New webpage explaining speaker impedances, etc.

gusfinley

New member
Hi,

It seems like I am always answering posts about speaker impedendances and what kind of problems a mismatch my give you - They seem to turn up every few days or so.

I created a webpage that should explain all this stuff, including diagrams and comouter simulations showing the effects of mimatched impedances.

Check it out and let me know if you have any suggestions or comments....

http://cc.usu.edu/~davbradshaw/amps/impedance.htm

If you find it helpful you may want to bookmark it and then paste the link in message boards when this question comes up....
 
Good idea. It takes effort to put up a page like that.


Section 1:


"So now, what does all this have to do with power? Well, Just as you have to work harder to get through a dense crowd, the more impedance a material has the more power is absorbed by it. Your amp itself has an equivalent impedance- this is what is labeled on the jack for your speaker- This is the impedance due to the internal components of the amp. That means that your amp not only provides power it also uses it up just trying to get the power to the speaker. As you can imagine you don't want your amp absorbing all that power."

The more impedance a material has, less power travels through it for a given voltage. It does not absorb power. It impedes it. A lower impedance lets more power through. I think you may be misinterpreting higher voltage drop across a higher impedance as an indication that it is "absorbing" power. :confused: Components need to be able to handle the current going through them, but they don't "absorb" it, except for the part that is lost as heat.

Your amp does not have an "equivalent impedance labeled on the jack". The impedance referred to on the jack is for the cabinet you want to hook to it. A jack labelled 8ohms on a guitar amp means it wants an 8ohm cab hooked to it. It does not mean the jack is another 8ohm load.

The analogy of having to work harder to get through a denser crowd, that is good. The crowd represents impedance.


"Now let's think about the speaker. It also has a characteristic impedance. Now, since the most power is absorbed by the component with the most impedance, If we were to match the impedances, each component would absorb the same amount of power. This is what we want as we will see in the following situations."

Speakers do not "absorb" power in the way you state. They transfer it from your amp to the air. The lower the impedance, the more power the amp can put out, and the more power gets to the air. A 4ohm speaker is louder at a given volume setting because it lets more power flow from the amp for a given input voltage, not because it absorbs less. Impedance is a measure of how much power can flow though something, not how much it absorbs. Power is lost as heat in the voice coils, but that is a separate issue.

Here is something to think about: The final impedance that any amp/speaker combo has to deal with is the impedance of the air in front of the speaker. Lower speaker impedance=more power from the amp=more power to the air.

Those little diagrams that show speakers as a resistance leave out the fact that those resistors have an output, ie they are coupled to the air. Any concept of power in an amp/speaker system needs to keep that in mind. The speaker is in essence a transformer, coupling the amp to the air. That transformer (speaker) has an impedance, which determines how much power the amp will produce for a given output voltage, and therefore how much is available to put into the air.

20V into 8ohms= 50W amplifier output, coupled to the air by the speaker
20V into 4ohms= 100W amplifier output, coupled to the air by the speaker

The impedance of the air doesn't need to be taken into account when dealing with system impedance, but the concept is important to understand.

You are correct, though, that if all the speakers have the same impedance, they will transmit the same power. But your reasoning as to why is flawed.

Section 2.

Section 2 builds on the misconceptions you have in Section 1, and adds a few more. All the info contained in it is incorrect, with the addition of the fact that you add power to the amp ratings to make up for your misconception that amps absorb power, and that higher impedance speakers use more power.

Your 10 watt amp was not producing 33W, nor absorbing 28W of power. It also most likely blew due to having too low of an impedance attached to it, not for the reasons you stated. Actually, in that diagram, you got the power levels to each load right, at least.

The idea of the amp "absorbing" power and figuring in the impedance listed at the output jack is incorrect.

For an amp producing 50W into an 8 ohm speaker, you do not figure in another 8ohm load to represent your amp because it says the jack is 8 ohms. The output jack is labelled with the proper impedance of the speaker to be connected to that jack. The amp does not "absorb" some of the power. If you hook up an 8ohm speaker to the 8ohm jack on a 50W amp, the speaker gets 50W, not 25W as you indicate. That is, if the amp actually can put 50W into an 8ohm load.

If you plug a 4 ohm speaker into an 8ohm jack, the speaker lets more power than an 8 ohm speaker, not less. It is louder, as well. Your diagrams and text indicate that the higher the impedance, the more power goes to the speaker. This is incorrect. More power flows through a lower impedance.


Also, people put different impedance speakers in cabs or on amps all the time. There is nothing inherently wrong with this. Think about it. If this were true, designers could only use one value of resistor in any given circuit.

In most bass cabs with piezo tweeters, for instance, the tweeters have a much higher impedance than the woofers. The tweeters are simply paralleled from the input jack or the woofers. No problems.

Many inexpensive PA and home stereo speakers also use this method. The higher impedance of the tweeter keeps the power level to the highs in an appropriate range. The piezo tweeters impede low freqs, and so the cab doesn't even need a crossover.

This concept is also used in some speaker clusters, where different impedance speakers are used to create what is called "amplitude shading". Using different impedances, different volumes can be had for different sections of a room, for instance.



"When impedances are correctly matched, the wave sees no difference in impedance when passing from one component to another, and all of the wave passes through the point where the components are connected. When the impedances aren't matched, they DO notice a difference and will react accordingly. When a mismatch occurs, part of the wave is reflected and part of it is transmitted. Not only does this mean that you don't get as much signal to you speakers, but part of you signal is sent back to your amp!! From there it will bounce back again (sometimes the wave will invert itself when it bounces) to the original mismatch and part of that reflection will pass through and part of it will be reflected AGAIN back to the amp, and so on until the signal that bounces back and forth and eventually dies down after a while. All of the bouncing waves in between points will add and subtract themselves from your original signal."

Again, think of how this would restrict all desgners to using one value of resistor for a circuit. Just not possible or practical.


Differences in tone from using two different impedance drivers more likely results from different response curves and impedance curves due to them being different drivers, not inherently from from being different impedances. Any reflections won't noticably be affected by that, nor will tone.
There is a phenomenon called "back-EMF" where a signal is sent back to the amp from a speaker as it vibrates after the initial signal. The speaker itself generates the signal. This can be a problem, but is usually well accounted for in amp designs. This is the spec referred to as "damping factor".



Section 3

No problems there.


Section 4

No problems there, even if it is worded a bit wordily. :)

I would put the formulas and their applications in one section, and the use of the calculator as another separate section. Trying to drop the calculator functions in there muddies up the explanation of the formulas, which is the important part.

Fixing the other sections is gonna be a lot more work, though.
 
Last edited:
woah!! -- I'm Under attack!!

Boingoman...

I have a found some errors in your rebuttal.... and will attempt to show that the reasoning I have used is correct...


First of all keep in mind that my analysis was from a steady state circuit
thoery/modelling standpoint. All the "other" crazy things that happen have been ignored such as the fact that impedance changes with frequency. This is an analysis of "ideal" components which do not exist!! I have presented them in this simplistic manner for those who are looking for info about how hooking up different values of speakers/ cabs to thier amps will affect it!!

Section 1:


"The more impedance a material has, less power travels through it for a given voltage. It does not absorb power. It impedes it. A lower impedance lets more power through. "

Power does not travel through anything!! Power is produced by a source in a limited amount. This source can be modeled as a thevenin equivalent circuit - a current source with a parallel impendance or a voltage source with a series impedance. I have used the Thevenin equivalent voltage source because it works better when it comes to power transfer scenarios....

Current is the only thing that travels in the electrical system.... You seem to confuse current with power in your statements...

First of all a component "impedes" current by absorbing power!! there are a few examples, based on your statement above.

For a given voltage... 50V, an impedance of 1 ohm gives 2500W, 10ohms gives 250W, 100 ohms gives 25W.... These results are found using the formula P = (V * V ) / R, which is a combination of P= I * R and ohms law, V = I * R. This is a measure of the power absorbed by the component itself. You will notice that the LOWER the impedance for this given voltage, the MORE power it absorbs, and therefore less power is available down the line....

"I think you may be misinterpreting higher voltage drop across a higher impedance as an indication that it is "absorbing" power. Components need to be able to handle the current going through them, but they don't "absorb" it, except for the part that is lost as heat."

Again using P= (V * V) / R as you increase the voltage drop for a given
impedance, indeed more power is being absorbed!! It is being absorbed in any way imaginable, usually heat.

"Your amp does not have an 'equivalent impedance labeled on the jack'. The impedance referred to on the jack is for the cabinet you want to hook to it. A jack labelled 8ohms on a guitar amp means it wants an 8ohm cab hooked to it. It does not mean the jack is another 8ohm load."

True the jack is not an "8 ohm jack", but the labeling on the jack is a thevenin equivalent impedance that is "seen" by the jack looking into the amp. It is really a stepped down value of the resistance of the plates from the transformer...

"Speakers do not "absorb" power in the way you state. They transfer it from your amp to the air. "

Speakers indeed abosorb power!! This is why they have power ratings!! When you blow up a speaker it has absorbed more power than it was made to absorb!! Also according to the laws of thermodynamics any time you change from one form of energy to another there is a loss of the energy (loss = absorbtion) the speakers transfer the electrical energy into mechanical energy to move the voice coils to creat the sound energy. Every step along the way power is being used up!!

"The lower the impedance, the more power the amp can put out, and the more power gets to the air. "

Again... There is a limited amount of power avaialble!!! The amp can only "put out" so much of it...

"A 4ohm speaker is louder at a given volume setting because it lets more power flow from the amp for a given input voltage, not because it absorbs less. "

A 4ohm speaker is louder at a given volume setting because it lets more CURRENT flow from the amp for a given input voltage... once again you have confused power for current.

Look at the equation P = I * I * R , indeed increasing the current will increase the power.... that is powere abosorbed which is power used, which means that your speaker is doing more work, making it louder for a lower impedance.

"Impedance is a measure of how much power can flow though something, not how much it absorbs. "

Impendance is an intrinsic porperty of matter!!! Even air has an impedance!! Impedance bounds power!! Notice that the equations for voltage ( V = I * R ) and current ( I = V / R ) are all dependant upon impedance, and therefore so is the calculation of power!! Once again Power does not flow.. Current does, and yes the lower the impedance of a component the more current will flow through it for a given voltage...


"Power is lost as heat in the voice coils, but that is a separate issue."

No, Power is power!! Once it is lost you cannot get it back!!


"Here is something to think about: The final impedance that any amp/speaker combo has to deal with is the impedance of the air in front of the speaker. Lower speaker impedance=more power from the amp=more power to the air. "

once again there is no way to get more power from an amp than it is capable of producing... Lower speaker impedance = more CURRENT from the amp= more power transfered from the speaker to that air (abosrbed by the speaker)....

"Those little diagrams that show speakers as a resistance leave out the fact that those resistors have an output, ie they are coupled to the air. Any concept of power in an amp/speaker system needs to keep that in mind. The speaker is in essence a transformer, coupling the amp to the air. That transformer (speaker) has an impedance, which determines how much power the amp will produce for a given output voltage, and therefore how much is available to put into the air."

The "rated' impedance of a speakers, doesn't care was is going on anywhere but at the speaker terminals, it is the equivalent impedance that will be seen at the terminals - the coupling with the air is all accounted for at this point and CAN be modelled only using the impedance (resistance) value...

"20V into 8ohms= 50W amplifier output, coupled to the air by the speaker
20V into 4ohms= 100W amplifier output, coupled to the air by the speaker"

Neglecting the coupling to the air and just using the equation P = ( V * V ) / R give the exact same results!! The coupling has already been taken into consideration in the rated impedance!!

"Section 2 builds on the misconceptions you have in Section 1, and adds a few more. All the info contained in it is incorrect, with the addition of the fact that you add power to the amp ratings to make up for your misconception that amps absorb power, and that higher impedance speakers use more power"

The idea that the higher impedance speakers use more power IS valid when there is more than one impedance at stake. The two speakers will form a voltage divider, and the larger of the speakers will have the larger voltage drop and therefore WILL absord more power!! P = ( V * V ) / R

"Your 10 watt amp was not producing 33W, nor absorbing 28W of power. It also most likely blew due to having too low of an impedance attached to it, not for the reasons you stated. Actually, in that diagram, you got the power levels to each load right, at least."

Its hard to say what was going on with the amp since I didn't take any measurements while it was still operational in this state... But again the voltage divider between the amp and the equivalent impedances of the speakers caused more power to be absorbed by the amp, a Direct result of the lower impedance attached to it, and that it why it burned up!!

"The idea of the amp "absorbing" power and figuring in the impedance listed at the output jack is incorrect."

This is an easy one... but not an obvious one... Let's look in the tube manual for a cathode biased class AB EL34 tube amp with a 375V supply voltage. Its maximum plate current is 2x95mA or .19 Amps. Using the equation P=V*I, we get the power produced by the tube. .19 * 375 = 71.25W !!! Yet, the "output power" listed in the manual is only 35W - about half of the produced power...

This clearly shows that half of the power produced by the tubes is being absorbed in the tubes, and this is because of thier impedance - or as the tube manual calls it, Load resistance. That load resistance ( 3.4K ohms ) is the impedance seen by the output transformer which gets stepped down to the 8 ohms that is seen by the output jack!!! This load resistance must be matched in order to have the most power (35W) tranferred to the load (speaker cabinet).

"For an amp producing 50W into an 8 ohm speaker, you do not figure in another 8ohm load to represent your amp because it says the jack is 8 ohms. The output jack is labelled with the proper impedance of the speaker to be connected to that jack. The amp does not "absorb" some of the power. "

Once again this has been proven mathematically above....

"If you hook up an 8ohm speaker to the 8ohm jack on a 50W amp, the speaker gets 50W, not 25W as you indicate. That is, if the amp actually can put 50W into an 8ohm load."

This is true!! I mistakenly labeled it as a 50W amp - it should really be 25W amp. In the model the amp is producing 50W, but only 25W is available for the load. An amp's output it rated as its ouput and NOT as its produced power. Thanks for pointing that out - I will update my page to reflect this....

"If you plug a 4 ohm speaker into an 8ohm jack, the speaker lets more power than an 8 ohm speaker, not less. It is louder, as well."

Once again, the speaker get more CURRENT, and therefore uses more power and is louder.

"Your diagrams and text indicate that the higher the impedance, the more power goes to the speaker. This is incorrect. More power flows through a lower impedance."

This would be true IF the amp was a pure current or voltage source - but it does have an equivalent impedance - and again, CURRENT flows.....


"Also, people put different impedance speakers in cabs or on amps all the time. There is nothing inherently wrong with this. Think about it. If this were true, designers could only use one value of resistor in any given circuit.
In most bass cabs with piezo tweeters, for instance, the tweeters have a much higher impedance than the woofers. The tweeters are simply paralleled from the input jack or the woofers. No problems.
Many inexpensive PA and home stereo speakers also use this method. The higher impedance of the tweeter keeps the power level to the highs in an appropriate range. The piezo tweeters impede low freqs, and so the cab doesn't even need a crossover.
This concept is also used in some speaker clusters, where different impedance speakers are used to create what is called "amplitude shading". Using different impedances, different volumes can be had for different sections of a room, for instance."

This is all a little more complex... but for someone wanting to hook thier guitar amp up to a cabinet properly, it doesn't really matter.......

"Again, think of how this would restrict all desgners to using one value of resistor for a circuit. Just not possible or practical."

Actually in "industry" a impedance "match" is indicated by a factor of two.
Therefore according to industry there would be no problem hooking up a 8 ohn amp to a 4 or 16 ohm cab, but they rarely are dealing with the kinds of currents that the guitar amp is pumping out!! The "2 factor" makes it both possible and practical for designers!!

"Differences in tone from using two different impedance drivers more likely results from different response curves and impedance curves due to them being different drivers, not inherently from from being different impedances. Any reflections won't noticably be affected by that, nor will tone."

Anytime there is an impedance mismatch there ARE reflection happening, the worse the mismatch the worse the reflections - better to be safe and just match them anyhow... It may be a negligable effect, but it is still happening... If you want to SEE it - mismatch your connection bewteen the TV cable wall-plate and the TV - you'll see all sorts reflection screwing up you formerly beautiful picture!! Does it effect tone? Hard to say...

"There is a phenomenon called "back-EMF" where a signal is sent back to the amp from a speaker as it vibrates after the initial signal. The speaker itself generates the signal. This can be a problem, but is usually well accounted for in amp designs. This is the spec referred to as "damping factor"."

Hmm, haven't ventured into this - probally due to a the electomotance produced when the speaker vibrations are tranlated to the voice coil and then through the terminals and wire back to the amp. It would most likely be a neglegable effect...

"I would put the formulas and their applications in one section, and the use of the calculator as another separate section. Trying to drop the calculator functions in there muddies up the explanation of the formulas, which is the important part."

Thanks for the suggestion. I think I'll keep them were they are as it not only teaches what the formulas are, but how to use them practially with a calculator...

Woah, That was exaustive!! I can see where a lot of the incorrect informations in your rebuttal was coming from - most of it would be true if the amp wasn't absorbing power, and didn't have a equivalent impedance...

Now you Know something that most people never will!!
 
gusfinley said:
Boingoman...

I have a found some errors in your rebuttal.... and will attempt to show that the reasoning I have used is correct...!!

It wasn't a rebuttal. It was commentary. You made the rebuttal. :D I'm not trying to attack you, it's just the page is fairly confusing.


gusfinley said:
First of all keep in mind that my analysis was from a steady state circuit
thoery/modelling standpoint. All the "other" crazy things that happen have been ignored such as the fact that impedance changes with frequency. This is an analysis of "ideal" components which do not exist!! I have presented them in this simplistic manner for those who are looking for info about how hooking up different values of speakers/ cabs to thier amps will affect it!!!!

There was nothing simple about your explanation, and you left much important information out. There was no mention of the fact of increased current draw from lower impedances, nor any mention of the fact that exceeding an amp's current capacity is the easiest way to kill it. You seem to be unaware of this. If an amp tries to put out too much voltage, it just clips. These are the three things people need to know most, IMO, besides how hooking up cabs affects impedance. When speakers are paralleled, this is a current divider, not a voltage divider. Most people, when hooking up multiple cabs, are dealing with paralleling. This is what happened to your PA. You dropped it below it's impedance rating.

The impedance rating on an amp is the minimum safe impedance you can hook to that amp. I didn't see that anywhere, either. Pretty important.

Too many speakers in series can be harmful, but it is a way less common occurence. People parallel speakers to take advantage of increased current flow with no voltage change.


gusfinley said:
"The more impedance a material has, less power travels through it for a given voltage. It does not absorb power. It impedes it. A lower impedance lets more power through. "

Power does not travel through anything!! Power is produced by a source in a limited amount. This source can be modeled as a thevenin equivalent circuit - a current source with a parallel impendance or a voltage source with a series impedance. I have used the Thevenin equivalent voltage source because it works better when it comes to power transfer scenarios........

Well, here in the real world of people hooking up guitar amps and power amps, for power transfer situations, we take the output voltage of the amplifier at the jack, and the speaker load, and figure out the power from that. What you speak of may be relevant for a designer, but not an end user.


gusfinley said:
Current is the only thing that travels in the electrical system.... You seem to confuse current with power in your statements..

LOL. You are correct. I should have said a lower impedance allows more current to flow, resulting in more power potential available for the speaker.

gusfinley said:
First of all a component "impedes" current by absorbing power!! there are a few examples, based on your statement above...
No, that is incorrect. Components impede current by being made of materials or designs which are more or less resistant to current flow. Silver is a better conductor than copper because it has more free electrons, not because it absorbs less power. Carbon is a poor conductor, as it has few free electrons. More carbon in a resistor means more impedance.
This is probably the most basic concept in electronics. If you do not understand this, you can understand nothing.

gusfinley said:
For a given voltage... 50V, an impedance of 1 ohm gives 2500W, 10ohms gives 250W, 100 ohms gives 25W.... These results are found using the formula P = (V * V ) / R, which is a combination of P= I * R and ohms law, V = I * R. This is a measure of the power absorbed by the component itself. You will notice that the LOWER the impedance for this given voltage, the MORE power it absorbs, and therefore less power is available down the line.......

Right data, wrong conclusion. The "component" in this case is the speaker, and the air it moves. More power to the component (air) is what you get when you use a lower impedance speaker. More is more, not less. More is indeed lost as heat, but more goes to speaker output as well. There is no "down the line". The speaker is the "end of the line".

And again, the speaker is not "absorbing" the power. It is allowing current and voltage to flow to a greater or lesser extent, depending on impedance. It dissipates the power as heat and movement.



gusfinley said:
Again using P= (V * V) / R as you increase the voltage drop for a given
impedance, indeed more power is being absorbed!! It is being absorbed in any way imaginable, usually heat.
More power is produced for a higher voltage, that is true. But the power is not absorbed by the component. It is dissipated. In the case of a speaker, it is dissipated as heat and movement.

gusfinley said:
True the jack is not an "8 ohm jack", but the labeling on the jack is a thevenin equivalent impedance that is "seen" by the jack looking into the amp. It is really a stepped down value of the resistance of the plates from the transformer..........

Even if this is true, you do not tack an extra 8ohm load on your amp when you are figuring out how much juice it puts out according to it's stated output. An 8ohm label on a jack on a 50W guitar amp means when you hook an 8ohm speaker to it, it can make 50W. All the internal stuff is taken into consideration in the power rating of the amp.

It is also an indication of the minimum safe impedance you should hook to that jack, to stay within the amp's current producing capacity.

gusfinley said:
Speakers indeed abosorb power!! This is why they have power ratings!! When you blow up a speaker it has absorbed more power than it was made to absorb!! Also according to the laws of thermodynamics any time you change from one form of energy to another there is a loss of the energy (loss = absorbtion) the speakers transfer the electrical energy into mechanical energy to move the voice coils to creat the sound energy. Every step along the way power is being used up!!

Really? In my universe the laws of thermodynamics say energy changes form, it is not gained or lost. Entropy is there, that's true, but in this case I feel safe saying it's not a factor. The voltage and current are changed to movement and heat. It is not absorbed by the speaker.

Speakers dissipate power. Speakers blow when they cannot safely dissipate the power they are receiving. The power rating on a speaker tells how much power the speaker can dissipate, not absorb.


gusfinley said:
A 4ohm speaker is louder at a given volume setting because it lets more CURRENT flow from the amp for a given input voltage... once again you have confused power for current. ..........

LOL. True enough. I should have said the lower impedance lets more current flow, which results in more power being generated.

gusfinley said:
Lower speaker impedance = more CURRENT from the amp= more power transfered from the speaker to that air (abosrbed by the speaker).........
Yes, which completely contradicts everything you implied on your site, and even earlier in this post.


gusfinley said:
The idea that the higher impedance speakers use more power IS valid when there is more than one impedance at stake. The two speakers will form a voltage divider, and the larger of the speakers will have the larger voltage drop and therefore WILL absord more power!! P = ( V * V ) / R.

Aaaahhh..finally I understand where you are coming from, and why you think the way you do.

Speakers hooked in parallel are a current divider, not a voltage divider. The one with less impedance gets more current, and the same voltage, and can produce more volume.
In series, yes, you are correct. In audio, most cabinets are paralled when hooked together. Wiring two speakers of different impedances in series would do what you say.

gusfinley said:
Its hard to say what was going on with the amp since I didn't take any measurements while it was still operational in this state... But again the voltage divider between the amp and the equivalent impedances of the speakers caused more power to be absorbed by the amp, a Direct result of the lower impedance attached to it, and that it why it burned up!! .

Again, no, even from your diagram. It shows the speakers wired in parallel. This makes a current divider, not a voltage divider. Your amp exceeded it's current capacity due to the low load, and fried.


All the stuff about amps absorbing power is true, I'll give you that. But it's not important or relevant for some guy just trying to hook up a cab to his amp. That was my real point.



gusfinley said:
"If you plug a 4 ohm speaker into an 8ohm jack, the speaker lets more power than an 8 ohm speaker, not less. It is louder, as well."
Once again, the speaker get more CURRENT, and therefore uses more power and is louder..

And you are contradicting yourself, as you spent a lot of time trying to convince me that a higher impedance speaker uses more power. I hope the current divider explanation helped you.


"Also, people put different impedance speakers in cabs or on amps all the time. There is nothing inherently wrong with this. Think about it. If this were true, designers could only use one value of resistor in any given circuit.
In most bass cabs with piezo tweeters, for instance, the tweeters have a much higher impedance than the woofers. The tweeters are simply paralleled from the input jack or the woofers. No problems.
Many inexpensive PA and home stereo speakers also use this method. The higher impedance of the tweeter keeps the power level to the highs in an appropriate range. The piezo tweeters impede low freqs, and so the cab doesn't even need a crossover.
This concept is also used in some speaker clusters, where different impedance speakers are used to create what is called "amplitude shading". Using different impedances, different volumes can be had for different sections of a room, for instance."

gusfinley said:
This is all a little more complex... but for someone wanting to hook thier guitar amp up to a cabinet properly, it doesn't really matter.........

And thevenin equivalents and power loss into the transformer aren't complex? Those things above are actual real-world examples people can relate to. You just read the numbers on the amp, and the cab, and don't go below the minimum impedance stated on the amp. Use the formula for parallel impedance to find out the load if you use multiple cabs.

gusfinley said:
Actually in "industry" a impedance "match" is indicated by a factor of two.
Therefore according to industry there would be no problem hooking up a 8 ohn amp to a 4 or 16 ohm cab, but they rarely are dealing with the kinds of currents that the guitar amp is pumping out!! The "2 factor" makes it both possible and practical for designers!!..

I was speaking of the fallacy of using drivers of different impedances together being a bad idea. Using that logic would restrict designers trying to use more than one resistor value in a design. I also wanted to point out that people paralleling drivers of different impedances is very common in audio.


gusfinley said:
Anytime there is an impedance mismatch there ARE reflection happening, the worse the mismatch the worse the reflections - better to be safe and just match them anyhow... It may be a negligable effect, but it is still happening... If you want to SEE it - mismatch your connection bewteen the TV cable wall-plate and the TV - you'll see all sorts reflection screwing up you formerly beautiful picture!! Does it effect tone? Hard to say!!..

If speakers are hooked in series, actually you are right. The inductance is different if you hook speakers together in series vs. parallel, even if they are of the same impedance. It does indeed affect frequency response. Reflections in guitar amps are much more of a problem at low frequencies, though. And most are due to back EMF.

gusfinley said:
"There is a phenomenon called "back-EMF" where a signal is sent back to the amp from a speaker as it vibrates after the initial signal. The speaker itself generates the signal. This can be a problem, but is usually well accounted for in amp designs. This is the spec referred to as "damping factor"."

Hmm, haven't ventured into this - probally due to a the electomotance produced when the speaker vibrations are tranlated to the voice coil and then through the terminals and wire back to the amp. It would most likely be a neglegable effect... !!..

You have the cause correct. But it is far from negligible. It is the most important factor affecting accurate solid "tight" bass response, aside from good basic design. Back EMF reflects like you said above and clouds bass. Damping factor is a measure of how well the amp controls the speaker. Essentially it measures how well the amp can keep the correct voltage during large current changes, and damp down back-EMF. It is a directly linked to the output impedance of the amp. Lower output impedance=better damping factor. Amps also use negative feedback to increase damping factor.


gusfinley said:
Woah, That was exaustive!! I can see where a lot of the incorrect informations in your rebuttal was coming from - most of it would be true if the amp wasn't absorbing power, and didn't have a equivalent impedance...
Now you Know something that most people never will!!

I knew that, but though it irrelevant and confusing to anyone just trying to hook up a cab to their amp.

Ah, the hell with it. I'm doing my own page.
 
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This discussion would be a lot clearer if the word 'power' was not liberally applied to both volts and watts. In one place, 'power' is taken to mean a voltage drop, in another, it's watts. No wonder there is confusion over the effect of different impedances.
 
hmmm...

Boingoman -

Its seems as though we keep going in around in circles trying to prove the same point - it really is kind of silly..

For example, speaker power absorbtion vs power dissipation - this is really power consumption and we have both shown that it happens but are just arguing over terminology and where it happens.. your idea of "dissipation" is really my idea of "absorbtion"

Also, In another forum someone pointed out that different amps use different equivalent circuits. It seems that most solid sate amps act as voltage sources, while tube amps operate as current sources...

I had assumed that for power considerations both of these models would produce the same effect, but it seems that modeling an amp as a current source produces the EXACT OPPOSITE of everything that I wrote about!! Looks like here again we were both right in our own reguard - the real question is - what kind of source is your amp.... because it DOES make a difference!!

I still have much to learn about all of this stuff.... Even after posting all I thought I knew I learned even more - looks like I wil have to update the page when I figure through all the details.. It was definately worth it due to all that I learned afterward!!

I guess there still is no easy answer for what is the "safer" mismatch, afterall..... I really depends on each indiviual amp!! Perhaps this is why there is som much confusion as to which the "safer" mismatch is...

I do hope that the example of power "dissipation" in the amplifier was helpful to you. I noticed that you had no response to that section. Without that fact things sure would be alot easier to figure out!!
 
gusfinley said:
Boingoman -

Its seems as though we keep going in around in circles trying to prove the same point - it really is kind of silly.. !!

LOL. I agree, sort of. Even then, though, the way you used the output impedance was way off base. It is an equivalent, not an actual component. You can't use it the way you did, as a discrete component to figure out what is going on inside an amp when you change the impedance at the output. As an equivalent, yes. Using it as an actual component, no. It has many components, in different places in the transformer.

Using it like you did causes the incorrect results for power with different speakers loads.

The change in dissipation in the amp is a result of lower impedance allowing more current flow, not the voltage dividing scenario you described.


gusfinley said:
For example, speaker power absorbtion vs power dissipation - this is really power consumption and we have both shown that it happens but are just arguing over terminology and where it happens.. your idea of "dissipation" is really my idea of "absorbtion"!!

Yeah, that was pretty much semantics. :) I'll go with you though- power gets absorbed as heat, and then dissipated.

gusfinley said:
Also, In another forum someone pointed out that different amps use different equivalent circuits. It seems that most solid sate amps act as voltage sources, while tube amps operate as current sources..!!

Aaahhh...I see. Different amps do indeed use different equivalent circuits. But.....

All amps are the same, as I see it, tube, solid state, signal, and power. They take a small input voltage, and turn it into a large output voltage. The only difference between a signal amp, like a mic pre, and a power amp, is the amount of current each is asked to provide into it's load. In power amps, current is the key.

Your page has two points, both correct.
Keep a tube amp well-matched, because
1) good matching provides max power transfer
2) proper matching keeps the correct impedance level on the transformer secondary.


Something was bothering me about those diagrams and your analysis, but I couldn't put my finger on it.

You can't use the output impedance in a simple way like you did. It does not function like a straight impedance with a voltage drop in the amp, not for figuring out the reason for dissipation in the amp, anyway. It's not a real thing. Using it like you did is how you got the (incorrent) result that an 8ohm load will "absorb" more power than a 4ohm speaker when it is hooked to an amp. Which ain't true.

The output transformer secondary is a voltage source. It's voltage level does not change with impedance, across what ever range it can handle. You put a speaker across it, current flows, it generates power. Put a lower impedance speaker across it, more current flows, more power is generated. Putting an equivalent impedance in series with the speaker, and basing your resulting ideas on that is where you went wrong.

Putting a lower impedance speaker on a tube amp lowers the impedance of the secondary/speaker circuit. This is the problem with the mismatch with a lower impedance speaker.

The increased power absorption/dissipation in the transformer due to impedance mismatch with a lower impedance speaker works like this:

The power tubes need the primary to have a certain impedance. The impedance of the primary is a reflection of the impedance on the secondary. Putting a lower impedance speaker on the amp lowers the impedance of the secondary. This, in turn, lowers the impedance of the primary. The power tubes now put their output voltage into the reduced impedance of the primary.

This results in more current flow in the primary, and more power into the transformer, which it must be able to dissipate, or it will be damaged. Too low of an impedance on the amp, and too much current will flow through the primary and tubes, resulting in.....smoke, baby.


So the problem comes from a reduced impedance on the secondary causing more current flow in the primary and tubes, rather than increased voltage drop causing increased current flow.



gusfinley said:
I had assumed that for power considerations both of these models would produce the same effect, but it seems that modeling an amp as a current source produces the EXACT OPPOSITE of everything that I wrote about!! Looks like here again we were both right in our own reguard - the real question is - what kind of source is your amp.... because it DOES make a difference!! ..!!

Your results were different due to the reasons above, not that tube and solid state amps act differently. The problem is that the output impedance is an equivalent, not what is actually happening in the amp.


gusfinley said:
I still have much to learn about all of this stuff.... Even after posting all I thought I knew I learned even more - looks like I wil have to update the page when I figure through all the details.. It was definately worth it due to all that I learned afterward!!

It was worth it to me, too. It's been a long time since I thought about anything before the output jack.

gusfinley said:
I guess there still is no easy answer for what is the "safer" mismatch, afterall..... I really depends on each indiviual amp!! Perhaps this is why there is som much confusion as to which the "safer" mismatch is...!!

Actually I think your point was valid, especially regarding tube amps. They do indeed want to see a specific load to ensure maximum power transfer and prevent transformer damage. Aside from potential damage, the power transfer issue alone is important to understand.

It does depend on the amp. Many Fenders, for instance, can handle a 100% mismatch either way and be happy, with good power transfer.

Tube amps are lucky they sound so good. Otherwise the hassle wouldn't be worth it. :)


gusfinley said:
I do hope that the example of power "dissipation" in the amplifier was helpful to you. I noticed that you had no response to that section. Without that fact things sure would be alot easier to figure out!!

Hopefully you will consider the above to be enough of a response. :)
 
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Gusfinley, and other Sound Gurus!
Hi there, I checked out your page, nice work. But I still have a question regarding speaker impedance. I thought it would just be easier to tell you what equip I have and your suggestions. I play in a band 3 piece, sometimes 4. I have a Crate 8DP+ dual power amp, It has two outputs for mains and two outputs for monitors (it has a switch where you can send the power from the mains/monitors to amp 2). Here is the rating on that thing: 200W At 4ohms and 300W at 2 ohms. I have two large cabs and two small cabs the large cabs are rated: 8 ohms 150 W RMS/600watts total power. They have one input and no output. The small cabs are rated 8 ohms, they have an in and out jacks and are 100 watts RMS, 200 watts peak 3khz. My bass player just picked up some new cabs that are 8 ohms. This would in effect give us 6 speakers to use for mains. ( I have a another power amp and eq that I use to run the monitorsfrom a line out on the Crate, usually 3 for all of us) My question/dilemma is this: I would like to run the 6 speakers for mains. I envision this type of set up. Using the four outputs on the Crate, hooking up one set of mains to one set of outputs on the Crate. Then, hooking up the smaller cabs to the other set of outputs and running those in "series" to the other larger cabs using the in and out jacks on the smaller cabs to run a cable to the other set of larger cabs. Will this give me 6 mains total but will this give me more power or less? Your thoughts?
THANK YOU DUDES!!!!
 
I think the concept here is good, but the actual implementation confusing at best.

The most common questions people have include:

1. Can I hook extra speakers to my amp
2. Will it hurt my amp
3. How do ohms work

Your first example is poor at best. A 25W amp will generally not produce much more than 25W. It can produce less with the wrong speakers and it can be damaged with wrong speakers.

Start with basics of power and load. Cover basics of series and parallel and resulting impedances. Then hit other areas like what makes it louder, things like 100W are not twice as loud as a 50W, etc.

Ed
 
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