Can output impedance measurement/plot be added to the amplifier test repertoire?

[solderdude]

As a result of this ratio, the speaker is unable to move the amplifier around by way of its back EMF, since a relatively high Z speaker is trying to push a near dead short around.

The damping factor is merely another way of describing output impedance. A parlor trick so manufacturers could state huge numbers and use it as a marketing tool.
You might remember the DF race with Sigma drive reaching numbers of 10000 and tales of 'control' or 'grip' on the drivers.

Consider a speaker with a resistance of 8 ohm and it is externally 'moved' up an down.
It generates a voltage (coil in a magnetic field).
When you load that voltage source (with an 8ohm resistance) with a short (D.F. infinite) the motion of the speaker is generating a current (Vvoicecoil/Rvoicecoil) so the current that 'damps' is U/8. Lets make the voicecoil voltage 8V (open coil voltage) this means the damping current is 1A.
When the output R is not 0ohm but say 0.2ohm the damping current is 8/8.2 = 0.975A so basically the same.
The fact that there is no voltage that can be measured with a 0ohm output R does not mean the driver cannot move. It just means you cannot measure a voltage when a current is applied.

A relatively high output R (opposite the driver) can noticeably reduce the actual 'damping current'.
This is not what is the actual problem though. What is audible is a simple voltage division issue (frequency dependent) which changes the FR of the signal arriving at the speaker terminals (R out + R speaker cable).

 

[mike7877]

The damping factor is merely another way of describing output impedance. A parlor trick so manufacturers could state huge numbers and use it as a marketing tool.
You might remember the DF race with Sigma drive reaching numbers of 10000 and tales of 'control' or 'grip' on the drivers.

Consider a speaker with a resistance of 8 ohm and it is externally 'moved' up an down.
It generates a voltage (coil in a magnetic field).
When you load that voltage source (with an 8ohm resistance) with a short (D.F. infinite) the motion of the speaker is generating a current (Vvoicecoil/Rvoicecoil) so the current that 'damps' is U/8. Lets make the voicecoil voltage 8V (open coil voltage) this means the damping current is 1A.
When the output R is not 0ohm but say 0.2ohm the damping current is 8/8.2 = 0.975A so basically the same.
The fact that there is no voltage that can be measured with a 0ohm output R does not mean the driver cannot move. It just means you cannot measure a voltage when a current is applied.

A relatively high output R (opposite the driver) can noticeably reduce the actual 'damping current'.
This is not what is the actual problem though. What is audible is a simple voltage division issue (frequency dependent) which changes the FR of the signal arriving at the speaker terminals (R out + R speaker cable).

To add to your last point, another thing that happens with increased output impedance is crossover points get shifted, and this can adversely affect driver integration - instead of hearing each speaker as a single source of sound, you begin to hear the different drivers. In addition, frequency response can (will) get peaky and/or have suck-outs (which way and how many points move depends on how many ways the speaker is, and which way the increased impedance affects each crossover point). Of course, this becoming an issue in the first place is dependent on the speakers being made properly in the first place. A cap that goes bad over time (higher ESR in addition to decreased capacity being potential problems) can cause similar problems - remedied by replacement.
Think of how low the ESR of a crossover cap is. Pretty low - likely lower than the speaker wire. Even this extremely small impedance is considered when (properly) designing a speaker's crossover network.

edit: I think a pretty good case has been made that amplifier output impedance/ damping factor is a good metric to measure and include in amplifier reviews. Some people don't seem care for it, but not many. Listening to them would be like listening to non-audio folks who say there's no point to ASR, so shut it down (a stupid thing to do). The people who don't care for audio, they aren't calling for ASR to be shut down, so I find it really hard to understand why people actually interested in the science of audio are actively trying to limit the collection of useful specifications during systematic review. They're tough, that's for sure.
I say this thread has been a success. We'll see!

 

[Deleted member 48726]

To add to your last point, another thing that happens with increased output impedance is crossover points get shifted, and this can adversely affect driver integration - instead of hearing each speaker as a single source of sound, you begin to hear the different drivers. In addition, frequency response can (will) get peaky and/or have suck-outs (which way and how many depends on how many ways the speaker is, and which way the increased impedance affects each crossover point). Of course, this becoming an issue in the first place is dependent on the speakers being made properly in the first place. A cap that goes bad over time (higher ESR in addition to decreased capacity being potential problems) can cause similar problems - remedied by replacement.
Think of how low the ESR of a crossover cap is. Pretty low - likely lower than the speaker wire. Even this extremely small impedance is considered when (properly) designing a speaker's crossover network

Diminishing returns set in very quickly reg. DF. There are a couple of comprehensive threads on the subject. There are some eye opening calculations.

The crossover network and cables pull down any high DF. Besides that you can think of the speaker drivers and the cabinet as an additional circuit too.

 

[mike7877]

Diminishing returns set in very quickly reg. DF. There are a couple of comprehensive threads on the subject. There are some eye opening calculations.

The crossover network and cables pull down any high DF. Besides that you can think of the speaker drivers and the cabinet as an additional circuit too.

Yeah. In my OP I added some stuff about it. I compared 800 vs 600 vs 400 at the amplifier vs at the speakers with 3m of 10ga wire. If I remember right, you keep about 75 of the first 200 you add from 400, and then 58 of the next 200 you add for 800. So even though you doubled amplifier impedance (+400), at the cablesspeaker terminals you only get about +125. Then there's what you lose in the woofer's crossover coil (its own coil has other effects lol). Basically what it works out to is, unless you have a really really good system, anything over 200 is going to sound about the same. So it's important to be able to separate DF 30 from DF 200. Less important above that. But we don't know 'til we measure!

edit: Personally I think systems designed so well that when an increase of DF to more than 200 improves sound quality, all that energy put into making those speakers should have gone into making them active - it'd be 10x better (an exaggeration, of course. You get the point)



People who think damping factor is a pointless measurement have been brainwashed, and incompletely.
Damping factor was hyped up a ways back, and manufacturers started striving for extreme numbers at the expense of actual performance.
So then most hi-fi amps had damping factors of over 200. At that point in time, for hi-fi amps, damping factor ceased being something to look at. But time has passed since then, and not all amps are hi-fi amps "...most hi-fi amps had damping factors of over 200", measuring the assortment of amplifiers in the varying devices they arrive as, is important. In this circumstance.
Will it be a useful metric for everyone? No.
But it will be for a good number. Especially those who are knowledgeable and own components more likely to be affected by an amp having a higher output impedance. And can't forget people doing research to understand - another data point.
Plus, one of the aims of this site is to hold manufacturers accountable. Some of them post damping factor specs - to get them to stop lying about them they need to know they're being checked!

 

[Mnyb]

Df is only a single number usually at 1kHz I assume ?

Tracking output impedance over 20-20000kHz says more , I’d like that.

My vaguely hold opinion is that about df 100 is where it ends

But it’s up to Amirm, how much work is it ? Can it be automatically calculated from already collected data

 

[solderdude]

Think of how low the ESR of a crossover cap is. Pretty low - likely lower than the speaker wire. Even this extremely small impedance is considered when (properly) designing a speaker's crossover network.

Say we have a 4.7uF cap and 4 ohm tweeter. This is a 8.466kHz -3dB point and 5kHz XO point.
At 20kHz the virtual resistance is 1.7ohm
When we add say 0.4ohm (DF10) at 20kHz the total load = 6.1ohm instead of 5.7ohm. The resistance wire is 4ohm (but impedance has probably increased already)
The cutoff point (-3dB) will have shifted from 8.466kHz 7.696kHz 10%. Assuming the tolerance of an real world capacitor is also around 10% it may even hardly shift or shift more. Does this belong to the charm of a tube amp ?
When considering DF40 the change is less than the actual tolerance.

Also consider the other drivers also shift a little in XO point. The XO freq of a woofer with 220uH inductor (4ohm 5kHZ XO) will also shift but upwards so there will be more overlap not a bigger gap so you would not hear individual drivers but get peaking and or phase issues with a higher output Z.

Now consider multiple inductors and capacitors and even resistor and they all have tolerances I would say that yes, DF will give some slight changes in XO point but component tolerance might likely screw up the actual filter frequency vs the designed one more so than the difference between a decent DF and high DF.

Besides.... people that use high output R (DF is really a nonsensical number) do want the sound to change and it will. XO point shift and voltage division all combined can surely lead to audible differences. But that's what they want and the funny thing is... it is always for the 'better'.

So yeah, output resistance for speaker amps is something that should be measured but varying impedance of the load (voltage division) is of far greater influence to the sound (talking about relatively high outputR+cable resistance) than the shift in filter frequency.

Whether or not you can get @amirm to include (properly made, without cable influence) output R plots is a different matter. It would be an addition.
That said... for Zout < 0.1ohm there is not much influence to the actual sound let alone < 0.05ohm.

When you want to see DF numbers the DF scale would need to be log type and you would need one for 4ohm and one for 8ohm or make a remark about it.
Would be 'better' to just post the output Z (over frequency).
The words 'Damping Factor' creates the wrong expectations.

 

[mike7877]

Say we have a 4.7uF cap and 4 ohm tweeter. This is a 8.466kHz -3dB point and 5kHz XO point.
At 20kHz the virtual resistance is 1.7ohm
When we add say 0.4ohm (DF10) at 20kHz the total load = 6.1ohm instead of 5.7ohm. The resistance wire is 4ohm (but impedance has probably increased already)
The cutoff point (-3dB) will have shifted from 8.466kHz 7.696kHz 10%. Assuming the tolerance of an real world capacitor is also around 10% it may even hardly shift or shift more. Does this belong to the charm of a tube amp ?
When considering DF40 the change is less than the actual tolerance.

Also consider the other drivers also shift a little in XO point. The XO freq of a woofer with 220uH inductor (4ohm 5kHZ XO) will also shift but upwards so there will be more overlap not a bigger gap so you would not hear individual drivers but get peaking and or phase issues with a higher output Z.

Now consider multiple inductors and capacitors and even resistor and they all have tolerances I would say that yes, DF will give some slight changes in XO point but component tolerance might likely screw up the actual filter frequency vs the designed one more so than the difference between a decent DF and high DF.

Besides.... people that use high output R (DF is really a nonsensical number) do want the sound to change and it will. XO point shift and voltage division all combined can surely lead to audible differences. But that's what they want and the funny thing is... it is always for the 'better'.

So yeah, output resistance for speaker amps is something that should be measured but varying impedance of the load (voltage division) is of far greater influence to the sound (talking about relatively high outputR+cable resistance) than the shift in filter frequency.

Whether or not you can get @amirm to include (properly made, without cable influence) output R plots is a different matter. It would be an addition.
That said... for Zout < 0.1ohm there is not much influence to the actual sound let alone < 0.05ohm.

When you want to see DF numbers the DF scale would need to be log type and you would need one for 4ohm and one for 8ohm or make a remark about it.
Would be 'better' to just post the output Z (over frequency).
The words 'Damping Factor' creates the wrong expectations.

Excellent example
Yeah, damping factor strongly suggests a large focus on woofer control specifically.
I'm going to rename the title. Nobody even plots damping factor over frequency, output impedance is plotted, and damping factor is calculated depending on one's speakers. Even for damping factor discussion, output impedance is more accurate.

 

[Piere]

Again: What should the magic number for DF be then for the tested amplifiers? And why? Without a clear reference value, it appears useless Amir incorporates it into his measurements.

 

[SIY]

Again: What should the magic number for DF be then for the tested amplifiers? And why? Without a clear reference value, it appears useless Amir incorporates it into his measurements.

There is no "magic number." For most speakers, lower source Z is better, but you hit diminishing returns very quickly.

 

[fpitas]

Again: What should the magic number for DF be then for the tested amplifiers? And why? Without a clear reference value, it appears useless Amir incorporates it into his measurements.

If that's a serious question (?):

Audio Myth - "Damping Factor Isn't Much of a Factor"

Myth - "Damping Factor Isn't Much of a Factor" Myth - "A Damping Factor of 10 is High Enough" Myth - "All Amplifiers Have a High-Enough Damping Factor" Where did these Myths Originate? These myths seem to trace back to a well-known paper (PDF) written by Dick Pierce. His analysis shows that a...

benchmarkmedia.com

 

[Piere]

Now we are on solid grounds. Yes it's a serious question. Although a serious answer seems fairly complicated. DF is just another number for the amplifiers output impedance. But its usefulness is not much different than the weight of an amplifier.

 

[fpitas]

Now we are on solid grounds. Yes it's a serious question. Although a serious answer seems fairly complicated. DF is just another number for the amplifiers output impedance. But its usefulness is not much different than the weight of an amplifier.

I'd argue that. The article I posted makes good points about the interaction of amp driving impedance and varying speaker impedance.

 

[mike7877]

I'd argue that. The article I posted makes good points about the interaction of amp driving impedance and varying speaker impedance.

There are people who form pretty solid opinions on things they know nothing about.
There are people who like to argue.
There are people who embody both characteristics - you can't convince them of anything.
Don't give them too much attention - it's mentally draining if you do.
Over time, this ends with you not being as passionate in discussion with people who are legitimately interested and always looking to improve their knowledge base.

I don't think there's anyone like that in this thread, but you never know... It's good to spot them early

 

[fpitas]

There are people who form pretty solid opinions on things they know nothing about.
There are people who like to argue.
There are people who embody both characteristics - you can't convince them of anything.
Don't give them too much attention - it's mentally draining if you do.
Over time, this ends with you not being as passionate in discussion with people who are legitimately interested and always looking to improve their knowledge base.

I don't think there's anyone like that in this thread, but look out!

Well...even engineers bicker. Get a bunch of experts in a room and they act like cranky old ladies.

 

[mike7877]

Well...even engineers bicker. Get a bunch of experts in a room and they act like cranky old ladies.

Experts become experts by having an open mind and working to expand their knowledge. As I said, I don't think there's anyone in this thread, but people who are opinionated and like to argue, it's a bad combination. Especially when strong opinions are formed and held in the face of simple facts being presented time and time again which refute their position

 

[fpitas]

No offense, but a lot of the arguing has come from you

 

[egellings]

Apart from telling fairy tales to my speakers, what's the use of it then?

Once you get to a reasonably low level of amplifier Zo, not much, outside of generating advertising copy. You need some, though. You wouldn't want to drive your speakers with a voltage controlled current source, either. Nice thing is that modern day S.S. amplifiers come with a plenty low Zo, so it's usually not an issue. With vacuum tube amplifiers, it can become an issue.

 

[mike7877]

No offense, but a lot of the arguing has come from you

I have been stating facts to inform a few people who are unaware of certain realities. A couple others seem to think that because the information isn't useful to them specifically there's no reason to collect it. When I sense trolling, I'm pretty quick to disengage. I thought I was being helpful, but I'm not always right (see that right there? self-awareness! the ability to correct previous misperceptions)

 

[fpitas]

I have been stating facts to inform a few people who are unaware of certain realities. A couple others seem to think that because the information isn't useful to them specifically there's no reason to collect it. When I sense trolling, I'm pretty quick to disengage. I thought I was being helpful, but I'm not always right (see that right there? self-awareness! the ability to correct previous misperceptions)

You made your case. And the Benchmark article explains things pretty thoroughly. It's a case of leading the horse to water etc. It is true, the vast majority of modern amps have good, low output impedance. But testing it is not a bad idea.

 

[mike7877]

Once you get to a reasonably low level of amplifier Zo, not much, outside of generating advertising copy. You need some, though. You wouldn't want to drive your speakers with a voltage controlled current source, either. Nice thing is that modern day S.S. amplifiers come with a plenty low Zo, so it's usually not an issue. With vacuum tube amplifiers, it can become an issue.

There's some good info in post 86, too.
Random thought in my head: Sometimes I wondered why some people don't read anything in a thread and keep asking questions to things that have been answered 5 different ways already. Then I realised trying to figure out illogical behaviour is illogical. Then I started ignoring people who exhibited this behaviour, once a pattern was established. For really serious offenders, there's the ignore button, too!