"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

"Rick Curtis" wrote in message
om...
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.

That will definately NOT work. You will protect the amplifers all right, but
you are effectively rectifying AC current, resulting in very bad to no audio
at all.

To get technical: in series with the output is a large capacitor that is
charged to half of the supply voltage of the amplifier. This voltage stays
at that level, because the average voltage and thus the current though the
speaker of an AC signal is 0. If you now put a diode in series with the
speaker, the average is not 0 anymore because positive current is passed and
negative current is blocked. First, this distorts the signal heavily.
Second, it causes this capacitor to charge to full supply voltage, until no
current is flowing at all anymore. No current = no audio.

Meindert

"Rick Curtis" wrote in message
om...
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.

That will definately NOT work. You will protect the amplifers all right, but
you are effectively rectifying AC current, resulting in very bad to no audio
at all.

To get technical: in series with the output is a large capacitor that is
charged to half of the supply voltage of the amplifier. This voltage stays
at that level, because the average voltage and thus the current though the
speaker of an AC signal is 0. If you now put a diode in series with the
speaker, the average is not 0 anymore because positive current is passed and
negative current is blocked. First, this distorts the signal heavily.
Second, it causes this capacitor to charge to full supply voltage, until no
current is flowing at all anymore. No current = no audio.

Meindert

In article ,
Dave wrote:

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?

The International Calling and Distress Frequency is Ch. 16 VHF.
VFH Ch. 9 is a Calling Channel for "Non-Commercial Users".
This was instituted to move "Non-commercial Calling Traffic" off
Ch. 16 in High User Density Areas in the USA.
If you are a Manditory Required User, you MUST Monitor Ch.16 while
underway, unless you are in a VTS Reporting System Area.
If you are a Volintary Equipped User, then you have a choice of
either Ch. 16 or Ch. 9.

Bruce in alaska
--
add a 2 before @

In article ,
Dave wrote:

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?

The International Calling and Distress Frequency is Ch. 16 VHF.
VFH Ch. 9 is a Calling Channel for "Non-Commercial Users".
This was instituted to move "Non-commercial Calling Traffic" off
Ch. 16 in High User Density Areas in the USA.
If you are a Manditory Required User, you MUST Monitor Ch.16 while
underway, unless you are in a VTS Reporting System Area.
If you are a Volintary Equipped User, then you have a choice of
either Ch. 16 or Ch. 9.

Bruce in alaska
--
add a 2 before @

What do they use for cell phone hook-ups where the cell call audio is
connected to your car's stereo system? Is that something that could
be adapted?

Eric

"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

What do they use for cell phone hook-ups where the cell call audio is
connected to your car's stereo system? Is that something that could
be adapted?

Eric

"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

I found one unit made by RF Systems - I think it would do exactly what
you want. A bit pricey, in my opinion, at about $100. Google
"automatic speaker switch" to find more.

See: http://www.rfsystems.com/si-2.htm
and http://www.homecontrols.com/cgi-bin/...1969/sesent/00

Eric

"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

I found one unit made by RF Systems - I think it would do exactly what
you want. A bit pricey, in my opinion, at about $100. Google
"automatic speaker switch" to find more.

See: http://www.rfsystems.com/si-2.htm
and http://www.homecontrols.com/cgi-bin/...1969/sesent/00

Eric

"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

"Meindert Sprang" wrote in message ...
"Larry W4CSC" wrote in message
...
Never, EVER connect two transistor amplifiers in parallel......

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.


First of all, to do what you describe (double the max. voltage) the amplifiers
would be connected in series not in parallel. Secondly, as you say, both
amplifiers are fed the same input signal in opposite phase, not different
signals as has been suggested. This technique is called "bridging" and has
been used for years in concert hall PA systems and other applications where
LOTS of decibels are desired (such as ridiculously over powered car stereos).
Connecting two transistor amps in parallel with different input signals is
only a good idea if both the VHF and the AM/FM radios are old and you'd like
to replace them but you wife won't let you because "they work just fine". ;-)
Don't do it.

Bob