Speaker Impedance

[Macbass68]

Hi Folks,

I am a newbie Bass player. I have a 500watt 2 ohm Yamaha Bass head - is it best to use 8 ohm speakers or 4 ohm (for 1 x15, and 2 x 10) 

I find the 'impedance thing' quite confusing? Thanks if you can help, Mac

[Dan Dare]

Hello Mac. This is not completely technically accurate, but it helps, if you don't understand impedance, to view it as your amplifier needing to see a certain amount of resistance from your speakers. If it doesn't see at least that resistance, it may attempt to deliver more power than it is capable of providing. This can cause it to overheat or even be damaged if you are running at high volumes.

 

Once again, for those who do understand this topic, I am trying to express it in a simple to understand manner.

 

From your post above, your Yamaha can deliver a maximum of 500W and must see a minimum impedance of 2 ohms. It will happily work into higher impedances, such as 4, 8 or 16 ohms, but not into lower. If it sees higher impedance than 2 ohms, it will deliver less power, but will be quite safe to use in that way. Note that it will only be delivering 500W when it is running flat out, which rarely happens in the real world.

 

As a rough rule, the power an amp can provide will halve when speaker impedance doubles. So 500W into 2 ohms becomes 250W into 4 ohms, 125W into 8 ohms and so on.

 

Speaker impedance reduces as you add more of them. So using two 8 ohm speakers means the amp sees a combined impedance of 4 ohms. And so on.

 

If you have one 8 ohm and one 4 ohm speaker, the total impedance they present to your amp will be about 2.7 ohms. You can run both together, but be aware that the 4 ohm speaker will get twice the amount of power as the other because it provides less resistance to the amp. So make sure it can handle it.

 

Hope this helps.

Edited Monday at 14:29 by Dan Dare

[TimR]

3 hours ago, Macbass68 said:

Hi Folks,

I am a newbie Bass player. I have a 500watt 2 ohm Yamaha Bass head - is it best to use 8 ohm speakers or 4 ohm (for 1 x15, and 2 x 10) 

I find the 'impedance thing' quite confusing? Thanks if you can help, Mac

 

As long as both cabs are 4ohm or both are 8ohm, your amp will be fine.

 

I am assuming you mean cab, as opposed to speakers, and calling the 2x10 a 'speaker'.

 

 

[Macbass68]

Thank you both for your very helpful replies. Why is it when i'm looking at cabs online, the majority are 8 ohms?

[prowla]

12 minutes ago, Macbass68 said:

Thank you both for your very helpful replies. Why is it when i'm looking at cabs online, the majority are 8 ohms?

Because two 8 Ohm cabs in parallel make 4 Ohms.

[godathunder]

most amplifiers will be unhappy to see a speaker load below 4 ohms (there are exceptions obviously)

consequently theyll offer half of their rated power into an 8 ohm cabinet with the option to add another 8 ohm cabinet (wired in parallel) meaning the amp will deliver it's full power into the resulting 4 ohm load, with each cabinet receiving half of that power.

 

very few people run more than 2 cabinets so this is usually the most effective setup

[itu]

@Macbass68:

Remember that power does NOT tell you about loudness. dB is the right instead of wattage.

 

When you double the power, you gain 3 dB in loudness. 100 W to 200 W. This amount of difference can be heard in studio surroundings.

 

When you want to get 10 dB louder (this can be heard on stage, too!), you need to multiply the power by 10. This means that a 100 W amplifier should be replaced by a 1 000 W amp.

 

Low impedance (Z) is also hard to the amp. 8 ohm is an easy one, and does not heat the amp that much. You can think Z as a stick that you push. If the number is big, the area of the stick head is big, and it is easy to push it with your thumb. The smaller the Z, the thinner the stick, and at some point like a needle.

[Dan Dare]

2 hours ago, godathunder said:

most amplifiers will be unhappy to see a speaker load below 4 ohms (there are exceptions obviously)

 

Class D amps are often more tolerant of low impedance. Both of mine will run into 2 ohms. The back panel of any amp will state the minimum speaker impedance it can be used with.

 

2 hours ago, Macbass68 said:

Why is it when i'm looking at cabs online, the majority are 8 ohms?

 

Because the majority of standard speaker drive units are 8 ohm. That's why cabs containing two drivers will often be 4 ohm (two 8 ohm drivers, parallel wired). They could be wired in series to give 16 ohms, but that would limit the volume they can produce with a given size of amp, so manufacturers choose parallel. Once cabs use more than two drivers, they are usually wired in a combination of series and parallel to give 8 ohms total impedance.

 

Things can get more complicated. Impedance can vary according to frequency and, as itu points out, speaker efficiency (how much volume a cab will give for a given amount of output power) will also affect how much volume your gear can produce. From a simple, practical point of view, as long as you stick within the impedance and power limits of your amp and cabs, you'll be fine.

Edited Monday at 20:03 by Dan Dare

[prowla]

4 hours ago, itu said:

@Macbass68:

Remember that power does NOT tell you about loudness. dB is the right instead of wattage.

 

When you double the power, you gain 3 dB in loudness. 100 W to 200 W. This amount of difference can be heard in studio surroundings.

 

When you want to get 10 dB louder (this can be heard on stage, too!), you need to multiply the power by 10. This means that a 100 W amplifier should be replaced by a 1 000 W amp.

 

Low impedance (Z) is also hard to the amp. 8 ohm is an easy one, and does not heat the amp that much. You can think Z as a stick that you push. If the number is big, the area of the stick head is big, and it is easy to push it with your thumb. The smaller the Z, the thinner the stick, and at some point like a needle.

Just a sub-note to the power to loudness; that's all other things being equal and the speakers not limiting, cables being transparent, etc.

Not saying it's wrong per-se, but just beware of things which don't state the assumptions being made.

There's another saying that 10dB is twice as loud.

But I've also seen 10x Watts = twice as loud (ie. 1000W = twice as loud as 100W); that's why a 50W valve amp can sound surprisingly loud!

 

[Downunderwonder]

It's not uncommon for a modern class D amp to be capable of running down to 2 ohm but deliver the same rated power to 4 ohm worth of cabinets. Over to OP to check the manual.

[hill76]

 Posted in error, my iPad is driving me crazy

Edited Tuesday at 01:10 by hill76

[TimR]

12 hours ago, prowla said:

Just a sub-note to the power to loudness; that's all other things being equal and the speakers not limiting, cables being transparent, etc.

Not saying it's wrong per-se, but just beware of things which don't state the assumptions being made.

There's another saying that 10dB is twice as loud.

But I've also seen 10x Watts = twice as loud (ie. 1000W = twice as loud as 100W); that's why a 50W valve amp can sound surprisingly loud!

 

 

It's logarithmic.

 

A 10dB (6dB) is a doubling of power. 2x watts. This gets confusing because to actually double the sound pressure (loudness) you need 10x the electrical power (watts).

 

Loudness is subjective. Don't confuse sound power with electrical power and doubling perceived loudness isn't just doubling sound pressure.

 

Anyway that's going off topic. 

Edited Tuesday at 10:44 by TimR

[TimR]

The OP states the Amp can operate at 2ohms. 

[chris_b]

22 hours ago, Macbass68 said:

Hi Folks,

I am a newbie Bass player. I have a 500watt 2 ohm Yamaha Bass head - is it best to use 8 ohm speakers or 4 ohm (for 1 x15, and 2 x 10) 

I find the 'impedance thing' quite confusing? Thanks if you can help, Mac

 

Hi, forget the science lesson, your amp can run. . . .

 

1 x 8 ohm cab (8 ohms)

2 x 8 ohm cabs (4 ohms)

3 x 8 ohm cabs (2.67 ohms)

1 x 4 ohm cab (4 ohms)

2 x 4 ohm cabs (2 ohms)

1 x 8 ohm cab and 1 x 4 ohm cab (2.67ohms)

 

That's it. You're good with these configurations.

Edited Tuesday at 11:04 by chris_b

[casapete]

Sounds like the amp is the Yamaha BBT500h?
IIRC these amps had matching cabs , which were 4ohm models and therefore good

to use either singly ( giving 250 watts ) or in pairs, unleashing all 500 watts. 
 

If you use the advice given by @chris_b above you won’t have any problems.

[jrixn1]

One thing to consider about getting two 4-ohm cabs is that while they will work fine with your current Yamaha amp head, if you want to change heads in the future you might find that your choice is restricted, since many (most?) heads don't go down to 2 ohms.

 

Another is that while the combination of a 1x15 and a 2x10 is not necessarily wrong, and some people are getting good results - it's also not generally the modern modular approach, which is two identical cabs.

 

While impedance and speaker diameters are quite important, there are other factors.  Describing your situation might get useful specific cab recommendations (e.g. living room vs pubs vs functions; jazz duo vs heavy metal nonet; any budget or weight considerations).

[chris_b]

11 hours ago, chris_b said:

 

Hi, forget the science lesson, your amp can run. . . .

 

1 x 8 ohm cab (8 ohms)

2 x 8 ohm cabs (4 ohms)

3 x 8 ohm cabs (2.67 ohms)

4 x 8 ohm cabs (2 ohms)

1 x 4 ohm cab (4 ohms)

2 x 4 ohm cabs (2 ohms)

1 x 8 ohm cab and 1 x 4 ohm cab (2.67ohms)

 

That's it. You're good with these configurations.

 

Duh!!!!  I missed one out. . . . !

Edited Tuesday at 22:54 by chris_b