The story of this amplifier began with the need to trade my beloved Hifiman Arya Stealth for closed-backs because I only use headphones when I am in the office and open-back headphones are disturbing others.I found a trade deal for ZMF Atticus with Eikon pad and I traded without even listening to them. As expected, they sound much less exciting than the Arya Stealth but they still sound good for closed-back headphones.
However, as their impedance is 300 ohms, I needed something more powerful than my portable Cayin RU-6 and the integrated amplifier in my Topping DX3 Pro. I had the RJM Sapphire v4.2 boards laying around from a group buy and also a Modu Shop enclosure I bought a while ago that was just the right size for this build.
The documentation in the RJM website is very informative and contains a detailed BOM file that includes formulas and example values for different gain values. If you still have some questions you can also search by keyword in the DIYAudio topic, which also has a lot of info.
I opted out for the open loop configuration with a 21 fixed gain as I will be using it with only one pair of headphones so far. That’s why JP1, R2B, R2C, R3X are not installed on the board. You can find info on those in the BOM file.
This photo was taken in the middle of soldering the components to the boards. Resistors are different types in the different positions, of course they are consistent between right and left channel board, just because that’s what I had in my parts bin.
The trickiest part of amplifier building, drilling the enclosure, front and back panels. I am getting better and quicker with each amplifier and of course with all the mistakes I made in the past.
In the next photos the amplifier is 99% ready and already tested with a cheap pair of headphones for stability, oscillations and temperature issues in any of the transistors.
Without matching any of the transistors or even measuring their hfe at all I was able to adjust the output DC to values within +-10mV which is good enough for unmatched parts.
I forgot to take one more photos when I earthed the chassis, the output and the potentiometer, but just to remind you if you are building it – always take care of your grounding.
I will also make sure to upload more photos when I 3D print a volume knob and close the lid.
As I expected, a more powerful amplifier was definitely needed with the ZMF Atticus, they sound much meatier and with better control with more power from the RJM Sapphire.