[Boatanchors] Speaker Switching

[brianclarke01 at optusnet.com.au]

Hi All,

I replied to Eugene alone - he suggested that there may be others who could be interested - so, here's my thortz.

>There are several ways of doing this.
>
>If you want to have both radios going at the same time, then you could use a, say, 25
Ohm 5 W rheostat with each of the radio's hot loudspeakers' leads connected to the
two ends of the rheostat, the loudspeaker lead to the wiper and all the common loudspeaker
wires connected together. Warning - this will work best if the loudspeaker wiring is
isolated from other circuitry, eg, feedback circuits; however, it will work if one side of
each loudspeaker connection is connected to the chassis and you make such leads your common
- and you can be sure that the chassis earths are at the same potential.
>
>Another way would be to use the headphone outlets as inputs to a simple resisitive mixer
in front of a low power audio amplifier. With many sets that offer a headphone outlet,
inserting the headphone plug puts a dummy load across the output transformer and inserts an
attenuation pad to feed the headphones. The output from the hedphone socket would them be
about 100 mV to about 500 mV. A simple 5 W AF amplifier with about 24 dB of gain should be
adequate.
>
>If you use a simple SPDT switch, you will leave the unaudited radio's output
transformer unloaded = very dangerous to the transformer. Better to use a DPDT, Make Before
Break [MBB] switch that puts a load on the unaudited set's output. I say MBB so that
there is no instant when the output transformers are not loaded. There is no need to use a
loading resistor - a valve amplifier is quite happy working into a short circuit on the
secondary of its output transformer. So, one wiper of your DPDT switch is connected to the
loudspeaker's hot lead and the other wiper is connected to the common. MBB is seldom
available in toggle switches, is available on some slide and wafer switches - always check
rather than assume.
>
>73 de Brian, VK2GCE.