Thanks to Bob and many others who gave some great responses to this
question. Multiple rigs coming simultaneously thru one speaker is an
intriguing thought - keeps the shack clean but still allows for dual
monitoring when necessary. Thanks everyone.
73/Tim NZ7C
----- Original Message -----
From: "Robert & Linda McGraw K4TAX" <RMcGraw@blomand.net>
To: <tentec@contesting.com>
Sent: Sunday, February 23, 2003 9:20 PM
Subject: [TenTec] Multiple rigs to one speaker?
> 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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