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RE: Need help calculating power output for headphone circuit...

Posted by Caucasian Blackplate (P) on August 29, 2017 at 09:40:20

In Reply to: Need help calculating power output for headphone circuit... posted by soulbrass on June 7, 2017 at 07:20:49:

The 470 Ohm resistor is a really crappy choice, and you should probably pass on using the headphone jack knowing this. It would have been soooo much better for them to use two resistors per channel as a low impedance voltage divider. With a 470 Ohm series resistor on both channel, you're going to get wonky bass out of most headphones since the 470 Ohm resistors add to the output impedance of the amp.

Having said that, you can build yourself a little box that takes the speaker level inputs and has a proper selection of resistors to pad down the signal. I have seen some companies make similar devices advertised as converting the output of powerful headphone jacks for use with IEM's.

Just to give you an idea of how bad this setup is, let's assume you have some 50 Ohm headphones that are somewhat power hungry. Your amp makes 10W into 8 Ohms, or roughly 8.9V of output. Now we plug in the headphones and we'll assume that the 8.9V of output is still present into the new load of 530 Ohms (it may rise a bit). You now get 0.83V average (this will depend a bit on the frequency of signal going to the headphones) into your 50 Ohm headphones which is 13mW of power :(

If we put a voltage divider on the output that is a 12 Ohm resistor in series with a 4 Ohm resistor, then take the output from the junction of that pair, we'll magically get 100mW and damping won't be an issue.

Back in the day it there was a standard for adding a headphone jack from speaker jacks, and I believe it was a pair of 110 Ohm resistors, but headphone impedances have dropped a lot since then, and just adding series resistance is no longer adequate.

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