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Nate42

Easy Math Free Impedance (Ohms)

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Intro

 

Ok. So every so often someone comes around here with a 'help what are ohms' or something similar thread. Invariably what happens is a bunch of well meaning people show up and start spouting a bunch of math. Then still more equally well meaning people who unfortunately have no clue what they're talking about show up and spout helpful hints and even more math. The end result tends to be that whoever originally asked the question gets scared off and assumes theres no way he/she can ever understand this stuff.

 

But I promise you its really not that bad. In fact, while I encourage people to learn more about electronics, the real truth is that you don't need to know most of it. It doesn't take much to be able to select speakers wisely and avoid harming your amp.

 

This essay has kinda evolved away from an explanation of impedance into an "everything you need to know about matching speakers to your amp" thread. It is divided into parts, and you can feel free to ignore the parts that don't apply to you.

 

Part I: Basic Terms

 

Heres some terms you need to know to understand the later parts and see which rules apply to you. Don't let the first three definitions scare you, it gets easier from there.

 

Voltage: Voltage is a difference in electrical potential between 2 points in a circuit. In other words, voltage represents the potential to do work, but nothing really happens until you have both voltage and current. Fear not, a thorough understanding of this is not required for this tutorial. Voltage is measured in volts.

 

Current: Current is the physical flow of charge through a circuit. People like to point out that its the current that really matters, but you can't have current without voltage. Current is measured in amperes or amps. Not the same amp you plug your bass into. :D

 

Ohm: A unit of impedance, sometimes represented by the capital omega symbol. Impedance is a measure of how much something restricts the flow of electrical current. Voltage, Current and Impedance (ohms to us) are all related according to Ohm's law. Again, don't sweat it because full understanding of these concepts is not required.

 

Power: Measured in Watts, power is the product of Current and Voltage. Your amp's power rating is a good indicator of how much noise you'll be able to make with it, and is also something to keep in mind when selecting a cab.

 

 

Cab: Short for cabinet, a box with speakers in it. The Cab will have an impedance rating measured in Ohms. Typical values are 4 Ohms and 8 Ohms. A cab will also have a power rating for the amount of power it is designed to handle.

 

Tube Amp: An amplifier with tubes for the power amp output stage. You know if you've got 'em, the big glowing glass things are hard to miss. If you have a tube preamp it doesn't matter, for our purposes only the output stage matters.

 

Solid State amp: an amplifier with transistors for the power amp output stage. Most amps are solid state. If you have a tube preamp it doesn't matter, for our purposes only the power stage matters.

 

Stereo Power Amp: A solid state power amp usually used to power PA systems, but can also be combined with a preamp to make a kickass bass rig. They have 2 or more 'channels' which essentially means 2 completely independant amps in the same box. Sometimes the channels can be 'bridged' to make them work together for more power output.

 

Bridged Mono: What you call a stereo power amp that has been bridged as desribed above. More on this in later parts.

 

 

OK, thats it for part 1. The next parts (coming soon) will be specific to certain types of amps. Feel free to skip parts that don't apply to you.

 

Edit: my omega symbol didn't show up properly so I took it out. I'm referring to the greek letter.

 

Edit: added definintions of voltage and current at big geez's recommendation, and power too while I'm at it.

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Part 2: Solid state amp with only 1 cab

 

This is about the simplest setup you can possibly have. Any solid state amp has an 'impedance rating' that tells you what impedance in ohms the amp can handle. You never ever want to go BELOW what the amp is rated for. So if your amps says it can handle 4 ohms, you can safely plug in either a 4 or 8 ohm cab. If it says it can handle 2 ohms, you can safely plug in either a 2, 4, or 8 ohm cab. Thats all there is to it.

 

A word on impedance and power: Solid state amps put out more power at lower impedance. You'll have to consult your manual to know exactly how much, but as an example, my Hartke is rated for 275 watts at 8ohms, and 350 watts at 4 ohms. So, if you are concerned with getting the most power out of your solid state amp, you need to run it at the lowest impedance it can handle. However, you should be aware that the difference in power you will get doesn't make as much difference in volume as you might expect. So I wouldn't sweat it too much.

 

Also, exercise care when running at low impedances, especially 2 ohms. The lower impedance you run an amp, the hotter it tends to get. If you're going to run at anything less than 4 ohms, go easy on the volume until you're certain things are stable. And again, don't even think about running at a lower impedance than your amp says it can handle.

 

What about running with no cab? Some people think this is bad for amps, which is actually true of tube amps (see part 4). But with a solid state amp, no cab is essentially infinite impedance, it won't try to draw any current at all and won't harm your solid state amp in any way.

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Part 3: solid state amps with 2 cabs

 

With more than one cab, you follow the same basic guidline as with only 1: don't run your amp at a lower impedance than it can handle. However, with more than one cab, the impedances combine in 'parallel'.

 

There is a nifty mathematical forumula to show you how impedances add in parallel, but unless you're doing something weird, you don't need to know it. Heres the basic combinations:

 

Two 8 ohm cabs = 4 ohms

Two 4 ohm cabs = 2 ohms

One 8 ohm and one 4 ohm = 2.667 ohms

Four 8 ohm cabs = 2 ohms

 

Thats it. Once you know what the combined impedance of your cabs is, just follow the guidlelines in Part 2, and remember, don't run your amp at a lower impedance than it can handle.

 

Some cabs have an extra jack for 'daisy chaining' to another cab. If you do this, the cabs are still in parallel, as far as your amp is concerned its the same as plugging them both directly into the amp, and all the same rules apply.

 

A note on cabs of different impedance:

As mentioned above, if you combine an 8 ohm cab with a 4 ohm cab, the combined impedance is 2.667ohms. There's one other thing to consider though. Because the 4 ohm cab has less impedance than the 8 ohm cab, its will recieve more current and therefore more power than the 8 ohm cab does. This imbalance in power could potentially effect your tone.

 

This doesn't have to be a bad thing though. For example, if you have an 8 ohm 4x10 and a 4 ohm 1x15, the 1x15 will get the most power. But you might actually want the 1x15 to get more power to help balance its tone when compared to the 4x10, because 4x10s are typically louder than 1x15s.

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Part 4: Tube Amps

 

Tube amps are a slightly different animal. Tubes like to run at high voltage and low current, they can't put out enough current to drive a speaker directly. So tube amps have a thing called an 'output transformer'. Transformers essentially allow you to decrease your voltage while at the same time increasing your current, or the other way around. All you really need to know though, is that your amps output transformer matches it perfectly to the speaker load its designed for. A nifty side effect of this is that a tube amp always puts out its maximum power into the load its designed to handle. However, you want to be sure and give the tube amp EXACTLY the impedance its expecting.

 

Most tube amps have some way of selecting what impedance of cab you will use, either a switch, or multiple output jacks (one jack labeled 4 ohms, 1 labeled 8 ohms, etc.). As long as you use the right output jack (or switch setting) for your cab setup, you can be confident that your amp is running safely and is putting out its maximum rated power.

 

If you have more than one cab, their impedance combines in parallel as described in part 3.

 

A note on running a tube amp with no cab: Don't do it. Edit: My original explanation for this was incorrect, and a proper explanation is likely beyond the scope of this essay. Just don't do it. :)

 

(Thanks Isaac42)

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Aye, Natey! A job well done!

 

 

 

But a couple of critique-ing points:

 

 

1) Ohms is not purely a measure of impedance. Ohms are a measure of resistance, and are also used to reflect impedance.

(Sometimes I wish there was a convenient measure of IMPUDENCE... But that's another story!)

 

For the purpose at hand, your statement is correct.

 

 

2) Paragraph 5, Part 3... I'm not finding fault, just commenting that it doesn't read as well as the rest of your otherwise brilliant synopsis.

 

As well, it would be useful for one to know WHY 2.667 ohms when a 4 & 8 are used together. The explanation of same should also QED the inevitable question of "why does my 4 or 8 ohm speaker measure x.xx ohms instead of 4 or 8?" (See point 1 above.)

 

 

3) Part 4: I wouldn't have said 'like to' as much as 'HAVE to', where tube amps are concerned. (Then again, Brett Farve would've double-bagged...)

 

Beyond that you introduced the terms 'voltage' and 'current' without prior definition. That also opens the door for continued confusion.

 

 

Carry on, my friend!

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Originally posted by the_big_geez

Aye, Natey! A job well done!




But a couple of critique-ing points:



1) Ohms is not purely a measure of impedance. Ohms are a measure of resistance, and are also used to reflect impedance.

(Sometimes I wish there was a convenient measure of IMPUDENCE... But that's another story!)


For the purpose at hand, your statement is correct.



2) Paragraph 5, Part 3... I'm not finding fault, just commenting that it doesn't read as well as the rest of your otherwise brilliant synopsis.


As well, it would be useful for one to know WHY 2.667 ohms when a 4 & 8 are used together. The explanation of same should also QED the inevitable question of "why does my 4 or 8 ohm speaker measure x.xx ohms instead of 4 or 8?" (See point 1 above.)



3) Part 4: I wouldn't have said 'like to' as much as 'HAVE to', where tube amps are concerned. (Then again, Brett Farve would've double-bagged...)


Beyond that you introduced the terms 'voltage' and 'current' without prior definition. That also opens the door for continued confusion.



Carry on, my friend!

All those are already explained in details in the FAQ.

I see NATE's essay as a rather synthetic view of it.

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According to my science class:

 

Resistance in Parallel = Rp

 

Rp = 1/R1 + 1/R2 + 1/R3.... to infinity

 

So, we have three speakers wired in parallel with one of the parallel cabs having another one in series (in straight line):

 

|-2Ohm-2Ohm--|

| |

---|------4Ohm----|---------

| |

|------8Ohm------|

 

Rp = 1/2+2 + 1/4 + 1/8

Rp = (find denominator) 5/8

 

Flip 'em around (dont know why exactly)

 

and you get total Resistance in circuit of

 

8/5 Ohms = 1.6 Ohms

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You got it wrong buchie.

The formula is 1/Rtotal = 1/R1 + 1/R2 + 1/R3 + ...

HEnce the flipping.

This has already been covered at large in the FAQ anyway.

The point of this thread (accordingly to its author) is to do without maths.

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Originally posted by Jazz Ad

You got it wrong buchie.

The formula is 1/Rtotal = 1/R1 + 1/R2 + 1/R3 + ...

HEnce the flipping.

This has already been covered at large in the FAQ anyway.

The point of this thread (accordingly to its author) is to do without maths.

 

Yup.

 

In parallel, Req = (R1 * R2) / (R1 + R2)

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Isaac caught me in the act of being stupid. What can I say, I've slept since my transformers class. Anyway, post has been edited, sorry.

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Originally posted by Nate42

Well, looks like I'm going to be heading out for the rest of the evening. Part 5: Stereo Power Amps will happen eventually though, promise.


Glad you guys appreciate this, and hopefully I won't screw up any more.
:)

I'll be watching! ;)

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Originally posted by Nate42

Part 4: Tube Amps


cab, their impedance combines in parallel as described in part 3.


A note on running a tube amp with no cab: Don't do it. Edit: My original explanation for this was incorrect (thanks Isaac42). In fact the idea that you can't run a tube amp with no cab may just be folklore. But to be safe I still recommend you don't do it.

 

Safer to say NEVER do it. Ever heard of flyback? Arced tube sockets? I'm confused by the edit, but this is a pretty simple rule, backed up by Ohm's law. When you have an infinite impedence, at the secondary you get a voltage spike at the primary. Sorry if I'm missing something here, but I thought this was Tube Amps 101.

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Originally posted by bassmantele



Safer to say NEVER do it. Ever heard of flyback? Arced tube sockets? I'm confused by the edit, but this is a pretty simple rule, backed up by Ohm's law. When you have an infinite impedence, at the secondary you get a voltage spike at the primary. Sorry if I'm missing something here, but I thought this was Tube Amps 101.

Why would you get a voltage spike? I can understand if you were playing and got unplugged (you'd get inductive kick), but that's not the question. Some say it's dangerous to power up a tube amp with no load. Is it really, and, if so, why?

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Originally posted by isaac42

Why would you get a voltage spike? I can understand if you were playing and got unplugged (you'd get inductive kick), but that's not the question. Some say it's dangerous to power up a tube amp with no load. Is it really, and, if so, why?

output transformers are the beefiest of the three(usually there are three transformers in a tube amp) transformers in a tube amp. they generate plenty of power and when there's nowhere for it to go, it heats up the transformer quite a bit. eventually, which is not THAT far off, this leads to failure of the transformer. if it's not too badly damaged, it can be rewound. if the core is shot too, the whole transformer is a write-off. Output transformers are in general an expensive part to replace. a replacement for an SVT can be $300 or more.

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Originally posted by isaac42

Why would you get a voltage spike? I can understand if you were playing and got unplugged (you'd get inductive kick), but that's not the question. Some say it's dangerous to power up a tube amp with no load. Is it really, and, if so, why?

 

 

Because E = IR

 

 

If you have no speaker connecting the OT secondaries then R goes to infinity. Which means that E - voltage - tries to go there as well. The spike often causes arcing between tube socket pins or within the tube itself. At worst, the insulation in the OT can burn through and short out.

 

 

Do a search for the alt.guitar.amps FAQ, or just ask your local tech.

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bump. There really is another part coming, promise. I've been busy with Christmas stuff and band stuff.

 

I see there's been some discussion on the tube amp thing since I last posted. I still haven't heard an explanation for why not to run a tube amp with no load that I like, and I'm not comfortable changing my original post until I do. If any of you tube experts can give me a good explanation feel free to PM me and I'll change it and credit you in the original post. I'd rather keep that sort of discussion out of this thread if possible, I'm trying to keep this thread newby friendly.

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I think this is the clearest understanding of this stuff I've ever gotten. Thanks Nate!

 

This definately belongs in the FAQ forum if it hasn't been added already.

 

One question...

 

You said on a tube amp, often they will have two outputs. One for 4 ohms and one for 8 ohms. Could you have a cab plugged into each outlet and be OK or do you need to stick with 4 or 8?

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Originally posted by Low Tone


You said on a tube amp, often they will have two outputs. One for 4 ohms and one for 8 ohms. Could you have a cab plugged into each outlet and be OK or do you need to stick with 4 or 8?

 

I wouldn't go doing that unless the manufacturer says its ok.

 

If you plug one into a 4 ohm output and the other into an 8 ohm cabinet, those cabs are still (sort of) in parallel. They're not strictly in parallel because of the presence of the output transformer, but the effective impedance seen by the output tubes is definately going to go down. I wouldn't feel comfortable doing that without an ok from the manufacturer.

 

If the amp happened to have a 2 ohm output, I would hook them up to that. 2.66ohms is close enough to 2 that you would likely be okay.

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Originally posted by Nate42



I wouldn't go doing that unless the manufacturer says its ok.


If you plug one into a 4 ohm output and the other into an 8 ohm cabinet, those cabs are still (sort of) in parallel. They're not strictly in parallel because of the presence of the output transformer, but the effective impedance seen by the output tubes is definately going to go down. I wouldn't feel comfortable doing that without an ok from the manufacturer.


If the amp happened to have a 2 ohm output, I would hook them up to that. 2.66ohms is close enough to 2 that you would likely be okay.

 

Thanks Nate.

That was more of a curiosity thing, but good to know just in case.

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