kx-amplifier: A very high performance 28 Watt class A Current Feedback Amplifier
The Hifisonix kx-Amplifier is a 28 Watt class A DIY Current Feedback (CFA) 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.
Kindly note, there have been some updates following feedback from a few builders – see slide 4 in the presentation below.
(Note: document updated 28 Nov 20-18: – The operation of J8 has been clarified OPEN for class AAB and CLOSED for class A)
Here is a link to an Excel file that contains the BOM. It was updated on 27-06-2018 (My thanks to builder Anand Raman from the USA for his corrections on the earlier drafts of the BOM). Note that some spec updates were made on 28 April 2020 to C2, C3 and C10 – Voltage rating increased to 150V or higher.
kx-Amplifier PCB’s are back in Stock!
You can buy a set of double sided through hole plated , Gold plated , and silk-screened boards from Jim’s audio at the special introductory price of $23.99. Here is the link for the PCB’s:
Here is a link to diyAudio member rocksteady40’s searchable PDF’s for the kx-Amplifier module and the ripple eater boards. (You must download the files for the searchable function to work).
Posted October 2018: 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 in my view unmatched. 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
Posted September 2018: 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.
I have worked closely with a local manufacturer in Norfolk, England called Tiger Toroids to come up with a suitable 400VA kx-Transformer. The part number is TT2276. This features a GOSS band and interwinding screen. However, it is also wound with a lower flux density than normal and on a stabilized core. This results in dramatically lower radiated noise, and very low/non-existent audible noise even with mains DC offsets, which is usually exacerbated with high(er) flux densities. The price is about £55 excluding shipping, but I highly recommend it. It measures 135mm diameter x 65mm height, cloth tape bound and comes with the central void potted and double dipped vacuum impregnated with 200mm wires. The contact is Stephen Foster and the website address is https://www.tigertoroids.co.uk/
Here’s a picture of the Tiger Toroid transformer in the kx-Amp
Anand Raman, a US based builder, also 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:-
Antek, based in the USA, also provide transformers with GOSS bands, interwinding screen and an optional steel cases for the transformer – you can see the details on their website
Disclaimer: I have not personally tried the Torroidy or Antek transformers.
Here are some notes and sims about the class A/AAB bias controller circuit around Q7 (Note, in this write-up, C17 as a proposed additional capacitor from the collector of Q5 to the anode of D1)
As always, if you have any questions, don’t hesitate to contact me. Happy building!
Posted December 2018: Picture below is of my 800W load. Its switchable between 2, 4 and 8 Ohms and incorporates a 40 dB attenuator
For distortion and noise measurements, I use a QuantAsylum QA401 high resolution ADC/DAC that resolves cleanly down to about 1~2ppm (without attenuation – it goes UP by a few ppm when the attenuator is switched in as the noise floor rises a bit) – it’s fully isolated, so there are no noise problems from the PC (an old Dell that is used only for measurements) – my newer slim Dell is too noisy.
Here are some distortion plots, taken using a QuantAsylum 401 distortion analyser. Load resistance in all cases is 8 Ohms and the test frequency is 1 kHz. I will add IMD and distortion vs power and frequency when I get around to it.
The plot below is of the kx-Amp delivering 22W RMS into 8 Ohms. It comes out of class A operation at about 15W RMS, so the increase in distortion is due both to the transition to class AAB and a bit of large signal non-linearity (LSN) arising in the output stage.
The plot below is 15W RMS (~28W peak) class A – i.e. with c. 1.2A standing current per channel.
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.
I listen to a wide range of music, and given the speakers I have (KEF LS50’s, B&W 703’s and a pair of Q Acoustic 320’s) I tend to use my amplifiers according to the music I listen to. I use the kx-Amp for acoustic music and light Jazz and one of my bigger commercial amps for large scale orchestral and rock/pop music. The LS50’s are not efficient – about 85dB, while the B&W 703’s fair a bit better at ~90dB. Of course if you have a pair of very efficient horn speakers (say >96 dB), the kx-Amp will quite easily deliver rock concert sound pressure levels.
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
Pictures taken during testing and development