SPEAKERS & OHMS!!!! Everything you need to know about SPEAKERS & OHMS!!!

mikey383 · #1 01.11.05, 3:32 PM

Maybe I can get this turned into a sticky...

Since it's one of the most frequently asked questions.

Ohms:
A value of electrical resistance. The lower the number, the less resistance. The higher the number, the greater the resistance.

Speakers are rated at certain ohms (or impedance), most commonly, 4, 8, and 16.

Your amplifier is also rated at a certain impedance, or has multiple taps for speaker configurations. It is important that you match
these numbers up.

EXAMPLE --- 8 ohm cabinet--->8 ohm jack on amp
EXAMPLE --- 4 ohm cabinet--->4 ohm jack on amp
EXAMPLE --- 16 ohm cabinet--->16 ohm jack on amp

It is typically safe to mismatch one step higher

EXAMPLE --- 8 ohm cabinet--->4 ohm jack on amp.
EXAMPLE --- 16 ohm cabinet--->8 ohm jack on amp.

Now for the do-not do's.

NEVER EVER hook up a cabinet with a lower ohm rating than the head.

EXAMPLE ---4 ohm cabinet--->8 ohm jack on amp...THIS IS A NO-NO!!!

You will cause your output transformer to overheat, and eventually, it will blow.

NEVER - Turn on a tube amp without a speaker load attached to it. This can cause an output transformer to blow in a short time.

The only instance where it would be safe to turn on a tube amp without a load would be on amps that have a standby switch (Class AB typically) This would only be for visual diagnostics (tube heater check, bulb check, etc) BUT, DO NOT take it off of standby. While it's in standby, only the tube heaters are working, and will not cause damage to the output transformer.

Also, NEVER use instrument cable to hook up a speaker cabinet. The speaker jack can put out a high enough voltage to burn the small wire in instrument cable, thus leaving the amp without a load, and frying the output transformer. ALWAYS use a good heavy speaker cable.

General Speaker Info

Sensitivity - This number indicates how efficient the speaker works. Generally, this number will be between 89 and 105. If you replace a speaker with one of a higher sensitivity rating, you will also increase the apparent volume.

Generally speaking, an increase of +3dB will double the overall loudness of the amp.

Wattage - How much the speaker can handle. Smaller, combo amps usually have a speaker rating close to the rating of the amp. If when cranked, you don't like the sound of the speaker breaking up, then you might look into changing it out with one of a higher wattage. Some like the way a speaker sounds while being pushed to it's limits, and some don't. It's all in your presonal taste.

Wiring up multiple speakers.

When wiring speakers, there are three ways to do it

SERIES: one wire from the input jack to the "+" terminal of one speaker, from the "-" terminal of the same speaker to the "+" of the next speaker, last wire from the "-" terminal to the input jack. When wiring speakers in series, multiply the ohms by the number of speakers used.
Also, when one speaker blows, it will cut power to the other speaker as well, leaving both speakers not working.

EXAMPLE: jack------"+ '8ohm' -" ----"+ '8ohm' -" ---jack=16ohms

8x2 = 16

PARALLEL: One wire from the input jack to the "+" terminals of both speakers. Other wire from the input jack to the "-" terminals of both speakers. When wiring speakers in parallel, divide the ohms by the number of speakers used.
In this case, when a speaker blows, the remaining speaker will still work.

EXAMPLE:
jack----"+ '8ohm'"------------ "+'8ohm'"
jack----" - '8ohm'"-------------"- '8ohm'" = 4 ohms

8/2 = 4

This is also the way you will want to hook up multiple cabinets to an amp.

EXAMPLE: (2) 8 ohm cabinets ---->4 ohm jack on amp.
EXAMPLE: (2) 16 ohm cabinets--->8 ohm jack on amp.

SERIES-PARALLEL:

This is the most confusing. The total speaker load will equal the ohms of one speaker. Most commonly used on speaker cabinets that contain 4 speakers. This theory only works with 4 speakers of the same (ohm) rating.
In this case, if a speaker blows, it will take out it's series counterpart, leaving two speakers working, and two not working, hence doubling the impedance.

Starting at the upper left speaker, you will run one wire from the input jack to the "+" of that speaker, then from the same terminal to the "+" of the speaker below it.
Then run a wire from the input jack to the "-" of the upper right speaker, then from the same terminal to the "-" of the speaker below it.
Finally, connect the "-" of the upper left speaker to the "+" of the upper right speaker, and the "-" of the lower left speaker to the "+" of the lower right speaker.

Wiring Multiple Cabinets

Sometimes, it's inevitable that we need to run multiple cabs of different impedances. Well, here's how you figure out how to solve that problem.

EXAMPLE:

The amp has 4, 8, and 16 ohm taps.

Cabinet A = 8 ohms
Cabinet B = 16 ohms

In this case,

1/8 +1/16 = 3/16

Divide the back number by the front number to get the impedance.

16/3 = 5.33

You would want to run both cabinets from the 4 ohm taps, or daisy chain them if (2) 4 ohms taps aren't available. Remember, it's safe to run a slightly higher mismatch.

...And just as a reference

(2) 8 ohm cabs = 4 ohms
(2) 4 ohms cabs = 2 ohms - *typically unsafe*
(2) 16 ohms cabs = 8 ohms

(1) 8 ohm cab + (1) 4 ohm cab = 2.66 ohms - *typically unsafe*
(1) 4 ohm cab + (1) 16 ohm cab = 3.2 ohms - *somewhat unsafe*

The two above examples would typically be unsafe to run at, although, the very bottom example would be better to run, since it's closer to 4 ohms. This would get by in a pinch on some occasions.

[disclaimer]

The information provided above is a general reference. In some circumstances, it may not be appropriate to mismatch impedances. Use common sense, and consult the manufacturer if possible. If you are in doubt, the safest bet is to find a cab that has a nominal rating of that of your amp. I am not responsible for damages caused from mismatching impedances.

[/disclaimer]

I hope this clears up any confusion about speakers and ohms.

-Mikey383

thurston · #2 01.11.05, 3:41 PM

Ohmage is not a word or measurement. Impedance is the word you are looking for.

mikey383 · #3 01.11.05, 4:13 PM

You sir, are correct. Thanks for the correction.

scxtt · #4 01.11.05, 4:18 PM

Quote:

thurston --
Ohmage is not a word or measurement. Impedance is the word you are looking for.

actually ... "ohmage" ...

thurston · #5 01.11.05, 4:20 PM

Quote:

Originally posted by scxtt
actually ... "ohmage" ...

I'll be damned. LEarn something new everyday.

mikey383 · #6 01.11.05, 4:22 PM

Now dammit, I felt like an ass for using the word, now I feel like an ass for changing it......

macmidi · #7 01.11.05, 5:10 PM

My physics professor called words like that (ohmage) "strictly bush league." Impedance is a way better word.

Nice post. The "wiring multiple speakers" part is on target.

It is worth pointing out that most players will only be concerned with the "parallel" impedance formula, which is what comes into effect when you simply connect multiple cabs to a typical amp's speaker jacks -- not counting Ampegs, Fenders, etc. with "EXTENSION SPEAKER" jacks, which treat a load differently.

SOOOOOOOOO...if you're hooking up your basic Marshall head (or similar), adding another cabinet will REDUCE the impedance load. You said this; I'm just repeating it in big letters.

thurston · #8 01.11.05, 5:15 PM

Doesn't impedence also imply alternating current where ohmage and resistance imply direct current?

mikey383 · #9 01.11.05, 5:32 PM

Quote:

Originally posted by thurston
Doesn't impedence also imply alternating current where ohmage and resistance imply direct current?

Correct. Impedance is a term relating to AC circuits, where resistance applies to DC circuits.

EDIT: It's impedance on speakers, for some reason. They still run on DC current though.

thegaz · #10 01.11.05, 7:14 PM

Quote:

Originally posted by mikey383
Correct. Impedance is a term relating to AC circuits, where resistance applies to DC circuits.

EDIT: It's impedance on speakers, for some reason. They still run on DC current though.

Completely incorrect. A speaker hooked up to DC makes no noise. A speaker moves in and out very fast, at the frequency of the note/notes being played. For a middle A, this frequency is 440 Hz, 440 times back in and out per second. The speaker is pushed by positive voltage, and pulled by negative voltage (this can be switched with out any effect on the sound, unless you have multiple speakers. With more than one speaker, they all must be moving the same direction at the same time, or they are out of phase and will sound bad.) DC voltage will either be constantly pushing a speaker, or constantly pulling. So the signal to a speaker is alternating, a big sine wave (thought almost never a perfect one because guitars never produce single frequencies, there are overtones and such with single notes, and chords are obviously many notes at once).

mikey383 · #11 01.11.05, 8:07 PM

I guess I stand corrected. Thanks for the knowledge.

Elkvis · #12 01.11.05, 8:53 PM

Quote:

Originally posted by mikey383
NEVER EVER hook up a cabinet with a lower ohm rating than the head.

EXAMPLE ---4 ohm cabinet--->8 ohm jack on amp...THIS IS A NO-NO!!!

You will cause your output transformer to overheat, and eventually, it will blow

this is only true on solid state amps. mismatching by 2x in either direction is perfectly acceptable on tube amps (for the most part... some marshalls are very sensitive to impedance mismatch), and in fact may affect the tone in a positive way, depending on personal taste.

plugging a lower impedance load into a higher impedance output will reduce power and shift the frequency response more to the bass end, while plugging a higher impedance load into a lower impedance output will reduce power and shift frequency response toward the treble end.

a tube amp is self-limiting because of the nature of vacuum tubes, and connecting a lower-than-rated impedance load to the output will not cause the transformer or tubes to blow. what kills an output transformer is the high reflected impedance on the plates when the output is connected to no load or a very high impedance load (usually at least several thousand ohms). this effect is the result of the fact that the transformer is basically a solid coupling between the tubes and the load. think of it like a gearbox in mechanical terms. the input shaft is turning very fast (high voltage), but has little torque (low current). the transformer 'gears down' the signal to a lower voltage, higher current form, which has the power to drive the speakers. in fact, if you had a 5000-ohm speaker, you wouldn't need a transformer at all if you were running on a single 6L6.

the reason why a high impedance load will blow an amp is pretty straightforward. as an example, we'll use a 50W (2x6L6) push-pull amp with an 8-ohm output. the natural impedance of a pair of 6L6's is usually around 5000 ohms. to get the voltage ratio of the transformer, you need to divide the primary impedance (5000 ohms) by the load impedance (8 ohms) and then take the square root. in this case, the transformer steps the voltage down by a ratio of 25:1. at the same time, the transformer will step up voltages it sees on its load side by 25x as well. what happens with very high impedance loads is that they naturally want to run at high voltages, and will therefore reflect that impedance back through the transformer to the tubes. in the case of a 1000ohm load, the primary side of the transformer would see a very high voltage (roughly 125x the normal voltage), which would very likely be enough to break down the insulation in the windings of the transformer and fry it. conversely, if you were to connect a 1-ohm load, the primary side would see 1/8 the normal voltage, and current through the tubes and transformer would actually drop, due to ohm's law. this is the reason why a lot of amp manufacturers put speaker jacks in their amps which have a shorting contact to make sure the amp doesn't see an infinite-impedance load (no load).

I hope this was clear enough to understand for everyone.

mikey383 · #13 01.11.05, 9:24 PM

I must say that I disagree with you on that. You are the first person I've ever heard say that you can run a lower load on a tube amp.

Taken from Mesa/boogie's manuals:

You can always have a higher resistance without causing problems, but too low of a resistance will likely cause problems.

Quote:

plugging a lower impedance load into a higher impedance output will reduce power and shift the frequency response more to the bass end, while plugging a higher impedance load into a lower impedance output will reduce power and shift frequency response toward the treble end.

Again, from Mesa/Boogie's manuals:

When running a slight mis-match (for example: 8 ohm output into 16 ohm cabinet.), a slightly different feel and response will be eminent. A slight mismatch can provide a smoother darker tone with a little less output and attack. This response is a result of the amplifier running a bit cooler.

I've never read in any amp manufacturer's manual where running a lower load is safe. I see where running a higher load is safe, but not a lower one.

thegaz · #14 01.11.05, 9:36 PM

intuitively, less resistance/impedence will let more current flow, which would in theory burn up the amp.
but it seems like tube amps are counter intuitive. in most electronic cases, a very high resistance, like an open, causes no problems. you could leave a regular solid state power amp used for a pa on with no load attached (an open) all day with no ill effect. but if there is no load on a tube amp, the result is not good.
i'm not sure on this one.
my mesa boogie manual says "you can always have a higher resistance without damaging effects, but too low of a resistance will likely cause problems." but an open is a much higher reisitance, and a speaker must be attached (unless the silent recording switch is being used).

joeinthebox1980 · #15 01.11.05, 10:14 PM

i went ahead and made this thread a sticky at the suggestion of the thread-starter.

let's always try to refer members' ohm questions to this thread before any follow-up threads are posted.

- joe(inthebox1980)
moderator

Thread Tools
Show Printable Version Email this Page
Display Modes
Linear Mode Switch to Hybrid Mode Switch to Threaded Mode

- Support GuitarGeek! Visit our Sponsors -