Hifisonix X-Altra HPA-1 Class A Headphone Amplifier

You can buy PCB’s for the HPA-1 Headphone Amplifier over  in the Shop.

There is also a Standard  PSU+HPA-1 bundle option as well in the Shop.

The HPA-1 is a compact, very 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 extremely low output impedance ideal for driving modern high performance dynamic headphones. It operates in full class A up to 13 V pk-pk into 32 Ohms and beyond 13V pk-pk into 32 Ohms, the OPS transitions to class AB operation. Note, for most 32 Ohm headphones, 5V pk to pk is 120 dB SPL – ie loud Rock concert sound pressure levels.

The circuit combines a dual ultra-low distortion opamp, the LM4562 or OPA1642, with a discrete power output stage to deliver outstanding performance both measured, and sonically. 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).

Attention: Listening to music at elevated sound pressure levels can permanently damage your hearing.

The document below discusses the design and presents the measurements (updated with latest PCB’s etc 15 May 2023):-

The slides below show how to wind the output inductors L1 and L2 over the 3.3 Ohm output resistors 

HPA1 May 2023 – How to Wind L1 and L2

For the BOM, kindly go to the X-Altra Mini II BOM linked to below, and look at the sheet marked ‘HPA-1’.  The X-Altra Mini II uses the same headphone amplifier and BOM

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.


8 responses to “Hifisonix X-Altra HPA-1 Class A Headphone Amplifier”

  1. Nickson says:

    Hi Andrew

    How about PCB for this project?



    • Bonsai says:

      Hi Nickson,

      I’m working with a guy from diyAudio who is designing one. Will take a few more weeks.



  2. Nick Smith says:

    Hi Andrew.

    Is X-Altra Mini II HPA-1 a replacement of Hifisonix HPA-1 ?
    And what’s wrong with the old version ?



    • Bonsai says:

      Hi Nick, thanks for your question. Its basically the same amplifier but slightly tweaked. The original design did not run the 2nd stage opamp properly in class A – the X-Altra HPA-1 does.

  3. Arlo says:

    Hi Andrew.
    Excellent circuit!
    I would like to ask about the output DC offset from power up to stabilized.
    For the first look of this circuit, I don’t see the part to control the output offset.


    • Bonsai says:

      Arlo, the output offset is controlled by the 2nd opamp. The typical offset is 1 mV, worst case 3.5 mV.

      Hope this helps to answer your question.

  4. Steve says:

    Hi, I’m interested in this board as well as some of others that you’ve designed.

    Do you offer combined shipping for purchasing multiple boards? I only see how to purchase one board at a time (each with its own shipping cost).


    • admin says:

      Hello Steve, thanks for your comments.

      Yes, we do combined shipping to save on costs and are working to bring this to the shop.

      In the meantime, just email me with your requirements to bonsai(at)hifisonix.com.



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