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Discussion Starter #1
I have a 2 x 10 cab with two different 8 ohm speakers in it.

I'd like to be able to select one speaker, or the other, or both, using a switch.

It took me a little while, sketching up different circuits, but I think I've got something that will work with just a simple DPDT switch.

Am I missing something here? Or is this actually going to do it?

20180315_141305.jpg
 

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Am I missing something here? Or is this actually going to do it?
View attachment 181993
Maybe I'm having a brain fart, but that doesn't look like it will work to me.

I see it working in the middle/series position (assuming you're using an on-off-on switch). The paths for the other positions don't even make it to the other end of the jack. Maybe I haven't had my coffee... What am I missing?

I'd use an ON-ON-ON switch and wire it as follows:
Wiring Diagram - 2S Split-Series.jpg

The "red" lines indicate the path of the middle position on a standard ON-ON-ON switch. You may need to wire it the opposite way, depending which type of switch you have.
 

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In an dpdt on-off-on type switch middle is no connection at all, and it is wired above to have both working in series, with no switch connections. The reason this does work in the other positions (e.g. down position) is because despite the diagonal jumper always feeding + from sp1 to sp2, and sp2 always being connected to -, the path to - is shorted (around/bypassing sp2) by the switch (in down position). Electrons will take that path of least resistance (approaching 0 ohms vs the 6+ohms DCR of sp2's voice coil). Personally, I don't like it (prefer to break the connection vs create a short around it) and would do it differently, but it works.

Also, wouldn't on-on-on be, by defenition, _P3T?
 

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Discussion Starter #6
Right, like GG indicated, the default signal path goes through both speakers in series, and then I use the switch to jumper across the + and - terminals of one speaker or the other (to bypass it) depending on the switch position.

If there's a better way to do this that is still relatively simple but disconnects the speakers completely, I'm wide open to that idea.

(edit - duh, I just saw jbeals diagram - Granny, are you suggesting that's a better design?)

I didn't realize there were different types of DPDT switches besides "center off". The ones I have are labeled ON-off-ON and they seem to do what i was expecting when I check them with a continuity tester.

Related question - how much current would be passing through here (ballpark)?

For the moment, I'll be using this with an old Blackface Fender Deluxe - a pair of 6V6 tubes at about 23 watts, I believe.

But I can imagine other scenarios in the future, and I might as well build it to handle up to 150 watts or so that the speakers can handle. I'm using an Eminence Ramrod and an Eminence Copperhead in this cab - they're both 75W speakers afaik.

The switches I have handy are quite large and beefy - they're all stamped like this:

10A 125VAC
6A 250VAC

Is that lots for this application?
 

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@jbealsmusic 's diagram would only work with the type of switch he is suggesting be used. But yes, I would consider it better as it eliminates any connection to the unused speaker.

Yeah, most DPDT switches are on-on with no middle position. on-off-on is kind of a cheat - more accurately it would be called 2p2.5T or something because there are more than 2 switch positions, but it's not a 3T because that third position doesn't make a contact.
 

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I am so hopeless at the 'logical thinking' required to sort out multi terminal switches and the associated wiring diagrams. This discussion is very interesting and helpful.
 

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Discussion Starter #9
@jbealsmusic 's diagram would only work with the type of switch he is suggesting be used. But yes, I would consider it better as it eliminates any connection to the unused speaker.
Right, understood. I tried sketching something out that would work with the ON-OFF-ON switch I have, but I haven't come up with anything that actually disconnects the unused speaker. Best I can figure out is a circuit that shortcuts past a speaker that is connected.

Yeah, most DPDT switches are on-on with no middle position. on-off-on is kind of a cheat - more accurately it would be called 2p2.5T or something because there are more than 2 switch positions, but it's not a 3T because that third position doesn't make a contact.
Wouldn't a DP switch with no middle position be a "single throw" switch? i.e. a DPST?

Maybe I don't understand what "throw" means in this context.
 

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Discussion Starter #10
Wouldn't a DP switch with no middle position be a "single throw" switch? i.e. a DPST?
Huh. No, it would not. (answering my own question, lol)

A DPST would connect or disconnect two independent circuits ("poles").

So the switch I have is probably called a "DPDT-center off" or some such label.
 

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Yeah, most DPDT switches are on-on with no middle position. on-off-on is kind of a cheat - more accurately it would be called 2p2.5T or something because there are more than 2 switch positions, but it's not a 3T because that third position doesn't make a contact.
Much respect brother, but I have to jump in here regarding switch naming. The number of throws and the number of switch positions, while intuitively synonymous, are not actually the same thing at all.

RE: How Switches Are Named
Pole = The number of separate/individual circuits the switch can control. A switch with one pole can only modify the chain of one circuit. A switch with 3 poles can modify the chains of 3 circuits.
Throw = The number of terminals each pole can be connected to. Or, practically, the number of viable connections that each "common" can make within the switch.

A DPDT switch can come in on-on, on-off-on, and on-on-on. All 3 would still be considered a DPDT switch because in all of those cases, the common lug can only connect to 2 other lugs, giving it dual throw options. An open/unconnected position doesn't count as a throw because no connection is made. A DP3T would actually have 8 lugs.

We spent countless hours researching switches for our "Inside Out" video on switches. Most of the info didn't even make it in there to keep the length more reasonable. Check it out if you get ever have 5 minutes to lose: WATCH HERE

Back to the wiring question...
(edit - duh, I just saw jbeals diagram - Granny, are you suggesting that's a better design?)
The light clicked on as soon as I read GG's initial response regarding taking the path of least resistance. Your initial diagram will work. Missed it on my first viewing, but it has clicked in now. Didn't even need that coffee! ;) As for which is better? Someone more qualified will have to answer that. Like GG, I break/open connections rather than creating shorts around them. Don't know for certain whether or not that is "better", or if that would even be the right word to use in this context.

Yes, the signal will follow the path of least resistance. However, some current will still flow to the unused speaker simply because there is a connection there. However, in all practical application, the amount may be entirely negligible. It'd honestly be interested in testing to see if there would be a difference between the two wiring schemes. Both audible and measurable. That would be interesting.

In any case, your existing diagram will work so wire it up and let us know how it works out!
 

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Yes, the signal will follow the path of least resistance. However, some current will still flow to the unused speaker simply because there is a connection there. However, in all practical application, the amount may be entirely negligible. It'd honestly be interested in testing to see if there would be a difference between the two wiring schemes. Both audible and measurable. That would be interesting.

In any case, your existing diagram will work so wire it up and let us know how it works out!
The only way any current could flow through the speaker is if there was a resistance across the short. Otherwise, the speaker is effectively out of the circuit. If one were to want to experiment to prove it, just replace the connected speaker with an 8 ohm load and plug it in to an amp.
 

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Much respect brother, but I have to jump in here regarding switch naming. The number of throws and the number of switch positions, while intuitively synonymous, are not actually the same thing at all.

RE: How Switches Are Named
Pole = The number of separate/individual circuits the switch can control. A switch with one pole can only modify the chain of one circuit. A switch with 3 poles can modify the chains of 3 circuits.
Throw = The number of terminals each pole can be connected to. Or, practically, the number of viable connections that each "common" can make within the switch.
Thanks for that. "throw" is misleading in that way because as you point out it litterally suggests positions vs contacts.
 

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Agree, the word throw is a bit misleading. Single throw would only have 2 lugs for a SPST or 4 lugs in a DPST. In one switch position the center lug(s) would connect to nothing.
 

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Discussion Starter #15
Trying to solve my own question about the amount of current passing through this switch...

I know that R = V/I (Ohm's Law) and Watts = Volts x Amps (by definition).

I believe R should be roughly 6 Ohms here for a single 8 Ohm speaker, and the power handling is 75W

V / I = 6 Ohms
V = 6 Ohms x I

V x I = 75W
V = 75W / I

Since V = V, I can expect that:

6 Ohms x I = 75W / I
I x I = 75W / 6 Ohms
I squared = 12.5
I = 3.54 A

multiplying both sides by two (for 12 Ohms and 150W) doesn't change anything - the current is still about 3.54A

I can also solve for V using either of the starting equations (just for interest's sake):

V x 3.54 A = 75W
V = 21.2V

6 Ohms = V / 3.54 A
V = 21.2V

So the switch needs to handle about 3.54 A at 21.2 V.

The switches I have are rated for 10A @ 125VAC and 6A @ 250VAC, so that should be MORE than adequate.

Right?
 

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LOL, sorry, but I just went through that worry last week. I've decided to stop worrying but since I haven't actually flipped the thing on yet to see if I release the smoke I'm not gonna comment any further.
 

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So the switch needs to handle about 3.54 A at 21.2 V.

The switches I have are rated for 10A @ 125VAC and 6A @ 250VAC, so that should be MORE than adequate.

Right?
Right. Or close enough. ;)
The 6 ohms is a DC resistance but you will be working with the speaker impedance. The impedance curve is all over the place depending on frequency (example on this page: Speaker impedance curve explained with examples - Audio Judgement ).
Your switch has enough leeway in the rating that you will be fine.
 

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Discussion Starter #18
Ok, so recalculating using the nominal impedance (8 ohms), instead of the DC Resistance, I should see about 3.06 A of current at 75W.

The idea that impedance varies by frequency is new to me.

Higher impedance shouldn't be a problem for this switch, since that will result in a lower current.

LOWER impedance is potentially problematic, since that will INCREASE the current, so I'll solve for a lower-than-expected impedance value to test my limits here.

The example given in the article shows a 4 ohm (nominal) speaker dropping as low as 3.2 Ohms at about 10Hz.

I can imagine that I might put 4 ohm speakers in this cab someday (not LIKELY, but it's possible), so I'll check the math against a 100 watt speaker with a 3 ohm min impedance... and the formula says... 5.77 A.

So.. the switches can take it.

I kinda figured they would - the way they look and the way they're stamped makes me think they're probably intended for interrupting AC supply voltage.

Interesting topic.

I had a fuzzy idea of how the output transformer took us from a high voltage / low current scenario on the tube side to a low voltage / high current scenario on the speaker side, but I didn't have any sort of ballpark idea of how MUCH current that could be.
 
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