Author Topic: Does cable impedance add to source or load impedance?  (Read 5602 times)

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Offline fonographTopic starter

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Re: Does cable impedance add to source or load impedance?
« Reply #25 on: February 15, 2019, 09:23:17 pm »
I have never seen a thread being derailed so hard  in my entire life. All I asked is if cable impedance adds up to source or load, a extremly simple question that can be answered in one sentence and somehow this turned into discusion about audio, a topic I foolishly chosen believing it will be obvious to everybody that I used it merely to give a example to help deliver my question and that this topic of  audio is in context of this thread  and purpose of my question completly irrelevant, and unimportant.

That is like if you had 6 year old son and he came to you asking how many apples does Bob have if he picked 10 from tree, gave away 2 to grandma and 3 to sister, a first grade homework question that any adult not suffering from brain damage should be able to clearly answer in one sentence yet you would then go on hour long rant about pesticides, fructose intolerance, global warming and its impact on apple  yields, stupid politicians making retard laws about imported apples, morals of genetical modification of apples, apple pie recipes and your neighbour who steals apples from your tree in garden by reaching over the fence.
 

Offline Audioguru

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Re: Does cable impedance add to source or load impedance?
« Reply #26 on: February 15, 2019, 10:06:56 pm »
Cheap intercoms and TVs sound very boomy at the resonant frequency of the speaker and its enclosure. I think because its amplifier has a fairly poor damping factor. I think they will sound much better if the amplifier has a high damping factor.
 

Offline Johnny B Good

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Re: Does cable impedance add to source or load impedance?
« Reply #27 on: February 16, 2019, 12:29:22 am »
Cheap intercoms and TVs sound very boomy at the resonant frequency of the speaker and its enclosure. I think because its amplifier has a fairly poor damping factor. I think they will sound much better if the amplifier has a high damping factor.

 I'm afraid to say that you're quite wrong in thinking that a low Z output amplifier will have any noticeable effect on the audio quality of such speaker arrangements. The poor quality is essentially down to enclosure resonances with just one significant contribution from the drive's own resonance which can only be damped by the voice coil resistance in series with the amplifier's output impedance which can easily be less than an ohm even with a cheap amplifier meaning an effective DF of just 4 or less regardless of how high a DF is claimed for the amp. The other resonances created by the enclosure will remain unaffected by the DF rating of the amplifier since they're effectively decoupled from the drive unit's voice coil and cone assembly.

 To give fonograph the short answer he was seeking, the loop resistance of the cable simply adds to that of the amplifier's impedance and the voice coil resistance since they're all effectively in series across the drive unit's impedance at resonance, typically  some three to five times its nominal impedance. It is this resonance impedance that's being damped, not the nominal 4 or 8 or 15 ohm (or whatever) given for the drive unit.

 The damping factor figures of merit often quoted in the glossy brochures' advertising copy back in the 1970s were/are simply marketing bullshit designed to emphasise out of all proportion the importance of the ultra low output impedance figures that could be so readily achieved with voltage negative feedback in these new fangled DC coupled solid state amplifiers. The importance of lo-Z was grossly overstated as it relates to any drive unit damping effect, limited as it is by the series resistance of the drive unit's voice coil.

HTH & HAND  ;D

JBG
John
 

Online Zero999

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Re: Does cable impedance add to source or load impedance?
« Reply #28 on: February 16, 2019, 10:55:43 am »
I have never seen a thread being derailed so hard  in my entire life. All I asked is if cable impedance adds up to source or load, a extremly simple question that can be answered in one sentence and somehow this turned into discusion about audio, a topic I foolishly chosen believing it will be obvious to everybody that I used it merely to give a example to help deliver my question and that this topic of  audio is in context of this thread  and purpose of my question completly irrelevant, and unimportant.

That is like if you had 6 year old son and he came to you asking how many apples does Bob have if he picked 10 from tree, gave away 2 to grandma and 3 to sister, a first grade homework question that any adult not suffering from brain damage should be able to clearly answer in one sentence yet you would then go on hour long rant about pesticides, fructose intolerance, global warming and its impact on apple  yields, stupid politicians making retard laws about imported apples, morals of genetical modification of apples, apple pie recipes and your neighbour who steals apples from your tree in garden by reaching over the fence.
Don't worry about it. This is fairly common, especially with audio. As I said above, as far as power audio amplifier are concerned resistance = impedance. Although some may find the extra information useful, it can be a pain filtering it.

From the amplifier's perspective, the resistance of the cable adds to the load resistance. If the cable is 1Ω and speaker is 8Ω, then the amplifier sees a 9Ω load.

From the load's perspective, the resistance of the cable adds to the amplifier's resistance. If the speaker is 8Ω, the cable 1Ω and the amplifier has an output impedance of 0.5Ω, then the speaker sees a voltage source with an impedance of 9.5Ω 1.5Ω.

I hope that answers the question.
« Last Edit: February 18, 2019, 08:29:09 am by Zero999 »
 
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Offline fonographTopic starter

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Re: Does cable impedance add to source or load impedance?
« Reply #29 on: February 16, 2019, 11:47:56 am »
I have never seen a thread being derailed so hard  in my entire life. All I asked is if cable impedance adds up to source or load, a extremly simple question that can be answered in one sentence and somehow this turned into discusion about audio, a topic I foolishly chosen believing it will be obvious to everybody that I used it merely to give a example to help deliver my question and that this topic of  audio is in context of this thread  and purpose of my question completly irrelevant, and unimportant.

That is like if you had 6 year old son and he came to you asking how many apples does Bob have if he picked 10 from tree, gave away 2 to grandma and 3 to sister, a first grade homework question that any adult not suffering from brain damage should be able to clearly answer in one sentence yet you would then go on hour long rant about pesticides, fructose intolerance, global warming and its impact on apple  yields, stupid politicians making retard laws about imported apples, morals of genetical modification of apples, apple pie recipes and your neighbour who steals apples from your tree in garden by reaching over the fence.
Don't worry about it. This is fairly common, especially with audio. As I said above, as far as power audio amplifier are concerned resistance = impedance. Although some may find the extra information useful, it can be a pain filtering it.

From the amplifier's perspective, the resistance of the cable adds to the load resistance. If the cable is 1Ω and speaker is 8Ω, then the amplifier sees a 9Ω load.

From the load's perspective, the resistance of the cable adds to the amplifier's resistance. If the speaker is 8Ω, the cable 1Ω and the amplifier has an output impedance of 0.5Ω, then the speaker sees a voltage source with an impedance of 9.5Ω.

I hope that answers the question.

This is excellent answer, thank you very much Zero999.
 

Offline macboy

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Re: Does cable impedance add to source or load impedance?
« Reply #30 on: February 16, 2019, 12:59:44 pm »
So cable impedance adds up to and counts as source output impedance? I expected it will add up to load impedance.
Actually it is both.
From the perspective of the load (speaker), the source has an additional 2 ohms. From the perspective of the amplifier, the load has an additional 2 ohms.
 
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Offline unitedatoms

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Re: Does cable impedance add to source or load impedance?
« Reply #31 on: February 16, 2019, 01:42:55 pm »
So cable impedance adds up to and counts as source output impedance? I expected it will add up to load impedance.
Actually it is both.
From the perspective of the load (speaker), the source has an additional 2 ohms. From the perspective of the amplifier, the load has an additional 2 ohms.
For completeness of model, you may choose to consider 4 impedances of 3 devices. Single port source Zs, two port cable with Zin and Zout, and single port load Zl.
My estimate is that typical speaker cable (two thick wires as air line) is about hundred ohms. With insertion loss of -0.00x dB at low Hz frequencies.

edit: I looked up for formulations.
For 1 meter loslsess cable with characteristic impedance of 100 Ohm, 4 Ohm active load the cascade at 1000Hz will look like 4 Ohm plus 0.002 imaginary ohms.
http://www2.ece.ohio-state.edu/~johnson/311/week3.pdf
https://www.wolframalpha.com/input/?i=z%3D100*((4%2B0.002096i)%2F(100%2B0.0000838i))
https://en.wikipedia.org/wiki/Vacuum_permittivity
lost link to value of mu, it is 1.257e-6Henries/meter

The reactance value can be represented as lumped component , inductance about 300nanoHenries
So to answer the topic. You may choose to add very small inductance to load impedance.
Also equally valid to add inductance to source impedance instead.

But to be absolutely correct, the cable should be considered a lossy line and must be not added to either and model should be of 3 devices.

edit 2: Coming from VNA metrology lingo. It looks like for cables with load there is a general intuitive rule. The loads lower than characteristic impedance of line will make input of a cable to look like inductance, the matching loads will look like active load and the loads higher than characteristic impedance will look like capacitance. That sounds very intuitive and with agreement to calibration standards story for VNAs.
With assumption that line is very short. Lines longer than some portion of lambda will look very different.
« Last Edit: February 16, 2019, 04:20:05 pm by unitedatoms »
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Online Zero999

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Re: Does cable impedance add to source or load impedance?
« Reply #32 on: February 18, 2019, 08:30:22 am »
This is excellent answer, thank you very much Zero999.
Except for the fact I made a mistake, which I've corrected.
 


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