The HeadAmp is a compact stereo headphone amp capable of driving headphones from 32 ohm to 600 ohm impedance to 100mW levels with extremely low and innocuous low order distortion, and beyond.  Most headphone listening is done at the 1mW level or below, so great care has been taken to ensure the more nasty distortions like crossover distortion and PSRR related spray of generated harmonics do not occur. The resulting sound is fast and accurate with excellent micro-detail retrieval.  
 
While the topology is a proven complementary output buffer driven by a chip (all within a global feedback loop), great care has been taken to ensure no discontinuities occur in the forward path, rendering feedback ineffective. In addition the FET cascode OPA2134 op-amp has its output stage deliberately biased into Class A to obviate possible crossover and +Ve PSRR distortions. The discrete complementary output stage is also biased into Class A to 10mW @ 32 ohms or 200mW @ 600 ohms as standard, but this may be increased with a simple parts change. The area of Class A operation can also be reduced for low consumption portable operation - as low as 12V battery operation. The socketed chip can be rolled (changed) if another op-amp is preferred, or as improved op-amps reach the market.

The unit is a simple low parts count compact build that should suit a first time DIY project, with onboard volume control and only requiring a regulated supply for highest quality use. Heatsinks are provided for use with the high level Class A option.    

 

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Input
up to 2.8V rms
 Input Impedance
10Kohms
Gain
<12 dB or 4 times.
Bandwidth
DC - 600KHz (input filtered)
Output
100mW+ into 32 to 600 ohms
Distortion
<< 0.0001% at 10mW mostly 2HD
 Supply
+/-18V at 100mA,down to +12V at 20mA
 Dimensions
50mm x 50mm x 20mm