General Asylum: DIY High Quality Headphone Output From Amp Output by Jon Risch

I have posted this one before, and will probably post it again in the future;

Adding a Headphone Jack to a Quality Power Amp

You may not be aware of it, but it is relatively simple to build a very high quality headphone jack that will work with your favorite power amplifier.

Qualifiers: The amp must be able to have the two stereo channels grounds connected together without ill effects. You will probably want to put a switch on the output of the power amp unless it is a spare not used to drive the main speakers. An alternative is to use high quality bannana jacks to connect and disconnect the speaker load when using the headphones. If your amp or receiver has A-B speaker outputs, and they are not placed in series, then you could hook this circuit to the B outputs, and switch to it for headphone use. A huge powerful amp is not required, and the smaller power amps that tend to sound sweeter, work/sound better anyway.

Details: Obtain some very high quality resistors (Vishay foil units are my top choice, other wise, Caddock or any good wirewound {Ohmite} should do. Try to make sure that the body or leads are not made of magnetic material) A value in the range of 270 ohms to 100 ohms and another 1/10 that value, (times two for stereo) is all you need. The power rating of the resistors should be approx. 1/30 that of the power amp to avoid excessive power dissipation becoming a problem.

Remember, the actual heat dissipation is due to long term heating, and not peak power on transients. A pair of ten watt resistors should do the job nicely. The larger value resistor is connected to the output terminal of the power amp, and the other end to the smaller value resistor. This junction of the two resistors is connected to the headphone jack hot terminal for the appropriate channel, and the other end of the smaller value resistor is connected to ground. Repeat for the other channel. The two amp channel grounds should be connected to the ground lug of the headphone jack as well.

Looking at one channel, we have a 100 ohm resistor coming off the amp hot terminal and going to one ofthe headphone jacks hot terminals, and then a 10 ohm resistor from that same jack hot terminal to ground.
COming off that same channel we have a wire coming off ofthe ground terminal ofthe amp, and going to the headphone jacks ground connection (the sleeve portion).

What you have created is a resistive divider that has a low output impedance to help damp the headphones diaphram. It will have an output impedance of around 10 to 27 ohms. depending on the value of the smaller resistor. The load seen by the amp is from 110 ohms to 297 ohms. What this voltage divider does is reduce the signal, and hence the noise and low level distortion from the amp by approx. 20 dB.

Connecting headphones directly to most power amplifiers is very
frustrating, not to mention dangerous, as the noise, hum and low level colorations are maddening on headphones, which usually only require milliwatts to be driven to loud levels.

Most inexpensive receivers and intergrated amps with headphone jacks merely use series resistors, which results in high output impedance and a corresponding increase in diaphram resonances due to poor damping. The circuit I describe maintains most of the damping while reducing noise and hum, etc. and allowing the power amp to drive to decent levels without a significant amount of load on it.

Using this type of headphone connection can allow you to hear the inherent and intrisnic quality of your power amp without any loudspeaker/cable interactions to mask what the amp itself is doing, and without having to drive any significant current. Some amps respond very well to this hookup, others can sound kind of rough, even though they drive a loudspeaker quite well. The cost is right, so experiment.

Jon Risch