The ax-Amp: a 60W RMS class A Current Feedback Amplifier

Click this link to order 2 off ax- Amplifier PCB’s

Part 2 of the ax-Amplifier has just been published in audioXpress, 15th April, covering the power supply, power control, mechanical assembly and wiring, followed by detailed measurements. Here is a short synopsis provided from the AX website:-

Click on the link to visit audioXpress

Part 1 of the ax-Amplifier was published in the April issue of audioXpress (out on 15th March), with Part 2 to follow in the May issue, out mid-April. You can order copies of audioXpress here audioXpress Magazine Subscription – CC-Webshop

The ax-Amp is a topologically simple 60 W RMS per channel class A current feedback amplifier employing Augmented Feedback Error Correction (AFEC) to achieve ultra-low distortion, high PSRR and low DC offset. Direct regulation of the class A output standing current results in stable bias over the full operating temperature and all three bias modes. The design uses 4 pairs of 200 Watt sustained beta output transistors per channel to manage the thermal dissipation and allow the amplifier to comfortably deliver peak load currents of >26A. The full power 1 kHz distortion into 8 Ohms measures 3.5ppm, almost all of that contributed by the test instrumentation, while into 2 Ohms, delivering >200 Watts, the distortion is just 12 ppm. The 19+20 kHz full power IMD 1 kHz residual is -118 dBr, attesting to the ax-Amplifiers very good HF linearity. The full power mains noise floor peaks are better than -126 dBr. Using a small 3 position (ON-OFF-ON) toggle switch mounted on the chassis underside just behind the front panel, the amplifier can be switched to class A, class AAB or class AB. To save power, a fast response ‘plateau’ bias circuit switches the amplifier to class A or AAB operation (if selected) when input signal is detected and reverts it to class AB operation 40 seconds after the absence of any input signal.

Here is a quick teaser of the article as it appears in audioXpress: –

The ax-Amplifier uses the deluxe 5U chassis that comes with predrilled heatsinks and drilled rear panel. US builders can order the chassis from, while European builders can order the chassis directly from Modushop in Italy, specifying ‘diyAudio deluxe chassis’. Here is the enclosure.

As always, if you have any questions, I’ll be very happy to answer them and provide any technical support you may require should you decide to build the ax-Amplifier.


7 responses to “The ax-Amp: a 60W RMS class A Current Feedback Amplifier”

  1. Andrej says:


    I have read your article and found out this project very interesting. Can`t wait to see part 2 😉

    My question: could this amplifier be build as AAB or AB class amplifier, reducing heatsink size? Output power probably stays the same?



    • Bonsai says:

      Hello Andrej, yes, you could use smaller heatsinks for class AAB or class AB. You will get about 15 W RmS into 8 Ohms with class AAB. I’ll check later and confirm it.


  2. Joe says:

    Hi, cool (or should I say “Class-A hot”?) project.

    Thanks for publishing the articles in AudioXpress.

    The BoM from the supplementary data over at AX:
    states the following with regard to the PCBs:
    Item 3 | Qty 2 | Set of 2 off | ax-Amplifier amp module PCB’s Order from; 2oz copper, gold flashed THP, silk screened PCB

    I can’t spot the boards on offer in your e-store. Did I miss something or are they one their way?
    If the latter, any hints as to timeline?

    • Bonsai says:

      Hi Joe, I’m back home again in 8 days and will contact you then about the ax-Amp boards. Thanks for your interest. They will be going up in the shop in the next few weeks.

      Kind regards

  3. Wolfgang Postl says:


    I’m looking for the PCB(s) of this ax-Amp. What’s the price for a set of 2 pieces.

    Kind regards

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