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How do I build an A/B switch for speakers
Old 27th September 2014
  #1
Here for the gear
 

How do I build an A/B switch for speakers

I'm going through a phase of trying different speakers in my guitar cabs. I want to use an A/B switch so I can switch between different speaker cabs instantly and more easily hear the difference between the speakers, can I use a standard A/B switch or should I make one with speaker cable inside the switch?

Thanks for any help
Old 27th September 2014
  #2
Gear Maniac
 

I would use a DPDT switch to totally A/B both positive and negative leads. Never switch with the amp turned on as this would momentarily unload the amp, possibly damaging it.

A 50w amp into an 8ohm cab is approx 2.5 amps and 20v (probably not that in the real world, not 100% efficient) - do a little math for your situation and buy a switch that can handle the amperage and voltage. Not exactly a job for small signal switches like a guitar A/B box.

I'm not aware of any prebuilt A/B box but I wouldn't be surprised if there is one for this application.
Old 27th September 2014
  #3
If you are switching the actual speakers using the same amplifier and are using cabs connected with the usual 1/4-inch TS speaker cables and can solder, the least expensive way is to connect a DPDT toggle switch to a couple of (cut) speaker cables. It's worthwhile to use a switch with a current rating of 10 or 15 amps in order to keep the resistance as low as possible. It's not the current rating of the switch that's critical, it's the low contact resistance, so that you don't affect the damping factor of the speaker/amp combo too much by the introduction of the switch itself.

If you have no soldering skill, you can buy an inexpensive speaker switch and again use a cut 1/4-inch TS speaker cable . Most speaker switches are designed with spring clip cable connectors, so are very easy to connect to a cut guitar speaker cable. You will need 3 pieces of cable with 1/4-inch plugs on one end and stripped bare leads on the other (2) cables cut in half will do it with one section left over.

There are TS (unbalanced) and TRS (balanced) switches designed for active speakers (like the Hosa SLW-333), but those typically use low current switches and thin wire so have too much resistance and won't take the current that is required for speaker-level switching. Don't use an active monitor switch for speaker-level switching, even if it has 1/4-inch jacks unless it specifies it's safe for passive speaker use. The Hosa is not.

DIY "Toggle-Switch" version shown below:
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Attached Thumbnails
How do I build an A/B switch for speakers-b-speaker-switch.jpg  
Old 27th September 2014
  #4
Here for the gear
 

Thanks this is all really helpful!

The load issue won't be a problem because I'll be taking the signal from an attenuator which acts as a load box, so it doesn't mind not having anything plugged into the speaker out on that. Sorry I should have said.

I think I'm going to go with a consumer speaker switch, I couldn't find one when I first looked, I imagined they would be more like the guitar A/B switches that I'm more familiar with.

One last question though, if I did decide I'd like to use that consumer speaker switch without the attenuator, would I have to power down the amp because of the load issue? Or does the switch protect against that somehow?

Thanks guys
Old 27th September 2014
  #5
Quote:
Originally Posted by mascis2000 View Post
...One last question though, if I did decide I'd like to use that consumer speaker switch without the attenuator, would I have to power down the amp because of the load issue? Or does the switch protect against that somehow?

Thanks guys
The inexpensive one in the link I posted, does use parallel resistors in the circuit all the time, so offers some protection against transformer damage caused by switching while the amp is running at high power levels.
It has separate switches to connect either or both speakers at the same. The best way to be absolutely safe is to "overlap" the speaker switching:

Start with Speaker A On
Switch Speaker B On (now both are being driven) The momentary lowered load impedance won't hurt anything).
Switch Speaker A Off.

Using that sequence will always present a load to the amp so is safe. Always have at least one of the output switches ON.

If you use a hard-wired toggle switch, you can jump the input terminals with a 120 ohm, 5 watt wire-wound resistor to protect the amp output transformer from possible no-load over-voltage spikes during the instant of switching.
Old 27th September 2014
  #6
Here for the gear
 

Fantastic thank you
Old 28th September 2014
  #7
Lives for gear
 

This sounds like what You are looking for. Would allow the instant switching You mention.
Old 28th September 2014
  #8
The radial switcher uses a digital timer and relays to perform an automatic overlapping switch: A only --> A+B together--> B-only. It's a great little box but kind of "pricey" at $350, if it's just being used to evaluate different speakers. It is ideal for regular stage use with multiple speaker cabs . The switching is not instant (A to B) as it would be with a toggle switch, but according to radial there is a "momentary" overlap. They don't specify how long "momentary" actually is in real time.

Have no idea why this single post repeated (7) times, but have deleted the extra copies. The Forum has been a little "flakey lately and is acting like the server is "maxed-out" often.
Old 2nd October 2014
  #9
Here for the gear
 

Β£300 here in the UK, not exactly what I'd call a bargain...but worth knowing about. Think I'll stick with my Β£20 switch I ordered and make some cables...then I have Β£280 left over for the new speakers for my Fender Twin!
Old 17th May 2017
  #10
Here for the gear
 

Hi,

It's been a while since this thread but...

I going to put together the following circuit.
It suppose to add a delay time when switching speakers (on delay relay) in order to avoid no-load to the amp (tube amps mostly).

Works as follows:

Each speaker is connected to NC relay terminal so there is protection when the switcher supply is off.

When 12V supply are engaged trough foot SW to one side, that relay goes on with some delay due the time imposes by R1 and C1 and the speaker is disengaged (again, the speaker is connected to NC terminal so energizing the relay means speaker off). The other Relay switches off faster because the discharge of C1 trough R2 and transistor's base, so there is an overlapping time that both are in NC, so both speakers are ON.

Also, there is another "BOTH" switch that disables 12V to use both speakers at the same time (obviously with some mismatched impedance).

The ON time depends on R1xC1 and both transistor's base-emitter voltages.


Please comment!
Attached Thumbnails
How do I build an A/B switch for speakers-ab-speaker-switcher.jpg  
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