On Thu, 5 Feb 2004 16:20:26 +0100, "Meindert Sprang"
wrote:

"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.

Better look closer. In "Bridged" mode, the amplifiers are in SERIES,
not in parallel....

There are four 200W amps bridged into 2-channels at 400W each in my
stepvan. These power two 12", 3-way speakers mounted to swing out on
either side of the back of the van on the full-width swinging rear
loading doors. They'll even turn around to the sides pointing the
speakers in opposite directions, if the job necessitates. Two more
speakers, massive JBL pro-sound speakers where even the 15"
bass-reflex woofers are mounted in horns, re-juvenated last year with
600W Eminence Kappa 15LFA beasts, the biggest that would fit into
these cabinets, are wheeled to go on either side of the truck, making
a wall-of-sound DJ machine I rent out to parties, car lots, anyone
needing independently-powered music/PA. A nice DJ mixer board
completes the system, powered from a 500W Tripplite inverter.

It all runs off 700AH golf cart "house" batteries I can separate from
the truck starting batteries for 12-14 hours of battery-operated
continuous music without running an engine or genset. The Honda
EU3000is 3KW inverter-technology super-quiet genset comes on to
recharge via the now-surplus 40A charger off the Amel Sharki ketch my
captain gave me.

It's all legally licensed by ASCAP and BMI to play their entire
portfolio, most of the music you hear.




Larry W4CSC

No, no, Scotty! I said, "Beam me a wrench.", not a WENCH!
Kirk Out.....

"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

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.....

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

(Rick Curtis) wrote in message . com...
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.


Well, that might sorta work. Properly wired it could protect the output
transistors of the two amplifiers, but I suspect that the result would be
horribly degraded audio from both sources. Audio signals are AC and by
putting a single diode in series with the speaker you effectively chop
off half the waveform resulting in distortion and attenuation.

The circuit that Dan suggested is probably the best solution. If you're
not handy with electronics or know someone who is, surely there is
someone kicking around the marina who could put together a simple
circuit like that for you.

Bob

(Rick Curtis) wrote in message . com...
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.


Well, that might sorta work. Properly wired it could protect the output
transistors of the two amplifiers, but I suspect that the result would be
horribly degraded audio from both sources. Audio signals are AC and by
putting a single diode in series with the speaker you effectively chop
off half the waveform resulting in distortion and attenuation.

The circuit that Dan suggested is probably the best solution. If you're
not handy with electronics or know someone who is, surely there is
someone kicking around the marina who could put together a simple
circuit like that for you.

Bob

"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

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

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