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Measurements on amplifiers with speakers and cables connected

Opus111

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#1
It seems a not very well known fact that when a speaker is connected to an amplifier the distortion increases. This being due to the speaker presenting a non-linear load to the amp. Here's an article which includes several FFTs demonstrating the kind of distortion increase that occurs with differing cables : http://www.eetimes.com/document.asp?doc_id=1274851

So yes its really true - different cables do measure differently when they're in circuit.
 

tomelex

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#2
Yes, the type of output circuits and feedback used in an amplifier, the length of cable and type of cable, and the type of load, all interact and all have an effect. But, that's been known for several decades now (really actually published stuff on this from the tube era). I have that book by the way, recommend folks buy it to learn a whole lot about audio and its easy reading actually. Touches on a lot of things. The Audio Critic did all these tests too, the audiophile subjective community slammed him, meanwhile, those more objective among us put on our reading glasses and learned. Good find and post Opus111!
 

Opus111

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#3
I agree its been known for a long time - but I got chucked off a science-based forum a few years ago for suggesting that amplifier distortion is going to increase into a non-linear load :) So not everybody knows it it seems.
 

tomelex

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#4
yep, you could do that test with your opamps and show the results of lead and lag reactive loadings, standard power amps like boulder and krell and ML are just big opamps basically.
 

Opus111

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#5
I am planning tests on opamps - I'll start off with resistive loads but into headphones would be the next logical step. Right now though I've got this tiny headphone amp I'm in love with the (lack of) sound of, so experiments are on the back burner to make way for building the second prototype. Its shown on my blog here : http://www.diyaudio.com/forums/blogs/abraxalito/1332-mighty-mite-bal-se-classa-headphone-amp.html

Given it sounds of nothing at all that I can hear, it should be a good reference for trying out new measurements on.
 

March Audio

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#6
This is interesting and well worth investigating. I am doing a series of tests with what I have available to me.

3 different amps, NCORE NC400, Tag McLaren 100x5R and Onkyo NR5010.

Play these into 8 ohm resistive load (initially at 2.83 V - nominal 1 watt) and Usher BE718 speaker via 1m Blue Jeans (Belden 10-gauge 5T00UP) cable and 10m Altobiwire 14 guage cable (just using single).

Plan to use 1 kHz sine tone and intermodulation tones.

I have made a start. Note that to keep the ADC in its more linear range I have used a resistor attenuator, so the values arent absolute. Was meant to be 20 dB but ended up about 30dB as I didnt read the resistor code properly :) I am more interested in differences than absolute values.

DSC02349.JPG

DSC02350.JPG

DSC02351.JPG


first plot is of the NCORE NC 400 noise floor . Note this is without the attenuator so is ref a FS of 4.2 volts rms. The amp is capable of much higher output voltages so this still isnt a true representation of its dynamic range.

ncore noise floor 8R.png


This is 1kHz @ 2.83 volts rms into 8ohm resistive load (measured with attenuator) 1m BJ cable

ncore 1kHz 8R 2-83volts.png


This is 1kHz @ 2.83 volts rms into 8ohm resistive load (measured with attenuator) 10m Alto cable

ncore 1kHz 8R 2-83volts alto cable.png


This is 1kHz @ 2.83 volts rms into BE718 speaker (measured with attenuator) 1m BJ cable

ncore 1kHz spk 2-83volts 1m cable.png



This is 1kHz @ 2.83 volts rms into BE718 speaker (measured with attenuator) 10m Alto cable

ncore 1kHz spk 2-83volts alto cable.png
 
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DonH56

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#7
Nice thread.

Bear in mind that, even with a very short, heavy gauge cable from amp right to speaker, you can see the same effects, and often worse without the cable's added impedance. The amplifier's output impedance working into the speaker load, with or without cable, is a big (if not "the" factor) at play.

IMO a lot of the actual distortion is masked when playing music, but the changes in frequency response are what lead to a lot of threads on how amplifiers sound different when it is really how the amplifier and speaker interact. My Maggies present a low-ish 4-ohm load but it is pretty flat over frequency. An ESL generally exhibits a falling response over frequency, and conventional drivers tend to have significant impedance excursions for various reasons (including port and crossover effects as well as from the voice coil impedances themselves).

IMO - I'll come up with a sig line saying it's all only in my mind later - Don
 

Purité Audio

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#8
Correct me if I am wrong but, shouldn't an ( solid state ) amps output impedance be very low, and 'most' cables have very low LCR values when used in domestic audio lengths , so not really a factor, the speakers impedance will have an effect , but not as great as when the amps output impedance is high ,for example in valve amps?
Keith.
 

DonH56

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#9
You're not wrong.

Yes, to point, but even SS amplifiers tend to have rising output impedance with frequency (since feedback loop gain decreases) and so will exhibit some load dependence. It is never zero in the real world, so that leaves audibility up for debate (natch).

Some cables incorporate components that raise their impedance and change their reactance.

And yes tube (valve) amplifiers generally tend to much higher output impedance than SS amplifiers.

I posted a thread on WBF showing some typical amp and speaker frequency response curves using simplified models.

One "gotcha" for speaker designers is that they have to consider a wide range of amplifiers that might drive their designs.
 

Purité Audio

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#10
Don and damping factor is really no more than the speakers impedance connected in series with the amp?
so is damping factor a bit of a red herring?
Thanks of your advice
Keith.
 

DonH56

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#11
Close, but it is a ratio of the two impedances:

Damping factor DF = load impedance / source impedance

Load = speaker plus wires (remember there are two wires, plus and minus, to each speaker)
Source = amplifier output impedance

https://en.wikipedia.org/wiki/Damping_factor

If your 0.08 ohm Zout amplifier is driving an 8-ohm speaker that is a DF = 100. Most spec sheets only report a single number and only magnitude, so how much it varies over frequency (can be a lot) and how sensitive to a reactive load the amp may be are hidden. Note amplifier stability depends upon a lot more than just Zout, however.

I would not say it is a red herring, but is one factor (parameter) among many, and like any other spec should be consider with everything else -- including your speakers. In the 70's and 80's super high DF and super wide bandwidth were the rage for a while. There are drawbacks to anything taken to extreme, naturally, including lower stability and transient distortion from very high feedback to get high DF, and reduced stability and additional noise from super wideband designs. All is compromise...

One thing I tend to do is consider the impedance looking back from the speaker terminals, not the amplifier's output, to generate the effective DF seen by the speaker rather than right at the amplifier. This is not the commonly-accepted DF equation, but provides a better idea of what the speaker is going to do since that is the driving-point impedance it sees. That is,

DF' = Zspeaker / (Zamp + Zwire)

I remember reading, and this is a reference I really wish I had at hand, that damping factor over 20 is inaudible in most systems. Way back when, I decided anything over 100 in the LF region was not worth the trouble, but was looking at high-power amplifiers that usually have pretty low output impedance. SS amps I took a quick look at seem to range around 100'ish at 100 Hz or so, and tube amps can fall to the single digits. The latter is what makes them more sensitive to the speaker they are driving.

Note 10 feet of 14 AWG cable (20 feet total) is about 0.05 ohms, so if the amplifier was perfect (0-ohm output) that'd be a damping factor of about 8/0.05 = 160 measured at the speaker terminal looking back... If your amplifier has about the same output impedance (a DF of 160), then the effective DF' at the speaker is reduced to 80. probably in the mud as far as audible effects. Where DF matters is low-Z speakers and/or long speaker cables that are relatively small. Back then zip cord of maybe 24 to 20 AWG wire was not all that uncommon in a lot of inexpensive systems. Since then a lot of manufacturers have jumped in and raised awareness so most people are using 18 AWG and up for speakers, at least based on what I see and read around (anecdotal).

FWIWFM - Don
 

March Audio

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#12
Nice thread.

Bear in mind that, even with a very short, heavy gauge cable from amp right to speaker, you can see the same effects, and often worse without the cable's added impedance. The amplifier's output impedance working into the speaker load, with or without cable, is a big (if not "the" factor) at play.

IMO a lot of the actual distortion is masked when playing music, but the changes in frequency response are what lead to a lot of threads on how amplifiers sound different when it is really how the amplifier and speaker interact. My Maggies present a low-ish 4-ohm load but it is pretty flat over frequency. An ESL generally exhibits a falling response over frequency, and conventional drivers tend to have significant impedance excursions for various reasons (including port and crossover effects as well as from the voice coil impedances themselves).

IMO - I'll come up with a sig line saying it's all only in my mind later - Don
Thinking about the speaker impedance variation, I should be able to use REW distortion plot to provide a distortion figure throughout the frequency range.

I will post FR also. Wow it's expanding!

The NC400 output impedance is 0.7 milliohms according to Hypex. I will try and dig out the info for the other amps.
 
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DonH56

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#14
Note frequency response variation is not "distortion" in the normal sense of nonlinearities, but variations in response due to amplifier/speaker interaction are distortions of the desired frequency response.

Class D amplifiers require an output filter, usually a big inductor, that causes the output impedance to rise with frequency. Actually, all amplifiers have a rising output impedance with frequency unless something sneaky is going on. I really wish more would publish the curve, at least magnitude, just for fun.
 
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DonH56

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#16
I was referring to a plot of the output impedance, not frequency response. Since you posted it, at least Stereophile does use a simulated loudspeaker load in addition to load resistors. I wish I had kept my load resistors (I had non-inductive 4, 8, and 50 ohm RF loads in a gallon oil can rated to handle up to 2 kW).
 

March Audio

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#17
Note frequency response variation is not "distortion" in the normal sense of nonlinearities, but variations in response due to amplifier/speaker interaction are distortions of the desired frequency response.

Class D amplifiers require an output filter, usually a big inductor, that causes the output impedance to rise with frequency. Actually, all amplifiers have a rising output impedance with frequency unless something sneaky is going on. I really wish more would publish the curve, at least magnitude, just for fun.
Dug out the NC400 datasheet

http://www.hypex.nl/docs/NC400_datasheet.pdf

upload_2016-4-10_10-41-55.png
 

DonH56

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#18
Thanks, pretty impressive! Less than 1 m-ohm and pretty flat to ~15 kHz then rise to only ~3 m=ohms at 20 kHz.

Odd THD performance at 200 W and around 7 kHz...
 

March Audio

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#19
Thanks, pretty impressive! Less than 1 m-ohm and pretty flat to ~15 kHz then rise to only ~3 m=ohms at 20 kHz.

Odd THD performance at 200 W and around 7 kHz...
Yes its impressive. To my ears they just sound clean and neutral with no real "sound".

Some people dont like that ! Its not what they are looking for ;)

I built my own NC400s. Its hard to ignore the Nord NC500 based amps with selectable input boards. DIY or nord, the amps are great value for the performance.

http://www.iqspeakers.co.uk/#!hypex-amps/avuyk
 

Mivera

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#20
Yes its impressive. To my ears they just sound clean and neutral with no real "sound".

Some people dont like that ! Its not what they are looking for ;)

I built my own NC400s. Its hard to ignore the Nord NC500 based amps with selectable input boards. DIY or nord, the amps are great value for the performance.

http://www.iqspeakers.co.uk/#!hypex-amps/avuyk
As a Hypex OEM, and you being a fellow member, maybe I could find some unit's for you for audio science experimentation applications. :)

IMG_4055.JPG
 
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