wrote in message
...
On Fri, 6 Feb 2004 21:07:45 +0100, "Meindert Sprang"
wrote:

wrote in message
.. .
On Fri, 6 Feb 2004 17:21:14 +0100, "Meindert Sprang"
wrote:
Based on what? Running a high powered amp without a proper load on it
can cause rapid failure.

Give me one reason why.... (audio amp, that is. HF/VHF is a different
story)


Heat

Please note that although a speaker is a dynamic load, it still has a
range of impedanve values as the voice coil moves. There are many high
powered amplifiers that MUST be used with speakers with a specific
nominal impedance or the amplifier will cook itself to an early grave.
This is particularly true of amplifiers that utilize fewer stages with
higher gain to achieve "good specs" at low cost. The typical
amplifiers used in mobile stereo systems fall into this category.

If an audio amplifier is not loaded with a speaker, it will simply not draw
more current than the quiscent current. It is a myth that an amplifier
should somehow overheat or whatever when it is not loaded. An amplifier act
like a servo system, due to it's feedback. This servo system tries to keep
the output voltage the same as the input * gain. This same system makes sure
that when the amplifier is loaded, more current is 'pumped' into the load to
keep the output voltage at the desired level. When the load is small, less
current is needed to achive that. When no load is present, no extra current
is needed that the quiescent current that flows through the output
transistors when in rest. When the load is too high, *then* you have a
problem when currents get higher that the transistors can cope with. That's
all there is to it.

Meindert

On Fri, 06 Feb 2004 16:13:59 GMT, wrote:


Using a toggle switch to suddenly remove the load from the output of
the Stereo probably isn't such a great idea. The problem I've seen in
all the responses so far is that they are trying to switch in the
wrong place. The switching needs to take place at the preamp stage,
not the output stage.

Meanwhile the one really useful comment I saw was that stereo speakers
really aren't ideal for VHF anyway.

BB


Running an amplifier with no load or suddenly disconnecting the
speaker leads will do nothing to harm an amplifier. The only thing
that will harm them is shorting the output leads together or using a
speaker with an impedance that is too low. This will cause the
amplifier to draw excessive current and damage the output transistors.

I think that there is a carryover on this myth from the tube amplifier
days when they had a transformer in the output. Also some of the early
transistor amplifiers had transformers too but those days are long
gone.
If you had no load on an amplifier with an output transformer and
turned up the gain you could get excessive voltages from the kick back
of the transformer winding that could damage the output stage or cause
the output transformer to fail.

As for switching at the preamp stage becomes tricky to avoid problems.
All DC references must be maintained with various inputs or you will
get huge transients in the amp as you switch.

Regards
Gary

wrote in message
...
On Fri, 6 Feb 2004 22:28:09 +0100, "Meindert Sprang"
current is needed to achive that. When no load is present, no extra
current
is needed that the quiescent current that flows through the output
transistors when in rest. When the load is too high, *then* you have a
problem when currents get higher that the transistors can cope with.
That's
all there is to it.

Meindert


Good Luck!!!

Thank you!

Meindert

"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..

Here in France there is an automatic commutator on the market. It
automatically switches the cockpit speakers to VHF if there is a
transmission, otherwise stereo is fed through. Works very well. From a
company called Thira I think - try Google.

Here's the link:
http://www.clubsnautiques.com/thira/...eurHiFiVHF.htm

Ric,
The price shown is 490F. What is this in $ dollars USA or Euro?
Doug K7ABX
"Ric" wrote in message
...
Here's the link:
http://www.clubsnautiques.com/thira/...eurHiFiVHF.htm

I think that is about $100. Someone else sent me a link to that same model
and it was priced at $100..

Too expensive for me.


Steve
s/v Good Intentions