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The Wire 


Again something that came along quite unexpected, OPC's headphone amplifier project out of the Diyaudio thread.  I usually do not follow all there is on Diyaudio and I wasn't too interested in another solid state / chip amplifier at the time. Still I was kind of intrigued by the super compact layout and the finesse ot those boards. As for the technical details I just quote from the first page of the thread on Diyaudio:


"The amplifier is basically an instrumentation amp using three LME49990 op-amps and an LME 49600 buffer nested into the last stage. These parts represent the absolute best you can get for this sort of application. I used all 0.1% Susumu thin film resistors, and all X7R ceramic bypass capacitors placed directly on the supply pins. Bulk caps are all solid polymer for the absolute lowest ESR and best HF performance. Layout was optimized for short signal length, low noise and low crosstalk.
The circuit provides differential input, or the option to ground one phase and drive the circuit with an SE input. Gain is set to 1, but can easily be changed to pretty much anything with just two resistors It runs on +/-5VDC up to +/- 15VDC and has enough drive to run anything you can throw at it."


The schematic and really compact and refined board layout make for shortest signal paths and extremely low distorsion and background noise levels with zero phase shift. The board uses smd parts extensively and is really tiny!



OPC's boardOPC's board


Since impressive numbers alone don't interest me much I jumped on the chance to get a board from Owen... in fact I was very lucky to look into Diyaudio just the very moment when OPC started a clean out sale... that way I scored the only assembled SE-SE board available!


I cased the amplifier up in a solid frame cut from Golden Madrona Burl with a small perspex top cover and a tiny semi-floating panel up front with the headphone jack and the name. Since I wanted a really compact case to reflect the board dimensions I built this one without volume control. I already have a relay based digital attenuator in my pre amp anyway. The matching psu, in fact a simple bipolar supply using the venerable LM317/337 combo to provide + - 15V, is sitting in a case with a top cover made of soft steel... and happily rusting away!  Rusty surfaces seem to be the trend when you look at hot rods, vintage bikes, architecture and so on, so I picked up on that fashion.


Soundwise this tiny thing turned out to be a real surprise. During initial listening the quality of the bass was the first thing that jumped on me... bass control is  exemplary and bordering brutality without ever being overdrawn. But it also offers a soundstage of great clarity without the drawbacks I usually associated with solid state... no harshness, no midrange glare or the glassy character on certain high pitched piano notes. It has detail in abundance and is very neutral and as such will be my first solid state amp to stay.




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