Ohms - How do they work?

[Pacodelivery]

Yeah it's important to recognize that this is a completely different conversation depending on what your power amp is. For a solid state, yes, you will get more power out of a lower impedance cabinet (or cabinets) down to its minimum.

I have an excellent tech for my tube amp and he maintains that you should always try to run your tube amp at the highest possible impedance, so that you use the entirety of your output transformer.

Also, I've found talk about the musical differences between different impedances, that they each have their own character. I can say for sure that my traynor yba-200.2 is different at 8 ohms than at 4, even with similar cabs.

 

[gumtown]

OHM is either a chant used by Buddhist Monks, or may this describes it best.

 

[wmhill]

 

[bobyoung53]

thing is re: SVTs, 4ohms. there's a 2 ohm selector on back for 2 4ohm cabinets. never ran a single 8 on it, some guys here will say it's ok...lots won't. I never have.

but it sounds like you know this...at least 2 8s

it does allow the amps to run wild.

An SVT head will work with an 8 ohm cab but you get less power and the amp won't sound as tight, also you'll wear out your power tubes a lot quicker, could even damage the output transformer but that is unlikely. The old SVT's had automatic impedance switching, there was a main speaker output and an extension output. The ex output was shorted unless you plugged in the other cab. One cab was connected to the 4 ohm tap and with both cabs plugged in you were connected to the 2 ohm tap. You could run 2 8 ohm cabs to an old SVT, you either had to daisy chain them or run both cabs from the main output jack with a parallel mono 2 female into one male jack bought at Rat Shack. You always had to be careful to plug your cab into the correct jack.

 

[tbirdsp]

This is aimed at home stereo, but still applies.

Invalid Link Removed

 

[BioWeapon]

Alright I don't know if anyone's taken the science-y approach yet, so...

Ohm's law states that a components voltage (V) is equal to the resistance in ohms (R) multiplied by the current flowing through the component (I).
This gives us V = R x I, Ohm's law.
Here's where amplifier's and cabinets come in : Your average audio amplifier acts as a nearly ideal voltage source. This is the (V) part of the equation. Your speaker's impedance, generally 4 or 8 ohms, is the (R) part of the equation. So, you can divide (V) by (R) to get the amount of current required out of the amplifier. The amplifier's output stage can only supply so much current without burning out components, mainly transistors, or output transformers. This is the limiting factor for how low your impedance can go.

Next there's the amplifier's power rating at a certain output. Output current at a certain impedance is again given by I=V/R. Using Ohm's law again, we can input maximum current output of the amplifier and maximum voltage output, and from those two find minimum impedance: V/I = R.

Note that this is very, very simplified version of things actually going on as we have to deal with phase angles and apparent, real, and reactive power as amplifiers output AC current and voltage into an inductor (the speaker coil). This means imaginary numbers are involved.

If any of this is actually incorrect, please chime in and correct me... I am but a mere student!

 

[Engle]

OHM is either a chant used by Buddhist Monks, or may this describes it best.

That's awesome! OP do you feel like you understand?

 

[JHAz]

Speaker output is controlled by sensitivity of the speaker. Sensitivity is the loudness related thing that is likely to be not equal among different speakers. It is possible for a four ohm speaker to be 3 dB less sensitive than a similar 8 ohm speaker. Often that can be hidden in the specs by stating spl at a different voltage for different speakrrs. At any given SPL, a 4 ohm speaker that hpasrns yo be 3 dB less senditive requires twice the power, erasing the added power the amp may be providing.

It all depends on a lot of stuff. Gets worse if you consider that different frequency responses of the speakers can make one sound louder.

 

[jmattbassplaya]

Learned a lot in this thread. Thanks guys!

 

[Hand slap]

I was curious if anyone could explain how ohms work and their role in amplification. The biggest question I have is if they play any role in how loud a cab can get. For example, say we have two identical cabs and amps but one is 4ohm and the other is 8ohm. Would the 4ohm cab be louder?

Thanks!

Here's the deal, the lower the ohm, like 4, as opposed to 8, lower ohms, allow for more current through the load, which in turn, gives you a bigger voltage drop across the load, and when you input the bigger voltage drop, labeled as E, and more current, because the load, which is the 4 ohm, as opposed to 8 ohm, current is called I, ok now P =IxE, that is how it is calculated, and why power, labeled P is more, with a 4 ohm load, as opposed to 8.

 

[Hand slap]

Here's the deal, the lower the ohm, like 4, as opposed to 8, lower ohms, allow for more current through the load, which in turn, gives you a bigger voltage drop across the load, and when you input the bigger voltage drop, labeled as E, and more current, because the load, which is the 4 ohm, as opposed to 8 ohm, current is called I, ok now P =IxE, that is how it is calculated, and why power, labeled P is more, with a 4 ohm load, as opposed to 8.

Hopefully that helps, and keep in mind, if you underrate your amp, you will probably have to turn it up, to keep up with a louder band, and you can start clipping your output, which is essentially producing a square wave, which will blow you speakers, cook your voice coils, so a 200 watt amp, can and will blow up 1000 watt speakers, if you have to push the amp out of normal operating parameters, to keep up.

 

[Hand slap]

Hopefully that helps, and keep in mind, if you underrate your amp, you will probably have to turn it up, to keep up with a louder band, and you can start clipping your output, which is essentially producing a square wave, which will blow you speakers, cook your voice coils, so a 200 watt amp, can and will blow up 1000 watt speakers, if you have to push the amp out of normal operating parameters, to keep up.

I am a tech, by day, struggling musician by night.

 

[PocketProtector]

So to add one more piece to the puzzle, I ask because I can get a good deal on an 8ohm cab. My Aguilar DB751 can work with that just fine, but I'm considering getting an SVT. I was curious if I would be better off going for the 4ohm model over the 8 due to the SVT only being rated at either 4 or 2ohms.

Some things to keep in mind. To increase you're volume by 3db it take doubling the watts.

 

[okcrum]

Here's the deal, the lower the ohm, like 4, as opposed to 8, lower ohms, allow for more current through the load, which in turn, gives you a bigger voltage drop across the load, and when you input the bigger voltage drop, labeled as E, and more current, because the load, which is the 4 ohm, as opposed to 8 ohm, current is called I, ok now P =IxE, that is how it is calculated, and why power, labeled P is more, with a 4 ohm load, as opposed to 8.

No.

The maximum voltage across the load in SS amplfiers is set by the power supply rails of the amplifier. The only thing that changes when the load impedance changes is the load current.

In contrast, for tube amplifiers you are matching impedance. You see less voltage across and more current through a lower impedance load for the same power output.

 

[Munjibunga]

I was curious if anyone could explain how ohms work and their role in amplification. The biggest question I have is if they play any role in how loud a cab can get. For example, say we have two identical cabs and amps but one is 4ohm and the other is 8ohm. Would the 4ohm cab be louder?

Thanks!

If you apply the same power (watts) into each cab, there shouldn't be a noticeable difference in volume. However, as others have pointed out, if you use the same amp to power the cabs, the amp will produce less power into the 8-ohm cab than into the 4-ohm cab and, at the same level control on the amp, the 4-ohm cab will be somewhere around 2 or 3 dB louder.

 

[PocketProtector]

Keep in mind 3dB is generally considered the volume difference that is noticeable. It takes doublING the watts to get a 3db increase. So the volume difference will be negligible. Having an 8 ohm cab leave the possibility of adding another cab.

 

[40Hz]

This video discusses it in the context of a PA system, but it's relatively easy to extrapolate to a guitar or bass amp. It's probably the most useful quick & dirty not too technical presentation I've seen on the topic.



What I've been told over the years by people who know a lot more than me about the technical details of this stuff:

A: Series vs Parallel.

When you add a cab you need to know whether the two cabs end up being wired in series or in parallel with each other.

Most times with an instrument amp, adding a cabinet puts the additional cab into parallel with the first cabinet.

When you combine speakers in parallel, you divide the impedance. So two 4 ohm speakers in parallel will have a combined impedance of 2 ohms. (4 ohms divided by 2 cabinets = 2 ohms total)

If you combine speakers in series, you add the impedances. So two 4 ohm speakers in series will have a combined impedance of 8 ohms. (4 ohms + 4 ohms = 8 ohms)

AFAIK no instrument amps expect multiple cabs to end up wired in series.


B: Impedance and amplifiers

Amps are most efficient (i.e. loudest and run coolest) when driving the speaker impedance they're designed for. So an amp that has a 4 ohm impedance rating will perform best when seeing a 4 ohm speaker load.

You can put a higher impedance load on an amp than its nominal impedance rating. So you can get away with putting an 8 ohm load on an amp designed for a 4 ohm load, but you'll make the amp work harder and get a drop in volume because there's more electrical resistance with the higher impedance.

You should never put a lower impedance load on an amp than its nominal output impedance rating, because you'll risk severely damaging your amp and your speakers.

Most of the today's amps are designed for multiple output impedances, usually assignable by a switch. So if your amp had a choice of 2 or 4 ohm output settings, some quick arithmetic will tell you the combinations of cab(s) that will work with that amp.

Rule: All cabinets must have the same impedance. Divide the impedance by the number of cabinets.

So on an amp with a selectable 4 and 2 ohm speaker impedance setting:

For 4 ohms: 1 cabinet at 4 ohms, or 2 cabinets at 8 ohms each. (i.e. 4/1 or 8/2 = 4)

For 2 ohms: 1 cabinet at 2 ohms, 2 cabinets at 4 ohms, or 4 cabinets at 8 ohms. (i.e. 2/1 or 4/2 or 8/4 = 2)

C: Power and speakers

Generally, you'll want to match your speaker cab's power handling rating fairly closely to your amp's continuous (or RMS) output power. There's some debate about whether it's better that the amp's power should exceed the speaker rating by a certain percentage. But I'll leave that for others who know more than me to argue over. I try to make sure my speaker's power rating equals or exceeds the output power rating of the amp I'm plugging it into. So if my amp was putting out 500 continuous watts, I'd want my cab (or cabs) to be rated to handle 500 continuous watts as well.

 

[gumtown]

Hopefully that helps, and keep in mind, if you underrate your amp, you will probably have to turn it up, to keep up with a louder band, and you can start clipping your output, which is essentially producing a square wave, which will blow you speakers, cook your voice coils, so a 200 watt amp, can and will blow up 1000 watt speakers, if you have to push the amp out of normal operating parameters, to keep up.

A speaker will quite happily run on a square wave, so long as each half cycle is equal.
The problem is when the rail centre/balance point moves toward one of the supply rails, and introduces D.C. into the voice coil.
When the output duty cycle exceeds 50% on one supply rail (because if it happened on both the amp would blow).
At that stage your gear must be getting severely abused, so you would deserve it if something blew up anyway ...

 

[dincz]

At most, this much difference:

https://www.talkbass.com/attachments/3db-mp3.315441/

 

[twinjet]

I was curious if anyone could explain how ohms work and their role in amplification. The biggest question I have is if they play any role in how loud a cab can get. For example, say we have two identical cabs and amps but one is 4ohm and the other is 8ohm. Would the 4ohm cab be louder?

Thanks!

More ohms = more electrical resistance. That's my only knowledge on resistance. Check this thread out, should address every question you have about them.

Ohms FAQ