Impedance Matching

n9352527

Joined Oct 14, 2005
1,198
Originally posted by n9xv+Feb 21 2006, 09:13 PM--><div class='quotetop'>QUOTE(n9xv @ Feb 21 2006, 09:13 PM)</div><div class='quotemain'>What I am saying about impedance matching in regards to audio power amps is based on the indicated or published output impedance of the amp. If a high power amps manual says connect output to 2, 4, 8, or 16-ohms etc, then thats exactly what you should do if you want to acheive expected published results. If you connect the amp to a higher load impedance than that specified, you'll denie the amp of its potential to deliver the rated power. If you connect the amp to a lower load impedance than that specified, you'll be overworking the power supply in order to deliver the "extra" current to the lower load impedance. In other words, think of a "lab test" if you will. Take the high powered amp and run it full bore into its "specified" load impedance. Take the same amp, run it into a load impedance of half the specified value. The amp running into the lower load impedance will tend to clip the signal peaks due to the power supply not being able to support the designed MPP levels any longer. So we have distortion and increased heat dissipation and soon much damage will occur. Running the amp into a higher load impedance wont "hurt" it per say, the amp is just not reveiling its potential in that case. It cant deliver the "rated" power because the higher load impedance "limits" the available current to the speakers.

I'am trying to point out the facts about the importance of impedance matching but all I've seen as a rebutle is that the output impedance of an audio amp is very low so it does'nt matter what you connect to it. That wont "hold up" in the court of science.

When I say matched impedance, I mean "matched" to the specified output impedance of the amp in question. If an amp has a connection on the rear pannel labled 8-ohms, then one understands to connect an 8-ohm load to that output. The load is now "matched" to the output and the amp is happy. It does'nt matter what you "think" the output impedance of the amp "really" is, all that is necessary for the amp to operate properly is that it sees it's specified load impedance. Why do you think the impedance matching phenomena is different for AF than RF? It all boils down to efficiency. You can run power into a grossly mismatched load, and, as long as you keep it cool enough it will operate. The efficiency will be close to nothing but if kept "packed in dry ice" it will indeed operate. The designers knew what they were doing when they designed the amp. They gave it ratings based on certain criteria etc. For an audio amp that criteria is load impedance. To say an amp can deliver X-watts is only true if the amp is connected to the specified load impedance. If the load impedance is different than that specified, then X-watts is no longer true. Again I'am speaking of "real" power levels. This is true and applies to any power level but the "damage" is prominant at the higher power levels. I'am not arguing that 1 watt into a walkie-talkie speaker will destroy the amp if it is "mismatched" to the load.

Load impedance < the specified value = heavier burden on the power supply, increased heat dissipation, clipping/distortion etc.

Load impedance > the specified value = amp unable to deliver the available power to the load.

How much of a "problem" this all is depends on the power levels involved.
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So basically what you are saying is that for an audio amp the best scenario is to use a speaker with the "specified" impedance. And observing this "specified" load impedance is what you termed impedance matching?

<!--QuoteBegin-n9xv
@Feb 21 2006, 09:13 PM
I'am trying to point out the facts about the importance of impedance matching but all I've seen as a rebutle is that the output impedance of an audio amp is very low so it does'nt matter what you connect to it. That wont "hold up" in the court of science.
[post=14256]Quoted post[/post]​
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Please could you refer the scientific proof. I could always use something that broadens my limited knowledge.
 

n9xv

Joined Jan 18, 2005
329
By "scientific" I mean respecting the published data for the amp. I've never seen an amp that said "the output impedance of this amp is very low so dont fuss over the load impedance". Thats not the way the professionals attack a $10,000 + power amp. The "big boys" go by the numbers on this stuff because they've invested alot of money into it. In the case of a power amp maximum power transfer to the load is the absolute goal. Without that "match" this goal cant be realized. Thats why there's so much extensive documantation on the various configurations of series/parallel speaker arrangements. You can be creative in the quantity of speakers you connect to the output of the amp. As long as you bring it all back to the specified load impedance. Until I can find otherwise, for now I'll trust the engineers.

As I pointed out earlier, when a power amp designer designs an amp, he starts with the load and works his way back towards the input. Lets say he wants 1000-watts (average power) across an 8-ohm load. This will be 11-amps rms current and 90-volts rms. This will be about 250-volt Pk-to-Pk. So, we'll say MPP for this amp is 250-volts. - 125-VDC rail & + 125-VDC rail (push-pull output configuration).
The power supply is built at +/- 125-VDC @ 11-amps. Now throw a monkey wrench into the design by changing the load impedance. Those numbers are no longer valid and the amp wont do what is expected at the origional specified impedance.

Just like in Amatuer Radio where the legal limit is 1500-watts. 1500-watts does'nt come cheap, so you can bet the owner knows the manual and specs like his own children (unfortunately maybe better!). When tuning those RF power amps there is a specified time you have to work with for a specified mismatch condition, plate current etc. If you dont go by the book, you have one really big toaster on your hands.
 

n9352527

Joined Oct 14, 2005
1,198
Two pages (page1 and page2) that explain why it is necessary to keep output impedance of an audio amplifier as low as possible for better efficiency, response and control.

And yes, for audophile low amplifier output impedance (also commonly referred to as damping factor taken together with speaker impedance) is an important parameter and is commonly advertised (see at the bottom of this page or do a google search on damping factor of audio amplifier).

Note that damping factor is the ratio of impedance mismatch between the load impedance and the output impedance. High damping factor is preferred, where output impedance is more than _100_ times smaller than the speaker impedance (100 times mismatch!).

Damping Factor = Load Impedance/Output Impedance
 

n9xv

Joined Jan 18, 2005
329
Excellent material on damping factor. But dont confuse that with the mismatch scenario. The mismatch that you have controll over is that between load and specified output impedance. Still, allways follow the amps specifications as to the proper load impedance. Damping factor demonstrates the fact that you want maximum voltage & current to appear across the load, ultimately resulting in maximum power to the load and not waisted in the source providing the power.
 

n9352527

Joined Oct 14, 2005
1,198
Originally posted by n9xv@Feb 22 2006, 08:16 PM
Excellent material on damping factor. But dont confuse that with the mismatch scenario. The mismatch that you have controll over is that between load and specified output impedance. Still, allways follow the amps specifications as to the proper load impedance. Damping factor demonstrates the fact that you want maximum voltage & current to appear across the load, ultimately resulting in maximum power to the load and not waisted in the source providing the power.
[post=14285]Quoted post[/post]​
Thank you, and it seems you do agree, at last, that it is not impedance matching between amplifier output and load (speaker) that is important in audio amp, but observing the specified load (speaker) impedance.
 

n9xv

Joined Jan 18, 2005
329
I agree in principle. In practice, the "match" is between speaker and specified output impedance of the amp, with dampning factor indicating how well the output stage delivers the power to the load without waisting it. I will sum it up to say that no matter what impedance is what, allways adhere to the recomendations of the amps specifications as to proper load impedance and you cant go wrong. There's more to audio than just hearing it!
 

pebe

Joined Oct 11, 2004
626
Originally posted by n9xv@Feb 23 2006, 01:26 PM
I agree in principle. In practice, the "match" is between speaker and specified output impedance of the amp, with dampning factor indicating how well the output stage delivers the power to the load without waisting it. I will sum it up to say that no matter what impedance is what, allways adhere to the recomendations of the amps specifications as to proper load impedance and you cant go wrong. There's more to audio than just hearing it!
[post=14293]Quoted post[/post]​
Can I add my two pennyworth? I am no audio buff but I understood that the purpose of a low source impedance at the output of an amplifier was to damp natural oscillations in the loudspeaker. If you feed a pulse into a speaker its mass will make it overshoot and ring at its natural resonant frequency. As it overshoots its correct position it will generate an apposing voltage and force a current back into the output stage. A low output impedance will load the speaker and tend to stop the ringing.

It is for that reason that heavy guage cables are always recommended for speaker leads.
 

n9xv

Joined Jan 18, 2005
329
Yea your on to something there too. I remember skimming over that recently in some articles on the net. Interesting how back emf becomes a factor with a really massive speaker coil. I wonder even how big the largest speakers available are, and how much power they can handle. Really gives meaning the idea of "feeling" the rythm!
 
You're talking about different things...

There is output impedance which is really low in amps (much lower the the speaker you connect them to).

There is output impedance rating which refers to what speaker you should connect a certain amp to.

Someone said that speaker impedance is non-resistive and depends of frequency. That's wrong. It's purely resistive. Connect an ohm-meter to the terminals of an 8 ohm speaker and it will show you 8 ohms.
 

Papabravo

Joined Feb 24, 2006
21,158
Someone said that speaker impedance is non-resistive and depends of frequency. That's wrong. It's purely resistive. Connect an ohm-meter to the terminals of an 8 ohm speaker and it will show you 8 ohms.
Nonsense. 8 ohms is the magnitude of the complex impedance of the speaker at some frequency of interest depending on the speaker. Please don't tell me that the impedance of a subwoofer is the same at 10Hz. and 20kHz. I'm just not buying it.
 

Gadget

Joined Jan 10, 2006
614
Of course its non resistive. Inductive for most..(or Capacitive in the case of Piezo)... a speaker is a coil moving in a magnetic field. The impedance can change quite drastically depending on the frequency.
If you measure a speaker coil with an Ohm meter, you will effectively measure the DC resistance of the wire...(which is normally substantually Lower than the actual AC impedance).
 
Well... it seems i was wrong. It's resistive and inductive. Speakers have a resistive impedance rating (like some 8 ohm have 6.2 resistance and mine - the one i measured - is 8 ohms and has 8.2 ohms resistance).
 
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