Mini Battery Boost Powered Headphone Amplifier DIY

Mini version battery boost power supply Headphone amplifier V2.0 Second version improvements:

1. Reduce the size of the housing and use RocketFish USB2.0/IDE 2.5-inch mobile hard disk box; 2. Change the headphone amplifier circuit to Beyerdynamic A1; 3. Use a transformer to isolate and boost the boost circuit ; 4. Add a lithium battery to increase the endurance of the headphone amplifier.

Layout: The internal dimensions of the hard disk box are 71×125×14. Layout: The PCB is placed in the front and back of the hard disk box, and the lithium battery is placed in the middle. Specifically, the headphone amplifier circuit is placed in the lower layer in the front, the voltage regulator circuit is placed in the upper layer, the boost circuit is placed in the back, and there are two lithium batteries in the middle.

Handmade, the most difficult part of this headphone amplifier - winding the step-up isolation transformer

The step-up transformer wound for this headphone amplifier uses EE16 type magnetic core (finished size 15×15×12.5mm), with 13 wires in parallel on the primary, 8 wires in parallel and 4 wires in parallel on the secondary, and 8-layer segmented sandwich winding (because the transformer is too small and needs to be layered and segmented with multiple wires in parallel).

Since the maximum height inside the headphone amp box can only be 14mm, and the skeleton pins are much larger than 14mm when pointing downward, the pin positions were slightly modified this time. A small drill was used to drill holes on the side of the skeleton and the pins were installed on both sides.

The wound transformer was tested. The voltage of two lithium batteries was boosted from 7.2V to ±15~±20V (the circuit is SG3525+MOSFET tube), and the positive and negative arms were connected to 50 ohm resistor dummy loads (total power consumption>10W, which is greater than the power supply required by Beyerdynamic A1). The boost efficiency was>75%. After half an hour of testing, the temperature was slightly warm and there was no sound. It was quite satisfactory. [page]

Headphone amplifier part:

Since it is still under testing, the LED has not been replaced and the heat sink of the driver tube has not been connected. Therefore, the voltage on the test is positive and negative 12V (a pair of 1200uF chemical capacitors are connected after dual 8.5 AC rectification). I dare not go high and have not measured the current of the tube. After running for more than 20 minutes, the temperature of the driver tube is about 55 degrees!

The midpoint voltage is 0.3mV for one channel and 0.5mV for the other channel. Except for a pair of BC556/546 tubes that were purchased as matched tubes, the others are not matched, and the chip op amps are not from the same batch. The white one is a disassembled chip Omron relay, which is used as headphone protection. The protection circuit is on the lower layer of the PCB. The protection part has not been soldered yet, and the op amp part of the circuit is also on the lower layer of the PCB.

The two solder points on the left side of the relay are reserved for the SMD LED indicator lights, which are intended to let light through the slow-plate acrylic.

Headphone amplifier op amp part and headphone protection circuit

The coupling size of the PCB is designed according to the Weimar 3.3/4.7uF 50V. Two BP capacitors were found to replace it. In addition, the PCB has been designed for coupling shielding, so just make a square shielding cover.

The two output 100 ohm resistors are said to be from the British WELWYN MFR4 with 1/2W power. When the dual box was originally designed, it was intended to automatically change the output resistor value according to the impedance of the headset. Now only one resistor can be fixed, but the direct-insert resistor is also used here for easy replacement. [page]

Protected voltage regulator board

Fix the final tube of the headphone amplifier (a 0.8mm thick copper sheet was removed from a discarded portable switching power supply and used as a heat transfer sheet to fix the final tube 139/140)

Replace with SMD LED

Dial Potentiometer Acrylic Edge

Front panel opening

Replace with Weimar 3.5uf/50V coupling [page]

Power board chip PCB

Power board chip PCB

Main power board PCB

Main power board PCB

Connect the power board and voltage regulator board

Finish:

Press and hold the power button for 3-6 seconds to turn on the device, and press and hold for more than 6 seconds to turn off the device. If the 4th and 5th pins of the inserted mini USB plug are the same, the internal headphone amplifier will lose power, and the 5V output from the USB port will power external devices such as mobile phones. At the same time, the mini USB port is also the internal charging port. The charging time of the current charger is about 4 hours. The battery life of the headphone amplifier is 4 hours.