Fixed Audio Output for the K2

I have had several requests to provide a fixed audio output from the K2.  After looking at the circuits that others have designed, I found that they used single ended audio input and were usually incorporated with some other circuits that were not useful to me.  I already have my external Digital Modes Switchbox built to automate digital transmissions, but I still lacked the fixed audio that would allow me to position the K2 audio gain anywhere without disturbing the gain settings on my soundcard.

I choose a LM386N-3 Audio Amplifier IC for its differential inputs, high available gain, small packaging, and low external parts count.  The –3 version works well from the 8 volt supply in the K2.  It seems not widely known that the LM386 works just fine into a high impedance loads as well as low impedance.  The amplifier is directly specified for voltage gain (Av) which then gives rise to the power available once the load resistance is known.  The power available increases as the resistance decreases (P = V2/R) but the voltage is independent of the load.

After building a couple of prototypes successfully on perfboard, I talked with Tom Hammond N0SS at Dayton 2004 and Tom generously offered to lay out a small PC board for this application.  Tom did more than just do the PC board layout, he made up a nice documentation sheet with the schematic and parts list, provided some sample boards for evaluation, and provided the negative layout to FAR circuits for final board production – then he even volunteered to shear the panels of boards for me too, I owe Tom a lot for his help on this project – without his assistance, the etched PC board would not have been possible.

I no longer have kits nor boards for this project.  The circuit is simple enough that it can be easily handwired on a small piece of perfboard, or even built Manhatten style.

This Fixed Audio Output should serve the needs of those who want fixed audio for digital modes as well as those who would like to drive a tape recorder or other audio device.

The following pages provide instructions for assembling and mounting my full kit.

The schematic, parts list and board layout are shown below (compliments of Tom Hammond).


 Assembly Instructions

o       If you are uncertain of the cable orientation, check with the markings on the PC board.  The marked conductor #1 of the 2 conductor cable will mate with the AF OUT pin and conductor #1 of the 4 conductor cable will mate with the DC IN pin.

o       You will have to separate the conductors for about ½ inch back to insert the terminals into the housing.


Connections to the control board

Routing and securing cable onto the control board

Installed Fixed Audio Board showing cable routing close to the RF Board.

Installing between the Front Panel Board and the Control Board

I have added a PDF document which shows how I mounted the board between the K2 Front Panel and Control Board, and is now my recommended mounting method even though the components must be laid flat.  If you have level controls in your external device, the variable pot can be replaced by a couple of fixed resistors.  Since this mounting may use short connecting leads, the connectors (the most expensive parts) may be eliminated since all but the output wires are contained entirely on the K2 Control Board.  Right click this link to download or left click to view here FP-CB mounting

NOTES for mounting in a K2/100

The side-panel mounting position becomes quite critical in the K2/100 with the upgraded shield and its speaker shield.  It depends on how the speaker shield cap was positioned.

To determine if you can use the side panel mounting, assemble the K2/100 and remove the left side panel.  Test fit the fixed audio board  to see if it projects above the plane where the inner surface of the side panel will be (use a straightedge from top to bottom).  You may have to position RF Board C60 a bit away from the edge, and place the Fixed Audio board so it overlaps the C60 position.

Measure the position VERY carefully and place the board on the side panel, or you might be successful if you remove the paper from the foam tape and lay the side panel in place (move it straight downward with no side movement) – if the board sticks to the panel it will be in the correct place, if it doesn’t stick, you will have to measure the position carefully and translate the mounting position onto the side panel.  Measure to the nearest 1/32 inch (1 mm) and “measure twice, stick once”.

Should you find inadequate room to mount it on the side panel, you can use an alternate mounting location near the center of the RF board.  Remove the KPA100 and locate the IOC chip – it is the one in a socket located to the left rear of the SSB option board.  Remove only a portion of the paper from the foam tape and stick the Fixed Audio Board onto the top of the IOC chip.