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Author Topic: Two radios... One extension speaker  (Read 330 times)
N1JZZ
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Posts: 7




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« on: November 27, 2004, 07:39:14 PM »

Howdy,
I have an Yaesu FT-7800 and a Uniden BCT8 (scanner) in my car.  I also have a Motorola extension speaker.  I would like to use the speaker for both radios.  I am wonderifing if I should isolate the speaker outputs for the two radios, or if I could just hook them together and then to the speaker.
Thanks in advance.
JJ
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KZ1X
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Posts: 3228




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« Reply #1 on: November 27, 2004, 07:48:58 PM »

I am surprised that manufacturers don't supply what you are actually asking for ... seems like a good idea.

If I were you, I'd resistively sum the two radios' AF outputs together, then feed that into an amplified external speaker.  If you wanted to get fancy, use illuminated switches so you could instantly mute either of the inputs.
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N1JZZ
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« Reply #2 on: November 27, 2004, 08:42:30 PM »

Do you think a ground loop isolater, say from Radio Shack, might work in this application?
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AE6RV
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Posts: 146




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« Reply #3 on: November 27, 2004, 09:14:38 PM »

There are a couple of risks as I see it.

- You are feeding power from one output transistor back through to the other output transistor.  This may or may not be a good thing.

- Either or both of the receivers' output stages may be directly coupled, which might not be good.

Of course, you may do it and have both radios last forever.  I just don't know.  One solution is to get a cheap stereo amp, feed one radio to left and the other to the right.  Couple the outputs of L&R of the sacrificial (i.e. cheap) amp together and to the speaker.

Bob
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K5LXP
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« Reply #4 on: November 27, 2004, 09:47:19 PM »

What I would do is make a passive mixer and drive an amplified speaker.  The headphone mixer shown in the ARRL handbook for about the past 10 years or so is what I have in mind.  I also use a motorola speaker, a 12W amplified one.  They also have a 7W one that is common.  With such a scheme you can safely add as many radios as you care to make input circuits on the mixer for.  To do it right, the radios need to be isolated from each other as a direct connect would likely damage the audio output stage.  New Communications Solutions makes a mobile mixer that also takes care of the microphone circuits as well and gives you additional selection and patching features, if you just want to buy a solution to your problem.

Mark K5LXP
Albuquerque, NM
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N6AJR
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« Reply #5 on: November 27, 2004, 09:59:24 PM »

use a double pole single throw switch, it will be either  one or t'other.. easy fix and no worries about backards power burning up stuff..
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N6AJR
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« Reply #6 on: November 27, 2004, 09:59:24 PM »

use a double pole single throw switch, it will be either  one or t'other.. easy fix and no worries about backards power burning up stuff..
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AC5E
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« Reply #7 on: November 28, 2004, 05:10:44 AM »

Considering how fragile many transistorized audio output circuits are, I would NOT tie two amplifiers together and expect either to live. That's an open invitation to "reverse drive" an audio amp and they often last no longer than a few milliseconds under those circumstances.

Nor would I use a switch to select one or the other, since that inevitably results in one or both amplifiers driving an open circuit. While you may get away with that for a while sooner or later you will have a much flatter wallet.

My local PROFESSIONAL electronics store - the place your local electronic techs buy parts - stocks a stereo to mono speaker transformer. 2 X 8 ohms or 70.7 V nominal input, 1 X 8 ohms output. Rated separation is 25 dB or so - plenty to keep expensive repairs to a mininum.

This very handy unit keeps the speaker output from each channel separate while providing full speaker output to drive a single speaker. Pros use them for many things, but any time you need to keep two speaker circuits separate and still drive one speaker they are indespensable.

73  Pete Allen  AC5E
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K0BG
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« Reply #8 on: November 28, 2004, 06:37:28 AM »

Mark has the right idea here. There are a ton of amplified speakers out there, and almost any of them can have their inputs summed by using resistors. I certainly wouldn't worry about loading the outputs of the radio amps since they all seem to be transformerless. In fact, using a transformer as a mixing device is not a good idea (sorry Pete).

Alan, KØBG
www.k0bg.com
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AC5E
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Posts: 3585




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« Reply #9 on: November 28, 2004, 08:05:19 AM »

Well, Alan, over the years I have used a couple of hundred of the isolation transformers, many on germanium transistor output amplifiers that were notorious for shorting with any excuse. Including a couple that would die when the speaker cone rebounded at the end of its travel and generated a small voltage.

I was in a business yesterday that uses a system I put in around 1970 and it's still working. With the original Scott stero amp, Bogen PA amp, and Quam speakers.

Personally speaking I prefer to employ the KISS principle instead of the extra complexity of summing the outputs of two amplifiers with resistive pads, loading the amp outputs with resistors, and then adding a separate amplifier. All of which are subject to catastrophic failure.

A single transformer is cheaper, more reliable, and consumes less power.

73  Pete Allen
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K1CJS
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Posts: 6045




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« Reply #10 on: November 28, 2004, 08:36:52 AM »

Why not keep it real simple and use two speakers?  No danger to either radio, no transformers to buy or bother with, and no fancy mixing circuits to design--or to fail, either.

A second speaker will take up no more room, and considering some of the available sizes and mounting abilities of commonly available speaker units, can be mounted almost anywhere there is a little space.



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AA4PB
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« Reply #11 on: November 28, 2004, 10:49:18 AM »

Using a transformer can isolate the DC (if any) on the two outputs but how does it prevent the audio signal from one amp from feeding back into the other?
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N6AJR
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« Reply #12 on: November 28, 2004, 02:29:51 PM »

use 2 speakers, problem solved..
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