Impedance...

[JimmyBlood]

Here's a Brilliant quote from John Phillips from over at the gear page. It eloquently sums up the impedance questions that always come up. Sticky time for this?

"This comes up so often but it seems that it needs repeating...

A mismatch in either direction by a factor of two is not dangerous to the amp, in almost any case. That means that running into half the correct load (eg 16-ohm amp into 8-ohm cab) is NOT dangerous.

For a TUBE amp. It's certainly dangerous for a solid-state amp, which is probably where the confusion comes in.

But in fact for a tube amp, running into too low an impedance is safer than running into too high, all things being equal, and provided you don't go outside about a factor of two.

Running into too low a load is harder on the tubes and will wear them faster, but is unlikely to do any sudden or serious damage. It's still not a good idea to run into under half the right load for any length of time though, since you do run the risk of tubes failing, which could in theory cause other problems (though not in a well-designed amp really).

Running into too high a load is potentially dangerous since higher than normal flyback voltages are developed and stepped up through the output transformer which can exceed the insulation rating of components. This can cause sudden, serious damage to the transformer, the tubes or the tube sockets depending on where the insulation breaks down first. There's no real time factor, the damage either occurs or it doesn't, and it may not necessarily happen immediately. But it's definitely unwise to run into more than double the correct load with any tube amp.

There is a good reason why many amps (eg BF and SF Fenders) have a shorting switch in the speaker jack - it's to deliberately short the output in the case of no speaker being plugged in. This is because although it's not a great idea for the tubes, it's far safer for the amp than being left with an open circuit. Really.

There are a few specific cases where OT ratios may make a too-low load more dangerous than normal (eg old Marshalls, but they have three-way impedance selectors so you shouldn't have to), but by and large if you have the choice of too low or too high, go with too low.

Tonally you will hear a difference too - both too low and too high mismatches reduce power (typically about 1/3 for a factor-of-two mismatch), but they sound different from each other, and from a match. Too low gives a more mushy, thicker sound; too high gives a flatter, more midrangy sound. A correct match always gives the most power and headroom and widest frequency response."
__________________
John P

[0.1 watts]

Thanks for that, it answered a million of my questions in one. I have the back panel of my amp plastered in labels saying 'ensure speaker plugged in' because twice before when I moved the amp, I forgot to plug in the speaker, and turned it on. Both times it was on for about 30 seconds before I realised and switched off.

One thing I still don't understand is the ratio of impedances in a 4x12 or 2x12 speaker cab, how do you work out what impedance is needed on your amp? And what difference does series or parallel wiring make?

[buliwyf]

can there be a "mod" where the amp won't turn on until a speaker is plugged in, much like a boss stompbox won't turn on unless there's something plugged to its "input?"

[Slipstream]

Yeah it's possible. I don't know why they don't do that. It's also possible to have an impedence-dependant relay that wouldn't allow the high power circuit to work unless a speaker was hooked up, AND the impedence matched. I can understand why mfg companies don't do that though, too costly. I could do it if I were properly motivated.

[JimmyBlood]

awesome, thanks stratman!

.1, let's take a hypothetical 2x12 cab loaded with 8 ohm speakers...

In series they add to be 16 ohms.

In parallel they become 4 ohms.


Now, take a similar cab loaded with 8ohms speakers but 4x12 instead.

Normally, the two pairs of speakers are wired in series (each pair making 16 ohms in series) then the two pairs are wired together in parallel, making the overall load 8 ohms. That is series/parallel wiring.

[0.1 watts]

Thanks JimmyBlood. As a physicist I should know this!

[SG_74]

As far as I know, most guitar cabs run at 16 ohms.

Great info JimmyBlood! Cheers!

[Cashew_halves]

As far as I know, most guitar cabs run at 16 ohms.

Great info JimmyBlood! Cheers!

I think thats so you can wire two cabs together for an 8 ohm stack. Would you be better of just buying an 8 ohm cab if you were just gonna have a half stack and your heads impedance was 8 ohms? Although I guess you would have to buy two new cabs if you wanted to go to full stack then.

[Mroberge]

When playing just a simple combo amp there isn't much guess work with impedance and what not, but when moving to hevier equiptment like Heads, Cabs, Power Amps, and Mixers, it's good to have a good fundamental knowledge of how it all works. I'll try to break it down but I'm no technician so if anyone notices any errors please correct me.

My understanding of things is as follows:

As stated above, Speakers Wired in series, increases the impedance-- Wired in parallel, decreases the impedance.

This is important when mounting speakers into guitar cabs, AND when daisy-chaining speaker cabs together, AND when hooking up to power amps.

On the back of most guitar heads, and power amps it'll say something like: Max total load 200 watts @ 4 ohms RMS per channel

That means that the amp can push 200 watts through a 4ohm load.
Roughly that translates to something like 100 watts through an 8 ohm load
or 50 watts through a 16 ohm load.
The lost power is converted to heat through the speaker coil.

(those numbers aren't exact, and I believe the 1/3 power ratio posted above is probably a bit more accurate. but for purposes of simplicity I'm posting it this way.)

Ideally you want to configure your cabs to match your amp's impedance recomendations for max power output. That means that if you have two PA cabs each at 8ohms, you could either A: Plug one speaker into each output jack of your amp. (This will give each channel an 8ohm load, thus reducing your power output.)
Or
B: daisy-chain the two speakers together, giving you a 4ohm load, thus maximizing your power output. (usually the plugs in the back of cabinets are parallel inputs so that you can daisy-chain cabs together to cut impedance in half.)

As a genral rule, NEVER cut your impedance below 4ohms, and NEVER increase it over 16.
ALWAYS check your amps and heads to see what the max total load is.
ALWAYS know what each cabinets impedance is.
Match accordingly...

[Reverend40oz]

I'm going to sticky this, and get a bunch of grief for it.
By the way, I think I'm going to get my way on a sub-forum here in tools for the how-to stuff. It's on the back burner now I have been told so stickies for now.

That'd be a pretty good idea. I'd really like to see a single post sum up the steps of setting up a guitar properly. Even if it just compiles other posts into one.

[SKEETER]

Impedences in series add. In paralell the formula is 1/ (1/r1 + 1/r2 + 1/r3, etc.). In other words, in paralell, divide all the individual impedences into ONE, then sum them together, then divide that value back into ONE.

Most cabs, to my knowlege, are wired to be 8 ohms now. I think MArshalls were always different, but not sure what they are now.
If you put four 12s in a cab that are 8 ohms each, and do them series/paralell, you still have 8 ohms because you run them in two banks of two in parallel ( two 8 ohm speakers in paralell make 4 ohms) then make the two banks of speakers in series to each other, which adds, so you end up with 2 in paralell (making 4 ohms) in series with two more in parralell which gives you another 4 ohm load, and because they are in series to each other the loads add together to bring it back to four ohms.
Often cabs are wired to accomodate the same impedence requirements that a single speaker in a combo would be valued at, which as often as not is 8 ohms.