Digital pre-amplifier
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A project begun some time ago (1988).
This is an up to date description of the project. So the baxendal parts (treble/loud and fine volume) are still missing. In fact I use the output of the pre-amplifier with a commercial amplifier and so don't have immediately the need for.
After this, I will also probably replace the digital part to use a PIC or a 8031.
The software zip files gives also the philosophy I use to build this pre-amplifier.
Project aim
A pre-amplifier with full digital control (no small level signal on front panel, no switch and no potentiometer,...).
Allows to independently connect the 4 outputs (main, aux, tb1 and tb2) on one of the 8 inputs (2 cd, tu, aux, 2 tb and 2 phonos).
General schematic
The general schematic of the actual state of this projet.
Baxendal, VU-meter, volume and headset outputs are still missing.
Analogic part schematic
To come.
...
Digital part schematic
The digital control part :
- The 8085 based heart with EPROM and RAM,
- The keyboards and displays part is built around a 8279,
- One (and only one) interface with the analogic parts.
I have done this part using wrapping.
If somebody want it, I have the component side schematic.
Displays/keyboards part schematic
4 small boards just behind the front panel :
- One for power on or off,
- One for the volume/baxendal,
- One for the input selection,
- One for the functions.
Interface part schematic
The unique interface between the digital and analogic parts.
All outputs use open collectors to avoid parasitics of the analogic part with the noisy digital part.
I have done this part on an small prototype board.
Power supply part schematic
3 power supplies :
- For other elements of the HIFI chain,
- For the analogic blocs,
- For the numeric parts.
Displays/keyboards part component side
All the links with the digital part is done with 1,27 mm flat wire :
- Directly sold on the display part,
- Through DIL connectors for flat wire on the digital part,
So some surprising numbering of the wires!
Displays/keyboards part PCB
The PCB for the keyboards/displays parts.
Click on the picture to display the full resolution PCB (176 ko) or here to download it.
It's a 6,2 by 5,6 inches board at a 600 dpi resolution.
Power supply part component side
Three small PCB with no particular difficulty.
Don't forget that for the upper left one, the component are connected to the mains. Apply required isolation with the box, and take care during tests.
Power supply part PCB
The PCB for the power supply parts.
Click on the picture to display the full resolution PCB (105 ko) or here to download it.
It's a 7,0 by 5,6 inches board at a 600 dpi resolution.
Software
The software (source and binary files) and its documentation in 1.6 version: PREAMP16.ZIP (8ko) (binary file in intel-32 format).
Inside
Power supplies are on the left side, analogic parts on the top right and the digital parts on the bottom right.
So analogic signals are well separated from the noisy digital ones.