Speaker Impedance Question

[ganttmann]

I was debating this with a tech at Protech (repair shop in Silver Spring, MD, where I used to be a bench tech and now work part-time doing ebay listings) and neither of us could find a logical answer. If you have two speakers, identical except for impedance - one is 8 ohms the other 4 ohms - and you plug them, one at a time, into the output of a solid state amp (tube amps have output transformers so I don't think this'd apply) would one be louder than the other? Theoretically, a 4 ohm load would allow an SS power amp to put out about twice the power. Would the 4 ohm speaker then be louder? I was thinking that a 4 ohm voice coil would have fewer windings and therefore need more current to move it in the magnetic field, which would make the SPL about the same for either driver.

Any thoughts?

Gantt

 

[mark norwine]

Provided that the ss amp in question has transistors that can handle the current associated with a low(er) load, then the amp will produce more power with a lower load.

More power is more power, period.

 

[ganttmann]

Ohm's law being what it is and the laws of physics being what they are I agree with that statement. So you vote for the 4 ohm speaker being louder?

Provided that the ss amp in question has transistors that can handle the current associated with a low(er) load, then the amp will produce more power with a lower load.

More power is more power, period.

 

[mark norwine]

Yep.

Not because it's a different Z in-and-of itself.....but because its lower, it will allow the amp to produce more power. More power into a load = more SPL.

 

[ganttmann]

Now, I'm not disagreeing - I'm just throwing this out as an idea: A 4 ohm voice coil would have half the wire of an 8 ohm coil. That could translate into less magnetic flux to move the coil in the field of the speaker's magnet, which could mean that it'd need more current to accomplish the same amount of motion as the 8 ohm coil. Which could, possibly, mean that they SPL would be the same in spite of drawing twice the power from the amp. What do you think?

My curiosity about about this stems from a project I had going a while back. I had a Peavey Session 400 that I bought mostly for the JBL D130 it had in it. I tried for months to get a 4 ohm recone kit for it thinking that then I'd have the whole 200 watts of the Session 400 available when I played. I ended up reconing it to 8 ohms and the Session 400 is long gone, tube geek that I am. But still, I wonder. Was I pissing up a rope the whole time?

Gantt

Yep.

Not because it's a different Z in-and-of itself.....but because its lower, it will allow the amp to produce more power. More power into a load = more SPL.

 

[tbone666]

i've wired up cabs to do this before i knew any better, and the 4 ohm was louder than the 8 - the speaker cab sounded very unbalanced.

 

[mark norwine]

What do you think?

I think you're over thinking...

Eminence Cannabis Rex.....available in 8 or 16 ohms. Either one has a sensitivity of 102dB @ 1W @ 1M. Point being, forget how they're made, mass of the coil, inductance of the coil, etc. Toss it all.

Either one, when presented with 1W will push the same amount of air as the other

Ergo, either one, when presented with 50W will push the same amount of air as the other.

Agree?

Hold that thought.

Power amp Crown D150.....great old, little amp. Put out 100W (per channel) into 8 ohms, and about 60 into 16

Clearly, a single C-Rex can't handle that, but follow the theory:

The crown amp, let's say "turned up half way" (with a 100mVac sine wave applied) will make about 50 into 8 but only about 30 into 16. So far so good, right?

Attach the 8 ohm speaker to one channel & the 16 to the other. Remember, they have the same sensitivity & will react "the same way" when presented with the same power.

But they're not being presented with the same power, are they? The 16 ohm speaker is seeing less. It won't be as loud. End of story.

 

[SatelliteAmps]

The speakers are both going to be equally loud.

The issue becomes how much is the amp pushing. If you wired up both speakers to their properly matched impedance amplifier, then they would put out the same volume.

The amplifier is going to be more efficient (and therefor louder) by pushing a smaller impedance load. But that wasn't the question.

 

[mark norwine]

If the amp is producing a constant 20Vac (remember...this is a SS amp; no OT, no "matching"):

Into the 8 ohm speaker that will be 50W. (20^2/8) With a 102 @1@1 speaker, that translates to 118 dB @ 1M

Into the 16 ohm speaker that will be 25W (20^2/16). With a 102 @1@1 speaker, that translates to 115 dB @ 1M

118 is louder than 115. Not much, but indeed louder, and that was the original question.

 

[ganttmann]

Now, that's a different situation. Two speakers of different impedance wired together - If wird in parallel the 4 ohm speaker will allow more current to flow which translates into more power and will be louder. Wired in series more voltage will drop across the 8 ohm speaker which means more power so the it will be louder.

i've wired up cabs to do this before i knew any better, and the 4 ohm was louder than the 8 - the speaker cab sounded very unbalanced.

 

[rockon1]

If the amp is producing a constant 20Vac (remember...this is a SS amp; no OT, no "matching"):

Into the 8 ohm speaker that will be 50W. (20^2/8) With a 102 @1@1 speaker, that translates to 118 dB @ 1M

Into the 16 ohm speaker that will be 25W (20^2/16). With a 102 @1@1 speaker, that translates to 115 dB @ 1M

118 is louder than 115. Not much, but indeed louder, and that was the original question.

+1 3db increase in volume. Bob

 

[ganttmann]

I thought that matching the impedance is more important with a tube amp because it has an output transformer. A solid state amp has a safe operating range based on the current handling ability of the output transistors. And the question was, in fact, whether using a lower impedance speaker (all other things being equal) would allow the amp to run more efficiently and therefore be louder.

Gantt

The speakers are both going to be equally loud.

The issue becomes how much is the amp pushing. If you wired up both speakers to their properly matched impedance amplifier, then they would put out the same volume.

The amplifier is going to be more efficient (and therefor louder) by pushing a smaller impedance load. But that wasn't the question.

 

[ganttmann]

It all makes sense. And +3 dB is about 50% more SPL, isn't it?

Gantt

If the amp is producing a constant 20Vac (remember...this is a SS amp; no OT, no "matching"):

Into the 8 ohm speaker that will be 50W. (20^2/8) With a 102 @1@1 speaker, that translates to 118 dB @ 1M

Into the 16 ohm speaker that will be 25W (20^2/16). With a 102 @1@1 speaker, that translates to 115 dB @ 1M

118 is louder than 115. Not much, but indeed louder, and that was the original question.

 

[ganttmann]

I'm good at over thinking. It does make sense. So - If I had been able to get a 4 ohm kit for my D130 and I had installed it in my Session 400 it would have been louder.

Gantt

I think you're over thinking...

Eminence Cannabis Rex.....available in 8 or 16 ohms. Either one has a sensitivity of 102dB @ 1W @ 1M. Point being, forget how they're made, mass of the coil, inductance of the coil, etc. Toss it all.

Either one, when presented with 1W will push the same amount of air as the other

Ergo, either one, when presented with 50W will push the same amount of air as the other.

Agree?

Hold that thought.

Power amp Crown D150.....great old, little amp. Put out 100W (per channel) into 8 ohms, and about 60 into 16

Clearly, a single C-Rex can't handle that, but follow the theory:

The crown amp, let's say "turned up half way" (with a 100mVac sine wave applied) will make about 50 into 8 but only about 30 into 16. So far so good, right?

Attach the 8 ohm speaker to one channel & the 16 to the other. Remember, they have the same sensitivity & will react "the same way" when presented with the same power.

But they're not being presented with the same power, are they? The 16 ohm speaker is seeing less. It won't be as loud. End of story.

 

[SatelliteAmps]

If the amp is producing a constant 20Vac (remember...this is a SS amp; no OT, no "matching"):

Into the 8 ohm speaker that will be 50W. (20^2/8) With a 102 @1@1 speaker, that translates to 118 dB @ 1M

Into the 16 ohm speaker that will be 25W (20^2/16). With a 102 @1@1 speaker, that translates to 115 dB @ 1M

118 is louder than 115. Not much, but indeed louder, and that was the original question.

The speakers do not get louder, the amplifier is producing more power.

 

[ganttmann]

The speakers do not get louder, the amplifier is producing more power.

I think this comment from Mark makes sense:

Eminence Cannabis Rex.....available in 8 or 16 ohms. Either one has a sensitivity of 102dB @ 1W @ 1M. Point being, forget how they're made, mass of the coil, inductance of the coil, etc. Toss it all.

Either one, when presented with 1W will push the same amount of air as the other

Ergo, either one, when presented with 50W will push the same amount of air as the other.

Based on that the lower impedance speaker should be louder. We all know that double the power does not mean double the SPL but it has to mean something, right?

Gantt

 

[rockon1]

I think this comment from Mark makes sense:

Eminence Cannabis Rex.....available in 8 or 16 ohms. Either one has a sensitivity of 102dB @ 1W @ 1M. Point being, forget how they're made, mass of the coil, inductance of the coil, etc. Toss it all.

Either one, when presented with 1W will push the same amount of air as the other

Ergo, either one, when presented with 50W will push the same amount of air as the other.

Based on that the lower impedance speaker should be louder. We all know that double the power does not mean double the SPL but it has to mean something, right?

Gantt

Yes. It means with twice the available power (4 ohms verses 8) it will be 3db louder. Done. Bob