Class A Amplifier Electronics & Audio
The K10A is a 10 watt class A amplifier with a 12AX7 twin triode for the input stage. It runs on a single polarity supply, with the output load (speaker) capacitively coupled. It uses TO-3 package, complementary bipolar output transistors capable of high power dissipation. The circuit boards for this amplifier and the design itself came from Glen Kleinschmidt, in Australia.
Sorry, no component values.
Plate voltage for the 12AX7 is supplied from a voltage doubler circuit that uses a ~24VAC transformer (I used a dual 30VAC unit). The main supply uses a single 24VAC transformer, with the rectifier and smoothing capacitors off-board. This complicates the build slightly, since it uses two different transformers for each channel of the amplifier.
With the boards in hand, the hard work was done: design, layout and troubleshooting of an amplifier circuit can take a lot longer for me to do than to build the case that houses it. This article covers how I designed and made the enclosure for this amplifier.
I had a specific look in mind for this amp: an all aluminum, open look. Industrial looking.
A class A amplifier produces a lot of heat, and to dissipate that efficiently, the entire case is made with thick aluminum. This starts with homemade heatsinks:
These are made from pieces of aluminum door threshold, cut to size and bolted together with thermal grease between each piece. These are then bolted to the angles that the boards are mounted on.
Raw material for the case is 4mm thick aluminum. This was a large angle that I cut “L” shaped pieces from: the bottom / back and top / front from:
The top / front cut to shape. Most of the cutting was done on a table saw, using a sled to hold the aluminum securely. The slots on top were done with a router, using a template:
The amp boards are fully assembled using high quality parts. I salvaged the NEC output transistors from an old receiver that I found at Goodwill. These are fairly rare and are an excellent choice for this amplifier:
Powered up and tested. Just some basic tests to make sure everything is working properly and the amps are running stable:
1K sine wave looks clean:
20K square wave shows excellent, stable performance:
Because the tube requires a higher voltage, each channel needs two transformers. To neatly organize these transformers, I made a sheet metal enclosure to hold them. There are two 24 volt, 60VA transformers, one at each end of the rectangular box, and these supply the main power for the amps. In the middle there is a small dual 30 volt 8VA unit, which provides the higher voltage for both channels. The box also has bridge rectifiers and smoothing capacitors for the main power supplies:
The smoothing capacitors are mounted on top of the box:
Since these capacitors will be seen in the finished enclosure, I removed the plastic covers, making sure I marked the polarity on the bottom first.
The leads go through oversized holes in the box and they are glued in place with clear silicone.
I painted the inside of the bottom and back panel copper, but left bare metal where the heatsinks attach for best thermal conduction.
The amp boards installed and connected to the power supply:
The wire that is coming through the hole in the front is for the power switch that is not finished yet:
The enclosure is very nearly finished at this point. The output binding posts are in place on the back panel, as well as the input RCA jacks:
With it all put together and running, I can test how hot it gets.
With each output running at 800mA (200mV across the .25 emitter resistor) for nearly an hour I have 48C at the top of each output transistor, 46C at back of the upright part of the mounting angle, 40C at the outside thickest part of the heatsink and the lid is nicely warmed up to 35C.
I think this will be fine.
Next, I made the power switch. Basically it’s a push button on/off mains rated switch with a box built around it containing two LEDs. In this case, high intensity red LEDs. The clear plastic ring illuminates. The switch has a new “button” (cut from 1/4″ thick hardboard and painted black) that fits snuggly inside the ring and is glued to the switch button:
I used RightMark Audio Analyzer to do some basic testing. First, frequency response:
Total harmonic distortion:
The test results:
Four samples were done, the first is the sound card looped as a baseline for comparison. “Stereo Crosstalk” is invalid, since I only tested one channel at a time. Both channels tested within 2-3% of each other.
Excellent numbers, but how does it sound? Well, I’m not one to go on about the subtle nuances that only a class A amplifier will reveal in the recording, since I actually can’t hear any. Granted, my hearing is certainly not what it used to be, but I can never recall hearing any difference from one well made amplifier to the next.
The amp has been used in my home office since it was finished, connected to a small pair of speakers to provide some reprieve from the boredom of doing office work.