RE: Speaker Switching

["Kevin Hawkins" <lists@xxxxxxx>]

Speaker switching is a little more complex than it first appears and
there
are good and bad ways to do this.

    An amplifier is designed to transfer the most possible
power to a
speaker when the impedance of the speaker matches the spec - typically
this
is designed for an 8 ohm or 4 ohm speaker. The lower the ohms the
greater
current can flow for the same voltage such that on manufacturers specs
you
will frequently see that power is maximised on 4ohm speakers. A short
circuit (a big piece of copper wire) is effectively 0 ohms and an open
circuit is an infinite number of ohms.
    Depending on the design of the amplifier it needs to be
able to protect
itself against the speakers being shorted or becoming disconnected -
poor
designs may not have this protection and some protections only activate
at
power on and not during playing of an audio source. Obviously shorting
some
speaker terminals could generate huge currents, open circuit can
generate
large voltages and current glitches. Supplying any impedance other than
the
designed one back to the amplifier impacts the efficiency of the
amplifier.
    Multiway speaker switches typically work by allowing the
connection of
speakers in parallel and series combinations such that the impedance back
to
the amplifier is kept as close to say 8 ohms as possible - two 8 ohm
speakers could be in parallel to reach 4 ohms etc etc The more
expensive
switches (& volume controls) use a transformer matching technique to
make
this much more predictable. Resistors are introduced in the lower cost
designs to help smooth the switching changes out and also to cater for
the
critical time when the contacts change when a speaker set are switched in
or
out. This has to be done typically 'break before make' in these designs
to
avoid shorting the amplifier. Some extra resistors sometimes exist to
smooth
the variations in some switching combinations.
    The resistor that is in your switchbox is really there
probably for this
reason. It is to cater for the brief time at which the switches are
being
changed over - it is probably a fairly high value say 100 ohms in
comparison
to a speaker impedance and is to avoid the terminals going open circuit
and
possibly damaging the amplifier. It is not designed to allow the
amplifier
to play into the switch when no speaker is attached or switch in (As
there
would be no point in this). Additionally the power that is output from
the
amplifier is effectively absorbed by this resistor and has to be
dissipated
as heat - when you consider that some amplifiers can give out huge
amounts
of power nowadays say 50W this is a big problem. At a higher impedance
this
would be reduced though. But typical small resistors can only dissipate
1/4
W although the resistor in that box is probably much larger and may
dissipate say 5W or 10W. If it does get too much power it will simply
burn
up and go open circuit effectively exposing the amplifier to risk.
   You are right in that if this resistor is effectively in
parallel with
your speakers the majority of the current will flow through the
speakers
still and therefore the resistor has little overall effect.
    You haven't mentioned what sort of power these sources
might be running
but...
    Your problem is two fold - firstly suddenly switching
the load off from
some speakers which is where this resistor comes in - you need to be wary
of
what sort of power might be being switched at the time - and also what
sort
of natural protection your source amplifiers have inbuilt. As I
understand
your switcher you might have to provide this on all six different
inputs
given they could all be switched out .Secondly you have the issue of how
to
connect all or any combination of the speakers to one source. You
mustn't
connect them all in parallel for example as you would then be presenting
an
impedance of under 2 ohms  back to the amplifier - well below the 8
ohms
typical and possibly damaging to the source or causing it to go into a
protection state. This is really why most complex switching/mixing is
done
at line level rather than speaker level. Switching large powers that are
not
capable of running in short/open circuit conditions is complex to do
correctly.

    Kevin

-----Original Message-----
From: Tony T [mailto:tony@xxxxxxx]
Sent: 23 December 2002 09:26
To: ukha_d@xxxxxxx
Subject: [ukha_d] Speaker Switching


  Hi all,

  Thanks to advise from people on/off list, I intend to go ahead and
try
  and build a speaker switcher (not matrix).

  It will have 6 x inputs going to 6 x outputs.
  With input1 going to output1 thru input6 to output6.
  Ok, I don't need a switcher for this bit!

  But there will be a 7th input, that can be switched to any or all of
the
  6 output's.
  This is the bit that needs switching.

  The thing I hadnt realised, until it was pointed out, was that when
a
  source was switched out to allow the 7th input access to the
speakers, I
  would need to switch in a load to the disconnected source.

  So I bought a manual switch from maplin's which has 2 inputs,
switchable
  to 1 output.
  And took it apart to see how it handled putting a load on the
  disconnected source.

  To my surprise the load resistor is always in place, which with
my
  (very) limited knowledge of electronics I immediately thought meant
it
  wouldn't even work. Of course it does.

  So I assume that when a speaker is attached it becomes the path of
least
  resistance?

  Is this a safe/acceptable way to do this, or is it just a cheap way
to
  do this?

  Many thanks
  Tony T



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