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