The bypass capacitor is seen on almost every amp out there but is it really necessary? As in many text book, it serve 2 purpose: 1. Act as short to ground at high frequency which shunt noise from other noise source that couple into the power supply through the power supply trace. 2. Act as a power source for fast transient current at high frequency. But in reality, i see many amps connect the power supply directly to the V+ or V- leg of the opamp, if we want to maximize effect (1), it should be power supply -> bypass cap -> ic legs. So i think most of the time, the bypass cap is there to serve for purpose (2) but, here is the impedance plot of a 0.1uf x7r capacitor from kemet: You can see that at audio frequency, the impedance of the capacitor is very high, so when a "high" frequency audio signal need to be produce by the amp, the current should flow from the power supply because it has much lower output impedance at audio frequency, not the capacitor. The capacitor is only good at 1->100Mhz which is far above audio frequency. So what is the real benefit of a by pass capacitor?
Edit: the bypass here is some 100nf or 1uf ceramic capacitor from V+ or V- of opamp to ground. I just want to disscus the benefit of bypass capacitor in audio application, especially with opamp as headphone amplifier. Here is my simulation which take account of trace impedance and inductance, the load is about 5cm from the power supply and is bypass by a 10uf 70mR esr tantalum and 1uf ceramic which is the bypass strategy often seen in headphone amplifier. The blue trace is without bypass and green trace is bypassed. As you can see, the bypass capacitor only has effect from 300khz which is far above audio frequency.