I'm using an audio op amp (LME49726) to combine two audio sources together, to be listened to with headphones at its output. Whenever I plug in the headphones, there is a large voltage drop in the supply voltage and it doesn't work. What could be the problem with having a headphone load at the output? What op amp parameters would this involve? I have also tried a general purpose op amp with the same result. This is datasheet of the audio op amp: http://www.ti.com/lit/ds/symlink/lme49726.pdf Here's the simple circuit:
That particular op-amp is capable of hundreds of mA output current. I think you should be able to deafen yourself if the headphones are the normal rare-earth ~30\$\Omega\$ variety.
You seem to have identified part of the problem- the supply voltage drops. You need to capacitively couple the headphones. What you have now attempts to put 2.5VDC across the headphones, which is at least 83mA, thereby dragging down your supply.
Try a 100uF electrolytic in series with the output (+ side to the op-amp, - to the headphones).
If your power supply voltage is dropping then it indicates that the power supply is not capable of supplying the current to the load.
Many headphones are only 8 ohms. Suppose you had an output signal level into the headphones of 1V you would need to supply up to 1V/8 ohms = 125mA. If your signal level is higher you will need correspondingly more current. For example at 4V you would need 0.5A.
The LME49726 is capable of driving an 8 ohm load at signal levels up to 2.8V with a 5V supply and is easily capable of driving typical headphones.
The parameters that would typically be considered when driving headphones from an op-amp would be...
1) Voltage headroom from each supply vs load current (also referred to output swing).
2) Maximum load current.
3) Maximum allowed wattage based on ambient temperature, junction temperature, and thermal resistance (theta-JA). The power dissipation is approximately...
W = Is * VCC + |Iout| * (VCC-Vout) when sourcing current out of the output pin.
W = Is * VCC + |Iout| * Vout when sinking current into the output pin.
4) Also with respect to audio outptut quality you would consider CMRR, input offset voltage, gain bandwidth product, and linearity.