January | 2021 | Plamen Todorov's Personal Website

After making more than 5 different DIY amplifiers in class AB I decided to try class D. After spending some time reading threads in diyaudio, audiosciencereview and asking in our local forum about what are the best class D modules I had to choose between Purifi, Hypex UcG, Hypex Ncore, ICEPower and a few others. Purifi 1ET400 has the best specs according to the datasheet but it is priced at around 600EUR for stereo set even without a SMPS. I didn’t feel like spending 900-1000EUR for a DIY amplifier just to hear what class D has to offer. There is a chance I won’t like class D sound at all. Seems that Hypex Ncore is a middle ground in high end class D modules, placed somewhere between Hypex UcG and Purifi according to several forum members.

I was initially aiming at NC252MP which is 2x250W@4R and MP means that it has integrated SMPS on the same board. Class D is all about high efficiency so why not spend a few more bucks and go for the NC502MP instead? It is rated at 2x500W at 4R and 2x350W at 8R which seems like a good match for my low sensitivity ATC speakers. I found a brand new NC502MP online so I ordered it. Keep in mind that you can’t buy NC252/502 from Hypex directly because they are OEM only, you can only buy NC400 and UcG series from their website diyclassd as a non-OEM customer. However, from time to time NC252/502MP boards appear on ebay so just look there.

The measurements of the amplifier look very promising, especially the THD. THD+N vs power plot for NC502MP:

Here is the full datasheet of Hypex NC502MP.

While waiting for it to be shipped I designed a small “interface” board to fit the balanced(XLR) and single ended(RCA) inputs, relays to switch between them and also route some connections to allow for an external board to monitor and control the Hypex amplifier. I will probably use an Arduino based board which will monitor clipping indicator, temperature and current sense which are all available on the NC502MP 16pin connector.

It hosts 2x Neutrik NC3-FAH0 XLR connectors, two RCA connectors (also Neutrik) and 100nF/100R RC network for the single ended inputs as recommended by Hypex in their great XLR pin1 grounding guide which relates to every amplifier with an XLR input too.

The enclosure of choice is once again from my friend Gianluca at Modushop(HiFi2000). They sent me a black front panel instead of the silver ones I use for my other amplifiers but I didn’t even bother to return it because I love how it looks.

Here are some photos of how it turned out. I still have to figure out the control board but the amplifier works without it with the inputs defaulted to XLR because that’s what I connected to normally closed contacts of the relay.

I still have to let it run for at least 20 hours before sharing my opinion on how it sounds compared to my best sounding class AB amplifier.

Once I’m ready with the control board I will update this post.

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:

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.

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:

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.

THE BLOG

First shot at class D with Hypex

Posted by ptodorov / Audio, Solid State

DIY Class AB Amplifier – Apex A40

Posted by ptodorov / Solid State

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