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#11
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Dual use of Cockpit Speakers??
Some of the newer radios will monitor both Ch 9 & 16 and some
will also have a third channel of your choice. Leanne s/v Fundy "Dave" wrote in message ... On Wed, 4 Feb 2004 09:59:04 -0800, "Steve" said: I would like to figure out a way to connect the VHF external speaker to one of these, so I can hear any calls on it in scan or monitoring of ch16. This brings to mind a question I've been wondering about. Used to be that when you had the radio on you were supposed to monitor channel 16. However from what I've read the general calling channel is now channel 9. So which are you supposed to monitor these days? |
#12
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Dual use of Cockpit Speakers??
Never, EVER connect two transistor amplifiers in parallel......
The output of a transistor amp looks like this: +V | transistor 1 | |--------------------------------output (0 VDC) | transistor 2 | -V The negative terminal of +V and the positive terminal of -V, two separate power supplies, are hooked to "ground" the other side of the speaker connected to "output". The 0 VDC comes because transistor 1 and transistor 2 are both conducting the same current, dropping their respective power supplies full voltage, leaving the junction "output" is connected to the balance of the two, 0 VDC to the common "ground" of the two power supplies. Audio is fed out of phase to the two power transistors, turning transistor 2 to less current and transistor 1 to more current. The difference current flows through the speaker to ground causing it to move in direction A. On the other half cycle of the audio, transistor 2 turns on harder and transistor 1 current is reduced. Current flows the OTHER way through the speaker hooked between "output" and ground. Now, suppose you come along and connect ANOTHER of these directly-coupled amplifiers to "output" by paralleling two radios. Call the second set transistor 3 (on top) and 4 (on the bottom of my drawing). If transistor 1 and 3 come on harder when transistors 2 and 4 conduct less, nothing bad happens and the speaker output current is fine. But, WXXX, HOT FM 93, doesn't have the same audio as Channel 68. So, many times per second, Transistor 1 conducts hard at the same time as transistor 4 comes on hard. You have just hooked +V through two hard conducting transistors to -V, who is MORE than happy to provide all the current you can imagine will flow. It doesn't flow through the protective impedance of the speaker. It flows from -V through transistor 4 to the common junction called "output" up through transistor 1 to +V. MANY amps flow, melting one or both transistor's junctions from the intense heat. You just blew BOTH transistor power amps.....and the two power supplies. On the next half cycle, now that there are two shorted transistors that have melted, transistors 2 and 3 conduct straight to + and - V of the other's power supplies. The fuse blows right after all four transistors have fused together and blown the power supply rectifiers....(c; NEVER, EVER HOOK TWO TRANSISTOR AMPS IN PARALLEL! See why? On Wed, 4 Feb 2004 09:59:04 -0800, "Steve" wrote: I have a pair of speakers in the cockpit coaming for the AM/FM radio.. I would like to figure out a way to connect the VHF external speaker to one of these, so I can hear any calls on it in scan or monitoring of ch16. Both radios are below deck so I can only hear them in the cockpit on a remote speaker.. I know I could just use a toggle switch, but would prefer to find a way to combine both outputs so the VHF would be heard over the AM/FM output. Is there a simple way to do this and prevent one unit from harming the other?? Steve s/v Good Intentions Larry W4CSC No, no, Scotty! I said, "Beam me a wrench.", not a WENCH! Kirk Out..... |
#13
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Dual use of Cockpit Speakers??
Never, EVER connect two transistor amplifiers in parallel......
The output of a transistor amp looks like this: +V | transistor 1 | |--------------------------------output (0 VDC) | transistor 2 | -V The negative terminal of +V and the positive terminal of -V, two separate power supplies, are hooked to "ground" the other side of the speaker connected to "output". The 0 VDC comes because transistor 1 and transistor 2 are both conducting the same current, dropping their respective power supplies full voltage, leaving the junction "output" is connected to the balance of the two, 0 VDC to the common "ground" of the two power supplies. Audio is fed out of phase to the two power transistors, turning transistor 2 to less current and transistor 1 to more current. The difference current flows through the speaker to ground causing it to move in direction A. On the other half cycle of the audio, transistor 2 turns on harder and transistor 1 current is reduced. Current flows the OTHER way through the speaker hooked between "output" and ground. Now, suppose you come along and connect ANOTHER of these directly-coupled amplifiers to "output" by paralleling two radios. Call the second set transistor 3 (on top) and 4 (on the bottom of my drawing). If transistor 1 and 3 come on harder when transistors 2 and 4 conduct less, nothing bad happens and the speaker output current is fine. But, WXXX, HOT FM 93, doesn't have the same audio as Channel 68. So, many times per second, Transistor 1 conducts hard at the same time as transistor 4 comes on hard. You have just hooked +V through two hard conducting transistors to -V, who is MORE than happy to provide all the current you can imagine will flow. It doesn't flow through the protective impedance of the speaker. It flows from -V through transistor 4 to the common junction called "output" up through transistor 1 to +V. MANY amps flow, melting one or both transistor's junctions from the intense heat. You just blew BOTH transistor power amps.....and the two power supplies. On the next half cycle, now that there are two shorted transistors that have melted, transistors 2 and 3 conduct straight to + and - V of the other's power supplies. The fuse blows right after all four transistors have fused together and blown the power supply rectifiers....(c; NEVER, EVER HOOK TWO TRANSISTOR AMPS IN PARALLEL! See why? On Wed, 4 Feb 2004 09:59:04 -0800, "Steve" wrote: I have a pair of speakers in the cockpit coaming for the AM/FM radio.. I would like to figure out a way to connect the VHF external speaker to one of these, so I can hear any calls on it in scan or monitoring of ch16. Both radios are below deck so I can only hear them in the cockpit on a remote speaker.. I know I could just use a toggle switch, but would prefer to find a way to combine both outputs so the VHF would be heard over the AM/FM output. Is there a simple way to do this and prevent one unit from harming the other?? Steve s/v Good Intentions Larry W4CSC No, no, Scotty! I said, "Beam me a wrench.", not a WENCH! Kirk Out..... |
#14
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Dual use of Cockpit Speakers??
Steve,
The easiest way to do it without a switch is to use 1N4004 diodes on each of the four speaker wires where you hook the two sources to one speaker. This will keep the signal from going from one source to the other source. If you don't know what I mean, let me know and I'll send you a simple wiring diagram. Rick "Steve" wrote in message ... I have a pair of speakers in the cockpit coaming for the AM/FM radio.. I would like to figure out a way to connect the VHF external speaker to one of these, so I can hear any calls on it in scan or monitoring of ch16. Both radios are below deck so I can only hear them in the cockpit on a remote speaker.. I know I could just use a toggle switch, but would prefer to find a way to combine both outputs so the VHF would be heard over the AM/FM output. Is there a simple way to do this and prevent one unit from harming the other?? Steve s/v Good Intentions |
#15
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Dual use of Cockpit Speakers??
Steve,
The easiest way to do it without a switch is to use 1N4004 diodes on each of the four speaker wires where you hook the two sources to one speaker. This will keep the signal from going from one source to the other source. If you don't know what I mean, let me know and I'll send you a simple wiring diagram. Rick "Steve" wrote in message ... I have a pair of speakers in the cockpit coaming for the AM/FM radio.. I would like to figure out a way to connect the VHF external speaker to one of these, so I can hear any calls on it in scan or monitoring of ch16. Both radios are below deck so I can only hear them in the cockpit on a remote speaker.. I know I could just use a toggle switch, but would prefer to find a way to combine both outputs so the VHF would be heard over the AM/FM output. Is there a simple way to do this and prevent one unit from harming the other?? Steve s/v Good Intentions |
#16
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Dual use of Cockpit Speakers??
This is a variation of a VOX (voice activated switch) circuit.
An example can be found at http://www.rason.org/Projects/basicvox/basicvox.htm This example is designed to be triggered by a microphone, not high powered speaker outputs, so the resister values will need to be tweeked on the input side. (Mike Input). The output of the circui is labeled PTT in the diagram. This would be connected to the coil of a relay that switches the the signal to the speakers. The other side of the relay coil would be connected to Vcc (+12v). You'll probably want to use a double throw relay and shunt the AM/FM radio output through a pair of 8 ohm high-wattage resisters as some amps don't take kindly to open circuits. These must be marketed somewhere as finished products, but I've no idea where. I bought one many years ago at a Radio Shack (I think) to switch between a CB and an 8-track player/radio (you old farts out there will know what I'm talking about - for the benefit of you youngin's, an 8-track was a predecessor of the cassette tape). Steve wrote: I have a pair of speakers in the cockpit coaming for the AM/FM radio.. I would like to figure out a way to connect the VHF external speaker to one of these, so I can hear any calls on it in scan or monitoring of ch16. Both radios are below deck so I can only hear them in the cockpit on a remote speaker.. I know I could just use a toggle switch, but would prefer to find a way to combine both outputs so the VHF would be heard over the AM/FM output. Is there a simple way to do this and prevent one unit from harming the other?? Steve s/v Good Intentions -- Dan Best - (707) 431-1662, Healdsburg, CA 95448 B-2/75 1977-1979 Tayana 37 #192, "Tricia Jean" http://rangerbest.home.comcast.net/TriciaJean.JPG |
#17
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Dual use of Cockpit Speakers??
This is a variation of a VOX (voice activated switch) circuit.
An example can be found at http://www.rason.org/Projects/basicvox/basicvox.htm This example is designed to be triggered by a microphone, not high powered speaker outputs, so the resister values will need to be tweeked on the input side. (Mike Input). The output of the circui is labeled PTT in the diagram. This would be connected to the coil of a relay that switches the the signal to the speakers. The other side of the relay coil would be connected to Vcc (+12v). You'll probably want to use a double throw relay and shunt the AM/FM radio output through a pair of 8 ohm high-wattage resisters as some amps don't take kindly to open circuits. These must be marketed somewhere as finished products, but I've no idea where. I bought one many years ago at a Radio Shack (I think) to switch between a CB and an 8-track player/radio (you old farts out there will know what I'm talking about - for the benefit of you youngin's, an 8-track was a predecessor of the cassette tape). Steve wrote: I have a pair of speakers in the cockpit coaming for the AM/FM radio.. I would like to figure out a way to connect the VHF external speaker to one of these, so I can hear any calls on it in scan or monitoring of ch16. Both radios are below deck so I can only hear them in the cockpit on a remote speaker.. I know I could just use a toggle switch, but would prefer to find a way to combine both outputs so the VHF would be heard over the AM/FM output. Is there a simple way to do this and prevent one unit from harming the other?? Steve s/v Good Intentions -- Dan Best - (707) 431-1662, Healdsburg, CA 95448 B-2/75 1977-1979 Tayana 37 #192, "Tricia Jean" http://rangerbest.home.comcast.net/TriciaJean.JPG |
#18
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Dual use of Cockpit Speakers??
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#19
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Dual use of Cockpit Speakers??
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#20
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Dual use of Cockpit Speakers??
"Larry W4CSC" wrote in message
... Never, EVER connect two transistor amplifiers in parallel...... snip large story Actually, in high power car systems, they do exactly what you just said that cannot be done. In such systems, each channel has two amplifiers, carrying the signal in opposite phase. The speaker is connected to both outputs. This way, the maximum voltage that can be created across the speaker is doubled. So based on this same principle, it would be possible to connect the speaker between the hot wires of the outputs of the VHF and the radio..... The only thing that spoils the fun is the fact that the DC blocking capacitor in series with the output expects the speaker to be connected to ground. So an extra electrolytic capacitor of 1000uF with the minus to one output and the plus to one side of the speaker is needed. Another possible problem is the power: both amplifiers must be able to withstand the power output of the other. In the above example of a high power car system, this is the case. However, the amplifier of a VHF will probably not tolerate the much higher power of an average car stereo. To make a long story short: take Larry's advice and don't do it. But technically it is possible. Meindert |
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