Introduction:
● B22 (beta 22) is a high-end, reference-quality DIY stereo amplifier for dynamic headphones. It could also be built as a power amplifier for speakers (18Wrms into 8Ω unbalanced, or 50Wrms into 8Ω fully-balanced), or as a preamplifier.
● The all-discrete (no integrated circuits), fully complementary and cascoded topology, high-current MOSFET output stage, full class A operation and carefully tuned operating points provide vanishingly low distortion, high speed, wide bandwidth, low output impedance, and high output power. All elements of the design were critically examined to create an amplifier that performs as well on the test bench as it does with music.
● The B22 is the result of a community service project for the audiophile hobbyist. The schematic diagram is open source. The circuit design, as well as the printed circuit board layout are the work of AMB with participation from the DIY community. It is designed to be relatively easy to build, but it is not recommended as a first-time project for the beginner.
● The B22 project is primarily centered around the main amplifier board. Each PCB supports one channel.
● The B22 amplifier board does not include a power supply (PSU). It needs a separate dual-rail DC supply to provide power to the circuit.
● The current draw from the β22 board is approximately 200mA per board as a headphone amplifier, depending on its quiescent current (bias) adjustment settings. As a speaker amplifier, the current draw could have peaks of over 5A per β22 board, while it idles at the same 200mA. It is a good idea to use a power supply with some headroom above the maximum draw.
● Always measure the actual output voltage of the power supply before connecting it to the amplifier for the first time, to make sure its output voltage is as expected.
● The recommended configuration for the β22 is to use an outboard power supply. This allows the power transformer and other AC components to be located far away from the sensitive amplification circuitry for minimum induced hum and noise. You may instead use a large chassis and put all β22 amplifier boards and the power supply in it, but be sure to position the power transformer and other AC components as far from the amplifier boards as possible. A toroidal power transformer is recommended in this setup for its lower magnetic field leakage.
Input voltage: DC +/-24V --- +/-30V.
PCB Size: 155mm*50mm
This is mono. If you want to build a stereo amplifier, you need to buy 2 PCS.
NOTE: This is the Bare PCB. You need to match all the parts yourself.
Commissioning steps:
Step 1: Adjust the 501 blue adjustable resistor at the bottom, and measure the voltage at both ends of the 0.25W 2K2 resistor to be DC 4.4V.
Step 2: Adjust the top 501 blue adjustable resistor, and measure the voltage at both ends of the 3W 2R2 resistor to be DC 440mV.
Step 3: Adjust the middle 502 blue adjustable resistor, and measure the voltage at both ends of OUT and GND to be close to DC 0mV.
