The single channel in, four channel out Headphone amplifier, is powered from a micro USB conector. A phone charger can be used as power source. The single 5V input supply voltage is converted to a low noise +/- 5V stabilized and isolated power supply.
The volume and balance of each channel can be controlled separately by four push buttons. All control is done using an ATMEGA controller. Volume values are automatically stored in eeprom approx. 20 seconds after the last Volume up/down action.
The input accepts headphone levels and line output levels. So, the amplifier can be connected to a regular headpone connector or a audio line output. For each channel, the signal gain can be set in a range between -89.5dB..+37.5dB in 0.5dB steps.
Measured 1kHz , 1.9Vpp output, Rload 600 Ohms@6dB gain: -83.2dB THD, SNR 72dB
Measured 10kHz , 1.9Vpp output, Rload 600 Ohms@6dB gain: -101dB THD , SNR 73dB
For programming the ATMEGA firmware, Atmel AVR studio 7 is used. Interfacing to the AVRISP mkII programmer is done using a small dual row 8 pin-50 mil male SMD connector. I use this connector more often, it takes less boardspace, even though this board has space enough for larger connectors.
Some second thoughts:
The headphone jacks seem to be through hole types, but they are not. This made it harder (but not impossible) to solder by hand, because of the narrow space between the connectors.
A micoUSB connecor is quite fragile, a mini usb or larger connector would have been a better choice.