From bruce@ashbysolutions.com Fri Oct 14 06:57:59 2005
Subject:Re: Headphones out?

Hi Niccolo,

>What happens when you plug a mono jack in the headphones out of a
>keyboard or a mixer straight to a power speaker?
>Does the stereo signal get summed to mono? Is it wrong in any way or
>potentially dangerous for the keyboard/mixer?
>Sometimes I have to provide my own monitor so I would like to use the
>main out for the PA and the headphones out for my monitor. I know I
>probably should just split the signal with an Y cable, but I'm
>curious anyway.
>I am completely ignorant when it comes to impedance, voltage, ohms etc.

When you plug a mono 1/4 phone plug into a headphone jack, one channel (usually, the right side) gets shorted to ground. MOST headphone outputs are prepared for this, but it's possible to damage the headphone circuit if the plug is used often in a headphone circuit that is not designed to compensate for this error.

If you plan to do this a lot, there are two things that you can do. Neither one of them is a perfect answer, and both unfortunately require at least a little knowledge of audio electronics:

- Radio Shack and others have stereo-to-mono adapters and cables that prevent this problem. However, the summed end is usually an RCA or 1/8" plug, not the 1/4" size that you want. This requires either cutting the wrong end off and replacing it with a 1/4" plug, or adding a second adapter/cable to change to 1/4" style.

- You could make a special cable that has a stereo jack on one end, but the middle connection (the "ring") is not connected to anything. This way, the second headphone channel goes nowhere. However, if your keyboard or mixer has balanced/TRS outputs, this could lead to hum or loss of output level.

Regards,

-BW

--
Bruce Wahler
Design Consultant
Ashby Solutions™ http://consult.ashbysolutions.com
978.386.7389 voice/fax
bruce@ashbysolutions.com

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