Table of Contents
An the age of 15 I started with my audiophile hobby and bought my first equipment. At that time I had neither the money nor the room for a loadspeaker. As a consequence I used a headphone to listen to the music – an electrostatical Jecklin Float. With this remarkable head loadspeaker I heard many wonderful hours music.
At the Analog Forum 2013 of the AAA I had the pleasure to hear with a serious headphone after a long time – a Sennheiser HD800 – and I was immediately excited from this form of music listening. Since then I thought about a design for a headphone amplifier which I want to build.
back to the top of the page
In the last two years I analysed a lot of concepts and at the end I decided to use a relative simple design based on integrated circuits. Due to my very positive experience with the OPA627 in my active crossover I designed a circuit around this operational amplifier. The basic topology of the amplifier is a normal not inverted type and therefore a of different operational amplifiers as a replacement are also possible.
The 230V/AC supply is connected to the flat male tabs J2 and J3. The earth is connected to J4. Directly after the input of the phase follows the fuse F5. It is responsible to safeguard the whole headphone circuit and shall have the lowest selectivity. The X2 capacitors C44, C47 and C48 act as an interference suppression for the mains voltage. With C43, C45 and BR1 potentially DC portions of the line voltages are filtered out (C46 are not populated). The ground of the analog circuit is connected with R15, D10, D11 and C49 to earth.
- Circuit Diagram of the 230V/AC Supply
The power supply – as usually in my designs – is totally oversized and is build in a dual mono topology. A common headphone with a 6.3mm plug has a combined ground for both channels and therefore I connected the two grounds of the power supply together. Both supplies are identical and so I have to describe only one channel below.
Per channel a 25VA toroidal transformer is used. For the secondary outputs I use fuses because the transformer is not short-circuit proofed. Directly thereafter snubber networks are placed (R1, C1, C2 and R2, C11, C12). They damp the resonance circuit – inductance of the secondary winding and junction barrier capacitance of the diodes. The alternating voltage is than rectified with a discrete bridge rectifier build with ultra-fast soft-recovery diodes.
After the 2× 22mF reservoir capacitors the operating voltage is generated with classical fixed voltage regulators. I installed types from New Japan Radio (NJM7815 / NJM7915). At the output of the regulators I use 4× 4.7mF Panasonic FC capacitors. The LED’s are used as an optical control for the operating voltage. They get their power from the unregulated supply.
- Circuit Diagram of the Power Supply
Of course both channels are identical and therefore I describe only the left channel below.
There are 2 inputs available. At LK1 a single ended and at LK2 a symmetrical source can be connected. The symmetrical input signal is transformed into a single ended signal with IC6 (INA134). With the relay Rel1 the input signal is selected.
The selected signal of the relay is routed to the connector J1. One half of the external Alps stereo potentiometer (50kΩ logarithmic) is connected with J1. The amplification of the voltage take place with IC5 (OPA627) and the output current is delivered from the buffer IC7 (LKE49600). The feedback for IC5 is taken from the output of IC7 and therefore both integrated circuits building one amplifier block. With the help of the DIP switch different amplification factors can be selected (3dB, 10dB and 20dB). The circuit around the OP177 (IC8) is the servo loop of the amplifier. It guarantees that no DC voltage at the output is available and a coupling capacitor is not needed.
Complete Board of the Headphone Amplifier HA1
back to the top of the page
As always in the last years I used a case from Modushop / Hi-Fi 2000 from Italy. The machining I have done by myself with a Datron M35 milling machine. All electrical components are located on the board of the headphone amplifier and therefore only the needed external operating elements and connectors have to placed additionally in the case.
Front Side of the Headphone Amplifier
On the picture above you see that I have used instead of a 6.3mm headphone connector a 4-pole XLR female connector. I modificate my headphone with such a plug so that I can use it also with a bridge amplifier. Apart from that the handling of the amplifier is very easy corresponding to the circuit. On the left side of the front you find the power switch and the input selector. On the right side the volume actuator. In the middle of the front above the headphone output a power LED is mounted.
Back Side of the Headphone Amplifier
The rear side is also very simple. Two inputs – symmetrical and unsymmetrical – and the main socket is all what is needed for this amp.
View from Behind in the Open Cabinet