Originally published in January 2020.
The hifisonix kx2-Amplifier features outstanding distortion and noise performance, coupled to switchable class A, AAB or AB operation. It delivers 28 W peak (15W RMS) class A into 8 ohms at less than 20 ppm distortion, and >80 watts RMS into 2 Ohms at 0.2% distortion. The compact PCB measures 100mm x 115mm (each channel). This amplifier is specifically targeted at systems using high sensitivity speakers, or more conventional speakers in a small listening space.
Here is the full pdf build document which covers a circuit description, measurements, and build details (c. 4.7 MB) – slide 18 formula updated on 25th November 2022 :-
Here is the Mouser kx2-Amplifier V2 BOM in Excel format, updated 21 November 2022:-
Here is a short YouTube video introducing the kx2Amplifier https://youtu.be/89RhwuGUb0o
Please note the kx2-Amplifier PCBs are supplied by Jim’s Audio. You can buy a set of two kx2-amplifier boards and a basic power supply board here:- kx2-Amplifier PCB Set
The kx2-Amplifier (hereinafter just called the ‘kx2’) takes the original kx-Amplifier released in 2018 and irons out some wrinkles reported by a few builders – full details in the presentation linked to above – and also enhances the overall performance significantly.
The KSC2690 and KSA1220 driver transistors have been discontinued by ON and are no longer available from reputable distributors like Mouser and Digikey. However, this is no problem for the kx2-Amp. Simply replace Q10 with a KSC3503 and Q11 KSA1220 with the KSA1381. These are the same devices used in the amplifier VAS stages but work perfectly as drivers in the kx-Amp without modification or change in performance.
The build document above includes a comprehensive set of distortion and noise measurements using the Quant Asylum QA401 Audio Analyzer.
The kx2 features even lower distortion than the earlier kx-Amp and now uses TMC (‘transitional miller compensation’) rather than TPC (‘two pole compensation’) used previously. This provides better behaved square wave response with no closed loop HF overshoot which TPC exhibits. Further, by opening a jumper on the PCB, the compensation can be changed to standard Miller compensation which some people prefer.
On the original kx -Amp, the output operating mode could be switched between class A or class AAB. On the kx2, it can be switched between class A, class AAB or class AB.
Improvements to the PCB layout and re-routing of some tracks resulted in a dramatic 15 dB reduction in the measured noise floor. In class AB mode, this is now -120 dBr reference 15W RMS into 8 Ohms, and in class A mode -115 dBr 15 W RMS into 8 Ohms (inputs shorted to signal ground in both measurements).
The bias servo design has been improved so that at high output powers the LF distortion is dramatically improved over the earlier kx-Amplifier.