Re: High Current Speaker Selector

[Josh Snider]

in article , Chris Hornbeck at
wrote on 9/14/03 20.07:

> On Sun, 14 Sep 2003 23:52:59 GMT, Parthenon West >
> wrote:
>
>> Does anyone know of a good option or product to switch between speaker
>> sets on the high current, post-amplifier wires? I have one very good
>> muscle amplifier and three pairs of speakers ranging from NS-10s to
>> 4x10 Quested mains and not much spare cash for two more amps. Is there
>> an existing product or does anyone have advice before I go to the
>> electrical supplies store?
>
> Two pole household light switches are excellent, and dirt cheap.
> Grab a three-gang box while you're there.
>
>

Yes, but two pole light switches don¹t impedance match. One wrong switch
and you subject your amp to a load small enough as to cause it quite a bit
of heartache...

J


--
josh.snider
cave.productions
416.524.6927

[Scott Dorsey]

Parthenon West > wrote:
>Does anyone know of a good option or product to switch between speaker
>sets on the high current, post-amplifier wires? I have one very good
>muscle amplifier and three pairs of speakers ranging from NS-10s to
>4x10 Quested mains and not much spare cash for two more amps. Is there
>an existing product or does anyone have advice before I go to the
>electrical supplies store?

Go to the electrical supply store. Make sure the switch you buy has
wiping contacts; many high current switches rely on the arc when the
switch closes to get a good connection and they will not work well
with low-level signals. Switches with wiping contacts will clean themselves
every time you switch them so there's no layer of oxide to break through.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."

[Parthenon West]

In article >,
Josh Snider > wrote:
> Yes, but two pole light switches don¹t impedance match. One wrong switch
> and you subject your amp to a load small enough as to cause it quite a bit
> of heartache...
>
> J

What do you mean? What would I have to do to cause this condition and
what might be the results?

Thanks,

Scott

[Mike Rivers]

Josh Snider > wrote in message >...

> Yes, but two pole light switches don¹t impedance match. One wrong switch
> and you subject your amp to a load small enough as to cause it quite a bit
> of heartache...

Does he want cheap and effective, or does he want idiot-proofing too?
If he has his wits about him, he can throw two switches together
closely enough in time so that any unusual load would be very brief.

Any power ampplifier that can't take an overload for a second or so
isn't worthy of a "high current" switch, because it can't supply a
high current.

[Josh Snider]

in article , Parthenon
West at wrote on 9/15/03 00.09:

> In article >,
> Josh Snider > wrote:
>> Yes, but two pole light switches don¹t impedance match. One wrong switch
>> and you subject your amp to a load small enough as to cause it quite a bit
>> of heartache...
>>
>> J
>
> What do you mean? What would I have to do to cause this condition and
> what might be the results?
>
> Thanks,
>
> Scott

When you put a speaker on an amp, you are loading the amp's outputs with a
certain resistance (Impedance). For most studio and home applications that
impedance is 8 Ohms. The less load you put on an amp's outputs, the closer
that amp gets to short circuit, and thus the more current it puts out and
the harder it has to work. The advantage is, that you get more out of your
amp if you load it LESS. Most good power amps can handle a 4 ohm load, and
the better ones can handle 2 ohms. Any less load then the amp can handle
and you run a very real risk of burning out a fuse, or worse, bits of your
amp.
When you put two 8 ohm speakers in parallel (such as with a speaker selector
that will allow more then one speaker pair on at a time) you are putting two
small resistors in parallel. Using the formula to calculate parallel
resistances (1/Rt = 1/R1 + 1/R2...) two 8 ohm speakers in parallel become 4
ohms. Three 8 ohm speakers in parallel become 2.66... ohms. Four 8 ohm
speakers in parallel become 2 ohms. Add another speaker and you drop below
2 ohms.
If your poweramp can't handle a 4 ohm load, then even two speakers in
parallel could damage the amp.

Most good commercial speaker switches that allow more then one pair of
speakers on at a time impedance match, that is, they add a resistance in
series so that the amp always sees a constant load of (usually) 8 ohms, no
matter how many speaker pairs are connected.

For a DIY switchbox, you'd have to make sure that either A) you built in an
impedance matching circuit, or B) ensured that no more speakers could be
turned on in parallel then the amp could handle.

J


--
josh.snider
cave.productions
416.524.6927

[Josh Snider]

in article , Parthenon
West at wrote on 9/15/03 00.09:

> In article >,
> Josh Snider > wrote:
>> Yes, but two pole light switches don¹t impedance match. One wrong switch
>> and you subject your amp to a load small enough as to cause it quite a bit
>> of heartache...
>>
>> J
>
> What do you mean? What would I have to do to cause this condition and
> what might be the results?
>
> Thanks,
>
> Scott

When you put a speaker on an amp, you are loading the amp's outputs with a
certain resistance (Impedance). For most studio and home applications that
impedance is 8 Ohms. The less load you put on an amp's outputs, the closer
that amp gets to short circuit, and thus the more current it puts out and
the harder it has to work. The advantage is, that you get more out of your
amp if you load it LESS. Most good power amps can handle a 4 ohm load, and
the better ones can handle 2 ohms. Any less load then the amp can handle
and you run a very real risk of burning out a fuse, or worse, bits of your
amp.
When you put two 8 ohm speakers in parallel (such as with a speaker selector
that will allow more then one speaker pair on at a time) you are putting two
small resistors in parallel. Using the formula to calculate parallel
resistances (1/Rt = 1/R1 + 1/R2...) two 8 ohm speakers in parallel become 4
ohms. Three 8 ohm speakers in parallel become 2.66... ohms. Four 8 ohm
speakers in parallel become 2 ohms. Add another speaker and you drop below
2 ohms.
If your poweramp can't handle a 4 ohm load, then even two speakers in
parallel could damage the amp.

Most good commercial speaker switches that allow more then one pair of
speakers on at a time impedance match, that is, they add a resistance in
series so that the amp always sees a constant load of (usually) 8 ohms, no
matter how many speaker pairs are connected.

For a DIY switchbox, you'd have to make sure that either A) you built in an
impedance matching circuit, or B) ensured that no more speakers could be
turned on in parallel then the amp could handle.

J


--
josh.snider
cave.productions
416.524.6927

[Josh Snider]

in article , Mike Rivers at
wrote on 9/15/03 09.29:
> Does he want cheap and effective, or does he want idiot-proofing too?
> If he has his wits about him, he can throw two switches together
> closely enough in time so that any unusual load would be very brief.

Touche. I was just bringing up the warning :-)


> Any power ampplifier that can't take an overload for a second or so
> isn't worthy of a "high current" switch, because it can't supply a
> high current.

Also an excellent point...

J

--
josh.snider
cave.productions
416.524.6927

[dt king]

"Josh Snider" > wrote in message
...

> Most good commercial speaker switches that allow more then one pair of
> speakers on at a time impedance match, that is, they add a resistance in
> series so that the amp always sees a constant load of (usually) 8 ohms,
no
> matter how many speaker pairs are connected.
>
> For a DIY switchbox, you'd have to make sure that either A) you built in
an
> impedance matching circuit, or B) ensured that no more speakers could be
> turned on in parallel then the amp could handle.

Adding a sturdy resistor in series seems like the cheap dirty solution. Is
there a way of putting a fuse in for extreme possibilities?

dtk

[Rich Wilner]

> Go to the electrical supply store. Make sure the switch you buy has
> wiping contacts; many high current switches rely on the arc when the
> switch closes to get a good connection and they will not work well
> with low-level signals. Switches with wiping contacts will clean themselves
> every time you switch them so there's no layer of oxide to break through.
> --scott

may I also suggest, if you get a 2P2T switch, you get a
break-before-make switch. if not, you'll be connecting both speakers
in parallel for a moment, which may be a load your amp is not happy
with and will make switching during playback annoying.
better yet, maybe get a 2p3t switch and leave the middle throws
disconnected, you can use it as a mute.

[Justin Ulysses Morse]

dt king > wrote:

> Adding a sturdy resistor in series seems like the cheap dirty solution. Is
> there a way of putting a fuse in for extreme possibilities?

I think the resistor solution is a little too dirty. It makes more
sense to arrange a switch that doesn't allow you to turn on more than
one set of speakers at a time. So rather than a separate on-off switch
for each pair, how about a rotary switch that chooses one pair from
several?

And don't worry too much about the break-before-make contacts. If you
have a choice, great, but it won't be the end of the world. If the amp
you're using isn't internally protected against shorts and overloads
&c. then it's probably not such a badass amp anyway. A split-second of
4 ohms instead of 8 isn't going to hurt anything on any even remotely
competent amplifier.

ulysses

[Scott Dorsey]

Justin Ulysses Morse > wrote:
>dt king > wrote:
>
>> Adding a sturdy resistor in series seems like the cheap dirty solution. Is
>> there a way of putting a fuse in for extreme possibilities?
>
>I think the resistor solution is a little too dirty. It makes more
>sense to arrange a switch that doesn't allow you to turn on more than
>one set of speakers at a time. So rather than a separate on-off switch
>for each pair, how about a rotary switch that chooses one pair from
>several?

Well, in the case of two pairs, if you are happy with household toggle
switches, you can get a three-way switch that is a SPDT type in the
bin next to the SPST ones.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."

[dt king]

"Scott Dorsey" > wrote in message
...
>
> Well, in the case of two pairs, if you are happy with household toggle
> switches, you can get a three-way switch that is a SPDT type in the
> bin next to the SPST ones.

Get three of them and you can wire up a binary logic selection of four
sets.

More than that, though, and you'll need a cheat sheet to remember the
settings for turning on a particular set of speakers.

"Let's see, speaker set five was up, down, up, down, down, down..."

dtk

[Justin Ulysses Morse]

dt king > wrote:

> "Let's see, speaker set five was up, down, up, down, down, down..."


And Justin Bailey's were up, down, up, down, B, A, B, A, <start>.

ulysses