The Workbench

3-in 3-out amp switcher: is there such a thing? If not, can it be m…

26

Do all the the 5-watt amps that you have in mind "expect" the same speaker impedance?

27

They do, and they work at 4 - 16 ohms.

28

So, you're probably thinking of, say, a single 8 ohm speaker in its' own cab?

Is this rig you envision just for testing purposes or is it to be "stage ready?"

29

Well, “stage” is going too far. The most I can hope for is some floor in the corner of a room, but it’s supposed to be seamlessly switchable, like for parts of a song performance, yes. Like stomping and un-stomping any effect pedal.

I have the cab, don’t know what impedance. I can look though.

30

AMATEUR ALERT - THEORETICAL SUGGESTION ONLY

I think this should do it. 8ohm ballast load permanently wired into each input, which means switching an 8ohm speaker in will drop the load to 4ohms; as already established, this will be acceptable for all three amps. It will operate as follows:

  • SW1 Position 1 = Amp 1
  • SW1 Position 2 = Amp 2 or Amp 3, depending on the position of SW2.

Inputs can all stay wired to all three amps so they won't need switching.

A couple of caveats though:

  1. The dummy loads (R1, R2 & R3) will need to be sufficiently rated as to handle the output from any amp likely to be plugged into it. I would recommend at least doubling the power rating (i.e., 10w amp/20w ballast load) to help accommodate transient peaks.
  2. The switches will need to be break-before-make type to avoid inadvertently connecting 2 amps to the speaker at the same time.
  3. I am not an expert by any stretch of the imagination. I offer this only as a wiring suggestion that, in my opinion, should switch the outputs of the amps as required. I welcome Billy, Josh, Powdog, or any other poster more competent than me to chime in and let us know whether this would work or not.

EDIT: Cleaner circuit drawing below.

31

That looks workable but, like you Deke, I'd want to hear from the amp gurus around here.

I still don't know how Tim can operate the changes from one footswitch though. How big is the speaker? If it's an 8" maybe you could just go with a cab for each amp?

32

You can't run it with 1 switch, it needs 2 switches, but they are in the same box. Here's a cleaner version of the circuit (I scribbled the other one in a spare moment at work today).

33

Each amp is only 1 audio click away, but you may need to pre-select either Amp 2 or 3, depending on where you left Switch 2 last time.

So for example, say you're in Sw1, position 1, you're then listening to Amp 1. Sw2 does nothing to the audible output, but will still switch between amps 2 and 3, albeit behind the scenes. So if Sw2 has amp 2 selected and you want amp 3, you can pre-select amp 3 before activating Sw1 to position 2.

Sounds way more complicated in the telling that it would be in operation.

34

That looks workable but, like you Deke, I'd want to hear from the amp gurus around here.

I still don't know how Tim can operate the changes from one footswitch though. How big is the speaker? If it's an 8" maybe you could just go with a cab for each amp?

– Yavapai

How about a 4x10 cab and one speaker for each amp?

35

Wow! That just brought back some memories. My Bird Golden Eagle had a cabinet with 3 x 10" elliptical speakers back in 1964.

36

Billy is right, tube amps don't like to be unloaded and re-loaded at high RPMs. SO that kinda kills the idea.

The Helix could make this a slam dunk, Tim!

37

But these aren't tube amps.

I have a Helix. And a Kemper. Cheap little micro-amps are more funnerer.

38

My design doesn’t unload the amps, not even briefly. There is always an 8ohm load on all amps that do not have the speaker in circuit. As far as the amps are concerned switching the speaker in is just the same as plugging in an extension speaker.

By the way, the clean drawing up above is transparent, so it doesn’t show up too well if you use the black theme here on the gdp.

39

Hi Deke, That is a REALLY cool circuit!! I envy your ability to just sketch out a complex signal path like that! Someone smarter than I will have to look at it, but I'm unsure that a fixed resistance will play well with a tube amp. It normally sees a reactive load. I think that a fixed resistance would be like the output hitting a brick wall. Impedance is a different thing, and even L-pad attenuators ideally don't absorb the entire output (without eventually smoking the amp) The Weber Mass attenuator uses a silent speaker voice coil as a reactive load, so something like that might work - but three of them would make for a very large box. This is all very interesting and thought-provoking.

Tim - I agree with you. I love the Helix, but I still love my amps a tiny bit more.

40

Thanks Josh. The type of ballast load is unimportant as far as the wiring principles go, this is just a simple way to wire it up so the initial goal was achieved. I was thinking a heavy-duty cement 20w resistor, but if that's not compatible with the concept then consider it a placeholder for a more appropriate component.

I understand all amps will be solid state, not tube, does that make a difference?

Parts list will be:

  • 4 x jack sockets
  • 3 x ballast loads (to be determined)
  • 2 x DPDT BBM latching footswitches
  • 1 x stomp box housing

May as well throw in a partridge in a pear tree while you’re at it.

Oh, and a couple of feet of hook-up cable.

41

Deke, you are a brilliant man! We could probably get this sorted for Tim. He'll probably want fancy LEDs, so 3PDT switches will be needed.

I think Solid State amps are more tolerant of load situations. I think. Billy?

43

Can't seem to edit a picture so I have to post it again. Here it is a bit better still. Should show up on black background now.


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