Question about Impedance on 2902

Power Amps
1

Hey all,

I was just wondering about the impedance selector on the 2/90/2. If I’m running each side of the power amp into separate 8 ohm cabs, do I set both impedance selectors to 8 ohms? And can I use one 8 ohm cab, and one 16 ohm cab?

2

Hello,

Welcome to the community!

Yes.

Yes, as long as you set the impedance selectors accordingly, this is absolutely safe.

Best regards,
Julian

3

Great, thanks! That’s what I figured. I saw some guys on a forum saying it had to be treated like an amp head running two cabs at once, but that just seemed wrong.

4

Depends on the context. Perhaps he was referring to two cabs connected to one channel.

5

Old thread but I figured it’s worth a perhaps ambitious inquiry. What if I wanted to run my 16 ohm 4x12 on one side and my two 8 ohm 2x12s on the other? Absolutely not gonna try unless I get the Fryette blessing but I figured I’d shoot my shot. Absolutely in love with my 2/90/2 and it running into these particular cabs would be utter sonic annihilation.

Thank you all!

  • Paul
6

You work it out like this: impedance = top / bottom

top = A * B
bottom = A + B

So A = 16 Ohms and B = 8

A * B = 128

A + B = 24

So 128 / 24 = 5.33 Ohms

It’s closest to 4 Ohms, so use that.

7

Actually there are limited combinations with two cabs:

\begin{array}{c|ccc} \| & 4\,\Omega & 8\,\Omega & 16\,\Omega \\ \hline 4\,\Omega & 2.0 & 2.7 & 3.2 \\ 8\,\Omega & 2.7 & 4.0 & 5.3 \\ 16\,\Omega & 3.2 & 5.3 & 8.0 \\ \end{array}

So if you are using these speaker combinations with an amp with a three-way impedance selector, you can either round up to the nearest value.

\begin{array}{c|ccc} \| & 4\,\Omega & 8\,\Omega & 16\,\Omega \\ \hline 4\,\Omega & 4 & 4 & 4 \\ 8\,\Omega & 4 & 4 & 8 \\ 16\,\Omega & 4 & 8 & 8 \\ \end{array}

Or just round to the nearest

\begin{array}{c|ccc} \| & 4\,\Omega & 8\,\Omega & 16\,\Omega \\ \hline 4\,\Omega & 4 & 4 & 4 \\ 8\,\Omega & 4 & 4 & 4 \\ 16\,\Omega & 4 & 4 & 8 \\ \end{array}
8

@dan I think he was asking what about running a single 16 ohm cab on channel A (“one side”) and two 8 ohm cabs on channel B (“the other” side). which would be 16 ohms for channel A and I’m assuming the two cabs for channel B would be in parallel so 4 ohms.

9

Oh yes, missed that crucial detail! You are right @choalla , thanks. So A side would be 16 and B Side would be 4. :+1: