Home > Studio Recording DIY Projects> Microprocessor controlled ADC Preamp (Analog Digital Control Preamp)
Intro
I have this idea brewing in my head... an all analog signal path preamp that is digitally controlled.
We retain the thick fat sound of analog while having the convenience of digital! Plus, it's got that digital display which has a lot of COOL factor.
Features
The 2 buttons on the left control the preamp level... it's like the
volume knob except you have a (+) and (-) for increment and decrement
volume. Maybe I'll replace this with a motorized pot so users can
still tweak the knob, or use the digital control and watch the knob
turn by itself!
The dB settings are displayed on VFDs (Vacuum Flourescent Display) so it can be read even across the room... unlike LCD displays.
The 2 buttons on the right of the VFD are for the menu functions... from these buttons, you can control phantom power, phase switch, pad switch. And of course, all settings are displayed on the VFD.
Settings can also be "saved" into the PIC memory for later recall...
The digital control module will be a modularized design... that way I can use whatever analog preamp I have in the backend.
To accomplish this project, I'll be using a microprocessor. So I need to brush up on my assembly programming. Or better yet, use a high level language like C or Basic and then compile to assembly. I'll also need to interface the microprocessor to electronic relays to control phantom power, phase, pad, etc...
Tools
I found a small company in Yugoslavia that makes compilers and development boards for PIC microprocessors. It's mikroElektronika. They also have 3 different kinds of compilers, Basic, Pascal and C. And they sell plenty of development boards at very cheap prices! Even their compiler is cheap compared to other US-based compilers. I bought their EasyPIC3 development board, pictured here.
I really like this board. Beside the excellent price, it is loaded with features like on-board USB programmer, interfaces for USB, serial, ps2... USB or external power supply support, LCD 2x16 or the bigger graphics LCD, 32 buttons, seeven-segment displays, etc... Their compiler also has several built-in routines in it's library so it makes development work fast and easy.
I'll be using the above development board to create and test my program that will be used in my Digital Preamp. But before I can run, I need to learn how to walk first.
This project will need to be broken down into several bite-size pieces... the main divisions being HARDWARE and SOFTWARE.
HARDWARE
For the hardware part, we'll need a board to house the XLR and relay connectors. We'll call this the interface board.
Also, we'll need another board to house the LCD display and switches. We'll call this the main board (or motherboard).
The mainboard will control the interface board using low-voltage 5volt signal lines. The main analog preamp board will connect to the analog inputs and outputs of the interface board. This way, the analog and digital signals aren't mixed too much.
Concerns: There is concern though for RF or EM interference. I'd just have to be careful on how I layout the parts and make sure put plenty of decoupling capacitors. Of course, we won't know for sure until the first prototpe is built.
The interface board will use PCB mounted solid-state to control phantom power switching, phase reverse, and padding.
I am still unsure if I should use digital controls to adjust the volume level of the preamp. I know there are chips designed for automatic gain control and I'm wondering if I can use those. If I can find a device that will let the audio signal go through "unchanged" while controlling the signal level digitally, that would be the ideal scenario. Something like a digital analog potentiometer...
SOFTWARE
For my current development, I'm using a PIC16F877A microprocessor from Microchip. I think this is too big, being a 40pin DIP package and too powerful for the final product and what I'm planning to do... but this chip has a built in A/D converter and it will be cool to feature a VU meter too on the LCD display using the on-board A/D converter. We'll see.
I'm using mikroBasic for development... Although I know Pascal and like Pascal better than Basic, the wide user base of the PIC Basic community convinced me I should use Basic instead. That way, I'd have lots of people to bug with my questions.
Like what I said earlier, before I can start running, I will need to learn how to walk first. On the next page, I'll breakdown the programming concepts I'll need to learn. Then by combining these little concepts and functions, I can create a complete OPERATING SYSTEM for my Digital Controlled Preamp.