4SQRP Bayou Jumper 40 Meter CW Transceiver, Assembly & Instruction Manual

Page: 45 / 47

revised 3 Feb 2021 Bayou Jumper revB Assembly Manual page 44 of 46
L. Connecting external audio devices (amplifier, filter, etc)
The Bayou Jumper’s audio amplifier device (NJM2113) employs a differential output stage
which is not referenced to the circuit ground. That is why only the tip and ring contacts of the
stereo jack are connected, and the sleeve is left floating. This type circuit does not require the
large series capacitor such as used with LM386 amplifiers, and as a result, the low frequency
response of the audio stage is improved.
But since both outputs of the IC are internally biased at approximately half the supply voltage,
connecting either output pin to circuit ground will produce a short circuit which will
irretrievably damage the chip.
If you want to use an external amplifier, or external narrow band audio filter (such as a
NEscaf or HI-PER-MITE) you must use a DC blocking capacitor in series with both the tip
and ring connections. Check the schematic of your external device. A blocking capacitor may
already be in place in the signal lead, so you would only need to add one in the common
lead.
Since most of these devices have a relatively high input impedance, the capacitance does
not need to be very large
– you may be able to use a common 0.1 µF capacitor. If an
electrolytic capacitor is used, the positive side should be connected to the tip and/or ring of
the headphone jack.
The diagram below illustrates a proper connection to a HI-PER-MITE audio filter (top) or
other generic active audio device (bottom).