X-Altra HPA-1 Class A Headphone Amplifier
The HPA-1 is a compact high performance class A headphone amplifier that will deliver up to 1.75W into a 32 Ohm pair of headphones. The amplifier features ultra-low distortion, very wide bandwidth and class A operation up to 13 V pk-pk into 32 Ohms. Beyond 13V pk-pk into 32 Ohms, the OPS transitions to class AB operation.
The circuit combines a dual ultra-low distortion opamp, the LM4562, with a discrete power output stage. The complete amplifier, including the opamps, runs in class A. Into 60 Ohm headphones and above, the HPA-1 remains fully in class A up to the rated 21 V pk-pk output swing (note, it will actually swing a few volts higher on higher load impedance headphones).
Here’s a short 3 1/2 minute YouTube video about the HPA-1
The document below discusses the design and presents the measurements:-
The Excel spreadsheet below contains the BOM with Mouser Part Numbers (updated 21st March 2021):-
The price of the HPA-1 PCB is $15.99 excluding shipping which is $9.50 to the USA, Canada, Europe, Israel and South Africa and $15 to the rest of the world. Shipping takes 3-5 working days for Europe, USA and Canada, and 5-7 working days for all other destinations. Kindly note prices do not include any taxes or duties applicable in your jurisdiction.
Here are a new set of measurements I did where I replaced the LM4562 opamps with FET input OPA1642 devices. The measurements are very good, and the capacitive load driving ability also excellent.