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 2 watt into a 75 Ohm load!
It is intrinsic short circuit proof.
Without the input filter (R2, C1) the power bandwidth is >1 MHz and the G.B-product = 15 MHz.
No nonsence like 'crossfeed' has been implemented. This amp is meant for decent recordings!

The Diagram

Explanation of the Diagram

The amp contains just a few components: an op amp and the brand new buffer: BUF634. 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 BUF634, which simply enhances the output current of the OPA134 and lowers the output impedance to less than an ohm if the buffer is part of the feed back loop.
The amp is completely DC coupled! Its offset is less than 3 mV.
The power supply voltage is limited to +18/-18 volt. This is enough to

make nearly 0,5 watt into 300 Ω being the maximum permitted power for a HD650 of Sennheiser. It will make 2 watt into 75 Ω so I must be very careful with my 64 Ω TR 55LX: 'The STRETCH'.
The amp is short circuit-, and overloading proof, simply because the BUF634 is.
R5 (and R15) serve the quiescent current in the BUF634 (~4 mA).
The BUF634 does not suffer from cross over distortion. Even without overall feedback it performs with 0.05 % distortion with the worst amplitude into a 300 Ω load. There is no reason to flee into class A.

The feedback

The 'power stage' does not really need feedback, so that the feedback could be restricted to the op amp itself (the 'local feedback'). Bear in mind that the BUF634 goes up to 180 MHz with R5 = 0!
With 'total feed back' the distortion is reduced to 0.003 % at full power even with less quiescent current.

R6, R7 and R8 dictate a gain of 10x if the balance potmeter is in the middle.
The feed back is >100 dB@0Hz, 70 dB@1kHz and >40 dB@20kHz!

Oscillations

In case 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 should solve the problem, according to the data sheet.

Power Supply

The power supply is rather simple: with a two times 15 volt toroide transformer, a rectifier quad, electrolithic capacitors and stabilisers 7818 and 7918 the supply could be constructed. In the diagram below a solution is given. Mind the 10 Ω resistors (R1 and R2) serve filtering the higher frequency noise from the stabilisers.....

Conclusions

Before building this amp, study the data sheet of the BUF634! It works well at VHF, so some mounting details should be obeyed to prevent oscillations.
The remaining question that occupies me is: whould it sound well? The OPA134 is said to be an excellent sounding op amp:
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 comes with the realisation of the amp and the listening to it with a well-behaving headphone like Sennhei-
sers HD650 or HD800.

It took quite a lot of time before I became familiar with headphone-sound. It is a total different adventure as listening to my hybride loudspeaker system (ESL with MFB). One of the first things that stroke me is the sound of a Steinway D. The live level can hardly be produced with loudspeakers at least not with ESLs, but the HD650 connected to the headphone amp did it! Also details in the middle freqency range become more clear. In general, the sound is round and clear with a pricise bass. No ringing, nothing unpleasants.

To me the HD650 sounds better with this amp than the HD800.

NB.: this amp could very well be used as pre amp for the 35 watt solid state amplifier.