[hpsdr] External speaker with Mercury

[Luke Steele]

When I originally set up my HPSDR, I used a pair of amplified computer
speakers. Whilst this was ok, I did experience RF feedback. In addition,
the speakers were still active when I used headphones.

So, I fitted a small amplifier module and loudspeaker inside the Pandora
enclosure.
The amplifier I used was a Kemo #M31 3.5W universal amplifier module. It
may be better to knock up an amplifier using an LM-380, or similar,  but
the Kemo module was there on the shelf in the electronics store so I
grabbed it.
The speaker was a small unit that I mounted behind some ventilation slots
in the side of the Pandora.
The amplifier had more gain than I wanted, so I placed a pi attentuator on
the input, which I derived from the Headphone output from Mercury. As I
wanted a 6.5mm switching headphone jack on my HPSDR, I ran a stereo line to
the panel mounted headphone jack from the headphone output from Mercury.
That way, I could use the binaural feature with headphones. For the
amplifier, I used a resistive combiner (1k I think), and a pi attenuator so
that I could drive the amplifier adequately, but not excessively, to suit
the speaker. I also RF bypassed the low-level side of the amp, and also
fitted a 0.1 uF ceramic on the speaker terminals.
Now it is like a "normal" transceiver, in that the speaker is disabled when
the headphone is plugged in.
One more thing, I still found RF getting in to the audio amplifier, so I
soldered a SMD capacitor across the headphone jack on Mercury, as described
in the forum somewhere. You will also have limitations on the speaker
mounting and sound quality when mounting inside Pandora. I mostly use
phones when using my HPSDR, so the loudspeaker setup is just a compromise.
The amplified speakers certainly gave better sound, but RF issues caused me
to go for the internal amp and speaker.
Luke VK3HJ

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