Impedance match with parallel cabs and solid state amps

[DaLaP21]

I think this is a very specific situation:

I have a VOX BC112 cab that runs at 8 ohms, and has 2 input/output jacks for 8 ohms each that have a parallel circuit (the owner’s manual states that if you run another 8 ohm cab there, you’ll reduce the impedance to 4 ohm).

I have a solid state amp that says “minimum 8 ohms” in the loudspeaker section.

If I want to run my cab with another cab in parallel (as the only available option), what should the impedance be on the other cab for it to equal to 8 or 16 ohms?

 

[Chocol8]

Is it a single channel or stereo amp?

There is no way to combine another (single) cab in parallel to an 8 ohm cab and not get less than 8 ohms. Two 8 ohm cabs in series would be 16 ohm but you will need to rewire the amp jack, use a serial box, or somehow wire them in series without using the parallel out jack on the existing cab.

 

[jay mitchell]

If I want to run my cab with another cab in parallel (as the only available option), what should the impedance be on the other cab for it to equal to 8 or 16 ohms?

If the first cab is 8 ohms, and that's the minumum impedance your amp can drive, you can't add a second cab of any impedance in parallel. No matter the impedance of the second cab, the combined impedance will be less than 8 ohms.

 

[DaLaP21]

Is it a single channel or stereo amp?

There is no way to combine another (single) cab in parallel to an 8 ohm cab and not get less than 8 ohms. Two 8 ohm cabs in series would be 16 ohm but you will need to rewire the amp jack, use a serial box, or somehow wire them in series without using the parallel out jack on the existing cab.

Single channel, it’s the Laney loudpedal.

Interesting. So if I were to “mod” it and change the way of the wiring, it could be paired up with another cab but only if I manage to change the wiring to series, right?

Also, what is a serial box?

 

[Nathan Explosion]

Also, what is a serial box?

Series Box

The official Mesa Boogie online store for factory replacement parts, custom parts, hardware, shirts, hats and accessories.

store.mesaboogie.com

 

[Chocol8]

Also, what is a serial box?

Autoincorrect version of a series box!

 

[laxu]

Autoincorrect version of a series box!

I eat Captain Crunch's serial boxes for breakfast!

Anyway, the best solution for OP would be two 8 ohm cabs in series for 16 ohms using something like a Palmer Cabmerger. You will lose output power on the amp though, but this might not be a too big issue.

 

[Baba]

I also have this concern with using my BluGuitar Amp 1, BUT, luckily my BluGuitar 8-ohm 1x12 has both serial AND parallel out jacks, so, not a problem for me to use that in conjunction with my 16-ohm H&K 2x12.

The series box would be a good solution for you OP.

 

[jay mitchell]

The series box would be a good solution for you OP.

Or it might not be (probably is not). Adding a second cab in series will reduce the maximum power the amp can produce. If the second cab is identical to the first, the power from the amp will be reduced by half. To oversimplify by a bit:

Power = V^2/Z
=> V^2/(2Z) = 1/2(V^2/Z),

where V = the maximum voltage the amp can produce (which won't change with the added cab), and Z = cab impedance. IOW, the amp will produce 3dB less power into two cabs in series. Best case, if if the two cabs are placed side by side, mutual coupling will add 3dB to the overall sensitivity, but only at low frequency, the net result being that low-frequency output will be essentially unchanged, and output at higher frequencies will be reduced.

Maybe it's just me, but I'm generally not looking for less volume when I add a cab....

 

[bucephalus]

I eat Captain Crunch's serial boxes for breakfast!

Anyway, the best solution for OP would be two 8 ohm cabs in series for 16 ohms using something like a Palmer Cabmerger. You will lose output power on the amp though, but this might not be a too big issue.

You might want to eat the contents instead of the box though. Game changer when I tried it.

 

[Baba]

Or it might not be (probably is not). Adding a second cab in series will reduce the maximum power the amp can produce. If the second cab is identical to the first, the power from the amp will be reduced by half. To oversimlify by a bit:

Power = V^2/Z
=> V^2/(2Z) = 1/2(V^2/Z),

where V = the maximum voltage the amp can produce (which won't change with the added cab), and Z = cab impedance. IOW, the amp will produce 3dB less power into two cabs in series. Best case, if if the two cabs are placed side by side, mutual coupling will add 3dB to the overall sensitivity, but only at low frequency, the net result being that low-frequency output will be essentially unchanged, and output at higher frequencies will be reduced.

Maybe it's just me, but I'm generally not looking for less volume when I add a cab....

1,000%

I actually know this, in the non-technical sense, by physically trying it.

I think my thoughts came from this excerpt from the Blugepedia: "The total output is reduced by increasing the total resistance, but the volume usually increases by increasing the speaker surface."

But again, I totally agree with you, even if I don't understand the math behind it, because I've heard it.


Side note: I'll never forget the time when I hooked up a 4 ohm 2x12 cabinet to my Line 6 Flextone amp's 8 ohm speaker out, and was wondering why my amp kept cutting out/off, at band volume

Thankfully, FOH was getting the XLR out from the amp, so the SPEAKERS were the only thing cutting out, but, (speaker resistance) lesson learned, the hard way, lol.