I see a couple of different but simple approaches to this issue.
First I will presume that one wishes to connect two receiver ouputs (or
more) to one speaker and be able to hear both (or all) receivers at the
same time. Some have suggested usng a mixer such as a Behringer or Shure
(or other favorite brand) and feed the speaker output of the receiver to the
microphone input of the mixer. Caution, one will be feeding a high level
output (typically 1 volt or so) into a low level input (designed to accept a
few millivolts). This is not a good conditon as overdirve and subseqent
distortion will most likely result. Yes, there are some mixer inputs that
do have adequate gain control and are capabable of accepting higher "line
level" signals. Correctly, the output of the receiver should go through a
suitable 20 to 40 dB loss attenuator before going into the mike input.
While it may be of minor concern, this approach does not correctly provide a
low Z load for the receiver output stage and certain issues with regard to
unstability or oscillaton could result. Additionally the mixer output
usually does not provide a power output circuit suitable to drive a speaker
directly. Thus an external power amp is required to develop the required
power and be able to drive the lo Z of a speaker.
Correctly, to passively feed two receiver speaker outputs to one speaker
requires a simple resistive combining network. This network is no more than
3 resistors for 2 outputs of 8 ohms each feeding one 8 ohm speaker. In
this case the value of the resistors will be 2.6 ohms each. Rb = (
n-1/n+1)Z where "n" is the number of sources, Z is the impedance. This
assumes all are of the same impedance. Most applications will be
satisfactory using 8 ohms as the correct source and load values. For a 4
ohm source/load system the values of the resistors will be 1.3 ohms. View
the circuit thusly: a series resistor from each output, connected to a
common point and then a series resistor from the common point to the
speaker. This assumes all outputs are unbalanced thus all grounds are
connected together and common with the lo side of the speaker. For 3
sources connected to one speaker the value of Rb with 8 ohm source and load
will be 4 ohms. Typical 1 or 2 watt rated resistors will be satisfactory as
the average power is much less than 2 watts. To figure power loss with this
circuit dB loss = 20log(N-1) where N = the number of sources being
combined.
A second approach is to use a switch. There are several suitable boxes on
the market designed to switch various outputs to speakers. These do provide
correct back load or termination to the un used (non selected) receiver
output. This switching method does not generally lend itself to listening
to more than one source at a time.
73
Bob, K4TAX
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