Crossfeed Headphone Amp With BUF634. The X-Feed

In this post I’m about to show my new BUF634 based headphone amp which also has a crossfeed filter. This gives a better listening experience. The BUF634 IC from TI is an exceptional piece of technology and offers excellent performance. It is very suitable for headphone applications. I have used it in another of my projects with very satisfactory results. For my application the amplifier will have the task of providing some hefty voltage swings because I use 600 Ohm headphones, namely the Sennheiser HD 540 Reference. These are simply gorgeous and I do love them. Check out the HD540 at Head-Fi.

The basic headphone amp concept

This headphone amplifier is an inspiration by Rod Elliot’s Project 109. My application however is aimed at “desktop” applications. I do prefer to use my HD540’s at home and portable applications are not quite my coup of tea. As you can probably guess I have decided to modify the output stage and remove the parallel opamp configuration. BB’s application bulletin is an interesting concept and I will give it a tray in future. I do believe that BUF634 is a much better choice for a headphone amp. A quick look at the datasheet reveals the advantages over the parallel opamp configuration. The BUF634 has a substantially higher output driving capability with its 250mA of “grunt”. The 2000V/us means the input opamps will define the overall amplifier speed.

The crossfeed filter

The crossfeed concept was originally shown by Ingvar Ohman in 1994 in his article “Den Lilla Stereo-kontrollboxen SP12” published in “Musik och Ljudteknik” dec. 1994. Mr. Rod Elliot uses this topology in his application as well. There are also few modification of the schematic including the enhanced bass natural crossfeed from Jan Meier. I’m going to be honest here. I did build the headphone amp with the original corossfeed and I didn’t really like it. My HD540’s have a superior sound stage. The original Ohman filter simply collapses the sound stage into one centered image. I needed a better solution.

I found a good article at HeadWize by Chu Moy. Basically I had two problems – the filter had a high output impedance and the channel separation was too small for lower frequencies. What I did was to modify the filter values in my crossfeed headphone amp. I used the excellent Excel spreadsheet by Guss Wanner and here’s the result:

The new filter channel separation is about 6dB from 20Hz to 200Hz. The delay is good at about 250us and dropping to about 50us at 2kHz.

A schematic for the crossfeed headphone amp

This brings us to the schematic for this project. Click on the picture to get a better view.

U101 makes the input stage. For my amp I use the NE5532 dual op-amp in a buffer configuration(unity gain). I also tried the OPA2604 here with excellent results. The filter sits at the output of the buffers. K101 creates a bypass path in case a no-crossfeed configuration is needed, for example in binaural recordings.

The signal then goes to the second stage. Here I also use the NE5532 opamp. I also tried the OPA2604 and this gave a much lower output offset. The overall gain of the stage is set to about 15.5dB (G=6). A higher gain is possible by changing the ratio between R109-R108 and R121-R120.

BUF 634 sits inside the feedback loop of the op-amps. This gives a better DC operating conditions of the output stage, a high drive capability, precision, speed and noise performance. The complete schematic in pdf is available for download here:

The PCB

We arrive at the the PCB. Both channels are located on a single PCB made of two-layers. All layers are shown below:

The complete PCB is available for download below in PDF

To do list :

We arrive to the things to do, once I have the working PCB’s

1. Measurements – THD+N, SNR. 600Ohm load
2. Enclosure for the compete project. I will come up with a full metal enclosure to keep the amplifier nice and solid.