"Zig-Zag" headphone amplifier

Hello, i would like to present my headphone amplifier/preamplifier project. Lets start with the name, it was diverted from my real name, and thus the name "Zig-Zag" was born. The idea was to make great sounding, low noise headphone amplifier, with lowest possible distortion. The basic idea of designing an amplification stage is opamp with diamond buffer. Input signal comes thru film capacitor, which doesnt allow dc offset of the input device, to be amplified. Later on, the low pass filter, filters out all the high frequency mess. The input signal goes to the non inverting input of the opamp. In my case i choosed the LME49710HA, tin can version. Negative feedback is composed of two resistor, wich change the gain and also the compensation cap was added, in case of opamp oscillation. Blocking capacitors were added to the power rails of opamp for best stability. The operational amplifier will amplify the voltage and diamond buffer, composed of LED current sources and BD139/BD140 transistors works as an current amplifier. To protect the headphones from dc offset in case of some problems, protection circuit was added, it also works as delayed load connection, which removes the pop when amplifier is being turned on. Time of relay switch is set to 5 seconds. Power supply comes from Walt Jungs designs. It is composed of an opamp, wich works as an error amp. Two resistors connected to the inverting input sets the output voltage. Led diodes connected to the non inverting input works as a voltage reference. Opamp compares the two voltages and difference is being amplified at the output, wich opens the power transistor. Such a power supply is considered low noise with low output impedance. All the components are high quality ones, like ONSEMI/FAIRCHILD transistors, WIMA film capacitors, NICHICON and PANASONIC LOW ESR electrolytic capacitors, gold plated IC sockets, NEUTRIK/REAN connectors, Talema PCB transformer and ALPS potentiometer. Case is custom made from aluminium and steel, with custom engravings on front and rear panels.

Amplifier was tested with headphones like KOSS PortaPro, SONY MDR-7506, Beyerdynamic DT 770 and more. On the input, i tested dacs like Topping D10S, SMSL SU-1. Sound was very dynamic, without any noise or hearable distortion.

Device can be used also as a preamplifier, output jack for headphones works as an manual switch, when headphones are plugged in, the rear output connectors are disconected, when you unplug headphones, rear output connectors connects a thus it can be used as preamp.

Full specs:
Dimensions: 130x45x235mm
Output power: 1,9W 33R, 118mW 670R
Quiescent current of diamond buffer: 14mA (diamond buffer works in class A with all loads)
DC offset: about 0,3mV per chanel
Output impedance: 5R
Gain: 4.2x/12,5