Pre Amplifier

 

After the completion of the loudspeaker system DiCOSS I hooked it up to my 35 year old Denon PRA-1000 preamp, that still works perfectly but
misses a number of features like a remote control and digital inputs and outputs.
One thing I definitively want in a preamp is a good Phono amplifier, which the Denon has, since I have many old vinyl records.
Hooking up my media player that only has an optical SPDIF output is currently done with a small interface box, not ideal. Same thing with a Bluetooth receiver.
If I retire and have enough spare time to digitize my vinyl collection, I would need a way to convert the albums and upload them to my server, hence a phono amplifier with a digital output would be handy.
With this whishlist in mind I started searching for my "ideal pre-amplifier" but this search proved to be in vain.
That was the moment of conception of my DIY pre-amplifier project.

I started breadboarding the phono amplifier as I expected most hurdles with that one. More details on that quest in the section "Phono Amplifier".
When I had a stable working phono amplifier prototype I started with the design of the powersupply revision 1, that worked fine with resistive loads.
The schematic design and PCB design of the complete pre amplifier started after a few breadboard test on the analog input stages, the input selector and the output line driver and proved to be reasonably straing forward.
The digital I/O's were already proven on the ADDA board of DiCoSS, apart from the coax SPDIF inputs.

After I finished the PCB design of the complete pre amplifier I hooked it up to my powersupply. Then I found out my first powersupply design wasn't able to cope with the excessive inrush currents.
My design phylosophy is to use 100nF ceramics plus 10uF electrolythics as powersupply bypass for each amplifier and converter supply and that added up to several hundreds of micro Fahrads of load causing excessive inrush currents at startup that damaged the powersupply DCDC converters I used.
Back to the drawing board.  More on that in the section "powersupply".


The Remote Control system is currently under design.
My initial idea to use a Logitech "Smart Control" remote and hub turned out to be the "easy" solution to control the pre-amplifier.
The Logitech Smart Control system however is not the easiest one to configure and the Logitech support center is not the quickest to implement additional RC5 commands for cursor control.
My MSP430 based RC5 decoder works now and I am able to read out both the Logitech remote and the iPhone-app commands.
 
Since I also want to have a manual control capability for my pre amplifier in case the remote fails I added a display with touchscreen.
The display driver and touchscreen interpreter are my next firmware excersise.
More details will follow in the section Display & Remote.

The final mechanics are not yet on my horizon and will be another exciting challenge, although I already have put considerable thought in the PCB designs to make the whole system as easy to assemble as possible.