I’m a fan of the projects on Rod Elliott’s ESP site as they come with lots of information to help you understand the circuit, and most are simple and robust enough for entry-level DIY hobbyists. Yet this does not mean these are low audio quality circuits, Rod specifies this P113 headphone amplifier as a very low noise, low distortion amplifier. It is a dual op amp based circuit with current boosted by pairs of transistors. I decided to have a go at laying out the circuit myself in KiCad and getting boards fabricated. The amp and other boards have been finished for a few months now and I finally decided to get it finished and cased up.
The board turned out quite well. I suppose I could have made it more compact but as it is still under the 100 mm x 100 mm board limit for cheap fabrication at JLCPCB, I’m not too concerned. More irritating is the mistake with the labelling I made – I’ve labelled one of the input GND labels incorrectly!
Rod gives some advice on the choice of op amp and for this project I used the LM4562 as I bought a few of these a while ago. I used small heatsinks from my stock for the transistors, monitored the temperature with another new toy and found these to be adequate.
I’ve started accumulating a number of ‘spare’ parts, partly a result of buying components in multiples, partly as some projects haven’t materialized or have changed and partly from buying the wrong part. In an effort to use up some of these I thought I would go through my stock and see if I could finish the project using only what I had. Although specified for a +/-15V supply, Rod sees no problem with lower voltages. I already had a Meanwell 20-24 IRM PSU, so I used one of the slowdiyer filtered PSU boards along with my own recent DC splitter to get a +/-12V supply.
The first test went well, the DC splitter giving a steady +/-12V DC within a couple of mV. Hooking up to the oscilloscope shows a nice 1kHz sine wave.
I used a similar 2204 chassis from AliExpress as that for my OPABUF headphone amp, but this time with a black front panel. It really is difficult to argue with the price/quality ratio of these, with all the drilling and milling already done for £24 including postage. Delivery is also very quick, arriving in under two weeks.
Once all hooked up (shielded cable to and from the pot and a twisted pair from the board to the headphone jack), I encountered a problem in that the volume only really kicked in almost at the limit of the pot. I tried it with a 20K Alps RK27 and a 10K Alps RK09 pot with the same results each time. When I bypassed the pots and used my phone’s volume control, it worked perfectly. After much head scratching and reading I had to resort to asking for help on DIYAudio. The reply led me to notice that I had wired the pot to the output rather than the input of the amp. Needless to say I felt quite stupid.
After rectifying my mistake, I tested the amp using my phone as a source and I must say, I’m impressed. I suppose it’s a testament to how much I enjoyed it that I sat listening to music for about two hours, roaming through 80’s punk (Zounds), 90’s electronic dance (Leftfield) and a modern classical concerto (Tonu Korvits), pausing only to get a glass of cognac! The 24V supply and 20K potentiometer is more than adequate and plays music plenty loud enough. As Rod notes, with no music playing, it’s dead silent. I’m quite pleased with the finished result as it’s my first amplifier PCB, albeit from a published circuit.
Using my dummy load I measured the output power at 883mW into 25Ω and 165mW into 330Ω. You can see how these measurements compare to my other headphone amplifiers here.