I have a guitar amp that recommends the following for connecting to an external cabinet:

Only use extension cabinets with an impedance of 8 Ohms or more (e.g. 16 Ohms). NEVER use a lower impedance (e.g. 4 Ohms) or damage could result. WARNING: If you use an extension cabinet with an impedance of more than 8 Ohms your Pathfinder 15/15R’s output power will be reduced – it only produces its full output of 15 Watts when presented with an 8 Ohm speaker load.

I have been offered an extension cabinet described as having "Impedance - Switchable 4/16 Ohms mono or 2 x 8 Ohms stereo." The cabinet has two inputs labelled 'input' and 'link'.

Please could someone tell me:

1) Can the speakers in the cabinet be setup such that the cabinet will be 8 ohms running off the one input (my amp)?

2) If the above is not possible and the cabinet is 16 ohms, will I lose a lot of power by running the 8 ohm amp into it?


1 Answer 1

  1. Yes, but it's not really practical. You can use a 2:1 transformer to drive the speakers in parallel (i.e. 4 Ω mode), but such an extra transformer is unwieldy and may well colour the sound substantially.
  2. You will lose 50% power. That may sound like a lot, but this is actually only a 3 dB reduction – if you've ever used a mixing console you'll know that it's actually a rather subtle change. (This is due to the way our ears perceive loudness logarithmically, and because power is quadratic in amplitude.) So, you may still be fine. 15 W is of course not that much in the first place, but even 7 W can be pretty loud, at least if you allow the amp to go into full distortion.

There's a third option that may be best for you: simply use only one of the cabinet's speakers (i.e. in stereo mode, but with only one side connected). This has then the optimal 8 Ω impedance, so it will actually be louder than if you use both speakers in series.

There's theoretically also the option of using the cabinet in 4 Ω mode, but in series with another 4 Ω power resistor. But that gives the same volume as the 16 Ω solution, while generating more waste heat...

  • 1
    Most likely, the cabinet has two 8-ohm speakers. In stereo mode, one speaker is connected the left channel and one to the right. In the mono modes, either they are wired in series (giving 16 ohms) or in parallel (giving 4 ohms). Unless you are using some effects that give a (pseudo) stereo output from the guitar, as @leftroundabout said the simplest option is just to connect one stereo channel to your amp and leave the other channel unused. Assuming the speakers can handle it, that will give you the maximum output power from your amp.
    – user19146
    Jul 24, 2016 at 1:33
  • If the OP really wanted to use all speakers, he could probably have the amp's output jack rewired so that it didn't disconnect the original speaker. Guessing that would have 8 ohms, so with the extension cab would give 12 ohms, I think.
    – Tim
    Jul 24, 2016 at 6:13

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