Impedance, current and volume levels

[vinnyb]

I'm a bit confused with all the speaker/amplifier impedance talk. Time and again I have come across discussions where it is advised not to pair a speaker with an impedance lower than the amplifier's lower impedance rating.

I understand that driving a lower impedance load, the amplifier would have to put out higher current than when driving a higher impedance load. The higher current output can lead the amplifier to run hot and go into protection mode.

However, here's what confuses me. A lower impedance load would result in higher SPL, whereas a higher impedance load would lead to a lower SPL. As a result if I were to connect a 16 Ohm load to the amplifier instead of a 4 Ohm load, I would have to dial the volume up considerably to achieve the same SPL that I heard with the 4 Ohm load.

The increase in the volume should technically increase the voltage output of amplifier and thus result in similar current output as in case of a 4 Ohm load. So the benefit we got from the increase in impedance is nullified. So if at the same SPL with different impedance loads if we are talking of the same current levels, then where's the benefit?

Do I make a sense, or am I completely wrong? I was planning to re-wire my speaker drivers from parallel to series to bump the impedance from 4 Ohms to 16 Ohms. Just to ensure my AV receiver runs cooler. I live in a place where the room temperatures can go as high as 110 Fahrenheit and I don't want my receiver to keep tripping.

 

[markw]

Before we get started on anything else, your postulation on volume levels being dependent on impedance are totally off base. The loudness of a speaker relative to the power used is dependent on the sensitivity of the speaker.

The higher the sensitivity, the more efficient it is. The more efficient a speaker is, the less power is required for the same loudness.

 

[vinnyb]

Before we get started on anything else, your postulation on volume levels being dependent on impedance are totally off base. The loudness of a speaker relative to the power used is dependent on the sensitivity of the speaker.

The higher the sensitivity, the more efficient it is. The more efficient a speaker is, the less power is required for the same loudness.

It seems my choice of the term, SPL was wrong in framing the question. I was referring to the volume output as the SPL instead of the true meaning of the term SPL.

In my given scenario, I'm referring to the same drivers, wired in parallel or series to present different impedance to the amplifier. Since, the drivers themselves do not change the SPL (true meaning) never changes.

I hope that clarifies my question?

 

[markw]

It seems my choice of the term, SPL was wrong in framing the question. I was referring to the volume output as the SPL instead of the true meaning of the term SPL.

No, not really. You're the one that mentioned volume as the crux of the issue. It's what hits the ears that matters, not what one measures.

 

[Swerd]

What speakers and receiver do you have? Perhaps we can answer your question about the possibility of receiver overheating and failing if we know what you have.

Mark is right. A speaker's sensitivity is directly measured as loudness when a 2.83 volt signal is applied, while a microphone is 1 meter away. If the speaker is a true 8 ohm load, 2.83 volts is about 1 watt, and if its a true 4 ohm load, that would be about 2 watts.

Ohm's Law is a simple equation that correctly applies only to DC electricity. Electrical music signals are AC, and a loudspeaker contains reactive elements such as drivers, capacitors, or inductors. In fact, a speaker's impedance varies depending on the frequency of the AC music signal.

To complicate matters further, when AC or time-varying voltage or current is applied, the relationship between voltage and current becomes the solution to a differential equation, so the simple Ohm's law does not directly apply. (Whenever differential equations are involved, I personally run for the door .)

I was planning to re-wire my speaker drivers from parallel to series to bump the impedance from 4 Ohms to 16 Ohms.

Are you saying that you intend to rewire the drivers that are internally attached in parallel to the crossover? Don't go there, you will void the warranty and possibly damage your speakers.

 

[vinnyb]

Given what I said, let me try and re-phrase the question.

It's said that an amplifier would put out a lower current for a higher impedance load and vice versa. However, for the "same setting of the volume knob" a higher impedance speaker would deliver lesser volume than a low impedance speaker. Remember, the drivers are the same and we are just re-wiring them.

Now if I want to hear the same level of volume with the higher impedance speaker, I would have to increase volume knob. Effectively, increasing the voltage output by the amplifier and thus the current (I may be wrong with this statement).

So, essentially to hear the content at the same volume on both the speakers (4 Ohm and 16 Ohm), I had to bump up the volume and thus increasing the current output. I see no benefit of having the amplifier drive a higher impedance load, since I would drive it harder to get the same volume level output thus nullfying the drop in the current output at the same setting of the volume knob.

 

[BoredSysAdmin]

http://www.htd.com/whole-house-audio/Impedance-Matching
http://www.amazon.com/AudioSource-AE100VC-Impedance-Matching-Control/dp/B003H3C94W

 

[vinnyb]

What speakers and receiver do you have? Perhaps we can answer your question about the possibility of receiver overheating and failing if we know what you have.

Mark is right. A speaker's sensitivity is directly measured as loudness when a 2.83 volt signal is applied, while a microphone is 1 meter away. If the speaker is a true 8 ohm load, 2.83 volts is about 1 watt, and if its a true 4 ohm load, that would be about 2 watts.

Ohm's Law is a simple equation that correctly applies only to DC electricity. Electrical music signals are AC, and a loudspeaker contains reactive elements such as drivers, capacitors, or inductors. In fact, a speaker's impedance varies depending on the frequency of the AC music signal.

To complicate matters further, when AC or time-varying voltage or current is applied, the relationship between voltage and current becomes the solution to a differential equation, so the simple Ohm's law does not directly apply. (Whenever differential equations are involved, I personally run for the door .)
Are you saying that you intend to rewire the drivers that are internally attached in parallel to the crossover? Don't go there, you will void the warranty and possibly damage your speakers.

Swerd, I have DIY floorstanders built several years ago that I now want to use with the Onkyo TX-NR545. Receiver is rated for minimum 6 Ohm load. I have no problem re-wiring the speakers. I'm also building a new crossover to match the change in the impedance.

I'm thinking whether the re-wiring is going to bring any benefit at all, considering that with higher impedance I'll have to set the reciever to higher volume to hear same loudness as with a 4 Ohm load.

 

[BoredSysAdmin]

Swerd, I have DIY floorstanders built several years ago that I now want to use with the Onkyo TX-NR545. Receiver is rated for minimum 6 Ohm load. I have no problem re-wiring the speakers. I'm also building a new crossover to match the change in the impedance.

I'm thinking whether the re-wiring is going to bring any benefit at all, considering that with higher impedance I'll have to set the reciever to higher volume to hear same loudness as with a 4 Ohm load.

a) you probably should've started with specifics and b) your AVR should be able to handle lower than 6 Ohms loads, but I won't go crazy with volume

 

[vinnyb]

a) you probably should've started with specifics and b) your AVR should be able to handle lower than 6 Ohms loads, but I won't go crazy with volume

Yeah, I feel I wasn't that clear with the question to begin with. The problem is that the receiver gets decently warm nowadays with two channels driven and low room temperatures. I'll be soon adding the rest of the channels and summer is coming in couple of months when temperatures soar.

I don't want the receiver to run out of power (all channels driven) at 4 Ohms considering it's a mid level receiver. Secondly, as the ambient temperatures rise I don't want the receiver to go into protect mode. Since I'm adding new crossovers I have the freedom to work with parallel or series wiring. Can't change the wiring later as it would effect the crossover point. Should I go for a 16 Ohm wiring? Would it really run the receiver cooler?

 

[TLS Guy]

First of all you can not wire the drivers in series. That would increase the impedance to 16 ohms in the low pass section. However to maintain the same crossover frequency you would have to quadruple the value if the chokes (inductors) and reduce the value if the capacitors by a value of four in the low pass filter of the crossover to maintain the same crossover frequency and slopes.

Your next problem is you would have poor output and high distortion likely to cause tweeter damage.

Now your receiver will be voltage limited. If your speakers are truly four ohm, then your receiver will deliver a quarter of the power to the bass drivers it did before you rewired it. So if you had a 100 watt receiver, it will no be a 25 watt one driving those speakers. I don't think that is what you want.

The best way to explain this to you is with the sensitivity spec.

Lets take a speaker that is 8 ohms and it has a sensitivity of 90 db with a drive of 1 watt at 1 meter.

Now let us take the same speaker and rewind the voice coils so it is a four ohm speaker. It will have a sensitivity of 90 db 1 watt 1 meter, because a watt is a watt, a power measurement.

Now let us use the 2.83 volts 1 watt one meter, the 8 ohm speaker will show a sensitivity of 90 db.

Now lets drive the 4ohm speaker with 2.83 volts 1 meter. The speaker sensitivity will now be 93 db 2.83 volts 1 meter.

So what happened. Well the four ohm speaker when driven at constant voltage took twice the power from the amp. So it did play louder by 3db.

Every time amp power is doubled to a speaker its output increases by 3db, until it suffers excursion limits or thermal compression.

This all came about, because tube amps were not good at maintaining voltage as impedance dropped. So the 1 watt 1 meter spec

Solid state amps on the other hand are good at maintaining constant voltage in the face of decreasing impedance.

This means they will deliver more power until they become current and or thermally limited.

The other thing to remember is that a speaker has wide swings of impedance with frequency.

Another issue is that speakers are inductive and voltage is not in phase with current and this factor called the phase angle also varies with frequency. The significance is that there is a gap between apparent power and true power. As phase angles go negative, the apparent power demands, which the amp has to meet, increase. This means the effective impedance can and often is lower than measured. You have to calculate the effective impedance from the measured impedance and the phase angle.

Now the point is that all amps are current/voltage and therefore power limited. However you can drive any speaker you want if you use your head and ears to limit volume and therefore current.

If your speakers are truly 4 ohm in the power range below 600 Hz, then you will have to be careful with an Onkyo receiver, which history tells us are more fragile than most.

You can not help your situation with your rewiring plan, you will make it worse by far.

Your only solution is to self censor and limit volume levels.

 

[BoredSysAdmin]

Onkyo's tent to run hot and best bet is to make sure there is sufficient ventilation. My TX-SR805 is one of the hottests ones Onkyo ever made, so I know thing or two.
I also going to take a minor issue with calling your new NR545 AVR "Mid-range"
If you look at Onkyo new models range - there are 2 models below your 545, but whole 4 above it... see where I am going with it? Also 65 Wpc@8ohm clean power spec on internal amp confirms it's clearly belongs on budget shelves...

 

[Swerd]

I agree with everything TLS Guy said in his previous post.

No manipulation of your speakers can get around the problem of an under powered amplifier trying to drive insensitive speakers. But we don't know how sensitive or insensitive your speakers are.

• Do you know their sensitivity?
• Do you have an impedance vs. frequency curve for your present design?
• What woofer(s) are in them? The manufacturer should have rated their sensitivity and nominal impedance.

Onkyo claims the TX-NR545 receiver can deliver 65 Watts/Ch (rated at 8 Ohms, 20 Hz-20 kHZ, 0.7% THD, 2 Ch, FTC), which is modest power, but let's assume honest. It's ability to produce power is limited by the capacity of its power supply transformer. If a poorly designed power amplifier attempts to drive a low-sensitivity and low-impedance speaker that has large impedance phase angle changes, it can overheat or go into oscillation. Onkyo (and other makers) deal with this by using protection circuits that shut the receiver down if it senses an overload. There is no getting around this without using speakers that really are more sensitive.

 

[Swerd]

… There is no getting around this without using speakers that really are more sensitive.

All this reminds me of the physics course I took in college (where I developed my fear & loathing of differential equations). Every exam had an extra credit question. They all were variations of perpetual motion machines where one of Newton's Laws of motion was violated, and we had to show what law made it impossible. As the year went by, and we got away from motion and into electricity, magnetism, heat, etc., the laws of conservation of mass & energy were added to Newton's laws.

Your idea of changing the wiring from parallel to serial sounds like one of those extra credit questions . You cannot make your amplifier more powerful (or less overloaded) by altering the wiring of your speakers. The power supply transformer in your receiver has only so much juice it can produce.

 

[vinnyb]

Thanks to everyone for all the replies. It's been really helpful.

TLS Guy: You nailed it with your explanation. I'll stick to the 4 Ohm rating of the speaker and watch out for high volume levels. Since, the 4 Ohm rating is the DC resistance, the actual impedance of the speaker may be around 6 Ohms in reality. But again you never know. I don't have access to the data sheet of the drivers to be sure. The whole point of asking the question was to limit the current draw by increasing impedance of the speaker and thus running the receiver cooler.

BoredSysAdmin: Since, the units below the 545 were lacking in features and as the 545 was almost near the features of the 646 I thought it would fall in the mid range segment. The unit is running with 6 inch open space above it. So the ventilation is good.

Swerd: The speakers are very sensitive. Coupled with a small room, I never feel the need to drive them hard. Unfortunately, the manufacture of the drivers shut shop long time back and I don't have access to the spec sheet. These were built almost 11 years ago. But still standing tall. If the speakers happen to introduce any problems with the receiver then I'm off to my new build.

 

[Swerd]

Glad we could help . The answer to your question isn't simple, and many others have asked something similar, expecting a simple answer. I'm glad you do understand the answer.

 

[TLS Guy]

Thanks to everyone for all the replies. It's been really helpful.

TLS Guy: You nailed it with your explanation. I'll stick to the 4 Ohm rating of the speaker and watch out for high volume levels. Since, the 4 Ohm rating is the DC resistance, the actual impedance of the speaker may be around 6 Ohms in reality. But again you never know. I don't have access to the data sheet of the drivers to be sure. The whole point of asking the question was to limit the current draw by increasing impedance of the speaker and thus running the receiver cooler.

BoredSysAdmin: Since, the units below the 545 were lacking in features and as the 545 was almost near the features of the 646 I thought it would fall in the mid range segment. The unit is running with 6 inch open space above it. So the ventilation is good.

Swerd: The speakers are very sensitive. Coupled with a small room, I never feel the need to drive them hard. Unfortunately, the manufacture of the drivers shut shop long time back and I don't have access to the spec sheet. These were built almost 11 years ago. But still standing tall. If the speakers happen to introduce any problems with the receiver then I'm off to my new build.

Hang around here. You should be a useful member as you clearly are educable and understood the explanations. You are also ready to accept advice with a reasonable explanation.

Unfortunately we have a lot of people wandering through here lately who can not understand any explanation no matter how simple and dumbed down. In the end there is a limit to how much you can simplify complex problems and a degree of foundation knowledge is always required. You seem to have the latter.

 

[vinnyb]

Hang around here. You should be a useful member as you clearly are educable and understood the explanations. You are also ready to accept advice with a reasonable explanation.

Unfortunately we have a lot of people wandering through here lately who can not understand any explanation no matter how simple and dumbed down. In the end there is a limit to how much you can simplify complex problems and a degree of foundation knowledge is always required. You seem to have the latter.

I'll be certainly around here.

Until recently I've been mostly into the DIY space where I never had to bother with impedance matching. The amplifiers I built had enormous power supplies (Almost half the weight of the receiver I have today) and as such had quite a headroom. It was the fair warmth of the receiver driving just two channels and forum discussions around the Onkyo heat issues that got me worried.

Thanks for the kind words and help in sorting this out.

Vinny

 

[PENG]

Ohm's Law is a simple equation that correctly applies only to DC electricity. Electrical music signals are AC, and a loudspeaker contains reactive elements such as drivers, capacitors, or inductors. In fact, a speaker's impedance varies depending on the frequency of the AC music signal.

Sorry Swerd, I think I know what you are trying to say, but for clarity, Ohm's Law applies to both DC and AC system. For DC, V=IR, R is resistance in ohms with phase angle=0 degree. For AC, V=IZ, Z is the impedance and is a complex number. Yes it varies with frequency, but Ohm's law still applies, the calculation gets a little more complicated and will usually involve complex numbers (the j term due to the inductive reactance and capacitive reactance), that's all.

http://hyperphysics.phy-astr.gsu.edu/hbase/electric/imped.html

 

[PENG]

Swerd, I have DIY floorstanders built several years ago that I now want to use with the Onkyo TX-NR545. Receiver is rated for minimum 6 Ohm load. I have no problem re-wiring the speakers. I'm also building a new crossover to match the change in the impedance.

I'm thinking whether the re-wiring is going to bring any benefit at all, considering that with higher impedance I'll have to set the reciever to higher volume to hear same loudness as with a 4 Ohm load.

For argument sake, if you wire two complete speakers (with crossovers) in series, then yes you will double the impedance. As you know the impedance will change with frequency but if the two speakers are identical (including the crossovers), then the impedance of each will vary with frequency the same way. If you do that, as TLS said, your little Onkyo will run out of voltage sooner or later. However, if you simply want to run the Onkyo cooler, it will help as long as you keep the SPL low enough because higher voltage for the same SPL will result in lower current. I know this is just theoretical, not that you, or anyone would use two speakers per channel and wired them in series.