A friend of mine designed a simple phono preamp for me to use with my newly purchased turntable. Once he was done with the design, I used EAGLE to design PCB layouts for both the power supply and amplifier boards. They were fabricated by OSH Park.
The power supply outputs 27V and uses a LM317LZ regulator. The amp section uses 2N5088 transistors.
The build was relatively unremarkable. I used an aluminum Hammond chassis, and point-to-point wired everything before, between, and after the PCBs.
It sounds great and has been trouble-free for years!
I bought a Fisher CA-272 100w/ch amplifier that had very noisy controls. Specifically, the amplifier had a built-in EQ and the sliders were deteriorated to the point of being unrepairable. Instead of ditching the amp, which sounded relatively good, I decided to transplant it into a new chassis.
The cool thing about this amplifier was the beautiful pair of heatsinks for the output transistors. I decided to show those off a bit in the new chassis. Since I was ditching the front panel, I had some power to play with as well.
Upgrades
I replaced any capacitors in the audio path with Nichicon KL series as these are known in audiophile circles to have the best performance in these applications.
For the volume potentiometer, I used an ALPS “Blue Velvet” RK27112A00CC.
Chassis
For the chassis I decided on a steel Hammond 1441. The power transformer of this amp was pretty hefty, so I needed a relatively stout panel to mount it. Also, working with steel is a bit easier than aluminum when it comes to machining.
Construction
After drilling holes for the board mounts, transformer, RCA jacks, and power cord, I mounted everything in place.
To show off the heatsinks, I cut rectangular slots in the top removable panel using a dremel.
LEDs
The front panel of the original amplifier was fed using +17.2VDC, so I used this source for LED lighting. I decided to illuminate the heatsinks that would be protruding through the top panel. Eight amber LEDs were connected in series along with a 470 Ω current-limiting resistor. The array draws about 30mA.
To position the board and heatsinks at the correct level, I used Keystone standoffs to offset the board from the bottom of the chassis.
Putting it all together
I was pleased with the end result, but I wish the lighting was a bit more diffuse. At some point I plan on addressing this. I’d also like to clean up the edges of the heatsink cutouts using grommet edging.
