I came across this headphone amplifier project about a year ago on DIYaudio.com. Built using the default choices, the class A output buffer provides 30 mW minimum output power into all headphone loads between 30 and 600 ohms. One of the things that impressed me was the continual input the designer Richard Murdey had on the thread, including continual development and updates and prompt replies to questions, regardless of whether you bought the boards from Richard or had your own made. The Sapphire project began as a buffered opamp circuit through three major revisions before Richard designed a discrete, current feedback version, the Sapphire 4. You can track the progress of the Sapphire from the beginning, or jump to the 4th iteration which is the project built here. The latest boards are available through Richard’s RJM Audio website and he will also put together a kit at a very reasonable cost. I took advantage of the Gerber files and this is a build of the 4.2b
The design is very flexible and your guide here is what Richard has termed the design manual – a multipage spreadsheet which covers the schematic, board layout, and offers various options to get the type of amp you require. The BOM is comprehensive and contains options for voltage gain and feedback configuration. The power options come in three flavours –
- Standard (37.5mA bias current) for headphones with 30-300ohm impedance
- High power (75mA bias current) for headphones with 15-150ohm impedance
- Line stage (5mA bias current) 10k or larger impedance
The output stage can run in open or closed loop feedback configuration (closed loop for lower distortion, open loop for higher 2nd harmonic distortion) and there is a discussion of the merits of either choice on the forum thread. There is a nice calculator (inputs are headphone impedance and sensitivity) to calculate gain and fix the gain resistor choices. A jumper allows you to choose between three gain settings but I wish I’d routed this to a selector switch. The final sheet of the manual contains build guidance and notes.
I had my amp boards fabricated at JLCPCB, and they are nicely designed and clearly marked.
I used dual Nuvotem Talema 2x12V 25VA encapsulated transformers mounted on custom matching power supply boards designed by DIYAudio member Withmatt.
I decided to route the wiring under the boards as much as possible so mounted the connectors underneath. Due to the proximity of the input wiring to the transformer I used a good quality shielded cable from the RCA inputs to the pot and twisted CAT5 cabling from the amp boards.
The enclosure is a 1907A type from eBay, about £41 including postage to the UK. As usual, it is well-made although I don’t care for the engraved writing. The only additions needed were the Rean RCA sockets, a headphone socket, an ALPS potentiometer (and mounting PCB) and an IEC power socket.
This amp really is flexible. I’ve tried all the gain settings, with two different sources (Galaxy S7 and TEAC UD-501) and two different headphones – 300ohm HD650s and 23ohm HD569s and found that with gain set at 15dB, I can comfortably interchange any of these components. This really is a superb amplifier and will be permanently hooked up to my TEAC DAC in my shed/office at home.