“I have a Roland EM-10 keyboard. It has only a stereo headphone output. I would like to use it with a regular on-stage mixer/speakers, but my little band’s sound man says it is putting out power and may burn up his gear which accepts only ‘line level’ input. Is there a cable or adapter I can buy that is easy to use for this?”
There are devices that will dissipate power and turn high powered signals into very low powered signals, but you don’t need to worry about that. Your sound man’s concern is based in good intentions, but there is nothing to fear here.
One definitely should not connect the output of a power amp (by power amp, we mean an amp used to drive loudspeakers, not headphones) to any line level input. This is almost always disastrous. But a headphone amp is usually a different animal. A little background is in order.
Power amps are designed to operate at high current and voltage levels, usually driving into impedances of not more than 16 ohms (very low impedance). Line level amps are basically voltage devices in that they drive signals into very high impedances (usually thousands of ohms). With such a high impedance very little current will flow at the low voltages we are dealing with (consult your Ohm’s Law math to prove this for yourself). A typical headphone amp isn’t much more than a line amp configured to be able to deliver higher currents into the lower impedances found in most headphones. Headphone impedance will typically range from 8 to 600 ohms, which is we now know is much lower than a line level input. A headphone amp has to be able to deliver the current necessary to drive these phones, but since the drivers are small, and right up against your ears, it doesn’t take much voltage to generate the power needed to make them loud enough. If you connect a headphone signal to a several thousand ohm line level input it will behave pretty much like a line level signal. The key here is the voltage. How much voltage does your headphone amp produce? If it’s less than 15 or 20 volts there is no danger of it doing any damage to a line input, and almost all headphone outputs found in typical audio gear are substantially less than this. About the only time you need to worry is if you are working with special outboard headphone amps, or power amps that have headphone outputs (and even then you are usually safe).
So how do you convince your soundman of this? If he’s not willing to take our word for it try the following experiment. Find a portable cassette or CD player that has both a line out and a headphone out (typically these are both on little 1/8 inch jacks). With any luck the player came with a special cable that has the 1/8 inch stereo plug on one end and a pair of RCA plugs (one for left channel, and one for right channel) on the other end. This cable is for use when connecting the player to a standard home stereo. Using whatever additional adapters are necessary connect this cable between the line out of the player and your sound man’s equipment. Establish a good, workable level. Now turn the headphone volume all the way down on the player and connect the 1/8 plug to the headphone jack. Your soundman has to acknowledge that with the output all the way down his equipment is not in any danger. Slowly raise the level of the headphone output. You will probably find that even with the headphone volume all the way up the signal getting to the mixer is no louder than the line level signal was. This should pretty much prove the point. Now you can turn the volume of your keyboard all the way down and do the same thing with it.
So can a headphone output be a workable substitute for a line level output in other situations? Sure, in a pinch it will usually work okay. Headphone amps usually aren’t as free from noise and distortion as good line amps, but in many situations they can be used without significant degradation in overall sound quality. Of course, your results will vary.