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nlinus 15th May 2011 03:13 PM

Budget "passive preamp"
Guess it's a couple of years since my last post. Have so many different projects and interests. One of the last larger projects was a pulse jet engine powered snowmobile...

Anyway, I really needed a replacement for my old Rotel preamp. I also wanted to be able to use a remote to turn the volume up/down. I decided to put toghether a simple "passive preamp".

An input selector is not necessary, since my DAC (MSB DAC Link III) takes care of that. It automatically switches between optical (from computer), coax (from CD player) and one analog input (record player).

Bought a cheap motorized Alps pot from EBay. It's not an Alps Blue, and to be honest, I don't know what difference it would make if I had one. Since this pot is PCB mounted, I made a simple PCB with solder pads for the wires, and a screw terminal for the motor.

The casing is a cheap Hammond aluminium box, which I polished a bit and the painted black. The knob is turned out of steel (a piece of Impax I happened to find in the junk bin).

Internal wiring is Van Den Hul Videolink 75 - what I had at the moment. Don't know the brand of the RCA sockets.

nlinus 15th May 2011 03:22 PM

Yesterday I put together an IR receiver/decoder to control the motor of the pot. It decodes the NEC IR protocol and works very fine when tested on the desktop. One component is missing, the integrated H-Bridge, LMD8200, but as soon as that has arrived I will put it in and install everything in the enclosure.

The board is based on a Microchip PIC16F628A, which runs at 4 MHz (internal oscillator) and decodes/extracts the address and command out of the IR bursts sent by the remote control. It sends these values out on the serial port, so I can easily see what data each button generates.
As soon as this has been tested in the "preamp", the schematics and source code will be available if anyone would be interested.

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