Question about speaker resistance and stuff...
June 2, 2010 4:42 PM   Subscribe

Using two cabs with my guitar amp- which outputs should I use?

I have a Peavey Butcher. Speaker outs each have two labels:
1: single 16 ohm or first 16 ohm
2: single 8 ohm or second 16 ohm
3. single 4 ohm or third 16 ohm
4. no single cab or fourth 16 ohm
I usually get by fine with a single 8 ohm 4x12, but I'd like to play around with my other cabs. In the manual for the head, under connecting multiple speaker enclosures, it basically says 'if you're using cabs that aren't 16 ohm then there's too many combinations, you do the math'.
So, if I want to use two 8 ohm cabs, which outputs do i use? What if i want to use an 8 and a 16?
I can always rewire my cabs to all be 16, but I'd rather not.
Thanks!
posted by gally99 to Grab Bag (7 answers total)
 
Try this link.

I was going to try to explain it but that site makes very easy work of it. Rock on!
posted by CoinOp at 5:27 PM on June 2, 2010


I understand resistance math when wiring speakers, I'm more confused about multiple speaker sets- each will be getting signal from a different output. I don't know if the outputs from the amp are in series or parallel or ???
And how does plugging into one output effect the needs of the next output down the line?
Thanks though!
posted by gally99 at 6:29 PM on June 2, 2010


It's parallel. Three 16-ohm cabs in parallel would equal 4 ohms, the value of the third output. Series wiring is usually reserved for the inside of a cabinet.

One key to remember when doing your calculations is that the ohm value on the label is a minimum. You should never go less than than the ohm value of an output, but you can go higher than that without any issues. Higher impedance (ohms) means the amp puts less power into it, kind of like underclocking a CPU, so the sound won't be quite as loud but it'll also run much cooler. On the other hand, lower impedance means that the power amp has to work harder. If it's expecting 8 ohms and you give it 4, it'll have to work twice as hard because it meets half the resistance, and you may end up overheating or burning out the power amp.

So the two principles are: 1) Each individual speaker cabinet in the combination should be plugged into a jack that's equal to or less than its impedance value (so don't put an 8-ohm speaker in the first output no matter what combination you use), and 2) the last cabinet in the chain should be plugged into a jack that is equal to or less than the total impedance value of the combination.

With two 8's, use the second and third outputs for the second cabinet, as the total impedance will be exactly 4 ohms. Don't plug an 8 into the first output since it's less than 16.

With an 8 and a 16 in parallel, the total impedance is 5.33 ohms. Sometimes parallel combinations will give you values that you won't see on the labels, so if you have an odd ohm result then just make sure that the last output jack used has a lower value than the total impedance. In this case, you'd plug the 16 into the first output and the 8 into the third output, so the amp would expect 4 ohms and would see 5.33. One thing to be aware of, though, is that if you're using different valued cabs then the volume will be different between them. In this case the 16-ohm cab will get half the power of the 8-ohm, so the 8 will be much louder.

Disclaimer: This is all just based on my knowledge of speaker impedance without any specific experience with your model of amp. I'm assuming that it works this way since that makes the most sense to me, and since they didn't give any specific instructions on mixing cabs but they did say it was possible. As long as you always plug each individual cabinet into an output whose value is equal to or less than the cab's value, the worst-case scenario is that you won't get any sound at all - you're not at risk of overloading the power amp or of blowing the speakers.
posted by relucent at 6:20 AM on June 3, 2010


Higher impedance (ohms) means the amp puts less power into it, kind of like underclocking a CPU, so the sound won't be quite as loud but it'll also run much cooler. On the other hand, lower impedance means that the power amp has to work harder. If it's expecting 8 ohms and you give it 4, it'll have to work twice as hard because it meets half the resistance, and you may end up overheating or burning out the power amp.

This seems to be a vacuum tube (valve) amp, which is different from the situation you describe. A tube amp will not tolerate no load and it can be bad to put too high of an impedance on the amp (ie a 16 ohm cab on a 4 ohm tap). But a tube amp can tolerate a dead short (0 ohms) which would fry a solid state amp.

Anyway, OP without a manual or, even better, a schematic, it's a little hard to figure out the best way to hook the cabs up. Using cabs of different ohms isn't usually a great idea, though. It's possible some output transformers are set up to do that well but for the most part the lower impedance cab will get a lot more power and likely the higher impedance cab will not contribute much to the sound. If you wanted to run two 8 ohm cabs it's probably okay to put them on the 8 and 4 ohm taps but possibly not.
posted by 6550 at 6:31 AM on June 3, 2010


Peavey Butcher manual PDF
Schematic PDF

Thanks for your help folks!
posted by gally99 at 11:53 AM on June 3, 2010


Awesome! So the output jacks are all wired in parallel with some switching to go to the different impedances. For reference, on the output transformer the O tap (prob an orange wire) is the 16 ohm, the W (prob white) is 8 ohm, and the GN (green) is 4 ohm, with BK (black) ground/common/earth.

16 ohm cabs is straightforward, as you know. Just hook one to four cabs up to the labeled jack.

One 16 ohm cab on J1 goes to O (16 ohm tap).

Two 16 ohm cabs in parallel is an 8 ohm load, so the way the jacks work is it switches J1 and J2 to the W (8 ohm tap).

Three 16 ohm cabs in parallel is 16/3 = 5.3 ohm, so J1, J2, and J3 are switched to go the GN (4 ohm tap). Most tube amps can tolerate a 1:2 mismatch, say an 8 ohm cab on a 4 ohm or 16 ohm tap, so 5.33 ohms on the 4 ohm is very reasonable.

As an aside, you have to be a bit careful. Some amps, most notably some vintage Marshalls, are more particular about mismatches, particularly too high a load on a tap (like a 16 ohm cab on an 8 ohm tap on an 8 ohm cab on the 4 ohm tap). So if you end up with a vintage high-dollar amp it'd be best practice to play it safe and not mismatch the cab to jack impedance. But no amp should have an issue with the 5.33 ohm load on a 4 ohm tap.

Four 16 ohm cabs in parallel is 4 ohm load. J1, J2, J3, and J4 are switched to go to the GN (4 ohm tap), same as with three 16 ohm cabs.

Now we'll do other impedance cabs. Single impedance you just hook up to the proper jack, (16 ohm, 8 ohm, or 4 ohm).

Two 8 ohm cabs in parallel is a 4 ohm load. Hook them up to J2 and J3 (the 8 ohm and 4 ohm jacks). This switches J2 and J3 to the GN tap (4 ohm).

Three 8 ohm cabs would be 2.7 ohms. Peavey wouldn't recommend this but if it were my amp I'd hook them up to J2, J3, and J4, the 8 ohm, 4 ohm, and 4th 16 ohm jacks, which would be a 2.7 ohm load on the GN (4 ohm) tap.

Four 8 ohm cabs or two 4 ohm cabs is a 2 ohm load. I wouldn't recommend this configuration. You could run the cabs in series (daisychained) or series+parallel configuration which would get an 8 ohm load. You'd either need to make some cables to do this, although some speaker cabs have jacks that can accommodate this type of wiring.

Mismatched cabs?

8 ohm cab and 16 ohm cab. I suppose I should give you the parallel resistance formula. Rtotal = 1 / (1/R1 + 1/R2 +...+ 1/Rn). So 8 and 16 ohm cabs = 1 / (1/8+1/16) = 5.33 ohms. Hook them up to the 8 ohm and 4 ohm jacks to put a 5.33 ohm load on the GN (4 ohm tap).

Keep in mind different impedances will see different power levels (the 8 ohm cab will get twice the power of the 16 ohm and the 16 ohm will be correspondingly quieter). Could be an issue if you have 50 watt cabs on a 100 watt head. Mismatch the cabs and one will see 66 watts and one 33 watt, so you would have too much power for one cab.

8 ohm cab and two 16 ohm cabs. Rtotal = 1 / (1/8+1/16+1/16) = 4 ohms. Run them on J2, J3, and J4, the 8 ohm, 4 ohm, and 4th 16 ohm jacks.

Other combinations: do the math, get the total impedance, check the schematic, and see if you've got a reasonable load for some combination of jacks.
posted by 6550 at 2:52 PM on June 3, 2010


Thank you much, sir!
posted by gally99 at 12:43 PM on June 4, 2010


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