Wirewound SQP resistors non-linearity

[SpaceAudio]

Wirewound SQP (cement) resistors are well-known to everyone.
They can be found everywhere in audio equipment, for example, in crossovers used in speakers ranging in price from $100 to $100,000.
At first glance they all seem the same.
But as it turned out, some of them exhibit high nonlinearity (several times higher than the nonlinearity of a good transistor amplifier).

To test resistors for linearity, I have assembled a test setup where the tested resistor is denoted as R_DUT. The voltage across R_DUT is about 2Vp-p.

Some SQP resistors add a significant amount of harmonics into the test signal, and the spectrum of this distortion is dominated by odd harmonics, which are well-known to be the most harsh for the human ear:

Metal film resistors do not produce measurable distortion in this test setup:

The reason for the nonlinearity of wirewound resistors is the use of ferromagnetic wire.


SQP resistor made with non-magnetic wire does not exhibit significant nonlinearity:

You can check these resistors with a magnet.

 

[Cbdb2]

Do the data sheets give any clue? If I order then online how do I tell the difference?

 

[Philbo King]

They do make noninductive power resistors, presumably wound half one direction and half the other direction.

Ohmite 50W Non-Inductive Power Resistor 10 Ω No.2002 | eBay

This item is an Ohmite 50W Non-Inductive Power Resistor 10 Ω No.2002

www.ebay.com

 

[DonH56]

Note that non-inductive does not necessarily mean low-distortion. Ohmite has a pretty wide range of power resistors for various applications, but you probably have to dig into their materials and/or ask them about any PIM (passive intermodulation) distortion test results.

https://www.ohmite.com/assets/docs/Ohmite%20-%20Resistor%20Product%20Brochure.pdf?r=false

Vishay has some nice albeit pricey options but I don't think they have real high-power resistors if you want to handle 500~1000 W or more. I got some nice non-inductive, low-Tc kW film resistors at work a few years ago but can't for the life of me remember the manufacturer.

 

[Cbdb2]

Anybody know what the cause of this distortion is? Its not TC, that only matters if the temperature is changing which doesn't happen in a steady state. Were these test done after the resistor reached a steady temperature?

 

[bachatero]

Anybody know what the cause of this distortion is? Its not TC, that only matters if the temperature is changing which doesn't happen in a steady state. Were these test done after the resistor reached a steady temperature?

Hunch: Metal coils create an inductor, which breaks an assumption from the surrounding circuits that the resistor is linear, leading to nonlinear behavior.

 

[NTK]

Anybody know what the cause of this distortion is? Its not TC, that only matters if the temperature is changing which doesn't happen in a steady state. Were these test done after the resistor reached a steady temperature?

You can check this thread out for more information. PIM (passive intermodulation) can have very minor but measurable effects for audio, but more important for RF systems.

Ferromagnetic materials in audio connectors

As far as my experience goes, negligible conductor resistance is the most influential factor on an audio connector's signal transmission quality, followed by a tight mechanical connection and a coating that is resistant to oxidation over time. But I recently read this interesting IEEE research...

www.audiosciencereview.com

This US Naval Research Lab report gives the full details. See chapter 5 for the list of PIM sources.

https://apps.dtic.mil/sti/tr/pdf/ADA086995.pdf

 

[SpaceAudio]

Anybody know what the cause of this distortion is?

The cause is not the inductance (inductance itself is linear), but some kind of the Hall effect in the ferromagnetic material from which the wire, used to wind the resistor, is made. If you heat this resistor to the Curie point, when its magnetic properties disappear, it will become linear.

 

[KSTR]

At first glance they all seem the same.
But as it turned out, some of them exhibit high nonlinearity (several times higher than the nonlinearity of a good transistor amplifier).

Thanks for bringing this up.
I once ran into this issue when designing a current drive amplifier, using a 0.22R sense resistor. It took us several days to find out that the sense resistor (metal band type, from a well-respected Japanese manufacturer) was the problem as we had assumed that wire-wounds are safe wrt distortion.

 

[audio_tony]

I thought this phenomenon was common knowledge amongst engineers?

Back on the 80's I built crossovers and amplifiers using multiple carbon resistors in parallel to avoid using wirewounds.

The same goes for dummy loads - always use carbon resistors.

 

[DonH56]

I would not say it's all that common as it is not something generally taught in school, at least with respect to resistors, connectors, and such. I learned of it on my own, building audio circuits for fun, and later on the job where it was a Big Deal for some of the systems incorporating ICs I designed.

Metal film resistors and carbon film resistors are also alternatives, though power handling is lower. Wire wound resistors typically have the lowest noise but have other drawbacks as well. Here is an article found after a quick search that focuses on metal resistors but touches on other types:

Metal Film Resistor - Electronics Reference

The metal film resistor is a type of electrical resistor made with a thin metal film coating a ceramic body.

electronicsreference.com

 

[Cbdb2]

Thanks for bringing this up.
I once ran into this issue when designing a current drive amplifier, using a 0.22R sense resistor. It took us several days to find out that the sense resistor (metal band type, from a well-respected Japanese manufacturer) was the problem as we had assumed that wire-wounds are safe wrt distortion.

Thanks for the link. looks like a few different mechanisms for IMD from feromagnetic conductors/connectors. From that paper.
"There are undesired effects other than the nonlinear permeability change which disqualify fer-
romagnetic materials for use as electronic connectors. Besides the low conductivity, an effect exhibited
by ferromagnetic metals is the anisotropy of electrical conductivity- different values for different current
and/or field directions. Also, magnetoresistivity, the change in resistance associated with a change in
magnetization, weakly found in all metals, is orders higher in ferromagnetic materials. Magnetostric-
lion, the change in physical dimensions of a ferromagnetic material in a magnetic field is another
undesired effect for an electrical connector."
And also magnetic hysteresis. The paper seems to indicate these are much worse at high frequencies.

 

[valerianf]

Thank you @SpaceAudio for this very useful measurement of ceramic wirewound resistors. I have always suspected that they were bad but never that bad.
I will use them to tune my passive speaker crossovers and replace them with a metal oxide power resistor once knowing the final value.

 

[dualazmak]

Maybe, all of you are well aware of this thread "ASR dummy load configuration".

I especially like the intensive measurements by @Langston Holland here #97 and #98 on ARCOL NHS300 (Non-inductive, 300W) 4Ω 1% power resistors.