Built a Chu Moy Headphone Amplifier
Posted by retromaster on March 21, 2010
Just before I started working on the TFE+ hardware/firmware combination, I thought that improving my working environment was in order, so I took up a little side project. The sound output of the desktop I use for listening to music while working on electronics projects is a little low (especially when driving large headphones) and I thought a headphone amplifier would help. So, I decided to build a Chu Moy Headphone Amplifier.
The original design by Chu Moy is a very basic design around the OPA134 audio opamp. I used the OPA2134 (the dual version) instead to save a little space and simplify wiring a little. I am not using a battery to power the circuit but the simple voltage-divider based power supply splitting scheme is there. The input capacitors are larger (in capacitance and size) than those used in the original design, which should help with the low end frequency response. I did not include any a volume control pot as the desktop sound card volume control will suffice.
The amplifier is able to reach very high (unlistenable) sound volume. It is even able to drive my AKG 240 DF headphones (notoriously high input impedance at 600 Ohms, very difficult to drive) without much effort. The frequency response seems OK to my ears, although I am not able to describe these things as vividly as some “audiophiles” are able to. Overall the results are quite good. Unfortunately, however, there is some background hiss that is noticeable during silence although it is not very disturbing. This could probably be attributed to the rather high opamp gain and my crude construction methods, and a better power supply could definitely improve things. That said, I think that most of the hiss might actually be coming from the audio source. Nevertheless, the thing sounds good enough that I think this warrants a dedicated PCB in a proper metal case.