Altair 8800 Clone Processor Technology Music System

Amp board mounted inside of Altair Clone

Back in May 2015, the Music System from Processor Technology was revealed on the Altair 8800 Clone web site (

The original system was made up of a simple S-100 based interface board and a software package. The concept was shown on a classic Altair 8800 and also on the Altair 8800 Clone. In the Clone’s case two wires needed to be solder to main board between ground and the driven end of the current limiting resistor that drives the front panel INTE LED.

Once I saw the concept, I quickly made a bread board for testing followed by a printed circuit board following the circuit found in the Processor Technology documentation. The idea was to make a nice PC board and install it in my clone . Then I got to thinking. After the PC board comes some form of audio amp. Why not just integrate an amp on to the basic design to complete the overall package.

[Click here to see the PDF for the full project including the schematic, parts locations and  bill of materials. ]

Processor Technology Music System Amp

The schematic, shown here, is the resulting version of the amp board for the Altair Clone. The design uses the old dependable and easy to find LM386N. The LM386N can drive an eight ohm speaker up to one watt at 12 volts DC. For flexibility, the design has a board mounted volume control, a power switch and both internal 2-pin and external 3.5mm speaker plugs. The external plug is switched to cut off the internal speaker when an external speaker is connected to the 3.5mm plug.

The resulting assembled board, shown above, is then mounted using both the power switch and the external speaker 3.5mm plug, to a serial infill panel. This final assembly is then bolted to one of the unused serial holes on the back of the clone.

Power to drive the amp assembly is derived by unplugging the clone's supplied fan from J5 on the Altair Clone’s 

main board and inserting a three pin though cable between boards. The fan is then plugged in to a three pin connector on the amp board which just passes though power to drive it. This allows the use of the clone’s front panel power switch to turn the amp on or off. The extra three pin plug is for another project that also uses the clone's front panel power switch.

The amp by its self, at full volume, draws less than 300mA at 12V in to eight ohms. So there should be no issues with just the amp. My test unit is plenty load using an eight ohm, two watt, 87mm square speaker. All my family members come to the lab in the basement to see what was going on during full volume testing.

I tried to place volume knob within the same serial hole as the switch and 3.5mm connector. There just is not enough room there. I ended up placing the volume on the board. For some, once you have the volume setup you may not need to adjust it again.

Back of Altair Clone with amp board

The hope was to find some pre-made infill panels on the internet and then modify them to add the two extra holes for the 3.5mm connector and the power switch. Back 30 years ago you could find such infills at almost every computer parts store. In today’s modern world with almost no RS232 serial being used, they are few easy to locate suppers of these panels that will allow you to buy just one infill. This only left fabricating them ourselves.

A trip to my local hardware store yielded a section of 3/4 x 1/16 inch aluminum bar stock. Unfortunately the shortest length I could get was 24 inches long. This would allow me to make more than ten infill panels. Guess I'll have some unused aluminum bar stock for my next project needing such bar stock.

If you wish to have access from the back of the clone, use some of the extra aluminum from above and create a second infill panel. This infill panel can be placed in a second serial hole located next to the PC board assembly. Mount a small potentiometer for an external volume knob. A short run of three 24-26 AWG twisted wires can be soldered between the board and the volume potentiometer. Larger pads under the board mounted volume potentiometer have been supplied to allow for this option.