Two-channel preamplifier board

The ultimate two-channel amplified version, the circuit principle is OTL circuit. The push-pull power amplifier circuit without output transformer uses a single power supply to isolate the DC output signal from the midpoint of the two series-connected outputs through capacitive coupling. The power amplifier circuit of the output transformer is omitted. The AD823 op amp temporarily used has low noise and low distortion. Fairchild BC337 BC327 precision matching tube for output push-pull transistor. Achieve the volume balance of each channel as much as possible. The input uses red Weimar film capacitors, and the output DC blocking power supply capacitors use top-grade Genicon green robe BP non-polar capacitors. The sound is purer. Each channel uses a separate dual op amp diode amplification. The first stage amplification has a gain of 2, which provides better buffering. The second stage uses a gold-plated dial switch and has 3 gain options, which are 6 times, 10 times, and 15 times. Increase flexibility and playability. The output also uses a dial switch, which can be adapted to high and low-end headphones respectively to better match the output thrust. The protection circuit uses an RC delay plus an adjustable resistor, and the delay time is adjustable. Avoid current impact on the speaker as soon as the power is turned on. In addition, high-speed voltage comparators LM393 and TL431 are used as the reference voltage to detect the output voltage of each channel respectively. The voltage valve directly TL431 is adjustable DC0-2.5V, tentatively set at 0.5V here. Protect the speaker from burning the speaker headset due to capacitor self-excitation, leakage, etc. The protection actions are all done through Omron relays to cut off the output. The professional speaker protection chip UCP1237 is not used here to avoid that this chip uses AC input, and the power supply does not use a separate winding and is prone to power supply noise. In addition, 10 LEDs are assembled on the board to create a music spectrum, which beats to the rhythm of the music! The sensitivity is also adjustable with an adjustable resistor. See the video below for the effect! These two big greens are really expensive, but you get what you pay for, and the actual measured results are directly proportional!