Speaker impedance - Bass amplifier

[SA90]

Morning all,

 

I've been sprawling the web and asking friends for an answer, but they are always differing.

 

4ohm bass amplifier into 8ohm bass cabinet...bad or ok?

And why?

 

Impedance has always confused me...is there a simple explanation out there?

 

Thanks

[Brian]

4ohm bass amplifier...

 

Valve with transformer output or solid-state?

[SA90]

4ohm bass amplifier...

 

Valve with transformer output or solid-state?

 

Solid state

[pete10uk]

I'm by no means an expert but in the real world yes it will work.

 

Your amp will be rated at a particular wattage @ 4 ohms, @ 8 ohms you will be driving twice the residence, so your amp will provide half the wattage. For instance if your amp is 150watt @ 4 ohms, it will give 75watt at 8 ohms. Depending what your speaker is rated at will depend whether it sounds any good.

 

I may stand corrected though!

[SA90]

The amp is rated to 900W 4ohm, and the cabs is 450W RMS 8Ohm...

So would that mean that essentially, the amp (at full whack) will be delivering 450W at 8ohms???

[pete10uk]

That is my understanding in basic terms. There is obviously more to is when you look at the calculations, but like you I'm not in to ohms in a big way. I would and have looked at it in the way I've described and it's been fine.

[MarkPAman]

4 ohm rated (solid state) amp > 8 ohm speaker is fine. Though as Pete says, you will not get all of the power your amp is capable of providing. That does not make it a bad thing though, as your amp is having an easy time.

 

Add a second 8 ohm speaker (in parallel) and the amp "sees" 4 ohms which is good. Now you're getting all the power the amp was designed to give. Each speaker only gets to use half of that power though.

 

Add a third 8 ohm speaker, and the amp now "sees" about 2.7 ohms which is bad. You're now trying to get more power out of the amp than it was designed to handle, so it will probably distort - even at very low volume, get too hot, and may let the smoke out. Or, if you're lucky, it may have some self protection built in and shut itself down.

[SA90]

4 ohm rated (solid state) amp > 8 ohm speaker is fine. Though as Pete says, you will not get all of the power your amp is capable of providing. That does not make it a bad thing though, as your amp is having an easy time.

 

Add a second 8 ohm speaker (in parallel) and the amp "sees" 4 ohms which is good. Now you're getting all the power the amp was designed to give. Each speaker only gets to use half of that power though.

 

Add a third 8 ohm speaker, and the amp now "sees" about 2.7 ohms which is bad. You're now trying to get more power out of the amp than it was designed to handle, so it will probably distort - even at very low volume, get too hot, and may let the smoke out. Or, if you're lucky, it may have some self protection built in and shut itself down.

 

This particular amp (Darkglass Microtubes 900) also has a 2ohm setting, so I'm guessing it should be fine whatever the load? (providing impedance doesnt drop below 2...which it never will in my case!)

 

 

[alistermorton]

The spec page for that amp specifically mentions driving an 8Ω cabinet.

 

Press when using a 2Ω load. You can use this control to lower the output if your 4Ω cabinet is not able to handle more than 500W (you can also get 350W with an 8Ω cabinet in this configuration).

[Chappie]

Running an 8Ω cabinet on an amp that has a specified (on paper) power rating at 4Ω will be fine (to a point). Forgive the simplicity of this, I hope this explanation is not too simple.

 

Loudspeaker impedance is a measure of resistance that the amp has to drive against. Each amp only has a finite amount of power it can produce; you put in a certain level of signal and it amplifies it by a set amount when pushing against a certain (average) impedance. If you raise the impedance, the amp has to push harder and the amount of power available reduces accordingly. If you lower the impedance, the amp doesn't have to work as hard and the amount of power increases. Obviously, there is a downside to this "free" power and that is the amount of heat created within the amp. An amplifier's ability to cope with lower impedances is basically (amongst other factors) based around its ability to dissipate heat. A well designed amp with big heatsinks and fans can obviously dissipate more heat than the same one with only a small heatsink and thus can generally cope with lower impedances.

 

A rather basic analogy is you pushing a large rock, which you can only push at a certain speed due to the weight. If you double the size of the rock, you can only push it half as fast as previously. If you halve the size of the rock, you can push it twice as quickly, but you will get hot because you are running. If you then start rolling it down a hill, it will run away from you and you collapse from exhaustion.

 

Amplifiers can therefore work across a range of impedances, the minimum of which has been defined by the manufacturer as the safe point below which the amp may cook itself and the maximum is the point where you can't get any useful level out of the speaker. Some amp manufacturers specify ratings at 4Ω, rather than 8Ω, because it produces a bigger number for their spec sheet. Others may spec it because that may be the prevailing standard in a given field.

 

I mentioned at the beginning that it will be fine to a point and this point is how loud you want said speaker. As you said, your 900W @ 4Ω amp is probably good for roughly 450ish @ 8Ω, but it's generally good practice to use an amp that is 1.5 to 2 times as powerful as the rated load you are driving to allow for the dynamics of the musical content (obviously this depends on the type of content running through the amp). If it's not loud enough because there isn't enough power, you may want to keep turning it up, or turn up the volume downstream, to increase the amount of signal going into the amp. At this point the amplifier may start to clip, and the further into clipping you drive it, the more heat is produced (both within the amp and the speaker driver) and the more likely you run the risk of damage. You are far more likely to damage something using an under-powered amp run hard than you are with an over-powered amp running with clear headroom.

 

So, in reference to your original question, yes, the amp will be fine, as long as you don't push it too hard given that it is slightly under-powered for the cabinet you are driving.

[kgallen]

pushing a large rock, which you can only push at a certain speed due to the weight.

 

Well, that's Newton's Second Law of Motion up the swanee then. http://www.blue-room.org.uk/public/style_emoticons/default/ohmy.gif

Edited May 10, 2017 by kgallen

[sunray]

Running an 8Ω cabinet on an amp that has a specified (on paper) power rating at 4Ω will be fine (to a point). Forgive the simplicity of this, I hope this explanation is not too simple.

 

Loudspeaker impedance is a measure of resistance that the amp has to drive against. Each amp only has a finite amount of power it can produce; you put in a certain level of signal and it amplifies it by a set amount when pushing against a certain (average) impedance. If you raise the impedance, the amp has to push harder and the amount of power available reduces accordingly. If you lower the impedance, the amp doesn't have to work as hard and the amount of power increases. Obviously, there is a downside to this "free" power and that is the amount of heat created within the amp. An amplifier's ability to cope with lower impedances is basically (amongst other factors) based around its ability to dissipate heat. A well designed amp with big heatsinks and fans can obviously dissipate more heat than the same one with only a small heatsink and thus can generally cope with lower impedances.

 

A rather basic analogy is you pushing a large rock, which you can only push at a certain speed due to the weight. If you double the size of the rock, you can only push it half as fast as previously. If you halve the size of the rock, you can push it twice as quickly, but you will get hot because you are running. If you then start rolling it down a hill, it will run away from you and you collapse from exhaustion.

 

Amplifiers can therefore work across a range of impedances, the minimum of which has been defined by the manufacturer as the safe point below which the amp may cook itself and the maximum is the point where you can't get any useful level out of the speaker. Some amp manufacturers specify ratings at 4Ω, rather than 8Ω, because it produces a bigger number for their spec sheet. Others may spec it because that may be the prevailing standard in a given field.

 

I mentioned at the beginning that it will be fine to a point and this point is how loud you want said speaker. As you said, your 900W @ 4Ω amp is probably good for roughly 450ish @ 8Ω, but it's generally good practice to use an amp that is 1.5 to 2 times as powerful as the rated load you are driving to allow for the dynamics of the musical content (obviously this depends on the type of content running through the amp). If it's not loud enough because there isn't enough power, you may want to keep turning it up, or turn up the volume downstream, to increase the amount of signal going into the amp. At this point the amplifier may start to clip, and the further into clipping you drive it, the more heat is produced (both within the amp and the speaker driver) and the more likely you run the risk of damage. You are far more likely to damage something using an under-powered amp run hard than you are with an over-powered amp running with clear headroom.

 

So, in reference to your original question, yes, the amp will be fine, as long as you don't push it too hard given that it is slightly under-powered for the cabinet you are driving.

 

Well I understand what you are trying to say but if the OP wasn't confused I bet he/she is now.

 

 

 

 

The combination is fine as long as the speaker is designed for bass work.

 

Realistically the difference in sound pressure level (volume) is hardly noticeable when you simply double the power, in fact most people would be hard pushed to tell the difference.

 

I have some 300W amps at 4Ω and generally run them at 8 or 16Ω, I prefer to add amps rather than run them flat out.

 

 

 

 

 

[paulears]

If you think this got complicated quickly - when they do it on bass guitar forums, mainly populated by Americans, I think it's good they're not in the same state as they'd shoot each other. There seems to be a camp that can cite solid science to back up their opinions - every way around you can work this one.

 

I'm a pragmatist. My bass amp is pretty powerful, and I work on the principle that it's designed to work down to 2 Ohms. I have a couple of bass cabinets - one is 4 Ohms and the other 8 Ohms. I cannot remember which one is which, and being very honest, even looking it up is pointless. They both work fine. The bigger one sounds louder - but maybe that's also because it's bigger. It also weighs over 60Kg - so maybe after wheeling up or down a ramp this is why I think it sounds louder.

 

Nowadays we seem to get 3 year warranties on many items. If an amp design was poor and the things failed when an ignorant musician simply connected up and played long and loud, they'd be getting lots of warranty claims, Facebook would report the unhappy people, and we'd know about it. You can make the science fit any scenario with a bit of craftiness and pseudo-science.

 

4 Ohms, let alone 2 Ohms, is as close to a short-circuit as you can get in reality with speakers that are not faulty - so any amp that requires exactly a specific impedance is rarely going to get it.

 

One thing I have noticed over the years is that when you use a meter to test a music cabinet, you rarely get identical readings. An 8 Ohm impedance cabinet might appear to a meter as a DC resistance of between 5 and 7 Ohms - but if you have two supposedly identical speaker cabs, they always differ. Different number of turns on the coil? Slightly different copper wire? high resistance joints?

[SA90]

Having checked my current setup, the head I am currently using is also rated at 4ohms, going into my 8ohm cab.

This amp is only 300W...so going into a 8ohm cab, it's only transferring 150W?

 

The 900W amp should, in that case, give the cab a far better run for its money.

 

Thanks for all the info...I had actually figured out eventually that the new amp I am looking into will be fine, but wanted to know more but impedance anyway!

[Simon Lewis]

Just to complicate things.... even when you know what the amplifier can deliver into a given load, bear in mind that an "8 ohm" speaker" is probably only 8 ohms over a very small part of its operating range...

Also, the impedance curve is greatly affected by the cabinet design and loading method.

 

 

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