Want to build an amp/cab switcher - advice please

[Greg_L]

Hello all, I want to build this for my home studio:



Instead of constantly reaching behind amps and cabs to switch things around, I think this little gizmo could make life much easier. The idea is to just leave everything connected all the time and just turn switches to select amps and cabs.

Most of my amps are 50w and 100w heads. One of them is 22w.


What kind of consideration do I need to give to the rotary switches? Voltage? Amp rating?

What kind of dummy load can I maybe incorporate into this thing to safeguard against operator errors, and how to do it? (I will be the only one operating it).

In my simpleton mind it seems very simple. That means I'm probably missing something.

Any ideas or help is appreciated.

[John_H]

I would suggest a patch bay. It would be much easier, especially when trying to protect the amplifiers by using switching jacks with a dummy load.

[Greg_L]

I would suggest a patch bay. It would be much easier, especially when trying to protect the amplifiers by using switching jacks with a dummy load.

But then I'd have to manually plug and unplug cables. I wanna just twist the dials and leave everything permanently connected.

[g1]

Are you talking about powering down to make changes? If not, it gets a lot more complicated.

[Greg_L]

Are you talking about powering down to make changes? If not, it gets a lot more complicated.

Yes, definitely. I'm not leaving all the amps on at the same time. It's still just one amp and one cab at a time. I just don't want to keep reaching behind everything all the time to switch speaker cables around.

I'm thinking keep all the amps plugged into the box, and all the cabs plugged into the other side of the box. One rotary switch to pick an amp, one rotary switch to pick a cab.

[eschertron]

At a bare minimum, you'd want enough switching contacts to short out or add a resistive load to any amp not selected to the cabinets. Otherwise you run the risk of mistakenly running an amp into an 'open' load. Potentially fatal for the OT.

Another way to approach it would be a set of contacts that allow power to the amp that is selected in the switcher. Requires relay control, but the individual components can be easily sourced.

[Greg_L]

At a bare minimum, you'd want enough switching contacts to short out or add a resistive load to any amp not selected to the cabinets. Otherwise you run the risk of mistakenly running an amp into an 'open' load. Potentially fatal for the OT.

Another way to approach it would be a set of contacts that allow power to the amp that is selected in the switcher. Requires relay control, but the individual components can be easily sourced.

I'm not worried about the "open load" aspect. Mainly because it truly will be just me using it. I'm not saying I'm mistake-proof, it's just my risk I'm willing to take for simplicity of design.


Can yall see my drawing in the first post? Since speaker cables will be connected to each jack at all times, I'm not sure how to wire in a safety dummy load. I really just want to be able to turn the knobs and select what amp I want going to what cab, and then power it up.

[John_H]

But then I'd have to manually plug and unplug cables. I wanna just twist the dials and leave everything permanently connected.

Another idea: Switch the amps with on/on toggle switches. One position going to the speaker cabinet switch, the other going to a dummy load.

The patch bay that I envisioned would be a row of 1/4 switching jacks for the for the amps, and a row of jacks for the cabinets. One little speaker wire patch cable going from the amp you want to the cab you want.

Doing this, you would be able to run two amps/cabs simultaneously if you wanted to jam with your friend John when he comes by, or plug any other amp in easily.

[eschertron]

One little speaker wire patch cable going from the amp you want to the cab you want.

I definitely think the visual feedback of a cable plugged into the jack from amplifier "A" would be a reliable way of ensuring the desired amp was selected.

Unless one envisions a panel space with a row of input jacks and a row of output jacks, and a couple 50s sci-fi set bakelite knob/indicators that can be seen from across a dimly lit room. That would be good visual confirmation too... and take up as much space if not more than the patchbay method.

edit: I'm not being flippant. Thinking about my circumstances, I'd be hesitant to rely on a

little gizmo

if it weren't in plain sight and clearly readable while I was in the heat of artistic fervor.

[Greg_L]

Hmmm. I guess a patchbay is a pretty good idea. Less moving parts - no rotary switches. And the positive fool-proof verification that things are connected correctly. Good ideas, thanks.

More jacks though. I'll probably need a bigger enclosure. Can all of those switching jacks just go to one dummy load? Or do I need a dummy load for each jack?

[John_H]

Hmmm. I guess a patchbay is a pretty good idea. Less moving parts - no rotary switches. And the positive fool-proof verification that things are connected correctly. Good ideas, thanks.

More jacks though. I'll probably need a bigger enclosure. Can all of those switching jacks just go to one dummy load? Or do I need a dummy load for each jack?

I would wire the cables directly to the jacks. It's just speaker wire, and it would make for fewer mechanical connections to get dirty. The dummy loads would need to be isolated. One for each jack.

[Greg_L]

I would wire the cables directly to the jacks. It's just speaker wire, and it would make for fewer mechanical connections to get dirty. The dummy loads would need to be isolated. One for each jack.

Oh yeah, okay. So I can make speaker cables and run them directly into the box through grommets or strain relief. The other ends will be connected to jacks for "patching". I'll have 7 on the amp side, 3 or 4 on the cab side.

Why can't I use just one dummy load though? I was thinking the shorting switch tabs on each switched jack can run to one dummy load for amp-side jacks that aren't being patched. Why does each jack need it's own dummy load?

[John_H]

Why does each jack need it's own dummy load?

If the dummy load was on a common bus, the result of turning on the wrong amplifier would feed current backwards into the other amps. They need to be isolated.

[Greg_L]

If the dummy load was on a common bus, the result of turning on the wrong amplifier would feed current backwards into the other amps. They need to be isolated.

Okay, I see. In essence they'd all be parallel connected to each other through the single dummy load.

So I'd need 7 dummy loads. Damn, that raises the cost and size and complexity. Good safety net though.

So far a starter parts list, I'm thinking

Some kind of enclosure
~ 100ft 16awg speaker cable/lamp cord
At least 10 1/4" TS male jacks
7 switched 1/4" jacks
4 standard mono 1/4" jacks
7 16 ohm 100w dummy loads
Grommets and/or strain reliefs bushings
And a partridge in a pear tree

So this thing will kind of end up looking like a stage snake - a box with a bunch of cables sticking out of it.

[eschertron]

If the 7 switched jacks short when the patch cable is pulled, the you have the same safety rating that a typical amp head has for its speaker output jack. You could consider not using load resistors in this case.

The added bonus from making the patchbay is that (as pointed out earlier by John_H) if you have 3 speaker cabs, you could have up to three amps all plugged in and running simultaneously.

I'm a little envious at this point. My music room is cramped enough that if I have more than one speaker cab pulled out from the equipment closet (literally, a repurposed clothes closet) then there's hardly room to swing a guitar. Sometimes I have to move a cabinet out of the room temporarily to swap stuff around. sigh.