"Trevor Wilson" wrote in
message
"Dave Plowman (News)" wrote in
message ...
Got an 'interesting' fault on an ancient Linsey Hood
amp. Switch on a light etc, and sometimes *one* channel
bursts into loud mains hum - but carries on working. And
the output transistors get very hot. It's not normally
susceptible to mains born interference - so any guesses
before I rip it apart?

Both channels seem to match ok before this happens.

**First off: The "Mains hum" is not mains hum.

Well, it is still mains-related.

It is 100Hz hum.

Yes, power supply ripple from a full wave rectifier.

It is the sound made by an amplifier when so
much current is consumed from the power supply, that
ripple appears on the outputs.

Agreed.

Second off: You need to ascertain if the output devices
get hot with no load connected. If they still get hot,
then look for a fault in the bias section, or further
down the chain. Something is causing the output devices
to conduct too much current.

Agreed. An open bias network component, possibly a pot is a common cause.

If the output devices run cool when no load is connected,
then it is likely you have a large DC Voltage at the
output. If so, the fault could lie anywhere in the amp.

IME large DC voltage on the output is often very detrimental to the load,
unless the load is a stout bench resistor bank.

"Arny Krueger"
"Trevor Wilson"

**First off: The "Mains hum" is not mains hum.

Well, it is still mains-related.

** The term " mains hum " includes 50/60 Hz AC supply frequencies plus ALL
audible harmonics, either originating from AC supply voltage and current
itself, magnetic fields associated with transformers connected to same AND
harmonic frequencies produced in various rectification processes whenever
these frequencies enter a signal carrying circuit.


It is the sound made by an amplifier when so
much current is consumed from the power supply, that
ripple appears on the outputs.

Agreed.


** There is generally no audible hum from the un-driven channel of a
correctly functioning stereo amplifier, even when the other channel is
driven to full output. This is true despite the fact that in the vast
majority of cases, the two channels share a common and unregulated PSU.


Agreed. An open bias network component, possibly a pot is a common cause.


** Even a very high bias setting in a channel also does not result in a "
loud" audible hum from the attached speaker - as was described by the OP.

Any functioning hi-fi amplifier has very high rejection of its own supply
ripple.


If the output devices run cool when no load is connected,
then it is likely you have a large DC Voltage at the
output. If so, the fault could lie anywhere in the amp.

IME large DC voltage on the output is often very detrimental to the load,
unless the load is a stout bench resistor bank.

** A large DC offset at the output of an amp will cause the voice coil and
cone of the bass driver to be move well away from its normal, central
position and ( if sustained ) for it to emit smoke signals.



...... Phil

"Phil Allison" wrote in message
...

"Arny Krueger"
"Trevor Wilson"

**First off: The "Mains hum" is not mains hum.

Well, it is still mains-related.

** The term " mains hum " includes 50/60 Hz AC supply frequencies plus ALL
audible harmonics, either originating from AC supply voltage and current
itself, magnetic fields associated with transformers connected to same
AND harmonic frequencies produced in various rectification processes
whenever these frequencies enter a signal carrying circuit.


It is the sound made by an amplifier when so
much current is consumed from the power supply, that
ripple appears on the outputs.

Agreed.


** There is generally no audible hum from the un-driven channel of a
correctly functioning stereo amplifier, even when the other channel is
driven to full output. This is true despite the fact that in the vast
majority of cases, the two channels share a common and unregulated PSU.


Agreed. An open bias network component, possibly a pot is a common cause.


** Even a very high bias setting in a channel also does not result in a "
loud" audible hum from the attached speaker - as was described by the
OP.

Any functioning hi-fi amplifier has very high rejection of its own supply
ripple.


If the output devices run cool when no load is connected,
then it is likely you have a large DC Voltage at the
output. If so, the fault could lie anywhere in the amp.

IME large DC voltage on the output is often very detrimental to the load,
unless the load is a stout bench resistor bank.

** A large DC offset at the output of an amp will cause the voice coil and
cone of the bass driver to be move well away from its normal, central
position and ( if sustained ) for it to emit smoke signals.


Interesting - his medication seems to be holding....??

In article ,
Keith G wrote:
** A large DC offset at the output of an amp will cause the voice coil
and cone of the bass driver to be move well away from its normal,
central position and ( if sustained ) for it to emit smoke signals.


Interesting - his medication seems to be holding....??

Have I mentioned Kitty's blog yet today? The need to comment on anything -
regardless.

--
*To steal ideas from *one* person is plagiarism; from many, research*

Dave Plowman London SW
To e-mail, change noise into sound.