Pre Amplifier (Updated March 2018)

 

After the completion of the DiCoSS loudspeakers I hooked them up to my 35 year old Denon PRA-1000 preamp, that still works nicely but misses a number of features like a remote control and SPDIF inputs and outputs.
Connecting my Home Theatre PC with its optical SPDIF output was done using a small interface box, not ideal.

I also want to play the audio of my TV-box and TV over the speakers and preferably have just one remote to control all equipment: TV, TV-box, Preamp, CD player and HTPC.
With this whishlist in mind I started searching for an "off the shelf" pre-amplifier but this search proved to be quite a challenge and the perfect excuse to start the design of my own Pre-amplifier.

 

I started my Pre amplifier design with the Phono amplifier.

In the section "Phono Amplifier" I will go through the process how I decided for a 100% discrete Phono amplifier design.
If you are interested in amplifier stability and low noise design, you might find this interesting reading, others find this boring theory.

When I had a stable, low noise phono amplifier prototype I started with the design of the power supply that I tested with resistive loads.

The schematic design and PCB design of the Preamplifier started after a few breadboard test on the analog input stages, the input selector and the output line driver which proved to be reasonably straight forward.
The ADC, DAC and Optical SPDIF input circuits are proven designs in DiCoSS that I simply copied.

The Optical output, coax SPDIF outputs, volume control and Sample Rate Converter were put directly on the PCB without prototyping and worked first time as expected.
For more details on the actual Pre-amplifier design like channel cross-talk measurements and PCB layout, have a look at section "Preamp".

After I finished the PCB design of the complete Pre-amplifier I hooked it up to my power supply.
I found out the hard way that my powersupply design wasn't handling the inrush currents well that are caused by the large amount of bypass capacitors on the +/-15V supplies.
My design phylosophy is to use 100nF ceramics plus 10uF electrolythics as power supply bypass for each amplifier and converter which added up to several hundreds of micro Fahrads of  load capacitance.
Back to the drawing board to design power supply version 2.0.  More on that in the section "Power Supply".

I took a different approach with the controls of the Preamp and implemented the input selector, balance and volume settings by means of a touch screen.
This allowed for easy implementation of other functionalities like input volume presets, physical to logical input selector translation and on/off switching of an auxiliary 5V power-supply at a specific input selection to power for instance a BT receiver or a wireless headset.
This aproach also made integration with the IR receiver for the remote control easier.
So began the development of the Display unit, containing an intelligent eDIP-240-7 graphical display from Electronic Assembly with touch screen.
To keep the Pre-amplifier appearance slick I decided for a black-white monochrome dot-matrix display with 240x128 pixels. More on that design in the section "Display & Remote". 

The Remote Control firmware was done prior to the display design on a MSP430FR2311 launchpad plus IR Boosterpack. I needed the functionality to develop the Display Unit IR receiver part.
The final Remote Control design details can be found under the section "Display & Remote".

 My initial idea to use a Logitech "Smart Control" remote and hub turned out to be the "easy" solution to control the Pre-amplifier and my other equipment with one remote.
The Logitech Smart Control works great for off the shelf equipment but gives you a hard time to learn DIY equipment, even with standard RC5 codes. More on that in the section "Remote Control".

The mechanics are reasonably straight forward.
I decided to use standard 4mm thick aluminium plating for front, back and side panels that I had cut out by a watercutting company. Top and bottom are made from 1.5mm thick aluminium plates.
10mm square aluminium rods and a bunch of M3 bolts keep the case together.
A black and transparant anodized finish gives the Preamplifier casing a professional look. More on that in the section "Mechanics".