Rookie Question - advanced apologies.

[Roadburner]

Hello one and all,

I work for a few small bands as a general tech, but mainly assistant TM etc, so whilst I basically assemble backline and line tech, I find matching speakers and cabinets to be confusing, and wanted to clarify this (I'm used to just matching inputs by memory).

So, I've read a lot into ohmage and impedance, and I understand what it does, but on a practical level, I still just forget. It's like when you forget a basic maths equation.

SO... I have any given guitar head and a cab. I match the 8ohm output to the 8ohm input, or 16ohm output to 16ohm input. But as soon as multiple cabs are involved, I get confused. If I have an 8 ohm output on the head, to two cabinets, to I put it to 4ohm, 8ohm or 16ohm?

I know to you masters of amplification this is entry level stuff, but that's where I am, I could really do with this being dumbed down as much as possible (do you know where I could find photos of the correct matching?). The guitar tech of band I usually work for is on holiday right now, and I'm heading out with a big[ger] band across Europe this Summer without him, so i need to know this.

Advanced apologies, and advanced thanks.

Many many thanks,
John.

[NickC]

http://www.audioplex.com/corkyscorner/c ... er%204.pdf


Series Formula:

Zt = Za = Zb (where Z is impedance in ohms, series connection with alternating (-) and (+) terminals)

Parallel Formula:

Zt = (Za x Zb)/(Za + Zb) (where Z is impedance in ohms, parallel connection where the positive terminal of one amp channel is wired to the positive terminals of spkr A and spkr B, and the negative terminal of the other amp channel is wired to the negative terminals of spkr A and spkr B)


Also be careful to match amp power with speaker power rating, to avoid frying them.


http://cie-wc.edu/Series_Parallel_9_14.pdf

[xtian]

Maybe you need a human-familiar way of thinking about it. Let's take your amp's 8 ohm output. It wants to push against an 8ohm load--that's where it's most comfortable. So if you have TWO speaker cabs attached to that 8 ohm output, the speaker load better TOTAL 8 ohms. (This is where you need to know that speakers wired in parallel provide a LOWER overall load than their individual loads, and wired in series a GREATER overall load.)

[Phil_S]

If you are plugging two speaker cabs into one amp (assuming typical one output transformer), then those cabs are parallel. Each is valued at half the stated amount of Ohms. IOW, two 8 Ohm cabs become 4 Ohms each and both need a 4 Ohm input. Typically, you don't have this available, a place to plug into two 4 Ohm inputs.

More typically, you might have two input jacks. Let's say you have an 8 Ohm jack and a 4 Ohm jack. If using only one cab, match 8 to 8 or 4 to 4. If you want to plug in two cabs:
--plug a 16 Ohm cab into the 8 Ohm jack and
--plug an 8 Ohm cab into the 4 Ohm jack.

This works because you have 16 || 8, which makes the 16 cab "worth" 8 and and 8 cab "worth" 4.

If you are working with one output jack and a Y splitter for 2 cabs, then, for example, plug in two 16 Ohm cabs to the Y to the 8 Ohm input. That's because 16 || 16 = 8.

There are some output transformers that have two (or more) independent secondary windings (the secondary connects to the speakers). Is this bifilar winding style, you don't do any math at all. You just plug in whatever matches. With 8 and 4 Ohm inputs, you'd match 8 to 8 and 4 to 4.

So, you have to know something about the output transformer to know what to do.

[NickC]

....... SO... I have any given guitar head and a cab. I match the 8ohm output to the 8ohm input, or 16ohm output to 16ohm input. But as soon as multiple cabs are involved, I get confused. If I have an 8 ohm output on the head, to two cabinets, to I put it to 4ohm, 8ohm or 16ohm?
...........................

If you have two guitar cabinets, each an 8 ohm load, when you daisy chain them together (which is in parallel) the two cabs present a 4 ohm load. (Daisy chain = implies the cabinets have two 1/4" jacks which will be in parallel). You would connect that 4 ohm load to the 4 ohm speaker output jack on the amplifier (or select the 4 ohm speaker setting using a switch if there is one, like many Marshall's have).

If the guitar cabs were each 4 ohms, daisy chaining them together would give a 2 ohm load. Many amps can not safely handle that.

If the two guitar cabs were each 16 ohms, daisy chaining yields an 8 ohm load.

A 16 ohm cab daisy chained with an 8 ohm cab gives a 5.333 ohm load (the math gets more complicated when the cabinet loads differ).

[Roadburner]

Cannot thanks you guys enough for this. A simple but thorough 'human' explanation is just what I needed. That's all I wanted!

Thank you ever so kindly!

Cheers.