I'm cleaning up some of the Planker docs in preparation for an article for QRP Quarterly, and will pop them up here with brief descriptions and notes. Since I like to start every project from the "back end", I'll start here with the audio board.
Planker audio board Rev 1.0
This board is fairly straightforward: On the left side is the transmit audio circuit, starting with the mic connector,followed by a two-stage mic amp using a J310 and 2n3904.
Toward the center, you'll see the common connection to the Product Detector/Balanced Modulator board, to the right of which is a 2N3904 amp driving an LM380-8. To produce an audible CW sidetone in the speaker/phones, the output from the sidetone circuit is introduced at the top of the AF Gain pot. The LM380 is powered on in both transmit and receive, but power to the driver transistor is disconnected while in transmit. Nonetheless, enough voice audio leaked through to the 380 that it was clearly audible, so I added a 2N7000 to shunt the output from the driver to ground while transmitting. Even though this places the 1uF at the top of the volume control at ground potential, enough sidetone still get's throug to be useful, and I see no reason to improve on it.
The circuit above delivers enough mic audio (using a cheap condensor) to drive the rig to full output, and the coupling caps were chosen to minimize low-frequency response. All stations worked so far have made positive comments about the audio, so I'm calling that a success!
The receive audio will easily deliver a couple of watts to an 8 Ohm speaker, and is pleasant to listen to.
The next board that I plan to document is an adjunct to this one, providing separate inputs for a computer soundcard interface for digital modes, and a functional VOX circuit. Maybe I'll have that up tomorrow, maybe not... We set goals, not deadlines!
73- Steve N8NM
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