Right after getting things up and running and promising more frequent updates, it has been quiet for some time. However, it has not been quiet in my home lab. I’ve had two major projects under development: balanced preamplifier with relay stepped attenuator and modular headphone amplifier where amplifier boards can be swapped. The preamp is not completely my own design but is based on work by Bruno Putzeys and Hans Polak. Mr Putzeys published his simple but genius balanced preamplifier design in Linear Audio volume 5, in the article “The G Word, or How to Get Your Audio off the Ground” that discusses what signal ground actually means in practice and how single-ended signal can actually be considered as differential signal. This design has a long follow-up thread in Diyaudio.com and one aspect some people did not like was the use of normal potentiometer. Therefore, Hans Polak gave his contribution to the project and designed a relay stepped attenuator that can be used in place of the potentiometer. What I have done now is to combine these two and throw in an Arduino-compatible microcontroller circuit with OLED display and rotary encoder, and design a PCB that suits my plans.
The preamplifier is now in software development phase and I expect to get it to the measurement bench soon. I made one major mistake with the PCB by ignoring the coil polarity of the small signal relays. Therefore, some relay footprints are wrong so I have mounted them on the bottom side on my prototype board. Otherwise it looks promising so far.
Another design is a modular headphone amplifier that consists of a motherboard with all connectors and separate amplifier boards that are stacked on the motherboard. Currently I have two simple amplifier boards: one TPA6120A2-based and the other that accepts BUF634 or LME49600 buffers. These boards are ready to be measured when I have the preamplifier and whole chain ready. More of that later.