Poor Quality mixing desk output

[Zulu]

Hi, folks.

I'm setting up for a production that includes a video and sound track being played during the performance. At present, I have really poor quality sound (very muffly speech, and a constant background rattle), and so I need to change something.

 

The videos and sound are on a laptop. The sound directly from the laptop is fine (Tested that with headphones to the laptop).

 

My set up is:

Headphone socket of laptop to one of the input channels on our 12 channel mixing desk, output from the desk into the Aux. input of an old stereo amplifier. Amplifier output to speakers. Input gain to the desk is adjustable, and with the amp, there is no problem with volume. Fiddling with the Graphics Equaliser and "Bottom end cut-off" on the desk changes the sound as you'd expect, but doesn't address the serious "blurryness" of the output.

 

We routinely use CD inputs to the desk, output through the amp and speakers, with great fidelity. So I'm not sure why the desk manages to damage the laptop sound.

 

Any thoughts?

 

many thanks, Zulu.

[Sound Man]

Try a DI box with the earth lifted between the laptop and mixer.

 

Search this forum, there are plenty of topics regarding problems connecting a laptop to a mixer.

 

David

[Kevin Ross]

You say input channel, do you mean a stereo channel or just a mono one using a jack input.

 

 

 

If it is the latter then that might be the source of your problems if you are using a balanced cable. The laptop is outputting a stereo signal but when it gets to the desk it tries to process it as a balanced signal and effetely takes one side of the stereo image, flips its polarity and combines it with the other one (common mode rejection).

 

 

 

Does it get any better if you unplug the jack half way? Or if you use an unbalanced cable

[J Pearce]

A similar problem to above can occur if the jack plug isn't quite all the way in, which happens very easily on a lot of laptops where the headphone port is sculpted into the case (like the dells at work where a neutrik minijack just will not fit...)

[Zulu]

You say input channel, do you mean a stereo channel or just a mono one using a jack input.

 

 

 

If it is the latter then that might be the source of your problems if you are using a balanced cable. The laptop is outputting a stereo signal but when it gets to the desk it tries to process it as a balanced signal and effetely takes one side of the stereo image, flips its polarity and combines it with the other one (common mode rejection).

 

 

 

Does it get any better if you unplug the jack half way? Or if you use an unbalanced cable

 

A concurrent post has been automatically merged from this point on.

 

I've got an unbalanced stereo cable with a 3.5mm jack into the laptop headphone output, and a 1/4 inch stereo jack into a mono channel of the desk. Maybe I need a mono cable. Or use a stereo channel on the desk. Unfortunately, the stereo channels require two 1/4 inch jacks. I'll try half pulling the plug as well.

 

Thanks for the responses, I've learnt a lot from this and other threads (thanks, Soundman!). Maybe I need a DI box. Or even an external sound card.

 

ps, what does "lifted" earth mean. Does it just mean "disconnected" ?

 

chhers, Zulu.

[paulears]

You've found the problem! The stereo jack has screen on the sleeve, and then left and right audio on tip and ring - However, the unbalanced jack on the mixer has signal on the tip, and, expecting an unbalanced jack to be in the hole, is probably taking the earth connection from the ring - which will often be either linked to the sleeve internally, or left 'hanging' using the mono jack longer sleeve to carry out the shorting process. The end product is that what you're giving the mixer is only the things that are different on left and right. Anything central between the speakers, as in the vocals and other important instruments, gets partially or fully cancelled. Incidentally, you just re-invented the karaoke vocal remover, by accident. The unpleasant sound is what happens when this cancellation occurs.

 

To fix it, you need to merge the left and right output from the laptop - just linking them together, with the pretty basic audio output of the typical laptop isn't good practice, and will of course be mono. What you want is a 3.5mm to 2 x jack plugs - an easily available made up cable, and then two mono mixer channels, or one stereo.

 

The earth lift switch often talked about simply disconnects the earth connection of the box, leaving just the signal. It's very handy for dealing with earth loops - where two separate earth paths exist. Think about an amp or other device that has a 3 core mains cable. This earth connection makes sure all exposed metal parts and the earth part of the input jack are at earth potential (or at least the mains cable earth potential) However, the earth connection is also carried on the cable leading to the other bit of audio kit - the screen in the cable. If that other item is also earthed via the mains earth in it's cable, then problems often appear. If this earth is at a slightly different potential - and it often is, then the audio cable joins the two together in a kind of loop - current passes, and creates a voltage that gets amplified - the familiar 50Hz hum! The dangerous and thankfully rare practice of removing the earth from one item solved the hum - but is frankly too scary to even consider nowadays - but lifting the earth of the signal cable at one end, often works - safely!

 

In practice, lifting the earth will make a hum worse or better, so having the switch is a useful thing - people just switch it and if the hum goes away, they're happy!

[MarkPAman]

I think the only thing that I can add to Paul's post is a link or two, to what I use.