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akis 2nd January 2013 10:43 AM

Mixer desk inverting output
 
Hello, my son bought a Eurodesk mixer unit and we put the scope on it, to determine the various output levels, when we noticed that the "main output" was inverted compared to its inputs.

Is this the normal setting? I would have imagined that various sound effect modules, stomp boxes, equalisers, compressors etc, should not invert (or phase) the signals, else when you plug random number of units in series you'd get some random output. Similarly I had imagined that an instrument amp should not invert the signal at the speakers because if you had multiple amps plugged in you'd also get some random cancelling.

Why is the EURODESK SX3242FX inverting it then? I presume it is to eliminate microphone feedback?

JMFahey 2nd January 2013 01:02 PM

Just curious: how did you determine it's inverting its inputs?
Which inputs?
Are you talking about channel/insert/main/other inputs?
Thanks.

akis 2nd January 2013 03:53 PM

We used various mic inputs and checked the output shape and level on the main outputs using the scope.

I think it should not be inverting any of its inputs (and it has a lot).

indianajo 2nd January 2013 05:44 PM

I would only worry if some inputs through were inverted and some were straight, so that they added to nothing. Music is inherantly AC. About the only sources where you are sure the speaker should go out at the first is the cannon shots in 1812 Overture.

sreten 2nd January 2013 06:12 PM

Hi,

Inversion of main out is not a problem, its just stereo.

Elsewhere on sends, returns and the like it can be
a nightmare, so most outputs will not be inverting.

Don't misuse main out and its not a problem.

rgds, sreten.

JMFahey 2nd January 2013 08:51 PM

Quote:

We used various mic inputs and checked the output shape and level on the main outputs using the scope.
Fine. You saw the output waveform.
How do you know it's out of phase with the input?
Meaning, how do you know your microphone input phase?
Thanks.

tomi 2nd January 2013 10:22 PM

Interesting. I suspect you've caught out Behringer doing something sly to save a few pence that they thought you wouldn't notice!
It'd be worth checking that all the outpus (L&R and any aux, mono or groups) are in phase with each other, so that you don't get caught out if you end up driving different zones of PA, subs etc from different outs.
Assuming that's in order, just keep a set of phase reverse leads (swapping hot and cold) to hand when you use the desk in case you need them.

JMFahey: I'd assume thay either used a dual-trace 'scope to observe input and output together, or sync'd the scope timebase to the oscillator.

Diligent work anyhow chaps!

Enzo 2nd January 2013 11:03 PM

I think that may be JMF's point, we cannot assume that. Too many people misinterpret what they see to make such assumptions. They may be perfectly accurate in the assessment, we just cannot assume so.

Behringer will usually put the phase relationships on the owner manual, like most OEMs.

Not everything is + to + as in to out. QSC amps being a notable example of inverted power amp outputs. JBL cabs too maybe?

akis 3rd January 2013 11:17 AM

Quote:

Originally Posted by JMFahey (Post 3308383)
Fine. You saw the output waveform.
How do you know it's out of phase with the input?
Meaning, how do you know your microphone input phase?
Thanks.

The mixer has a ton of "outputs" along the signal path, and we tracked it there. Basically it has an out/in post-pre-amp but pre-eq and pre-fader, then it has aux send/return and in addition it also has FX send/return. We tracked it as near the mic as is possible, at the pre-eq. This is where you are meant to add your own effects per channel, eg compression, harmoniser, delay whatever.

JMFahey 3rd January 2013 11:27 PM

Quote:

We tracked it as near the mic as is possible,
Fine, thanks, but it still does not answer my doubts. :(
1) if you scoped some point "upstream", you can't be sure about microphone phase.
2) I still don't know how you determine phase just looking at a scope image.
I mean, there's one way to do it, but I'm not so sure you used it.
So far, you seem to be saying (I may be mistaken, of course) "I hooked a scope to the output, looked at the image and knew it was out of phase".
Out of phase with what?
In fact, I do not even know *what* sound source did you use for the test :(
Or how did you hook your scope up.
Sorry, nothing against you, it's just that I still find a little info missing to solve the puzzle :)


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