Depends actually....
Do you want to just mix the audio, or do you need the
power from one or both radios to drive something? For
this example, we know both radios have amplifiers with
high output power levels (vs small receivers with lm386
type outputs).
If you want to use the combined output to another
single speaker, you should follow the mixed output
with another speaker audio amplifier circuit.
If you want to drive the input of a low level
circuit, a simple resistor capacitor mixer circuit
is all that's required. Sometimes I follow the
mixed audio with a basic emitter follower transistor
circuit.
Each radio gets a 22 ohm 5 watt resistor on the
external speaker leads, paying attention to what
is considered the negative - lead and what lead
is the positive + lead.
The outputs of many/most radios and car stereos
are now hybrid or bridged type outputs, meaning
no ground. The no ground output is a big deal,
if you short the negative lead to ground, you
might blow the audio output amplifier.
The 22 ohm resistor provides a load and developes a
higher voltage at lower volume levels. The amp
should like the light load just fine.
Each resistive load negative speaker lead side
is connected to a single following audio amplifer
input ground through a capacitor (dc isolation).
Each resistive load positive speaker lead side
is connected to a single following audio amplifer
input (audio high + side) through a series 10K
resistor and a capacitor (dc isolation).
If the trailing audio amplifier has enough power
to drive your speaker at the desired volume level,
you're done. If you need more information, I
might be able to put up an example circuit on
the sonic web page.
The ARRL Handbook had a similar resistive speaker
audio mixer circuit, but you had to include caps
for dc isolation on the newer radios. FAR circuits
still sells the pc board for not much money. The
circuit has an on board lm-386 amplifer, which is
probably not enough for mobile operation. Small
5 to 15 watt audio amplifier modules are pretty
cheap.
You can do it, just be carefull not to dc ground
any side of the speaker output leads.
cheers,
skipp
www.radiowrench.com/sonic
> DCFluX <[EMAIL PROTECTED]> wrote:
> Anyone had experience with mixing the speaker output of 2 radios,
Say
> Motorola GM300's to one speaker?
>
> I originally tried a couple of resistors but I may have the wrong
> values as they got hot as hell and one started smoking, I was using
2
> .82 ohm at 2 watt resistors for each radio, one resistor in each
> speaker lead and at the center the speaker. My next best guess is
> using a multiple winding transformer with three windings of 4 ohms,
> but finding information on how to wind a transformer to do that is
> impossible these days. Any Ideas?
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