Preamplifier and volume controller

Quite a few years ago I had a plan to build a Vacuum tube amplifier, but I also wanted to add some modern features not usually included in the old school valve amps, such as digital volume control via a rotary encoder, input selection via a serial RS-232 connection, as well as an LCD display, or possibly even a VFD (vacuum fluorescent display).

I finally got around to designing and manufacturing a printed circuit board. My first design from scratch actually.

I chose to base the design from a PGA2310 IC and started from there, I ordered a DIP package from Digikey. I also had an Arduino mega2560 laying around, so I was able to connect them up with a breadboard, and do some basic testing.

I love the look of SMD boards, and a lot of the new chips these days are only being manufactured in SMD packages, so I didn’t want to waste time making a through hole board. Also soldering SMD components is not a problem for me at all, as I actually prefer them over through hole.

Although, in hindsight I should have made the electrolytic capacitors through hole, as the SMD versions are just too fragile, and can quite easily rip the pads straight off without much force at all (which I had to learn that the hard way.)

altium 3d top

The final design, rendered in 3D with component bodies. I couldn’t figure out why the QFP100 was purple with green pins though.

altium 2d top

2D view showing the top layer and ground planes.

altium 2d power plane

2D view showing the +5 V power plane, and +15/-15 V polygons

large_top

The rendered image from OSH Park

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Printed a 1:1 paper model to double check footprints

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Finally received the purple package

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Test fitting the through hole components. They all fit perfectly!

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Inspecting the tracks and solder mask through the microscope. The solder mask is slightly offset, but nothing to worry about.

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Yes, it actually works. Doing some initial tests with Arduino to test the hardware is working, uploading the program using ICSP header. I plan to write the final code using Atmel studio.

I also tested the audio section, and it’s working perfectly so far. I also plan to do some audio measurements at a later stage, and see how it really performs.

Although this process has taken me quite a long time to do, I have learnt so many things, as well as now when I start on a new design, I will do many things differently. For example, I would use smaller connectors, and populate the board more densely to save on space (and therefore money.)  I would also try to use a smaller microcontroller if I was going to make more than a few boards.

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Preamplifier and volume controller

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