ICM Controls G8NJJ User manual

G8NJJ Audio Mixer and Recorder
This unit allows two operators to use amateur radio transceivers for contest or field day operations. It allows PC type
headsets with boom microphones; one operator can transmit but both can hear each other. 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
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 microphone, selected by the rear panel switch. The audio mixers are all “virtual earth”
mixers created using simple operational amplifier circuits. 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. 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 as a homebrew PCB, but 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.
Connections
Front Panel
There are front panel connections for two PC-type headsets. These have conventional 3.5mm stereo jack sockets.
Erase Next Record Play Enable PTT
TX VU
Mic Gain
TX Mic
1 2
Spk Vol
RX in volTX out level
G8NJJ
Mixer &
Recorder
1 2 Mic
in Headset 1 2
1 2
Microphone inputs: Electret type microphone. Bias
supply voltage provided. (The bias may be
removed by removing an internal jumper if
required).
Headset output: stereo audio. (Be aware there is
no volume control –use headsets with an in-line
volume adjustment)
Indicators:
The green LED flashes while an audio record or replay operation is occurring
The yellow LED is lit when the PTT circuit is “armed” to key the radio during audio replay
The red LED is lit when PTT is active (note not lit when an external PTT switch is used).
Rear Panel
+12v
DC PTT
Yaesu
Dynamic
Mic in
TX To
Yaesu
Radio
TX Mic outExt Spk
Radio
RX in
G8NJJ Audio Mixer
Mic 2
select
Front
Rear
- +
Radio RX in: Receive audio. Connect to your radio’s
headphone out or similar connector.
TX to Yaesu radio: PTT and TX audio, wired as per
Yaesu 8 pin microphone connectors. (Make up a
cable for other radios).
PTT: Connector for an external PTT switch (e.g. a
foot switch).
Power: +15v-18V DC, positive centre.
Less commonly used connections:
Ext Speaker
PC-type amplified speakers can be connected here.
Yaesu
Dynamic mic
in
Allows a standard Yaesu dynamic microphone to be used on the “microphone 2” channel. The
adjacent switch selects the “microphone 2” channel between this input and the electret input.
TX Mic Out
TX audio for connection to other devices. Signal level OK for dynamic microphone inputs.