DIY Class AB Amplifier – Apex A40

After listening to the PeeCeeBee V4H for a while I decided it is time to finally blow the dust out of my soldering iron and take a new challenge. In our local DIY audio forum an interesting schematic started a very enthusiastic discussion and a few members swore that it sounded amazing. The schematic was called “A40” and it was created by Mile Slavković (Apex Audio), a neighbour from Serbia. Our community members did modify it a little and my friend Hristo Elefterov created a really good PCB design for it because the one that was kindly shared on DIYAudio had some design flaws especially with grounding. Here is the modified schematic and PCB design:

Apex A40 Schematic
Apex A40 PCB 3D
Thanks Hristo for the work. Please drop me an email if you like to get the PCBs.

I decided to build it in dual mono configuration, that’s basically two completely separated units for each channel in one enclosure.

Transformers of choice are two great looking Breve Tufvassons 200VA 2x40VAC toroidals from TME, one for each channel. PSU boards are very trivial thus I didn’t take any close up photos of them. Each channel is supplied by a DIY PCB which consists of a 25A bridge rectifier and four 12,000uF Cornell Dubilier SLPX, two for each supply rail. Total of 96,000uF for both channels.

Another small 6VA transformer is used for the speaker protection boards and is bolted to the enclosure using a custom drawn and 3D printed stand. I love how my 3D printer allows me to make such things in less than an hour. Speaker protection boards are from Aliexpress but I did modify them. I am using two stereo boards which I have converted to mono to stick to my dual mono concept by removing one relay from each one and also upgraded the relays to high quality Panasonic relays.
Transformer for speaker protection boards, IEC inlet, fuse holder

All wiring is 4connect oxygen free copper 2×1.5mm2 and 2×2.5mm2. Input RCA jacks are Neutrik NYS367. Output terminals are Cliff TP/6. Output terminals are one of the hardest things to find nowadays. Always use good quality RCAs and output terminals in your amplifiers. Don’t buy the ones sold on Aliexpress if you don’t like your precious music distorted by a terrible choice of metal.

NB: I am not kidding. A friend was trying out to figure out a high THD of an amplifier and tried almost everything but it was still measuring bad. After a few days of experiments he found out that the conductance of the metal of the output terminals was so bad it affected the THD measurements negatively.

Bias is set at around 120mA per pair or 60mA each.

Here are a few photos from the finished amplifier:
Apex A40 boards
Apex A40 inside
Apex A40 top view
Apex A40 rear panel

Conclusion: I am not good at recreating my listening experience in words but I must say the modified Apex A40 schematic built properly in dual mono configuration raises the bar of DIY class AB amplifiers very high. +-56VDC supplies gets me 156W at 8R which is just enough to make my low sensitivity ATC speakers sound amazing.