DIY 7.1 Audio Upgrade: PT2256 Chip & TDA7498E Amp

If you're already using a 7.1 surround sound system like I am but don't want to invest in a new set, and you also need an amplifier that can work with a 24V UPS system while operating on DC power, consider creating a digital control circuit for more convenient volume adjustment.

Let's break it down:

Finished Product Showcase

Chip Selection

There are several methods for volume control, broadly categorized into three types: analog potentiometers (including precision resistor multi-stage knobs), digital control integrated resistor chips, and digital resampling control (like volume output control on computers and phones). The first two are lossless (in theory), while the latter has some loss (reducing the sampling bit depth of digital signals lowers the signal-to-noise ratio, but for audio above 10 bits, differences are barely noticeable).

The PT2256 is a dual-channel volume control chip that uses an internal resistor array. It employs digital control to select the connected resistor, achieving volume control. The external circuit is simple, providing good audio quality and user-friendly usage.

To indicate the volume of each channel, leverage the chip's 8-level voltage output feature to drive an LED display bar. Input the voltage into an LM3914, and different voltages will light up a different number of LEDs.

To address interference and noise caused by the common ground of various chips and signals, use the BA3121 ground isolation amplifier for noise elimination.

Finally, feed the processed audio of the 8 channels into the TDA7498E amplifier chip, amplifying and driving the speakers. This digital amplifier chip boasts a maximum voltage gain of 35.6dB (60 times), high efficiency, excellent audio quality, and powerful output at an affordable price. At an input voltage of 36V, each channel (1% THD, 4Ω) can output up to 125W, delivering impressive bass. (For a 24V power system, it's approximately 60W per channel.)

Circuit Construction

Use pre-existing 2.0 and 5.1 amplifier boards for the audio part of the 7.1 setup. The real challenge lies in creating the volume control circuit. First, verify the circuit's correctness on a perforated board, and once confirmed, customize the PCB.

PCB

Transfer each component from the perforated board to the PCB, conduct a power test, and ensure that all functions work flawlessly.

Completion!

Now, enjoy the immersive audio experience brought by the 7.1 surround sound system to the fullest!