Mixing speaker impedances in same cab

[kyldh]

Hi, I've been thinking about building a guitar small guitar cab for practice purposes and I'm going over my options. I'm familiar with some of the basics when it comes to speaker impedance, but not sure how to add it up when you're dealing with a mix.

I have: Two 10" Greenbacks (8ohm) and One 12" Vintage 30 (16ohm)

I figured I'd probably just build a 1x12 cab for the V30, but I got to wondering how it'd sound to have a custom cab that incorporated the 2 greenbacks as well. Curiosity killed the cat I guess.

If I wired all 3 of them up, what would the resulting impedance be? Would parallel or series be better?

OR, if I'm set on this, would it be smarter to just have 2 inputs on the back of the cab? 1 for Greenbacks and 1 for the V30?

Lastly... is this just a terrible idea? I've never heard of many (any?) people doing this with a speaker cab, and maybe that's for good reason I suppose.

If it matters, I'll be using this cabinet with a Mesa Roadster head.

 

[Jackie]

It's completely possible. Though the V30 will probably tonally overpower the two Greenbacks because it's a 12 inch speaker, it has higher sensitivity (100dB versus the Greenback's 95dB) and is more high-mid prominent.

As far as the wiring goes, it's simple, wire the 8 ohm Greenbacks in series to get 16 ohms, and wire the two Greenbacks together in parallel with the V30 to get a total of 8 ohm mono.

 

[MrMarkIII]

I think you mean 16 ohms in parallel with 8 ohms = 5.33 ohms.
Parallel wiring is too math intensive for me, so I looked it up.
The equation for parallel connections is:
Total Load = (S1 x S2) / (S1 + S2)
Where:
S1 = the impedance of Speaker load # 1
S2 = the impedance of Speaker load # 2
( 16 x 8 ) divided by ( 16 + 8 ) = 128 / 24 = 5.33 ohms
Science!

 

[Jackie]

All fine and dandy and the formula is correct, but no, I don't mean 16 ohms in parallel with 8 ohms. I mean two speakers wired in series and the two together wired in parallel with a 16 ohm speaker for a total of 8 ohms.

By wiring the two 8 ohm speakers in series you basically get "one" 16 ohm "piece".

Imagine resistors. You wire two 8ohm resistors together in series and then piggyback a 16ohm resistor across both. Total resistance = 8ohms.

 

[kyldh]

Awesome, thanks for the info guys.

If I go through with this I think I'll either A) Wire all 3 speakers up to 8ohms like you suggested or B) have 2 separate 16ohm inputs on the cab so I can chose to use just the greenbacks, just the v30, or both at once.

If I have a change of heart, the V30 might just go into a 1x12 cab and greenbacks into my old Gibson Medalist 4x10 Combo that badly needs some new speakers.

Thanks again

 

[Jackie]

You could have both - wire two jacks on a mono/stereo switch (like the Marshall 1960 cabs use). I don't know how exactly that is done, but it's a great solution.

Keep in mind however that if you do decide for this option, you might want to be careful with the power handling of your cab. The Roadster is a 100W amp, and the V30 can only handle 60W (the Greenbacks in series, likewise). Though IIRC you can drop the power to 50W on a Roadster.

 

[kyldh]

Good point, yeah I had figured on switching to 50w. This cab would only be used around the house, maybe when recording, or at lower-volume band practices. We sometimes practice with an electronic drumset and turn down the amps. So basically, no situation where I'd need to have the Roadster set to 100w.

Good call on the mono/stereo switch. I'll have to look into that. Like I said, I'm definitely a novice when it comes to wiring up speaker cabs (I've wired up guitars before, but that's it) so I didn't know such an option even existed. Very cool.