[Boatanchors] Speaker Switching

[Gary Schafer]

It is generally a bad idea to allow one radio to feed audio to another's
output. With the pot connected to two radios and one speaker some of the
audio from one will couple into the other. The voltages from both
transformers can add in the transformer and cause it to arc. 

73
Gary  K4FMX


> -----Original Message-----
> From: boatanchors-bounces at mailman.qth.net [mailto:boatanchors-
> bounces at mailman.qth.net] On Behalf Of brianclarke01 at optusnet.com.au
> Sent: Sunday, August 06, 2006 9:58 PM
> To: Eugene Hertz
> Cc: Boatanchors at mailman.qth.net
> Subject: Re: Re: [Boatanchors] Speaker Switching
> 
> 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.
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