Tweaks
Tuning the Gain
In an op-amp circuit, the resistors in the feedback loop (R3 and R4 in this case) set the gain: the amount by which the output voltage is multiplied compared to the input voltage. The formula for gain in this amp is
To change the amps gain, you can change whichever of the two resistors is most convenient. You should pick values somewhere in the 1?k? to 100?k? range. Lower is better, for a couple of reasons. First, high values will increase your circuits susceptibility to stray RF and electromagnetic noise. Second, higher resistor values are inherently nosier. (You can explore this phenomenon with my op-amp noise calculator.) If you go much below 1?k?, the current wasted through the feedback loop will become significant.
The default gain of 11 is fine to start with. It wont let you use the full sweep of the volume knob unless you have very inefficient headphones or a very weak source, but it does provide good, stable operation of the op-amp. As you lower the gain, you make it more and more likely that the op-amp will become unstable. Indeed, some op-amps wont work reliably at low gains. The OPA13x series chips are unity gain stable, meaning that you can take them down to a gain of 1, if your circuit is built soundly. (You get a gain of 1 by connecting the output straight to the inverting input.)
I have used a gain as low as 2 with inefficient Sennheiser 500 series and AKG K x01 headphones, when the source is strong enough. Similarly, a gain of 2 or 3 with efficient headphones should work fine even with weak sources. A gain of 5 or 6 is practical for almost all combinations of sources and headphones.
For your first amp, build it with a gain of 11 at first. Then once its working correctly, try out different feedback resistor values, checking carefully for audible distortions with each change. Once you get to a gain level you like and its stable, box it up.
Quieting Noises (R5)
Normally the R5 position in the CMoy amplifier is shorted. (0??) This gives the amplifier the best control over the headphone drivers, and results in the best sound in most circumstances. Sometimes, however, putting some resistance in the R5 position is necessary to quiet some sort of noise you hear in the headphones.
Before I go into more details, please read all the information on noise in the companion article Basic Troubleshooting for Headphone Amplifiers. The solutions in that article, if they work, are superior. Adding R5 should only be done as a last resort.
Having tried everything else to get rid of noise, try putting 10?? to 100?? in the R5 position. Higher values will be more effective, but because lower values have fewer bad side effects, your incentive is to use the lowest value you can get away with.
