Laurence Payne wrote:
On Sun, 28 Oct 2007 14:44:10 +0000, Rob
wrote:

I've got two amplifiers and I'd like to switch between them. At the
moment I'm plugging/unplugging at the speakers. For reasons of cable
management, convenience and neatness I'd like to use a switch. QED did
one a while ago but it's discontinued, and replaced with a wall switch
which is too 'embedded' for my liking, and pricey.

I was having a look in Wickes for a suitable switch, and it looks as
though a four gang light switch would do it (8 wires in, four wires
out), but this has four switches.

Ideally I'd like one switch - amplifier A, or amplifier B.


For several reasons, can I persuade you to make up a little box with
banana plugs and sockets?

I suspect your main reason is the possibility that I'd run a valve amp
into open circuit, and also, possibly, my competence at wiring anything
up in the first place. Good points, noted.

Eiron wrote:
Rob wrote:
I've got two amplifiers and I'd like to switch between them. At the
moment I'm plugging/unplugging at the speakers. For reasons of cable
management, convenience and neatness I'd like to use a switch. QED did
one a while ago but it's discontinued, and replaced with a wall switch
which is too 'embedded' for my liking, and pricey.

I was having a look in Wickes for a suitable switch, and it looks as
though a four gang light switch would do it (8 wires in, four wires
out), but this has four switches.

Ideally I'd like one switch - amplifier A, or amplifier B.

Any ideas?

Thanks, Rob

I used a couple of 12v DPDT relays powered by an old psu from a cordless
phone.
One amp has a switched mains-out socket on the back; the psu connected
into that.
That then was the master amp so when it was on, the speakers were switched
to it, and when it was off, the speakers were switched to the other amp.

If neither amp has a switched mains outlet, you could use an 'intelligent'
mains extension block with one amp connected to the master socket.


Interesting, thanks. An issue here is that the valve amp is supposed to
be 'soft started' - one switch wait 30s, then the main switch. And it
would still mean running two sets of wires to the speakers I think?

Adrian C wrote:
Woody wrote:

Maplin JK30H @ £3 each. You only need to switch the live side of the
speaker feed - the ground sides can stay connected in parallel
throughout.


Though the OP should check (with a DVM) before doing this, that that
negative speaker outlets of *both* amplifiers concerned are actually
internally connected to ground. Ye don't want to upset a
bridge-tied-load amp, if ye have one of those in your setup ...

http://en.wikipedia.org/wiki/Bridge-tied_load


Thanks both, and Woody in particular for those links.

Just out of curiosity, why has it always been the case that four wires
run to speakers, when three would do (link the ground on a stereo pair)?

Owain wrote:
Rob wrote:
I've got two amplifiers and I'd like to switch between them. At the
moment I'm plugging/unplugging at the speakers. For reasons of cable
management, convenience and neatness I'd like to use a switch. QED did
one a while ago but it's discontinued, and replaced with a wall switch
which is too 'embedded' for my liking, and pricey.
I was having a look in Wickes for a suitable switch, and it looks as
though a four gang light switch would do it (8 wires in, four wires
out), but this has four switches.
Ideally I'd like one switch - amplifier A, or amplifier B.
Any ideas?

You need a 4pole double-throw [changeover] switch (4PDT) which you can
probably get from Maplin, FH08J is 5A at 12V so okay for low power
applications.

Bigger switches have tags which take either soldered wires, or blades
for crimp terminals (which you can also get from Maplin).

Try www.rswww.com part nos 274-0441, 320-972, 316-995, or 266-2212,
ratings 3 to 20A

Add some terminal strip for the speaker wires, and encase the lot in a
small plastic case (also from Maplin)

Owain


Thanks muchly. Plenty to go on here - thanks to everyone.

On Mon, 29 Oct 2007 08:36:41 +0000, Rob
wrote:

Adrian C wrote:
Woody wrote:

Maplin JK30H @ £3 each. You only need to switch the live side of the
speaker feed - the ground sides can stay connected in parallel
throughout.


Though the OP should check (with a DVM) before doing this, that that
negative speaker outlets of *both* amplifiers concerned are actually
internally connected to ground. Ye don't want to upset a
bridge-tied-load amp, if ye have one of those in your setup ...

http://en.wikipedia.org/wiki/Bridge-tied_load


Thanks both, and Woody in particular for those links.

Just out of curiosity, why has it always been the case that four wires
run to speakers, when three would do (link the ground on a stereo pair)?

Cos your speakers stand ten feet apart. Kinda hard to run a single
ground wire to both of those.

d

--
Pearce Consulting
http://www.pearce.uk.com

In article , Rob
wrote:


Just out of curiosity, why has it always been the case that four wires
run to speakers, when three would do (link the ground on a stereo pair)?

The two speakers will normally be placed well apart, so if using just one
'ground' lead you'd have to run it to one of them, and then to the other.
This would mean that the lengths of ground cables for each differ.

A) The main result would be a difference in cable resistance for the two
speakers, possibly causing an imbalanced change in frequency response.

B) Unless you ran both 'live' cables along the same path, one speaker
connection would have a very wide spacing between its live and ground. The
result would be very high inductance. Again possibly producing a
significant difference in frequency response for the two channels. Might
also make RF pickup a problem.

C) If you *did* run the 'live' wires together then this would mean the
'live' to one speaker would be longer than the other - making effect (A)
worse.

D) Even if you used a symmetric 'Y' shape of cable for the ground to try
and avoid some of the above, using a common ground connection increases the
level of crosstalk as the common resistance will cross-connect the signals
to some extent. Aso, (C) would increase EM crosstalk.

Should I go on?... :-)

Basically, it isn't a good idea to try and skimp by using one ground cable
for the two speakers. Probably saves almost no money, and can cause all
sorts of problems. Although the problems may be small enough that it can be
done if there is no easy alternative.

Best to use distinct cables for the two speakers, have them of similar
lengths, and run them well clear of each other.

Slainte,

Jim

--
Electronics http://www.st-and.ac.uk/~www_pa/Scot...o/electron.htm
Audio Misc http://www.audiomisc.co.uk/index.html
Armstrong Audio http://www.audiomisc.co.uk/Armstrong/armstrong.html

In article ,
Rob wrote:
Just out of curiosity, why has it always been the case that four wires
run to speakers, when three would do (link the ground on a stereo pair)?

If it were easier wiring wise to do this then no reason why not. But in
the majority of cases the speaker wires go off in opposite directions from
the amp.

--
*Modulation in all things *

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

On Mon, 29 Oct 2007 08:36:41 +0000, Rob
wrote:

Just out of curiosity, why has it always been the case that four wires
run to speakers, when three would do (link the ground on a stereo pair)?

As on the face of it this is such a stupid question, perhaps he's
talking about bi-amping or bi-wiring?

"Rob" wrote in message
...

Just out of curiosity, why has it always been the case that four wires run
to speakers, when three would do (link the ground on a stereo pair)?

The alternative would be a 3-wire system. The common wire would have to have
twice the copper per unit length. In general, the savings in terms of copper
and wire would be negligible.

If you had a typical bookshelf system with the amp or receiver in the
middle, and a speaker on each side, I can see where a 3-wire system would
make the wires longer, than separate grounds for each channel.

Laurence Payne wrote:
On Mon, 29 Oct 2007 08:36:41 +0000, Rob
wrote:

Just out of curiosity, why has it always been the case that four wires
run to speakers, when three would do (link the ground on a stereo pair)?

As on the face of it this is such a stupid question, perhaps he's
talking about bi-amping or bi-wiring?

Nope, no bi-anything - that'd be eight cables. As you face it, it
remains stupid.