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Can anyone explain in laymans terms head/cab ohms etc?
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dougal engineer
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#1
5th February 2010
Old 5th February 2010
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Can anyone explain in laymans terms head/cab ohms etc?

I need to learn matching heads with cabs regarding ohms etc?

Both for tube amps and transistor amps as I understand this differs for these.

If you cannot match the cab/head identically do these two rules apply?


If running a tube amp the ohms on the amp should be more than the cab?
If running a transistor amp the ohms on the amp should be less than the cab?

Thanks for any explanation / advice.
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5th February 2010
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from what I understand (and I'm no expert), you should always try to match the (amp head - tube) to the cabinets ohm rating. You can run a 4 ohm head into an 16 ohm cabinet but you should never run a 16 ohm head into a 4 ohm cabinet. I have no idea about solid state.
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5th February 2010
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As was just said, you should always match the impedance. This can be confusing and controversial. However there are exceptions under certain conditions.
VP
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5th February 2010
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Dont think thats correct, guys!

Tube, lower = safer

Transistor, Higher = safer.

So an 8ohm output on a tube amps head going into a 4ohm cab is gonna be ok. Its really hard to damage the output transformer by pulling too much, the tubes will maybe work a little harder but they reach a point where they just start to limit the power they give out.

The same happens if you are running too HIGH a load from a tube amp but now other gremlins creep in causing voltage spikes which have the potential to fry the OP transformer.

I can't remember the technical reasoning, but I have always believed the opposite when thinking about transistor amps (and had it verified a few times). So a transistor amp doesn't like to go into too low a load but is ok going into a higher one. Need to re-learn the reason why...

for lots of info about tube amps, including a whole section on ohmage check out this page
http://www.geofex.com/tubeampfaq/TUBEFAQ.htm#matchspkr
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5th February 2010
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I'm pretty sure the above post is incorrect.


[top]"What is an "ohm" rating anyway?


All speakers have a characteristic known as impedance which is measured in units called ohms. The most common values for speakers are 8 ohms and 4 ohms. Many older speakers have ohm ratings of 16 and even 32 ohms (this is because in the old days amplifiers used vacuum tubes, and higher impedance speakers were more compatible with the output impedance of vacuum tube amplifiers). A speaker with a lower ohm rating represents a more demanding load for an amplifier to drive."


It's your transformer. A 4 ohm load demands more than a 16 ohm load. Don't connect a head output of 16 ohms to a 4 ohm load. Don't do it!
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5th February 2010
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Hmmm, did you read the link I posted above? Its a pretty in depth site that explains a lot about the construction and inner working of tube amps (in great detail!) and they disagree. The reason, electronically, makes sense too.

The site agrees that a lower rated load is more demanding to drive, but goes on to say its almost impossible to damage the transformer that way as the tubes reach a point where they pretty much refuse to give out any more current to a lower ohmage load. Result is the transformer is not that much more taxed than normal. However too high a rated cabinet can cause extra problems and voltage spikes which MIGHT be enough to "punch through the insulator in the transformer and short the windings". Ie its not so hard on the tubes but a lot harder on the OT which is the icomponant most likely to go!

I'm happy to be proved wrong, but I would be surprised if the people who wrote that site had got it wrong as well, given there knowledge of all other things 'tube'!

However, bottom line is to keep things matched if at all possible, and going too far in either direction is likely to cause damage to your amp!

EDIT - heres a link to a similar discussion on the orange forums, with a very decisive post at the bottom of page 1, including lots of techy links!
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5th February 2010
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Quote:
Originally Posted by Victory Pete View Post
As was just said, you should always match the impedance. This can be confusing and controversial. However there are exceptions under certain conditions.
VP
Better to be safe than sorry.
See the controversy, I will do more research.
VP
#8
9th February 2010
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Just make sure the head impedance is lower or equal to the cabinet impedance.
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9th February 2010
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The head impedance should be equal or bigger than the cab impedance.

An output transformer needs to see the adequate impedance, if the impedance of the cab is smaller than the impedance of the output transformer (head) the transformer won't see the proper resistance and will work harder to compensate for the lower impedance of the cab which might eventually blow the transformer.

Here's an example from mesa boogie:

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10th February 2010
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Quote:
Originally Posted by matskull View Post
The head impedance should be equal or bigger than the cab impedance.

An output transformer needs to see the adequate impedance, if the impedance of the cab is smaller than the impedance of the output transformer (head) the transformer won't see the proper resistance and will work harder to compensate for the lower impedance of the cab which might eventually blow the transformer.

Here's an example from mesa boogie
Thank you!
That's what I was saying. I think you can cause $$$ damage to your amp if you get it wrong. I would hate to see someone blow their amp because of some erroneous information.
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10th February 2010
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Quote:
Originally Posted by Tapeworm View Post
Thank you!
That's what I was saying. I think you can cause $$$ damage to your amp if you get it wrong. I would hate to see someone blow their amp because of some erroneous information.
You're welcome, I think mesa boogie know what they are talking about.
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11th February 2010
Old 11th February 2010
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Mr. Science to the rescue...

An amplifier is like a light bulb. It's a circuit through which electricity is run. The electricity flowing thorugh the light bulb is resisted by the filament and that's why it glows. The electrons are being forced through it against the resistance and it heats up and disperses that electrical current as heat (which stimulates electrons in the filament's atoms, which then release that absorbed energy as light.) If you pull the light bulb out and stick a piece of metal in there, that has very low resistance, you'll blow a fuse in the fuse box because there's no resistance to the flow and it starts sucking way too much power through the circuit.

An amp is the same. It's pushing electrical current through a speaker. The speaker is acting like the filament in the light bulb. It's dispersing the energy that's coming from the amp, in this case through a conversion from electrical energy to physical motion (and hence to heat in a different sort of way.) The speakers have a certain amount of resistance to the flow of electricity, measured in ohms. If you use too low an ohm'age for the amp, then it's not absorbing enough electrical current and the amp can blow just like the fuse blows, because too much current is now flowing through it.

Each amp can handle some range of resistances. You generally don't want to try to use a lower ohm'age (smaller number) than the amp is rated for, or bad things could happen. You can use a larger ohmage since that just means it's sucking up more power, which just lowers the volume of the amp a bit.
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12th February 2010
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real world application question--

Here's a real world application question-- what if you are gigging at a club that has some beat up old no-name 4x12 with no ohm markings, and you bring your JCM800 head. You ask the sound guy what the ohms of the cab are and he says "I think it's 8." So you set the Marshall to 8... and the cab is really either a 4 ohm cab or a 16 ohm cab. Can that blow the transformer or will something bad happen? Can you set the amp to 8 ohms and not worry too much about either one or the other error, up or down?

Also, why is my own Marshall 4x12 a 16 ohm cabinet, and lots of cabs are 8 ohm or 4? Why is there not some sort of standardized thing for 4x12's? (I guess that could be a divergent question maybe?)
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13th February 2010
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Quote:
Originally Posted by xopeth4 View Post
Here's a real world application question-- what if you are gigging at a club that has some beat up old no-name 4x12 with no ohm markings, and you bring your JCM800 head. You ask the sound guy what the ohms of the cab are and he says "I think it's 8." So you set the Marshall to 8... and the cab is really either a 4 ohm cab or a 16 ohm cab. Can that blow the transformer or will something bad happen? Can you set the amp to 8 ohms and not worry too much about either one or the other error, up or down?

Also, why is my own Marshall 4x12 a 16 ohm cabinet, and lots of cabs are 8 ohm or 4? Why is there not some sort of standardized thing for 4x12's? (I guess that could be a divergent question maybe?)
I wouldn't plug any of my amps into a cab if I didn't know what his impedance is because yes, it could eventually blow your amp up.
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13th February 2010
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If you wan to be safe, buy an inexpensive ohm meter and keep it in your gear bag. You can test it and be sure what it is.
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14th February 2010
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OK, SS and tube amps are completely different in how they react to impedance mismatches.

SS is pretty straight forward: with most amps, any impedance higher than the rated impedance will result in less power but no damage. Most amps have a minimum impedance, less than that will overtax the power supply and burn something out. The exception is some class D amplifiers, like the Crate Powerblock which didn't tolerate a larger load very well for some reason (it'd shut down intermittently when running into a 16 ohm cab from the 8 ohm output).

Tube amps are tougher because there's a lot going on in there. First off, they deal with VERY high voltages, like up to 650v DC. This means that you have to be careful what you do, it CAN come back to bite you. Secondly, the way a tube amp works is that the internal resistance of the tube (usually in the 4-6000k ohms range) needs to equal the external resistance (4/8/16 ohms) approximately. I say approximately because the range that a tube can operate in can safely vary quite a bit, so a tube that's biased in the middle of it's operating range can live with output impedances that are somewhat higher or lower. As the output impedance goes up the tube essentially acts as if it's biased colder and it's output drops. As the impedance goes down, it's like it's biased hotter and eventually the tube plates melt (not a good thing). The limit on how low an impedance a tube amp can stand is mostly determined by how close the tubes are to their bias limit, but for most amps it's OK at 1/2 the rated impedance (like running an 8 ohm Fender amp into 4 ohms, which most of them were built to tolerate, or even running a 2 ohm Super into 1 ohm!). The other problem with impedance mistmatches comes from the fact that a speaker is a motor. It turns electrical power into physical movement using a magnet and an coil. When the power is turned off (when the sine wave of input voltage crosses 0) the speaker cone collapses back to the neutral position. This coil moving in a magnetic field results in a "back EMF) voltage, equal and opposite to the voltage that the tube originally supplied. It's multiplied thru the transformer, so a few volts at the speaker becomes 100s or thousands at the tubes. If the impedance mismatch is bad enough, this back EMF will cause arcing thru the transformer, tube sockets or capacitors. This is even more destructive to the amp than melting tubes. Most amps will survive a 2X mismatch upwards, too (a 8 ohm output into a 16 ohm speaker) but most authorities don't recommend going any further than that.
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25th June 2010
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Lots of Amps often state on the exterior of the cases, something like

100 watts into 8 ohms.
200 watts into 4 ohms.

This is really often stating that the Amp will be half as efficient with a 4 ohm load.
people often interpret this the other way round.

so it means here, that the Amp circuitry is 8 ohms.. because it will use less electricity with an 8 ohm speaker.

what should be investigated is the Amps ohm design.. this is often stated (inside) the Amp not the outside of the Amp.

you can ruin an Amp by getting this wrong because you are burning out the Amp to achieve the same Volume.
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28th June 2010
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Ive always gone by the saying "High(head) to Low(cab) will not go, Low to High let if fly."
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2nd July 2010
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The easiest analogy I've ever heard with regards to ohm matching is that your head's the coffee and the cab's the cup. Filling it half way's fine. Filling it full is ideal. Don't overfill it.

The most noticeable negative effect of a "half full" setup I've experienced is when I was playing bass briefly in a band. My setup was a 4 ohm Aguilar head into an 8 ohm Ampeg 4x10 cab. I played it through a 4 ohm setup one time at a club using the backline cab, and the difference was unbelievable. The head runs 250 watts at 8 ohms, 500 at 4 ohms, so there ya go. I've never noticed the difference nearly as much with guitar heads / cabs.

As an aside, one thing I've noticed in particular is that vintage Fender combos (Twins, etc) hate having replacement speakers with mismatched ohms. When I bought my '68 Twin, someone had replaced the original speakers with incorrectly matched Celestions, and it sounded like garbage. Popped in some C12Ns, and voila... back to great sound.
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3rd July 2010
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Quote:
Originally Posted by popvulture View Post
The easiest analogy I've ever heard with regards to ohm matching is that your head's the coffee and the cab's the cup. Filling it half way's fine. Filling it full is ideal. Don't overfill it.
nice one
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3rd July 2010
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I gotta jump back in here. Please, please, PLEASE remember there's a difference between tube and SS.

Tube=output transformer=no more than a 2:1 mismatch, doesn't matter up or down.

SS=direct coupling=as far as you want to go up (it just cuts power) NO margin for error down (if it's rated for 4 ohms and you put it thru 2 ohms, kiss it goodby).
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3rd July 2010
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ok, so if i have an 8 ohm head and two 4ohm cabinets i need to chant om 16 times during warm-up, right?
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Actually, that's not trivial. Most speaker jacks are wired in parallel, so a pair of 4 ohm cabs is going to be 2 ohms. Using the 8 ohm speaker output is a dangerous mismatch. Either make a serial cable or use a 4 ohm or 2ohm tap.
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3rd July 2010
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I have rarely seen a thread with so much misinformation about a basic topic.
The OP asked for a "layman's" version of amps, speakers, and Ohms. Let's see if I can do this simply and concisely:

An amp will deliver current depending on the load put on it. If you put a 4 Ohm load on an amp it will output a signal impeded by 4 Ohms. When the number of Ohms is higher there is more "impedence " to the signal flow--so the same amp would have an output impeded by 16 Ohms creating a lower signal because of the higher impedence.
When an amp puts out more current than it was designed to deliver problems occur. This is true of both tube and solid state units although different issues may happen in either design. If the amp had no impedence or too little it would overload and fail. If the amp had too much of a load it would not acheive the volume required. You are not going to blow up an amp by putting a higher-impedence load than rated --just lose efficiency and volume. Danger occurs when the impedence is lower than rated output.
Various guitar amp heads have selectable impedence which taps the transformer at different points to best match the intended load.
To calculate the total impedence of your rig remember: speakers connected in series have a higher impedence when combined while speakers connected in parallel produce a lower number. For example: Two 8-Ohm speakers wired in parallel equal 4-Ohms total impedence, but in series they would be 16-Ohms. Two 16-Ohm speakers wired in parallel equal 8-Ohms. In series they would be 32-Ohms. In 4-driver boxes series and parallel are often combined to get the desired rating: Two sets of parallel-wired 16-Ohm speakers combined in parallel would equal 4-Ohm total impedence. If the pairs were series together(32-Ohms) then combined in parallel you get 16-Ohms total. So a four-speaker cabinet with 16-Ohm drivers will either be 4 or 16-Ohms--never 8. A cabinet with 8-Ohm speakers will usually be wired series/parallel for 8-Ohms since 2-Ohms is way too low for most guitar amps to handle and 32-Ohms doesn't allow enough juice to flow.

Parallel wiring has all of the positive terminals connected together and all of the negative terminals connected together. Series wiring has the positive terminal of the first speaker connected to the positive input wire but the negative terminal of that speaker is connected to the positive terminal of the next speaker/s in the chain. The negative terminal of the last speaker is then connected to the negative wire of the input. It is quite obvious which technique is employed when looking inside a cabinet.
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7th July 2010
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10th July 2010
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Ears2thesky, you are DANGEROUSLY wrong. I'm sorry, but check out the tech section on The Gear Page, there are some well respected amp techs there. Read or ask Gerald Weber. Heck, get a decent electronics theory book that includes tubes. A tube amp uses a transformer to change the high output impedance the tube needs to match the low output impedance the speaker generates. When the speaker returns to rest it produces a kickback voltage nearly equal and opposite to the voltage induced in it, if the transformer is designed for a much lower impedance, that kick back or fly back voltage will be multiplied to many thousands of volts and it WILL fry the output transformer and the tube sockets. I've never done it myself, but I'm aware of people who have. Please, just leave it like this:

1. Tube amps: no more than a 2:1 mismatch in EITHER direction.
2. SS amp: go up as high as you want, but never less than the rated impedance.
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