5986
Joined: Sun Sep 09, 2001 11:00 pm
Location: The wilds of Hampshire

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


Re: PA mixer powered output to line level safely

Postby audioloud » Wed Jun 18, 2014 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.
audioloud
Poster
Posts: 12
Joined: Wed Aug 17, 2005 11:00 pm

Re: PA mixer powered output to line level safely

Postby James Perrett » Wed Jun 18, 2014 1:01 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.
James Perrett
Moderator
Posts: 5986
Joined: Sun Sep 09, 2001 11:00 pm
Location: The wilds of Hampshire

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


Re: PA mixer powered output to line level safely

Postby Exalted Wombat » Wed Jun 18, 2014 2:02 pm

Why do you want a rather noisy spring reverb?
Exalted Wombat
Jedi Poster
Posts: 5652
Joined: Sat Feb 06, 2010 12:00 am
Location: London UK

You don't have to write songs. The world doesn't want you to write songs. It would probably prefer it if you didn't. So write songs if you want to. Otherwise, dont. Go fishing instead.


Re: PA mixer powered output to line level safely

Postby chris661 » Fri Jun 20, 2014 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
chris661
Poster
Posts: 11
Joined: Thu May 22, 2014 11:00 pm

Re: PA mixer powered output to line level safely

Postby audioloud » Tue Jun 24, 2014 6:20 pm

Thanks Chris
audioloud
Poster
Posts: 12
Joined: Wed Aug 17, 2005 11:00 pm


Who is online

Users browsing this forum: molecular and 1 guest