Using the Rear Panel speaker jack

(Thanks to Brian Lloyd WB6RQN for posting this info)

You can plug powered stereo speakers or a stereo headset into the
speaker jack on the back of the radio with no damage. In this case the left
audio channel will go to both speakers or ear-speakers but the speakers
will be out-of-phase. If you are using powered speakers or stereo
headphones you are better off using the front-panel headphone jack since
both the R and L channels are present on that connector.

If you plug an unpowered speaker into the rear speaker jack you must use a
three-conductor (TRS) 1/8" (3.5mm) plug with the speaker wired to tip and
ring. You will not hurt anything if you wire the speaker to tip and sleeve
("ground" conductor on the plug) or ring and sleeve but audio level output
will be down by 6dB. But you must not plug a two-conductor (tip and sleeve)
plug into the rear speaker jack
as the sleeve will short the ring terminal
of the jack to ground thus shorting out the Left '+' amp output.

The rear panel and accessory jack speaker outputs are actually push-pull or
"bridged" outputs. You can think of each of these as having TWO audio amps,
one for the '-' terminal (tip or ACC pin 19) and one for the '+' terminal
(ring or ACC pin 6). Since each of these is the output of a completely
separate audio amp you do not want to connect any of these pins to ground
as you will be shorting out the output of one of the amps. Bridged output
is a way to get 4x the audio output power from the same supply voltage.

So, for best results, plug stereo headphones or powered speakers into the
headphone jack. If you wish to use unpowered speakers, wire the left
speaker to the tip and ring of a 1/8" (3.5mm) plug and plug that into the
speaker jack on the back of the radio. Wire the right speaker to pins 6 and
19 of the 25-pin Accessory jack.

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