Parts 1 and 2 of the ax-Amplifier were published in the April and May 2024 issues of US based audioXpress magazine.
Click this link to order 2 off ax- Amplifier PCB’s https://hifisonix.com/shop/2-off-ax-amplifier-module-pcb/
You can order copies of audioXpress here audioXpress Magazine Subscription – CC-Webshop
Here is a short YouTube video introducing the ax-Amp https://youtu.be/Tr4ledT5DLs
The ax-Amp is a topologically simple 60 W RMS per channel (140W peak class A at <7 ppm distortion) class A current feedback amplifier employing Augmented Feedback Error Correction (AFEC) to achieve ultra-low distortion, very 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. At the rated 60W RMS power, 1 kHz distortion into 8 Ohms measures 3.5ppm, almost all of that contributed by the test instrumentation, while into 2 Ohms, delivering >220 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 8 ohm load 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 Part 1 article as it appeared in audioXpress: –

The ax-Amplifier uses the diyAudio.com deluxe 5U chassis that comes with predrilled heatsinks and drilled rear panel. US builders can order the chassis from diyaudio.com, while European builders can order the chassis directly from Modushop in Italy, specifying ‘diyAudio 5U deluxe chassis’. Here is the enclosure. When ordering the chassis from Modushop, be sure to specify ‘diyAudio pre-drilled hole specification’.
I have done a lot of listening with the ax-Amp to a wide range of music covering jazz, classical and rock on KEFLS50s paired with a B&W sub, B&W 703s (the big originals back from 2003/2004) and Dali Oberons. The bass is clean, and goes exceptionally deep which is great for rock and fusion jazz, but that is matched with a fantastically open and detailed mid-range and a delicate, expansive top end, particularly evident on the LS50s. I have a particular penchant for 19th and 20th century classical music, for example Stravinsky’s Firebird Suite and Sibelius’ 3rd, 4th and 5th symphonies. This complex, multilayered music, demands clean open amplification free of IMD and harmonic distortion of any sort if every nuance in the recording is to be reproduced faithfully. Claudio Abbado’s LSO recording of the Stravinsky Firebird Suite emerges out of absolute silence, followed a few minutes later by the climactic crescendo at the end of the first movement that demands huge reserves of power, dynamic range and soundstage breadth, all of which the ax-Amp reproduces confidently and with the utmost finesse, no doubt a lot of this resting on the class A bias. My musical journey into jazz covers everything from Ella Fitzgerald, Chick Corea through to Miles Davis and Dave Brubeck. On playback with jazz recordings, and especially small ensembles, I look for the space between the musicians which creates that 3D ‘being there’ sonic canvas, and Brubeck’s ‘Take Five’ 1959 recording on the album ‘Time Out‘ has this in bucket loads which the ax-Amp delivers superbly through the KEF LS50’s. Many of Chick Corea’s early recordings like ‘The Mad Hatter‘ (1978) hint at over-zealous compression by the recording engineer but are nevertheless well recorded and sound at their best through the B&W 703s which are able to reproduce the ‘in your face’ musical experience. Again, the ax-Amp is able to reproduce every nuance and instrument with clarity and when required, the power this genre demands.
Physically, this is a BIG, imposing, amplifier weighing in at about 23 kg (50 lbs). It isn’t a cheap amplifier to build, but it will look the business in any system, and most importantly deliver the goods no matter what type of music it is asked to reproduce. The ax-Amp is best suited to medium efficiency speakers of around 86 dB sensitivity in average sized living/sound rooms which will enable it to confidently reproduce orchestral crescendos. My build used 10mm feet to ensure the amp stood well off the ground, allowing good air circulation. After continuous use for about an hour, the heatsink temperatures will be about 56C, with the ambient at c. 21C. As discussed in the technical write-up, the plateau bias circuit means the amp will revert to class AB if no music is detected for 40 seconds or more. It should be noted when operating in class AB, the ax-Amp will deliver 5W peak class A before transitioning to class AB – so it is a very generous class AB.
As always, if you have any questions, I’ll be very happy to answer them and provide any technical support or other enquiries you may require should you decide to build the ax-Amplifier.
Hello. I am looking into purchasing and building the ax-amplifier. However, I was looking at the BOM and everything seems in order except the resistors listed in it are no longer available in Mouser and the alternative parts listed are available only for orders in multiples of 1,000. I am looking into replacing all the resistors with KOA Speer brand resistors. From what I can tell, they are exactly the same except that the KOA Speer resistors are considered Semi-Precision instead of Fixed Resistor. I intend to measure all resistors upon arrival, but can you please confirm that these resistors would be able to work for the ax-amplifier?
Original resistors are 273-1K-RC and I intend to replace them with MF1/2CCT52R1001F. All resistors in the design would be changed into these.
If these are inappropriate, can you please point me towards the right ones?
Thank you in advance.
Hello Antonio, I will be updating the BOM in a few weeks as soon as the PCB supply issue is confirmed as resolved.
You can use any 9 or 10 mm long by 3 to 3.5 mm diameter metal film resistors 1% tolerance in place of those specified in the BOM.
I will put a further update on the News page a little later today.
Thanks
Andrew