I struggled with knowing what to call this. An audio recorder? A microphone mixer? A headphone amplifier? It's all of those, and more. It allows two operators to use amateur radio transceivers for contest or field day operations, and to play out pre-recorded "CQ" calls. It allows PC type headsets with boom microphones; one operator can transmit but both can hear each other. You can use a "fist" microphone too. Its features include:
- Two microphone inputs, with individual gain controls
- Two identical headset outputs (note no volume control – use the headset volume control)
- Select switch to choose which microphone is routed to the transmitter
- TX level control, to set “mic gain” on radios with no separate adjustments
- RX level control, to set a comfortable radio receive signal level
- Internal speaker for RX audio, with volume control
- VU meter to equalise signal levels for the radio TX path
- Audio recorder, for recording & replaying “CQ” type calls
- PTT button, with latch
- “ERASE” erases the current recorded message. If pressed for around 3 seconds it will erase all messages. The green Led will flash 7 times when that happens.
- “NEXT” skips to the next recorded message. If on the last one, it will skip to the start.
- “RECORD” records a new message while it is pressed. The front panel microphone is used for record audio.
- “PLAY” plays the current message.
- “ENABLE” turns on or off automating PTT keying during replay of a recorded message
- “PTT” can be pressed on or off as normal to key the radio. Alternatively if it is pressed momentarily, it will latch PTT; press it again to unlatch PTT.
About the Unit
Each microphone input has its own preamplifier, and a bias circuit for the electret microphones found in PC headsets. The bias can be disabled by removing an internal jumper. Channel 2 can have the microphone input provided by a handheld "fist" microphone, selected by the rear panel switch: the gain is higher to allow dynamic microphones to be used.
The audio mixers are all “virtual earth” mixers created using simple operational amplifier circuits. There are several mixers, for different audio paths. The audio recorder uses an ISD1760PY audio recorder IC; the simplest way to buy these is to buy a PCB on ebay and throw away the PCB. The VU meter uses a simple LM3916 LED display driver.
The major design challenge was getting all of the controls into a front panel within an acceptable size envelope. In the end I chose a two PCB solution, with the audio path on one PCB and the recorder and VU meter in the other. The main audio board was too complex for a homebrew PCB, so I had a batch of 5 made in China, very cost effectively. The VU meter and recorder were made on veroboard, and later as a homebrew PCB; if I made more I’d make some of the PCB pads a little larger.
The whole fits an extruded aluminium box, making a compact unit. The front and rear panels have laser-printed legends, making an attractive end product. By far the longest mechanical activity was drilling the holes for the speaker: on reflection, cutting out an oval hole might have been faster.
Operation
Mixer
The unit mixes audio to the headsets from the radio, both microphones and the audio recorder. The radio input is stereo, for those radios that have two independent receivers (e.g. for dual polarisation antennas). “RX in Vol” adjusts the RX audio input level.
The speaker output does not replay the microphone audio. This minimises opportunity for audio feedback.
The unit has been designed for two operators, with only one operating the transmitter. Both operators can hear each other. The front panel switch selects which one is connected to the transmitter.
Audio Recorder
The audio recorder permits for repetitive “CQ” type calls. It is capable of recording around 30 seconds of audio. Each time “play” is pressed, the stored message is played; the unit can key the PTT at the same time.
- “ERASE” erases the current recorded message. If pressed for around 3 seconds it will erase all messages. The green Led will flash 7 times when that happens.
- “NEXT” skips to the next recorded message. If on the last one, it will skip to the start.
- “RECORD” records a new message while it is pressed. The front panel microphone is used for record audio.
- “PLAY” plays the current message.
PTT
The PTT function is simple. It is controlled by two buttons:
- “ENABLE” turns on or off automating PTT keying during replay of a recorded message
- “PTT” can be pressed on or off as normal to key the radio. Alternatively if it is pressed momentarily, it will latch PTT; press it again to unlatch PTT.
VU
The VU meter shows the transmit audio level on 10 LEDs. It has been provided so that the signal levels can be equalised between microphones and the audio recorder. The last two LEDs are red: it is suggested you should aim for the red LEDs just not lighting. (Not that it shows the signal level for the microphone selected with the front panel switch – not both)
Once the signal level has been set with the VU meter, adjust the TX level control to achieve the correct TX signal level. If an ALC display is available, adjust so that ALC is just being indicated.
This has proved to be a good piece of kit. The audio recorder in particular transforms "CQ" calls in contest, particularly at quiet times. We've used it for the SSB field day, and achieved solid results from it under field conditions.
There's always more you can do with a design, but I've reached the point where I consider it "finished". The TX audio feed could do with being buffered: the TX audio level at the moment depends on the input impedance of the microphone input to the radio. The audio recorder should ideally us the headset microphone signal, rather than the front panel electret microphone; but that needs registers in the audio recorder chip changing. Finally the record/repplay interface would be better if there were separate buttons for several messages. One day!