Yesterday I decided to bite the bullet and trace the fault on my right hand
channel old speaker wire that is for the other room. Having discovered a
spiky bit of wire shorting the wire several feet down at a junction block
inside a QED headphone socket and volume box, I thought it was solved, but
the sound, though it was there, was weak and crappy. Move along wire to
another QED box that switched speakers from a local sauce which worked to
the remote one which was grotty, I eventually traced it to a three position
rocker switch for the two inputs. Would you believe it, this beefy switch,
presumably originally made for mains use had gone intermittent on one
contact.?
After a lot of squirting of stuff and movement of the switch it seems to
be working again.
Why is it that often gremlins strike at the simplest of systems at the same
time? Its not been disturbed in maybe 10 years and has been working all that
time.
Brian

--
----- -
This newsgroup posting comes to you directly from...
The Sofa of Brian Gaff...

Blind user, so no pictures please!

In article ,
Brian Gaff wrote:
Would you believe it, this beefy switch,
presumably originally made for mains use had gone intermittent on one
contact.?

Not that uncommon. Mains switches may just have brass contacts which
corrode. The relatively high current with mains tends to keep them clean
(the spark as they operate)

Signal switches usually have plated contacts to prevent corrosion.

--
*Don't worry; it only seems kinky the first time.*

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

Ah yes. One thing I never did understand was that the one common fault on
torches was poor contacts in switches or between battery and the rest of the
circuit. You would have thought that would have been solvable.

Maybe in the case of my switch then what I shouldhave done is to take it out
and use it on mains for a few days.
Not practical.
Brian

--
----- -
This newsgroup posting comes to you directly from...
The Sofa of Brian Gaff...

Blind user, so no pictures please!
"Dave Plowman (News)" wrote in message
...
In article ,
Brian Gaff wrote:
Would you believe it, this beefy switch,
presumably originally made for mains use had gone intermittent on one
contact.?

Not that uncommon. Mains switches may just have brass contacts which
corrode. The relatively high current with mains tends to keep them clean
(the spark as they operate)

Signal switches usually have plated contacts to prevent corrosion.

--
*Don't worry; it only seems kinky the first time.*

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

Brian Gaff wrote:

-------------------


Maybe in the case of my switch then what I shouldhave done is to take it out
and use it on mains for a few days.


** Bonkers idea - the switch needs replacing.

Switches are very much "horses for courses" and ones rated at mains voltage may fail to work at 12V or lower. Speaker signals require switches that have low on resistance at the millivolt level which implies the use of silver alloy contacts or better.

There are two major issues with large mains rated toggles when used with less other than mains voltages.

1. The contact material ( often plain brass) tarnishes with a non conductive layer. Regular use at mains voltage usually prevents this layer building up.

2. Grease applied to the moving parts of the switch migrates to the contacts, hardens and stops conduction.

In case 2, a squirt of WD40 often restores good operation, far as being a mains switch is concerned.

IME, speaker switches fitted to amplifiers are either rotary or slide types, the latter may have a toggle like action to fool the user. Both use silver alloy contacts and are self wiping in use, essential to keep tarnish build at bay.



..... Phil

In article ,
Phil Allison writes:

There are two major issues with large mains rated toggles when used with less other than mains voltages.

1. The contact material ( often plain brass) tarnishes with a non conductive layer. Regular use at mains voltage usually prevents this layer building up.

2. Grease applied to the moving parts of the switch migrates to the contacts, hardens and stops conduction.

In case 2, a squirt of WD40 often restores good operation, far as being a mains switch is concerned.

There's a third issue specifically with switches used on DC circuitry,
which is that current flow is constant and unidirectional and so the
arcing that occurs as a contact is made and broken will always be in
the same direction and will not be broken by the alternation of the
current, only by the contact gap getting large enough.

--
Mike Fleming

In article ,
Brian Gaff wrote:
Ah yes. One thing I never did understand was that the one common fault
on torches was poor contacts in switches or between battery and the
rest of the circuit. You would have thought that would have been
solvable.

It is. But may cost. Like decent gold etc plating. Not only do you need to
prevent the contacts corroding, but you also need any plating to be
thick/strong enough to last well too.

Maybe in the case of my switch then what I shouldhave done is to take it
out and use it on mains for a few days. Not practical. Brian

Why not just use the correct type of switch? Rotary or sliding contacts
tend to work best for signal levels.

Got a switch story from my old car. The window switches control relays.
The exact same Lucas switch is used on other makes with no relays. On
mine, the switches need regular cleaning. In the high power use, not.

--
*It's a thankless job, but I've got a lot of Karma to burn off

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

In article ,
Phil Allison wrote:
IME, speaker switches fitted to amplifiers are either rotary or slide
types, the latter may have a toggle like action to fool the user. Both
use silver alloy contacts and are self wiping in use, essential to keep
tarnish build at bay.

Often wonder what the spec is for a relay you so often find on amps - to
prevent speaker thump at switch on etc. They look pretty small for the job.

--
*I tried to catch some fog, but I mist.*

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

Mike Fleming wrote:

-------------------


There are two major issues with large mains rated toggles when used with less than mains voltages.

1. The contact material ( often plain brass) tarnishes with a non conductive layer. Regular use at mains voltage usually prevents this layer building up.

2. Grease applied to the moving parts of the switch migrates to the contacts, hardens and stops conduction.

In case 2, a squirt of WD40 often restores good operation, far as being a mains switch is concerned.


There's a third issue specifically with switches used on DC circuitry,
which is that current flow is constant and unidirectional and so the
arcing that occurs as a contact is made and broken will always be in
the same direction and will not be broken by the alternation of the
current, only by the contact gap getting large enough.


** Switches have *distinctly different* voltage ratings for AC and DC currents for exactly this reason.

240VAC switches ( and relays) are normally down rated to 24V or 32V when used with DC current of the same magnitude.

https://www.mouser.com/blog/Portals/...ig1-Switch.png

Not much relevant, however, to our present topic.


...... Phil

On Wed, 20 Dec 2017 00:57:46 +0000, Dave Plowman (News) wrote:

In article ,
Phil Allison wrote:
IME, speaker switches fitted to amplifiers are either rotary or slide
types, the latter may have a toggle like action to fool the user. Both
use silver alloy contacts and are self wiping in use, essential to keep
tarnish build at bay.

Often wonder what the spec is for a relay you so often find on amps - to
prevent speaker thump at switch on etc. They look pretty small for the
job.

I prefer to design bridge output amps which eliminate the need for such
add on measures. Also, it halves the supply rail voltage needed to drive
any particular load to a given power level which allows the use of lower
voltage rated output devices. It also improves immunity to strong RFI
field strengths and allows a simple fuse in the DC supply rail, whether
actual or, much better, an electronic circuit breaker (perhaps making use
of the speaker voltage to modulate the tripping point to make it detect
impedance overloads rather than relying on detection of a maximum current
limit alone which could still burn out the output devices at moderate
volume levels[1]) to provide protection without introducing unwanted
distortion when a fuse is used in series with the speaker load in
unipolar designs.

[1] In which case you need to include clamping diodes on each output
terminal to ground and 'fused' supply rail (four in all for each output)
to prevent inductive overloads from generating destructive voltage spikes
back into the output terminals when a fuse or fast electronic circuit
breaker is employed on each channel's DC supply rail for protection.

--
Johnny B Good

Johnny Bull****s Good wrote:

----------------------------




I prefer to design bridge output amps which eliminate the need for such
add on measures.



** Hogwash, it does no such thing.

Power up and down noises mostly originate in the pre-amp stages.

A speaker relay with a few seconds on-delay and fast off catches them PLUS any from the power stages too.



Also, it halves the supply rail voltage needed to drive
any particular load to a given power level which allows the use of lower
voltage rated output devices. It also improves immunity to strong RFI
field strengths and allows a simple fuse in the DC supply rail, whether
actual or, much better, an electronic circuit breaker (perhaps making use
of the speaker voltage to modulate the tripping point to make it detect
impedance overloads rather than relying on detection of a maximum current
limit alone which could still burn out the output devices at moderate
volume levels[1]) to provide protection without introducing unwanted
distortion when a fuse is used in series with the speaker load in
unipolar designs.


** A bridge mode output stage is *highly vulnerable to damage* if there is a short on the speaker line.

Normally, shorts on amplifier outputs go to ground *NOT* from one amp's output to another. The latter scenario is impossible to protect against with any form of VI limiting and amp makers offer no warranty against it.




..... Phil