Speaker Cabinets and Ohms Ω

Difficulty: Spectrum

I've just been out and bought an amplifier head (an old 1973 Marshall JMP50, sounds lovely) and realised that I STILL did not really understand how all this Ohm stuff works when it came to speaker cabinets... after playing electric guitar for more than 20 years and having had it explained to me a dozen times!

So I've just spent a few hours reading up on all this and I'm going to try and explain it to you and hope that you will actually understand it :)

The basic idea is that when an amplifier sends a signal out it is important to match the speaker and the amplifier so they are set with the correct Ohm setting. It is REALLY important with valve amps, you can do lots of damage if you get it wrong. Transistor (solid state) amps are more forgiving.

If you only have one speaker box, then I guess it's pretty easy - most amp heads have a switchable Ohm (Ω) setting, so you look at your speaker box, and see what the Ohm setting is, and set your amp to that. But it get's a bit more complex when you have more than one speaker box and there are other things to consider.

AMP RULE: Always use SPEAKER cable, NOT GUITAR cable.

The may look the same but they are not. Use the right tool for the job.

VAVLE AMP RULE #1: Don't turn the amp on without the speaker plugged in.

Ignoring the rule above can screw up your amp pretty quick, so just don't do it. There are some "power brakes" that you can use (THD Hot Plate is great) but you should still never just turn it on without speakers plugged in.

Impedance (Ohms and stuff)

Impedance measures the resistance that occurs when electricity is flowing to a load (speaker). It is shown by the greek character Omega Ω (like the watch logo!). The name kinda gives it away, the signal is impeded.

Amps are designed to work with a set amount of resistance (impedance) but many amps have switches for different ohmages on the back so can work at various levels. It is something you need to check when you are putting different heads and cabs together.

The higher the number of Ohms in a speaker or cabinet's rating, the more resistance to the flow of power and less power will reach the speaker.

VAVLE AMP RULE #2: Make sure the Ohm setting on the amp is the same as the speakers (or cabinet).

A valve amp with the wrong Ω load might still sound ok, but may be causing excess wear on the transformer, cathode resistors and valves = expensive repair bill.

TRANSISTER AMP RULE: Read the manufacturers guide cos they are all different :)

With solid state amps, setting the Ohm rating is not quite as important. It effects the volume. Plugging in a speaker with a higher Ohm rating (and leaving the volume the same) will produce less volume (a 16Ω speaker will be quieter than an 8Ω speaker when plugged into the same amp at the same volume setting).

Speakers in Series vs Parallel

Speakers can be wired in series or parallel. You can also load your amplifier the two ways as well.

Wiring in parallel means the signal is sent to the speakers at the same time. Or from the amp into cabinet 1, and from the amp to cabinet 2 - of course you will need an amplifier with two speaker outputs for this to work. This is the most common way to load up your amp, and most amps that have an "extension speaker" output will be offering it as a parallel output - but you need to read the owners guide (or see the web site) to make sure.

In series the signal goes from the input to one speaker then out into another speaker. Or from the amp into one cabinet and then out of that one and into another cabinet.

Why is this important? because it changes how the loads add up.

Parallel: Two 8Ω cabinets both plugged into an amp = 4Ω load

In Parallel you simply divide the total Ohm rating by the number of speakers (or speaker boxes). So if you had 2 16Ω Boxes then you would have a total of 8Ω. Got it?

Series: One 8Ω cabinet plugged into another 8Ω Cabinet then into the amp = 16Ω.

In a series you just add up all the values of the speakers or cabinets. Kinda easy to work out that one...

So if you have two 8Ω cabs and your amp will run at either 4Ω or 16Ω which should you choose?

I have NO IDEA :)

OK, rang my old mate Pete Whittard (the man who knows about all this stuff) and he said... try it and see :) His initial thought was that parallel might be better because if one speaker blows the other would still work (if they were in series they would both stop). But then in Parallel the impedance will go wrong, and you'd have to stop and change the Ohm setting anyway!

So we still don't know which is better. If you do - them please let me know on the forum! Can someone please ring up Eric Johnson and ask him, cos he is sure to know...

He also said that it's better to have the impedace too high on the amp than too low - having the Ω set too low on a valve amp means it will overhead and get screwed up very quick... but my advice - match them!! Don't even think using them if the ohm settings don't match up.

Have further comments, questions or suggestions to do with this topic then please leave them on the forum!

Hope that helps. J.

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