This preamplifier consists of six TL074/TL072 preamplifier stages and PT2258 electronic volume control. The input stage of this preamplifier consists of six TL074/TL072 buffer stages. The PIC16F886 microcontroller manages the PT2258 electronic volume control IC. All the volume levels and menu options are displayed on HD44780, 16x2 character LCD screen.
All the menu navigations and volume controlling can perform using a 5-pin, EC11 series rotary encoder. An additional push switch provides to mute and unmute the audio outputs.
The firmware of the PIC16F886 has been developed using Microchip's MPLAB XC8 toolchain. The provided firmware supports two volume control modes. In default mode, the user can control simultaneously control volume levels of all 6-channels. In the second mode, the user can control the volume of the individual channel. These volume control modes can select from the menu system.
The firmware can directly upload to the MCU through the ICSP terminal provided on the PCB.
The PCB of the preamplifier is designed as a double-sided board. Many of the components used in this PCB are in the form of surface-mounted packages.
We use a dual-line package for the PT2258 due to the difficulty of obtaining the SMD package.
The provided PCB design contains a preamplifier, buffer stages, digital volume control circuit, driver circuit, and power supply unit.
The recommended transformer for this circuit is 6V×2 (1A) step-down transformer. The given power supply circuit consists of three voltage regulators, including LM317, LM337, and L7805. Both the LM317 and LM337 regulators are configured to provide 4.5V dual-rail output.
The project was sponsored by PCBWay. The accurately configured PCB of this preamplifier can directly order from PCBWay through their website. The dimension of the PCB is 151.45 × 50.76mm. The video related to this article clearly shows the steps of assembling, flashing, and mounting this preamplifier.
As mentioned at the beginning of this article, we successfully tested this preamplifier with an ASUS Xonar AE sound card. This preamplifier provides significant and clean output with the above-mention sound card and speaker system. We construct our prototype build using a commonly available 230mm × 150mm × 84mm ABS project box.
This preamplifier is an open hardware project. All source codes, KiCAD design files, and compiled binaries are available at the GitHub project repository. The complete documentation of this project is available at the GitHub project wiki. The PCB can order from PCBWay. The PCB Gerber files are also available at the release section of the above GitHub repository.