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'[OT]: Audio Amplifier'
2002\07\01@125318 by Anand Dhuru

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Please excuse the completely OT nature of this post; its just that I have seen such great talent in various branches of electronics on this list, I feel tempted to put my query here.

I have a TV which came without a earphone socket. I just wired in a socket the usual way (speaker automatically gets disconnected if you plug in a headphone), and it seemed to work fine...for a few seconds.

After that, there was a bang, and the amplifier chip within blew up.

The chip used is Philips TDA 8945S.

The TV speaker has an impedence of 8 ohms; the headphones are of course, a much larger impedence.

This being the case, why would the chip blow?

Do some amplifiers also get damaged if the load impedence is *higher* than the one specified?

In which case, would a resistor network that still offerred about 8 ohms to the amplifier even when the headphone is connected help?

I just might have inadvertantly shorted the output wires (for not more than a second or two, I'm sure). This chip is supposed to have both, thermal and short circuit protection. Could the possible short then still have caused the problem?

I have replaced the chip, but before trying out the headphone again, I would like to get some advice from you folks out there.

Regards,

Anand Dhuru

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2002\07\01@143249 by Rick C.

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A higher impedance load won't damage the output amplifier on a solid state amp. You have to remember that the ground in a TV receiver may not necessarily be earth ground. It could float as high as your line voltage if not properly polarized.
If you add a jack, you could be in for some sparks. This is why you don't see many headphone jacks on most cheap TV's. Those that do have a jack have an isolation transformer isolating the chassis ground from the headphone ground. Isolating
with just capacitors won't meet UL approval.

Also, putting a 100 ohm in series with the output will protect the amp from possible shorts damaging the amp without loss of signal to the headphones.
Rick

Anand Dhuru wrote:

{Quote hidden}

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2002\07\01@144052 by Rick C.

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I failed to mention why. If you had a hot chassis, and you were wearing a headphone or earphones, you would have a lethal potential right next to your head. Any leakage in the wiring would be lethal. If you added a jack and ran your signal to an
amplifier that was grounded to earth ground...... POP!!!!!!!
Rick

"Rick C." wrote:

{Quote hidden}

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2002\07\01@145105 by Peter L. Peres

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On Mon, 1 Jul 2002, Anand Dhuru wrote:

>Please excuse the completely OT nature of this post; its just that I have seen such great talent in various branches of electronics on this list, I feel tempted to put my query here.
>
>I have a TV which came without a earphone socket. I just wired in a socket the usual way (speaker automatically gets disconnected if you plug in a headphone), and it seemed to work fine...for a few seconds.
>
>After that, there was a bang, and the amplifier chip within blew up.
>
>The chip used is Philips TDA 8945S.

I dunno about that chip but if it is a bridge amplifier (I think it is)
then the results you describe match the expected behavior, especially if
you grounded one of the headphone wires. The other possible solution
involves self oscillation caused by the unexpected low load and long
wires.

Normal phones sockets use a resistor in series with the socket and another
in parallel with the phones. Also inexpensive headphone sockets briefly
short the amp on insertion and removal. Most amps do not like this. Use a
relay to switch the socket if you can't afford a good quality female
socket (that is specifically designed not to short on insertion). And be
careful, the usual reason for a missing phones socket is an unsafe
chassis. You could electrocute someone or start a fire.

Peter

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2002\07\02@010424 by Anand Dhuru

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Thanks Rick, Peter, for the information.

I did some checking after reciving the posts from you guys, and what I find
is this:

The TV has a 2 pin mains plug (no earth); neither of the two seem connected
to the PCB ground (the ground on the Audio IN, Video IN plugs). Also,
neither of the speaker wires seems directly connected to the PCB ground.

On this basis, am I right in assuming that a workable solution might be:

1) a 100 ohm resistor in series with the speaker / headphone socket, always
in circuit regardless of whether the headphones are plugged in, for short
circuit protection, and

2) an audio transformer to drive the headphones; this comes into circuit
only when the speakers are cut out, and would give complete isolation to the
headphones?

The only issue I foresee is that I still would short the circuit at the
socket while inserting the headphones, but now it would be thru' the 100 ohm
resistor. Would this work? And, would it be safe?




{Original Message removed}

2002\07\02@020949 by Jinx

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> 1) a 100 ohm resistor in series with the speaker / headphone socket,
> always in circuit regardless of whether the headphones are plugged in,
> for short circuit protection, and

h/p will be only a few mW so you must have that resistor unloess you
like your sound unusually loud for a brief time

> 2) an audio transformer to drive the headphones; this comes into
> circuit only when the speakers are cut out, and would give complete
> isolation to the headphones?

Shouldn't be necessary. A transformer is normally used for
impedance matching - what you are looking for is voltage
matching (or rather voltage dropping, therefore a resistor)

> The only issue I foresee is that I still would short the circuit at the
> socket while inserting the headphones, but now it would be thru'
> the 100 ohm reistor. Would this work? And, would it be safe?

The socket shouldn't short. Not in the way you think perhaps. The
tip and ring contacts on the plug shouldn't be able to be shorted to
the barrel of the plug via the socket contacts, nor should the tip
or ring be able to short out the contacts inside the socket. However,
if you think there is shorting then the 100R will really help

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2002\07\02@142847 by Rick C.

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Jinx wrote:

>
> Shouldn't be necessary. A transformer is normally used for
> impedance matching - what you are looking for is voltage
> matching (or rather voltage dropping, therefore a resistor)
>
>

For most headphone impedances a 600ohm/600ohm transformer will give you the
isolation you need without impedance matching problems. Jameco has them for a
buck or so. You could use the phone line transformer out of an old modem card
too, but you might suffer just a little high frequency loss but no so bad.
Rick

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2002\07\02@164145 by Andre Abelian

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-----Original Message-----


Please excuse the completely OT nature of this post; its just that I
have seen such great talent in various branches of electronics on this
list, I feel tempted to put my query here.

I have a TV which came without a earphone socket. I just wired in a
socket the usual way (speaker automatically gets disconnected if you
plug in a headphone), and it seemed to work fine...for a few seconds.

After that, there was a bang, and the amplifier chip within blew up.

The chip used is Philips TDA 8945S.

The TV speaker has an impedence of 8 ohms; the headphones are of course,
a much larger impedence.

This being the case, why would the chip blow?

====================================================================
the only way you can damage the chip is to short the speaker wires
for some reason it happened that. If you didn't add resistor because
of your headphone is low powerful in this case first headphone will
get damaged then the amp.
====================================================================

Do some amplifiers also get damaged if the load impedence is *higher*
than the one specified?

==================
never
==================

In which case, would a resistor network that still offerred about 8 ohms
to the amplifier even when the headphone is connected help?


===============================
the help is to lower the power to match with
your head phone
===============================


I just might have inadvertantly shorted the output wires (for not more
than a second or two, I'm sure). This chip is supposed to have both,
thermal and short circuit protection. Could the possible short then
still have caused the problem?

=====================================
that is the only way you can damage the chip
I do not think the chip has short circuit protection.
==========================================


I have replaced the chip, but before trying out the headphone again, I
would like to get some advice from you folks out there.

===========================
make sure when you plug it in it is not making any
short connection for a moment or so and add resistors
to lower the power
=============================

Andre Abelian



Regards,

Anand Dhuru

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2002\07\02@230923 by ards, Justin P

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<snip>
Do some amplifiers also get damaged if the load impedence is *higher*
than the one specified?

==================
never
==================
Just thought I might clarify, the post was with regards to audio amps but
thought it best to mention I *think* RF power amps like to see a load or
they can fail.

Is that accurate?
Justin

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2002\07\02@235156 by Rick C.

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No. It's like saying, do I need a load on an opamp? A higher impeance load
will just mean a lower power transfer to the load.

Back in the vacuum tube days when you had a plate transformer matched to a
low impedance speaker, if you took the load (speaker) off and drove it hard,
huge amounts of voltage would be generated from inductive reactance that
would literally arc and burn the transformer up.
Rick

"Richards, Justin P" wrote:

{Quote hidden}

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2002\07\03@005845 by Herbert Graf

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> <snip>
> Do some amplifiers also get damaged if the load impedence is *higher*
> than the one specified?
>
> ==================
> never
> ==================
> Just thought I might clarify, the post was with regards to audio amps but
> thought it best to mention I *think* RF power amps like to see a load or
> they can fail.

       IIRC there are some audio tube amp designs out there that will get damaged
if no speaker is attached.

       As for RF amps yes, quite a few of the high power ones can self destruct if
they don't have an antenna attached. TTYL

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2002\07\03@035512 by Russell McMahon

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> > Do some amplifiers also get damaged if the load impedance is *higher*
> > than the one specified?

> No. It's like saying, do I need a load on an opamp? A higher impeance load
> will just mean a lower power transfer to the load.
>
> Back in the vacuum tube days when you had a plate transformer matched to a
> low impedance speaker, if you took the load (speaker) off and drove it
hard,
> huge amounts of voltage would be generated from inductive reactance that
> would literally arc and burn the transformer up.

While in most cases the above answer is correct, there are situations where
a load is needed.
(Incidentally to this, the transistor amp equivalent of a valve amp open
circuit is a short circuit - valve amps tended to survive these whereas
transistor amps need to be protected against them.)

Some amplifiers perform 'differently" under little or no load. An LM324
opamp is a specialist example - it has an "interesting" output stage which
runs in (AFAIR somewhere between class A and AB under light load and
switches to full class B under heavier loads. Under certain conditions of
light load it has nasty output distortion which vanishes as the amp is
biased up to run in true class B.

A voltage regulator is a specialised amplifier with extra circuitry added to
do a particular job. SOME voltage regulators become unhappy at very light
loads.

An amplifier could be designed (and some are) to rely on output current draw
to bias itself correctly.



       RM

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2002\07\03@045115 by Alan B. Pearce

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>Some amplifiers perform 'differently" under little or
>no load. An LM324 opamp is a specialist example - it
>has an "interesting" output stage which 0runs in (AFAIR
>somewhere between class A and AB under light load and
>switches to full class B under heavier loads. Under
>certain conditions of light load it has nasty output
>distortion which vanishes as the amp is biased up to run
>in true class B.

Seen this happen with 741 op-amps driving 600 ohm headphones on a project we
did as an exercise for apprentices. Solution was a 1k resistor from output
to ground before the coupling capacitor so the op amp always had to drive
current through the crossover region.

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2002\07\03@080148 by Rick C.
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Yes, RF amps must have a load on the output and be properly matched to the
load or there could be problems. Most properly designed RF amps have a
"foldback" circuit to detect any mismatch by measuring the reflected power
(power coming back) and thus reducing the RF drive back to a safe point to
prevent self destruction. Better amps will actually shut down the output if
the condition continues. I work with RF amps from 5 watts to 50,000 watts,
all of which have foldback circuits and you'd be amazed at the extent the
design engineers go through to provide this  safety feature.
Rick

"Richards, Justin P" wrote:

{Quote hidden}

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2002\07\03@085652 by Dale Botkin

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On Wed, 3 Jul 2002, Herbert Graf wrote:

>         As for RF amps yes, quite a few of the high power ones can self destruct if
> they don't have an antenna attached. TTYL

A lot of low power amps will, too.

Dale

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2002\07\03@141848 by Peter L. Peres

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On Wed, 3 Jul 2002, Richards, Justin P wrote:

><snip>
>Do some amplifiers also get damaged if the load impedence is *higher*
>than the one specified?
>
>==================
>never
>==================
>Just thought I might clarify, the post was with regards to audio amps but
>thought it best to mention I *think* RF power amps like to see a load or
>they can fail.

It depends which kind of RF amp. Medium power and up certainly. Low power
usually not unless it's really badly built.

Peter

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2002\07\03@141851 by Peter L. Peres

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On Tue, 2 Jul 2002, Rick C. wrote:

>No. It's like saying, do I need a load on an opamp? A higher impeance load
>will just mean a lower power transfer to the load.
>
>Back in the vacuum tube days when you had a plate transformer matched to a
>low impedance speaker, if you took the load (speaker) off and drove it hard,
>huge amounts of voltage would be generated from inductive reactance that
>would literally arc and burn the transformer up.
>Rick

Afair from 100Volt line adventures, removing the load allows the
transformer to develop maximum undamped inductance and this transforms the
amplifier into an oscillator (the feedback is usually from a secondary on
the transformer) at the s.r.f. of the final transformer. This is due to
the phase shift induced into the feedback loop. This is one of the reasons
why tube amps work best with as low as possible feedback. Good units had a
resistor across one of the transformer windings to prevent this.
Feedback-less tube amps do not have this problem. Thay also have a snubber
across the anode winding however.

Peter

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2002\07\03@231246 by Michael Rigby-Jones

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{Quote hidden}

I think that may be slightly backwards, true class B has no biasing of the
complimentry output stage which leads to crossover distortion.  Class AB
deliberataly biases both transistors into conduction which virtual
eliminates crossover distortion at the expense of efficiency.

IIRC the way to achieve minimal distortion with the LM324 was to use a
resistor from the output to Vcc to bias it into Class A operation (again at
the expense of efficiency).

Regards

Mike

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2002\07\04@063744 by Alan B. Pearce

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>>Just thought I might clarify, the post was with regards to audio amps but
>>thought it best to mention I *think* RF power amps like to see a load or
>>they can fail.

>It depends which kind of RF amp. Medium power and up certainly. Low power
>usually not unless it's really badly built.

When I was dealing with VHF power amps we were using transistors rated for
25W output (for a pair in push-pull) and they were rated for infinite VSWR
on motor vehicle supplies (13.6V).

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2002\07\04@131304 by Dwayne Reid

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At 10:04 AM 7/3/02 +0100, Michael Rigby-Jones wrote:

>IIRC the way to achieve minimal distortion with the LM324 was to use a
>resistor from the output to Vcc to bias it into Class A operation (again at
>the expense of efficiency).

Actually, NS shows the resistor from the output to ground.  The app notes
suggest 6K2.

dwayne

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