Some speaker impedance theory (Twizza, Trasher)

This may bore the pants off many people so click that "back" button :wink: We've got a "Right, thread's spread" situation with Twizza and Trasher posting Qs about matching/adding cabs to amp outputs. I thought I'd put what I think I know here, so those interested can have a read. I'm not a techno, but I picked up some useful info from people like Walter Harley, Bill Bolton and Jack Read. If this stuff is useful; it's thanks to them. If there's a glaring cock up; that'd be my fault. Do chime in with corrections/additions, people.

The impedance of a speaker driver (the coney thing with a magnet on it) and the impedance of a cab are the same thing as far as your amp is concerned. What I mean is, joining two (or more) speakers together is no different to joining cabs. The amp has no idea about the wooden boxes and couldn't care less.

Speakers can be connected in series or parallel and (with a little trick) so can cabs. Here comes a bit of maths (but it's not difficult). The total impedance, Ohm(total) is really easy to work out when linked in series:

Ohm(total) = Ohm(S1) + Ohm(S2) + Ohm(S3) +...

where S can be either a speaker's impedance or the overall cab's impedance. So, two 16ohm speakers or cabs linked in series presents a total of 32ohms to your amp. A 16 and an 8 in series gives 24ohms total.

When wired together in parallel, the (cheat) maths is slightly harder but not much:

1/Ohm(total) = 1/Ohm(S1) +1/Ohm(S2) +1/Ohm(S3) +...

don't be scared. So four 16ohm speakers wired in parallel makes a four ohm total. Every guitarist learns that 1/4 = 1/16 + 1/16 + 1/16 + 1/16 (from your dealer, if not from your teacher :wink: )

The problem is when you have either drivers or cabs of different impedances. Say a 16ohm cab and an 8ohm cab:

1/Ohm(total) = 1/16 + 1/8 = 3/16. So Ohm(total) = 16/3 = 5.33ohm. Where the heck do you plug that in :?: Actually, Weber makes an output tap for 50W and 100W valve amps that covers 2, 2.4 (2ohm+4ohm), 4, 5.3, 8 and 16ohms - neat, huh?

So, supposing you have a SS amp head that has a 8ohm out and a 16ohm out. For max output, choose one cab that is 8ohm overall or chain (using the extra spkr out jack) two 16ohm cabs together and plug it into the 8ohm output. So how come a Marshall four by 12" cab has 16ohm speakers but the cab's overall impedance is 16 ohms? Well, wire up the pairs in parallel and are now 8ohms each pair. Connect the two pairs together in series and you have a 16ohm cab. The Slash cab that Trasher mentioned; I haven't seen it, but it'll simply be set up to allow plugging in as all-parallel four ohm, standard series/parallel 16ohm or 2 x 8ohm pairs for stereo. I've just found this site by the excellent David Szabados (piccies too!):http://www.legendarytones.com/all_about_ohms.htm

Yeesh! I could've saved a lot of typing just by putting in that URL :oops: :cry: However, I have the opposite op to him on mismatching speaker impedance to OPT. David knows a lot more than I do, so I'll try and check with him on that.

David's site shows you how to wire speakers in parallel and series, but how do you connect two cabs in series? If you connect the extra speaker jack to the input of the other cab, it'll just be parallel, right? The answer is to have made a special cable that goes +ve tip, -ve sleeve on one end to -ve tip, +ve sleeve on the other (clearly label it or, better still, have the insulator in something like bright orange PVC). Otherwise, just reverse the wiring on the other cab's input socket. BTW, David's site makes the excellent point that anything connected in series is a trades union - "one out, all out". If connected in parallel, one component going down doesn't take everything else out with it.

One other thing. Two cabs isn't "stereo", even if you plug the two cabs into the amp's two outputs. If it's a mono amp, it's still a mono amp - the two outputs are there either for different loads or, on older amps, are identical, parallel wired outputs for old cabs that only had one jack socket. You'd switch the tap if you plugged a cab into each. If you have a stereo amp (or use two heads) then that's different; the two cabs are stereo if you plug one into L and the other into R. It's possible to wire a 4x or 2x cab so that each speaker or pair has its own input jack. Trouble with this is that the cab is "stereo" like your little, portable ghetto blaster is "stereo". The L+R separation is not very much and Joe Audience won't get it.

:shock: End of lesson. Sorry about the
lengthy post. Even sorrier I hadn't just looked up that other site first. Hope somebody reads this and makes use of it.

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