Fancy making your own Grado RA1 amplifier clone. Well the circuit below is all you need. About £10 worth of electronic parts minus the case and regulated supply. Also very easy to make on veroboard or protoboard. The original circuit came from diyaudioprojects.com who I understand reverse engineered an original RA1. The design can be easily altered to use other better quality op-amps and more importantly E24 series preferred resistor and cap values.
The opamp used is the LM4562 or better still the OPA2134. You can use the cheaper original JR4556 but the OPA2134 should be easier to buy and is much higher quality than the JR4556.
Put it in a nice case with a power supply like Grado do and you have neat little headphone amp for little cost. Probably one of the simplest circuits to make based around an op-amp as it is a standard non inverting circuit.
Battery or Regulated Supply
I guess you could run it off 2 x 9v batteries for a split rail supply but a small regulated +- 9V DC supply is a much better option for long term more stable use without the voltage drop you would get from batteries as they discharge. The 2 batteries could and very likely discharge at different rates meaning the split rail supply becomes un-balanced.
Gain is Vout / Vin, or 1 + (Feedback Resistor / Neg Ground Resistor) [1 + Rf / Rn]
= 1 + 470 / 120 = 4.92
We are aiming for a gain of 5
The gain can be lowered by adjusting the negative feedback resistor and using the next preferred value down (430K) giving a gain of 4.6. I would keep the gain under 5. Also some op-amps sound better at lower gain levels. You can try and tweak the gain between 2 and 5 by adjusting Rf.
The grado circuit is the same topology as the popular CMoy circuit that uses a OPA2132. Most non inverting opamp based audio pre-amp circuits just follow the same design rules but set resistors and input cap to suit the IC and application in question.
Some audio op-amps have enough current drive to handle a pair of headphones directly but it is possible to feed this voltage gain circuit into a current buffer. This can be another op-amp selected for the purpose or into a mosfet follower.