A Solid State Headphone Amplifier

Introduction

Another Headfone Amplifier Yes, against all HiFi-religions, I offer a class-B amplifier with overall feed back which performs excellent (< 0.003 % distortion) with a 300 Ω resistive load like a Sennheisen HD650.
The amp could provide nearly 500 mW, limitted by the voltage of the power supply.
It is intrinsic short circuit proof.
Without the input filter the power bandwidth is >1 MHz and the G.B-product = 15 MHz.

The Diagram

Explanation of the Diagram

The amp contains just a few components: an op amp and two tran- sistors with their biasing components. Henk ten Pierick measured the OPA134 as the best sounding op amp in high input impedance applications. With his research method, active components can be ranked in sound quality. Many listening sessions approved his method.
The op amp drives the NPN-PNP-pair BD139-BD140 which has a fabulous reputation for many years.
The power supply voltage is limited to +18/-18 volt. This is enough to make nearly 0,5 watt over 300 Ω being the maximum permitted

power for a HD650 of Sennheiser.
The amp is short circuit proof, simply because the op amp can't supply a current which could drive the BD139 and the BD140 into a region which should harm them. For all sake the emitter resitors are taken 10 Ω.
R7 and R8 serve the quiescent current in the BD's (~6 mA).
This type of power stages hardly suffer from cross over distortion. Without overall feedback it performs with <0.05 % distortion at 10 V into a 300 Ω load. There is no reason to flee into class A.

The feedback

As I stated above, the power stage does not really need feedback, so that the feedback could be restricted to the op amp itself (the 'local feedback'). The consequence is that the power stage should be corrected for the output offset voltage (preferably < 5 mV). With the 'overall feedback' the loop gain for DC is equal to the open loop gain of the op amp (120 dB) because of C2! The output offset melts away to some micro volt, so the small amount of distortion does!

It is reduced to 0.003 % at full power even with less quiescent current.
R13 and R14 dictate a gain of 11x. The input circuit R11 and R12 reduces the total gain to 10x. R12 - C2 keeps out input > 60 kHz
(if the source impedance is adequate).
The feed back is >110 dB@0Hz, 70 dB@1kHz and >40 dB@20kHz!

Conclusions

MicroSim has proven to be a nice rather easy to learn simulator, at least for audio applications. The developed amplifier circuit is quite simple and certainly not new, but did the very good performance be common knowledge?
The crux of the matter is: Does the model MicroSim cover the design in all aspects? One of the problems: 'oscillations' (which occurred in practice) had NOT been predicted. A small 10 nF-capacitor directly in parallel with the output terminals, sometimes transforms the amp into an oscillator!
The remaining question that occupies me is: whould it sound well? The OPA134 is said to be an excellent sounding op amp but
is this still the case if it is driven so high?

The data sheet depicts:

.... in combination with high output drive capability and excellent dc performance allows use in a wide variety of demanding applications. In addition, the OPA134’s wide output swing, to within 1V of the rails, allows increased headroom making it ideal for use in any audio circuit.

However, the ultimate answer will come with the realisation of the amp and the listening to it with a well-behaving headphone like Sennheisers HD650 or HD800.
To me the HD650 sounds better with this amp as the HD800.

Oscillations!

The remedy should be: put a dominant pole in the amp, somewhere over 20 kHz. To make a long story short: 220 pF mica or styroflex parralel to the output of the op amp solves the problem. (This update has not been shown in the diagram.)