Hifisonix kx-Amplifier

Hifisonix kx-Amplifier

The Hifisonix kx-Amplifier is a 28 Watt class A DIY power amplifier featuring low distortion, wide bandwidth and fast rise/fall times.  It can be operated either in pure class A mode, or in class AAB mode with a lower output standing current and slightly higher supply rails – full details in the write-up below. 

kx-Amplifier Aug 2018

Here is a link to an Excel file that contains the BOM. It was updated on 27-06-2018

kx Amplifier BOM – Amplifier module – Anand1

You can buy a set of double sided through hole plated and silk-screened boards from Jim’s audio at the special introductory price of $20.99.  Here is the link for the PCB’s:


There’s been lots more testing and sound evaluation in the lab over the last few weeks. I’ve had it driving a pair of B&W 703’s (a hugely underrated speaker in my view and I’ve been listening to quite a few £10 000 plus speakers over the last 2 or 3 months) and a pair of the fabulous Kef LS50’s that John Atkinson of Stereophile fame felt were amongst the finest he’d ever heard (and here are reviewer Sam Tellig’s comments about the LS50). I am very pleased with the final result – the kx-Amplifier, within its power capability – has a wonderful ‘grown-up’ sound.  I’ve done the evaluation using  some Yo-yo Ma recordings (great because the recording quality is always good), Steve Gadd Band and Fourplay amongst others. The bass for a small amp is surprisingly well formed, while the mid and top end are certainly a step up from the sx-Amplifier. The higher loop gain on the kx-Amplifier means distortion is typically about 50x to 60x lower than its predecessor, while the two pole compensation means the loop bandwidth is also wide (about 12 kHz)  i.e. the feedback across the bottom octave of the audio band is flat at about 60 dB. 

BTW, just like the sx-Amp, I am happy to report that there is absolutely no turn-on or turn-off thump when powering the kx-Amp ON or OFF!

Some Notes About the Power Transformer

Anand Raman suggested builders look at Toroidy Transformers, Poland, who manufacture stock transformers especially for audio – these come as standard with a GOSS band (aka flux band or ‘belly’ band) and an interwinding screen.  Here is the link:-

Toroidy Transformers, Poland


Antek also provide transformers with GOSS bands, interwinding screen and an optional steel cases for the transformer – you can see the details on their website

The recommended transformer size for the kx-Amplifier is 300 to 400 VA for a 2 channel amplifier – its quite ok to use a 500 VA as well.

As always, if you have any questions, don’t hesitate to contact me.  Happy building!

The picture below is of the kx-amplifier driving my Kef LS50s’




The kx-Amplifier uses my original sx-Amplifier chassis, but I had a top plate and nameplate made at Front Panel Express in Germany.



Above you can see the Low/High bias switch (red) just to the left of the right hand foot


The finished amplifier weighs 10 kg’s