Help! Need a check on my wiring diagram for a 2x12" cab please

Gorgon90

Gorgon90

Harmonic Dive Bomber
I have a home built 2x12" cab, currently wired in stereo with 100w 8 Ohm Celestion drivers. I want to wire it so that it can be 4 Ohm Mono, 8 Ohm Stereo, or 16 Ohm Mono. My school level electronics means I'm aware to get 4 Ohms I need to put them in parallel, and to get 16 Ohms they need to be in series, off one jack in both cases.

I've tried googling for a decent circuit diagram to but not found one that covers all three options so far. I believe I can do this with a switching jack and a 2 way double pole switch and I've (badly) drawn up the following and if we've got any wiring experts around, who can confirm this is logically right I'd appreciate it.

Full wiring:

Speaker Wiring All.png




4 Ohm Path
Speaker Wiring 4 Ohm.png



8 Ohm Path
Speaker Wiring 8 Ohm.png



16 Ohm Path
Speaker Wiring 16 Ohm.png


If I'm right the 2 way switch would need to be one way for 4 Ohm Mono / 8 Ohm Stereo and the other way for 16 Ohm Mono. 4 Ohm and 16 Ohm Mono use should always be through Jack 1. It would be possible to use a single speaker (speaker 2) as 8 Ohm Mono with the switch in the first position and using jack 2 only.

If all this is correct, what rating do I need on the double pole switch as I've no clue about what amount of amps is likely to be going through this?

If there's a better/safer way to wire this up, I'd also greatly appreciate that info as well.
 
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Your design visualisation is... unorthodox, lol. I think you've pretty much figured it out, but have you accounted for both jacks being plugged in and the switch being in the 16ohm position? I think you need to ensure nothing bad can happen there. Here's what I've scribbled down, btw, which I think is what you were going tor. Still doesn't account for the switch being in the 16 ohm position and both jacks being in use.
 

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irishstu said:
Your design visualisation is... unorthodox, lol. I think you've pretty much figured it out, but have you accounted for both jacks being plugged in and the switch being in the 16ohm position? I think you need to ensure nothing bad can happen there. Here's what I've scribbled down, btw, which I think is what you were going tor. Still doesn't account for the switch being in the 16 ohm position and both jacks being in use.
Click to expand...

I knocked up mine (badly) in Visio as I'm not up on the right electronic squiggles, yours appears to be the same thing if I understand it correctly.

I am aware that the switch being in the 16 ohm position, only Jack 1 should be used as I've drawn it.

If only Jack 2 was used when the switch is in the 16 ohm position, there would be no circuit as the red switch would break it on one side before getting to speaker 2, while the other connection would go through both speakers, back to Jack 1 but then be broken by having a plug in Jack 2.

Plugging into Jack 1 and Jack 2 with switch in the 16 Ohm position would create a no load path from Jack 2 to Jack 1 and probably a load of other bad things that would do nasty stuff to both amps and speakers probably!

What I haven't figured out, is if there is a safe way to wire this so that is protected automatically? Otherwise it's a case of remembering to use Jack 1 only for mono.

Wiring this for just 4 and 8 Ohms would make this simple as I wouldn't need the switch and the jacks would take care of it. The worst that could happen then is I get one speaker at 8 ohms rather than 4 if I plugged into Jack 2. However, it seems a waste not to give myself the option...
 
I was in the middle of writing something about your switchable jack maybe being able to provide a safety by breaking the 16ohm circuit somewhere, but I'd need to think about that. Another possibility would be to forget about the switchable jack and doing all three positions via the switch. Now that I think about it, maybe you could still use the switchable jack and a (still double-throw) switch with three (or more) poles in each position and wire the extra poles to account for mistakenly having the switch in the 16ohm position with both speakers plugged in. Again, I'd need to think about that.
 
So... just after an initial look, you have three options here:
1. If you accidentally have two speakers plugged in, and the switch is set to 16ohms, you force it to stereo 8ohms - this is the trickiest to achieve I think.
2. If you accidentally have two speakers plugged in, and the switch is set to 16ohms, you force it to mono 16ohms - this is easier to achieve I think.
3. You forget about a switchable jack and have a three position switch and you always have both speakers plugged in and it just works and you don't need to worry as much (as long as you don't start unplugging speakers, which you should never be doing anyway even regardless of this particular project).
I think I'm trying to sell option 3 to you, ha. I do think 2 is not difficult to achieve, though. 1 probably is doable, but complicated.
 
Actually there might be something else that makes option 3 complicated, now I think about it. When the second speaker is plugged in, the amp is expecting to output stereo. Messing with that may mess with the amp itself. That might also make option 2 complicated.
ALSO, the way the amp knows whether or not to go with mono or stereo output is quite possibly controlled by a switchable jack already, which might complicate option 1! Sorry to piss on your cornflakes, @Gorgon90 :undecided:
 
Thanks for having a think about it @irishstu. To be honest, given different amps need different impedances anyway, if the switch was in the wrong position, it could all wrong anyway. On that basis, and as my main use is going to be for 4 ohm mono and 8 ohm stereo, I think I might just go for my original design and put warning labels on it!
 
@irishstu - any idea on what type of rating I need on the switch? Looking at CPC and I know I need a 2 way switch with 6 terminals (effectively 2 sets of 3 being worked at the same time) but no idea what volts/amps/wattage ratings I need on this. As the speakers are 100w 8ohm each, I think that means the cab would work as follows if that helps:

Mono/Parallel - 4 Ohm - 100 Watt
Stereo - 8 Ohm each side - 100 Watt each side
Mono/Series - 16 Ohm - 200 Watt

Cheers.
 
Hey @Gorgon90 , I'd say for wattage, go for something over 200W (honestly I'd just double that, even though that's probably entirely unnecessary).
To calculate amps (I) from wattage (P) and resistance (R), use the formula I = √(P/R). I guess do that for the three different outputs and use the highest one. Again, go for a value healthily bigger than that.
To calculate voltage (V) from wattage (P) and resistance (R), use the formula V = √(P × R). Same goes for working out all three, choosing the largest and then increasing it a bit. You're interested in AC voltage ratings, btw, not DC.
I did see a thread on reddit where people were discussing something similar to this and really getting into the quality of one switch brand over another. I'm not sure how pro you want to go with this, but it might be something you want to look into. I'm the sort of person that would probably go for the cheapest "perfectly decent enough" option, but that's just me.
One thing I'm sure you're aware of. I wouldn't ever advise using the switch while the amp is on.
 
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