Mixer to Mixer Gain setting

[KBToys82]

I'm getting ready to start hooking up the sound system for our musical.

I'm running a mixer from the Back of the House to our on-stage audio setup. Back of the House mixer is a Mackie ProFX22 and the onstage is the ProFX16.

My question is since I'm running XLR's out of the main out, what should the gain setting be on the On-stage mixer? Having it at 12 o'clock seems to high. Since they are 2 cables, I'm not running them through the stereo channels.

I'm guessing I should have the gain fully down.

 

[jkowtko]

I daisy-chained Mackie CFX boards in the past ... but my boards were side by side and I used the group outs instead of main outs on the chained board. I don't remember anything special about the gain. I suggest you work this like you would any other line source, i.e. set your chained board to the normal output levels (mains around zero), then adjust channel input gain on the main board until you get to a comfortable level.

 

[JD]

Chances are, the XLR outputs on the board are line level and quite hot signal wise. If you were planning to plug these into the mic jacks on the second board, you may have a significant overload problem. If there is enough range in the pad and input gain settings on the stage board, you may be able to get it to work. If not, you will end up with significant distortion.
An alternative would be to patch these into the line inputs at the stage. Chances are, that means you will be moving from XLR's to 1/4 TRS connectors, so you may need to adapt them.

 

[KBToys82]

Chances are, the XLR outputs on the board are line level and quite hot signal wise. If you were planning to plug these into the mic jacks on the second board, you may have a significant overload problem.

I triple checked to make sure this was correct, but according to Mackie, both Mixers XLR inputs accept Mic or Line level inputs.

 

[FMEng]

Turning the gain trim way down, to something like 9 o'clock should work fine. Some Mackies have a "U" marked on the gain control for "unity." It's a good idea to turn off phantom power, if you don't need it, on the on-stage console.

 

[Max Warasila]

Here's a technique that will work quickly and effectively. Make sure to turn the gains at FoH on the channels from backstage all the way down before you start:

If you run noise into the backstage console, and set it so that the meters on that console show 0dB at the outputs, you have a reference level to use for setting the gain staging at FoH.

At FoH, turn the gain all the way down, solo one of the two channels from backstage, and bring your gain up until you've reached 0dB on the meter. Match the other channel, and you should be optimized.


My guess is that you'll end up around 10 o'clock, if the circuitry is set up to receive line input on the XLRs. If its, not, you'll notice that even turned all of the way down, the signal is about 3-4dB too hot. This is because most professional gear (including the ProFX series) outputs signal at +4dBu. There are a few reasons why it's standard, the most important being cable losses and noise floor from interference, but in this case, it means the following to you in your situation: the signal is running through the pre-amp when you plug into the XLR inputs, and there is nothing you can do about it.

It's not a bad thing (it will affect the sonic quality and you'll run your fader slightly lower), it just means that the line input circuitry is only inline with the TRS input. Some mixers allow you to switch, usually through a pad (effectively just pushing it through a resistor which makes the XLR a line input rather than a mic input... I digress), but the ProFX series does not. Just a design choice by Mackie to save costs.

The easiest solution (besides just leaving it as is) is building or getting a female XLR to male 1/4"TRS adaptor. I have several for when I go to installs I don't know by heart, as they are easy to build and great insurance.

GL;HF!

 

[KBToys82]

You know whats funny, when I was running audio from a different board into a an M-Box, I would run a 1khz tone at 0db from my phone to match the levels, yet I never thought to do that here.

Thanks!

BTW, I have 2 XLR-M to 1/4 TRS, but what would the advantage be over just keeping it as an XLR?

I'm also running a 4 speaker setup, so 2 are going to be coming from the main outs and 2 are going to be coming from the sub-outs, although each will be routed to a feedback destroyer first before reaching the on-stage mixer.

Now, I have to find the best way to get rid of the ground-loop. Debating on getting a Ebtech Hum X Ground Loop Exterminator and plugging the on-stage mixer into that or 2 Pyle-pro PHE400 Hum Elimator, though I'd rather not add another piece of electronics to the setup.

 

[FMEng]

The two differences between the TRS line input and the XLR mic input is the input impedance is higher and there is no phantom power on the line input. Some line level outputs may smoke with phantom applied, but the Mackie console probably has adequate DC blocking so it won't care. Unless the signal is being distributed to several place, the input impedance isn't important with a low impedance source.

The simple way to deal with the ground loop is to cut the shield connection, at pin 1, on one end of the run. Use tape or shrink tubing to make sure none of the shield or drain wire can come in contact with pin 1 or the connector shell. If you make or modify a cable to do that, make sure to mark it as such. Once modified, that cable cannot be used for microphones.

The sub outs are unbalanced or pseudo-balanced , which may cause more hum headaches. Assuming that the feedback eliminators have true, balanced outputs (most do), put them at the house console to drive the long lines.

 

[JD]

Yea, watch out for that phantom power back-feed. You can make up a DC block box by using a male and female XLR and putting a .5mfd cap between pin 2 in and pin 2 out. Same with pin 3 in and out. Use a paper/Mylar cap (just not electrolytic) rated at 100 volts. Since most inputs have a current limiting resistor on each input, usually phantom will not damage equipment, but it makes one heck of a pop if you happen to disconnect the line with the system on!

 

[FMEng]

JD's concept is OK, but the component value would wind up causing a significant amount of bass rolll-off. That circuit makes a high pass filter at 96 Hz, because the mic preamp input impedance is only 3300 ohms. Since the house console already has blocking capacitors in its output stage, I'd want to use capacitors of at least 10 uF (micro-Farad). Those would be electrolytics with the + side toward the mic input. A voltage rating of 63 volts or higher would be fine.

Or, switch the phantom off. Or, use the best option, the TRS line input.

 

[KBToys82]

I understand. I was triple checking before testing the system the last few days to ensure the phantom power was off. Fortunately on the stage-mixer, I'm only utilizing it for those 4 channels and not having any other inputs be used so no need to even apply it from that mixer. Everything is being run into a long 16x4 XLR snake.

I'm definitely having the Feedback eliminator at the house mixing mixer, just so that any and all equipment can be seen by the student running the board during the show and not being hidden behind the locked stage mixer.

 

[Max Warasila]

Don't take this the wrong way, but if you need a feedback destroyer for your mains, something else is wrong - be it mics, speakers, or gain staging. I sold off our school's because it just cases more sonic trouble than it's worth. Investing in a good front end and output is the best decision I've ever made. If you have the two TRS adaptors, just go ahead and use them.

 

[KBToys82]

More like size of stage and location/direction of speakers which will not change. If I didn't need to have boundary mics, I wouldn't really need it.

 

[Max Warasila]

We have a 60x16 foot apron. Even with 20 headworns, three PCC-160s and a few assorted hanging mics I'm usually fine. Best of luck to you - I've worked places where it's not always sensible.

 

[wolfman005]

I've had instances where I need to go mixer to mixer as well and I ended up picking up a line input adapter made by Shure. It takes a line input and spits out a mic level. My only complaints are that it cuts out a bit of the low end and I usually end up running 2x XLR even if the distance only needs one. It sticks out a fair ways from the mixers and I'm paranoid about someone destroying the XLR jack because of that. That being said, I always run a TS/TRS if the mixers will be next to each other.