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'[EE]: Unloaded outputs on audio amps.. will they b'
2000\08\17@220605 by Damon Hopkins

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On rec.audio.car there is a recent discussion on whether or not unused
speaker outputs on amplifiers should be loaded or not.
so I figure there are far more knowledgeable people on this list when it
comes to the actual how's and whys then on the car stereo newsgroup so I
come to ya'll looking of answers :)

               Someone suggested that if left unloaded the output transistors could
oscillate and burn themselves out it not protected (i.e. thermal
disabling or some other method)
I talked to my father and he explained the theory behind this and I
believe it basically follows along the same thread of the grounding of
unused PIC pins... hence my idea to post it here.
someone also asked if the outputs on the headunit itself would also be
prone to this or is it all a bunch of urban ledgend type stuff.

       I guess I'm looking for yes, no or a maybe and sometype of description
that I can relay back to the interested people.

                       Thanks,

                               Damon Hopkins

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2000\08\17@222106 by David Duffy

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Damon Hopkins wrote:
>On rec.audio.car there is a recent discussion on whether or not unused
>speaker outputs on amplifiers should be loaded or not.
>so I figure there are far more knowledgeable people on this list when it
>comes to the actual how's and whys then on the car stereo newsgroup so I
>come to ya'll looking of answers :)
>
>                 Someone suggested that if left unloaded the output
> transistors could
>oscillate and burn themselves out it not protected (i.e. thermal
>disabling or some other method)
>I talked to my father and he explained the theory behind this and I
>believe it basically follows along the same thread of the grounding of
>unused PIC pins... hence my idea to post it here.
>someone also asked if the outputs on the headunit itself would also be
>prone to this or is it all a bunch of urban ledgend type stuff.
>
>         I guess I'm looking for yes, no or a maybe and sometype of
> description
>that I can relay back to the interested people.

I can't see any good reason for it in modern amplifier designs. The amplifier
output stage should not rely on the speaker loading to determine its stability.
I'd say it might be a left-over from the old days of valve (tube)
amplifiers which
were unstable if the output transformer was left unloaded. I remember sparks
flying (internal to the amp) a long time ago when I accidently turned my old
Sansui HiFi (valve) amplifier way up with the speakers not connected. As you
say, what happens to the speaker outputs of a car audio head unit that is only
feeding a bigger external amplifier via the line level outputs? (Hint:
nothing !)
As you've probably guessed, a lot of car audio people are badly informed.  :-)
Regards...

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2000\08\17@222513 by Bob Blick

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Leave them floating, no need to load them.

Unloaded power amps are stable. Shoot any engineer that designs an audio
power amp that is unstable into no load!

-Bob

At 10:07 PM 8/17/2000 -0400, you wrote:
>On rec.audio.car there is a recent discussion on whether or not unused
>speaker outputs on amplifiers should be loaded or not.
>so I figure there are far more knowledgeable people on this list when it
>comes to the actual how's and whys then on the car stereo newsgroup so I
>come to ya'll looking of answers :)
>
>                Someone suggested that if left unloaded the output
transistors could
>oscillate and burn themselves out it not protected (i.e. thermal
>disabling or some other method)

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2000\08\17@223538 by hard Prosser

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I remember that some of the early transistor amps with transformer output
had a similar problem - unloaded outputs could generate enough volts to
cause insulation breakdown in the transformer or high dissipation in the
output transistors.


Shouldn't be a problem these days I would hope!





Richard P



                   David Duffy
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Damon Hopkins wrote:
{Quote hidden}

I can't see any good reason for it in modern amplifier designs. The
amplifier
output stage should not rely on the speaker loading to determine its
stability.
I'd say it might be a left-over from the old days of valve (tube)
amplifiers which
were unstable if the output transformer was left unloaded. I remember
sparks
flying (internal to the amp) a long time ago when I accidently turned my
old
Sansui HiFi (valve) amplifier way up with the speakers not connected. As
you
say, what happens to the speaker outputs of a car audio head unit that is
only
feeding a bigger external amplifier via the line level outputs? (Hint:
nothing !)
As you've probably guessed, a lot of car audio people are badly informed.
:-)
Regards...

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2000\08\18@051217 by Simon Nield

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It depends... well that's a pretty generic answer.

- Unused outputs of valve amplifiers need to be terminated with the correct load due to the fact
that 99.9% of them have an output transformer configured as a current transformer. With no load the
voltage on the output tends to 'infinity', only limited by real-life things like magnetic
saturation, huge sparks and so on...
- Car audio amps are made to drive into very low impedance loads, often as low as 2 ohms. Like all
consumer goods component cost is very important. With even a 'classic' design (dc-dc and linear amp)
a designer may decide to make the (not totally unreasonable) assumption that all outputs are always
connected. This might lead to a design which is not stable with no load connected, leading to
(usually) high frequency oscillations of the output stage and destruction of that stage.
- Car audio amps are also a good place to find innovative amplification technology, such as 'pure'
class c amplifiers; i.e. the whole amp is in fact a cleverly controlled switch mode power supply.
Some SMPS are unstable with no load, usually they don't oscillate in a destructive manner, and those
that are vulnerable to an unloaded output are usually step-up converters with no (or a very small)
internal output capacitor... not a likely design for an amplifier.

Thankfully you can pretty easy check if an amp is stable with no load by looking at the output with
no load on an oscilloscope. Remember that both outputs on a car amp are floating, and that the scope
probe _is_ actually putting a small capacitive load on the output, which if you are really unlucky
will be enough to damp out the horrific oscillation that is occurring the rest of the time.



or: stick the amp on a bench hooked up to a car battery with a load (speaker) connected, running at
a reasonably quiet volume. Let it run for 'a while'. put your hand over the amp and try and get a
feel for how hot it is. Now try the same thing without the speaker connected. If it's cooler then
you don't need a load. If it is a lot hotter then you probably do.



Well I couldn't let this fascinating debate actually be _resolved_ now, could I ?



Regards,
Simon

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2000\08\18@081427 by M. Adam Davis

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Ultimately it comes down to how the manufacturer designed the unit.  While it
would be better and not much more costly to design a unit which is not prone to
this, it is reasonable for a designer to assume that if you aren't connecting
speakers then there is no point in even powering the amp.

So, in short, RTM!  (Read the Manual)

If having an unconnected speaker is going to destroy the amp, then you can bet
your money they have a note in the manual saying, "Never run the amp without
speakers or a suitable load resister attached."  Otherwise they have a customer
service nightmare.

But, I imagine, even in amps that may oscillate to infinity they probably
install some sort of protection which keeps it just barely within spec.  You may
find that your amp consumes as much power as though a speaker were connected.

-Adam

Damon Hopkins wrote:
{Quote hidden}

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2000\08\18@133401 by Peter L. Peres

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According to legends, some ancient car radio designs with transformer
coupled output used to self-destruct when run without a load for some
time. But I have never seen any modern unit do that, nor did I hear of
one.

It is possible that this urban legend originated there and has been
repeated from speaker to microphone to this day.

Peter

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2000\08\18@135229 by David VanHorn

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For only $79.95, I'll sell you my precision output terminator.

This keeps reflected energy from the unterminated outputs from coming back
through the supply, and causing distortion on the other channels.  If you
prefer, the Ultra-Terminator is available, at $129.95, for higher end
systems, where exact matching is important. There is an adjustment you can
trim to exactly match your system, and remove that confused soundstage that
results from reflected energy distortion.  The standard model is adequate
for automotive systems, since the tiny bit of remaining distortion after
termination is inaudible in the automotive environment.

You will need one per unused channel.

(did I miss my calling, or what? :)
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2000\08\18@135925 by W. K. Brown

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Love It <G G G>
Keith
       -----Original Message-----
       From:   David VanHorn [SMTP:RemoveMEdvanhornTakeThisOuTspamCEDAR.NET]
       Sent:   Friday, August 18, 2000 1:50 PM
       To:     spamBeGonePICLISTspamBeGonespamMITVMA.MIT.EDU
       Subject:        Re: [EE]: Unloaded outputs on audio amps.. will they
burn out?

       For only $79.95, I'll sell you my precision output terminator.

       This keeps reflected energy from the unterminated outputs from
coming back
       through the supply, and causing distortion on the other channels.
If you
       prefer, the Ultra-Terminator is available, at $129.95, for higher
end
       systems, where exact matching is important. There is an adjustment
you can
       trim to exactly match your system, and remove that confused
soundstage that
       results from reflected energy distortion.  The standard model is
adequate
       for automotive systems, since the tiny bit of remaining distortion
after
       termination is inaudible in the automotive environment.

       You will need one per unused channel.

       (did I miss my calling, or what? :)
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2000\08\18@140704 by Andrew Kunz

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>(did I miss my calling, or what? :)

Nope.

<VBG>

Andy

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2000\08\18@142136 by David VanHorn
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At 02:00 PM 8/18/00 -0400, Andrew Kunz wrote:
> >(did I miss my calling, or what? :)
>
>Nope.
>
><VBG>


I shoulda been in marketing. And I hear they have a two drink minimum too :)


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2000\08\18@150957 by Bob Ammerman

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rotflol!

----- Original Message -----
From: David VanHorn <TakeThisOuTdvanhornEraseMEspamspam_OUTCEDAR.NET>
To: <RemoveMEPICLISTspamTakeThisOuTMITVMA.MIT.EDU>
Sent: Friday, August 18, 2000 1:50 PM
Subject: Re: [EE]: Unloaded outputs on audio amps.. will they burn out?


> For only $79.95, I'll sell you my precision output terminator.
>
> This keeps reflected energy from the unterminated outputs from coming back
> through the supply, and causing distortion on the other channels.  If you
> prefer, the Ultra-Terminator is available, at $129.95, for higher end
> systems, where exact matching is important. There is an adjustment you can
> trim to exactly match your system, and remove that confused soundstage
that
{Quote hidden}

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2000\08\18@162858 by David VanHorn

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At 03:06 PM 8/18/00 -0400, Bob Ammerman wrote:
>rotflol!


Picture a dale 100W 8 ohm aluminum case resistor, with a quartz crystal
duct-taped to the side. :)
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2000\08\19@084509 by Peter L. Peres

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>For only $79.95, I'll sell you my precision output terminator.

Please add the words 'gold plated' 'exclusive' and 'highly appreciated' at
least 3 times each in your message and post it on usenet on the high end
audio groups. You will make a killing sale. I want 0.01% if you exceed
$1000 in revenues. ;)

Peter

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2000\08\19@185520 by Stuart

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I'll Take 5.1 of them for my new surround system.
Stuart

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From: David VanHorn <dvanhornEraseMEspam.....CEDAR.NET>
To: <EraseMEPICLISTspamMITVMA.MIT.EDU>
Sent: Saturday, August 19, 2000 3:50 AM
Subject: Re: [EE]: Unloaded outputs on audio amps.. will they burn out?


> For only $79.95, I'll sell you my precision output terminator.
>
> This keeps reflected energy from the unterminated outputs from coming back
> through the supply, and causing distortion on the other channels.  If you
> prefer, the Ultra-Terminator is available, at $129.95, for higher end
> systems, where exact matching is important. There is an adjustment you can
> trim to exactly match your system, and remove that confused soundstage
that
{Quote hidden}

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2000\08\19@195451 by David VanHorn

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At 08:57 AM 8/20/00 +1000, Stuart wrote:
>I'll Take 5.1 of them for my new surround system.
>Stuart

I'll ship the moment your check clears.
BTW: Where is the bank of langoornia?
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