I have attached the circuit of a JLH-style amplifier I built for headphone use. It uses an opamp and a +/-18 volt power supply to eliminate the DC-blocking capacitor at the output. The NPN driver transistor of the original was replaced with a PNP emitter follower configuration - the original design would require the output of the opamp to venture too close to the negative rail to be stable.
With no load but a Zobel network, the amplifier is totally clean to within a volt or so of the rails. As soon as a load is added, however, the positive-going signal clips at about half the supply voltage. This is largely independent of the output stage bias current.
From memory, the feedback resistor is 100k, the resistor to ground is 10k, and the emitter resistor varied between 4.7k and 10k (depending on bias current being tested).
Does anyone have any ideas as to why this topology would clip only on the positive-going signal, and only when the load is present? Any suggestions would be greatly appreciated.
-Nick
With no load but a Zobel network, the amplifier is totally clean to within a volt or so of the rails. As soon as a load is added, however, the positive-going signal clips at about half the supply voltage. This is largely independent of the output stage bias current.
From memory, the feedback resistor is 100k, the resistor to ground is 10k, and the emitter resistor varied between 4.7k and 10k (depending on bias current being tested).
Does anyone have any ideas as to why this topology would clip only on the positive-going signal, and only when the load is present? Any suggestions would be greatly appreciated.
-Nick