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Power Resistor Distortion
Power Resistor Distortion
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Old 7th May 2017, 01:39 AM   #1
Conrad Hoffman is offline Conrad Hoffman  United States
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Default Power Resistor Distortion

There have been various posts on resistor distortion, but this is a new one on me. A friend reported increasing distortion at high frequencies when he used a large tubular "non-inductive" load resistor. It's a 4 ohm 100 watt light green ceramic type available on the 'bay and advertised as low inductance for audio testing.

He sent me one and I ran some tests. It's low inductance, as advertised, but sure enough when placed in series with a "good" resistor (forming a voltage divider), the distortion rises as you approach 20 kHz. As an example, in series with a good 16 ohm part, and driven with a 20 kHz signal of 0.0158%, the THD across the green one is 0.092%, almost six times worse than the input. It would be even more dramatic with a lower THD power amp.

Scraping away some of the coating, it appears the resistor is wound with 1.5 mm wide resistance tape in two layers. The first winding spirals one way, and the second spirals the other, cancelling the inductance, as they're in parallel. The outer coating has a distinctive appearance lightly embossed double triangles.

My guess is the windings (which are highly magnetic) act as cores for each other, and almost any inductance with a core, intentional or accidental, will have a distortion problem. A prime example is the low value output inductor of a power amp, which always has to be air core.

Anyway, that's my current theory, unless somebody can suggest another source of non-linearity that only affects high frequencies. IMO, these resistors are OK for simple power measurements, but they wouldn't be my first choice for other audio testing.
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Old 7th May 2017, 08:59 AM   #2
JonSnell Electronic is offline JonSnell Electronic  United Kingdom
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The source of your distortion is probably your amplifier not able to work into a non inductive load causing a reactance.
Resistors do not distort, they reduce current, (in series) or sink current (as a load).
All non inductive wire wound resistors are wound with two layers, one going one way round and the other opposing.
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Old 7th May 2017, 09:21 AM   #3
traderbam is offline traderbam  Canada
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Conrad, that's very interesting. Are you saying the ribbon material is highly magnetic? Nickel?
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Old 7th May 2017, 11:24 AM   #4
simon7000 is offline simon7000  United States
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Quote:
Originally Posted by JonSnell Electronic View Post
The source of your distortion is probably your amplifier not able to work into a non inductive load causing a reactance.
Resistors do not distort, they reduce current, (in series) or sink current (as a load).
All non inductive wire wound resistors are wound with two layers, one going one way round and the other opposing.
Actually all resistors distort. The best resistors may have thermal coefficients of 5 PPM. The more power the resistor dissipates the more the distortion. It also increases as the frequency decreases.

In wirewound resistors the usual source of distortion is where the resistance wire is connected to the connection terminals or leads.

I would ask if a magnet sticks to the resistance wire?
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Old 7th May 2017, 12:01 PM   #5
googlyone is offline googlyone  Australia
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Power Resistor Distortion
I had a similar issue with a nichrome wire dummy load I use. It simply comprised coils of heavy gauge nichrome wire bolted to terminals.

I spent hours trying to debug an amplifier which appeared to generate high distortion under load!

The nichrome wire dummy load (and I imagine the bolting arrangement of the wire in particular) had some funky non linearity in its resistance.

So no - the load itself did not generate distortion but... because this non linear resistor was is series with the amplifiers zobel Network, when I measured the distortion across the nichrome wire there was elevated distortion. Checking at the amplifiers feedback point showed normal low levels of distortion.

With the zobel being outside the feedback loop, and especially with its rising impedance with frequency, the amplifier was unable to correct for the effect of the non linear resistance of the nichrome wire and bolts.
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Old 7th May 2017, 12:19 PM   #6
KSTR is offline KSTR  Germany
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Years ago I spent hours in tracking down a level- and frequency-dependent distortion issue in a current-drive amp. It turned out the current sense resistor was the culprit, it was a typical 0.22R MPC-"brick" style, chinese origin. Replacing it for a standard wirewound in a different, bigger form factor cured the problem by 100%
I did not try to find the root cause why this resistor distorted but it's likely it was some inductance-related nonlinear parasitics...
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Old 7th May 2017, 12:53 PM   #7
Conrad Hoffman is offline Conrad Hoffman  United States
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Not amp THD, as I compared the amp output with the junction of the 2-resistor divider. I tried various combinations of resistors, with and without the offending wirewound, and established that it's only that one with the horrible problem. It measures wonderfully in every other way, at lower frequencies. I'm guessing the problem is made worse by the use of wide resistance tape, rather than wire, so the crossed area if high. Or, if it's something to do with the actual material, I've not a clue, as I've no way to determine what it is. It is strongly magnetic, as are the metal end pieces. I've never seen a non-inductive resistor done this way with tape, only wire, so have no experience with them. I believe it's Chinese in origin, but can't imagine their resistance alloys are that different from anybody else. IMO, understanding the root cause of this should be pretty important when designing low THD amps, so one can simply avoid parts of known bad construction. Ha, were life that easy.
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Last edited by Conrad Hoffman; 7th May 2017 at 12:55 PM.
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Old 7th May 2017, 01:20 PM   #8
Mark Johnson is offline Mark Johnson  United States
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Power Resistor Distortion
In the hopes this may be useful:

http://www.diyaudio.com/forums/parts...resistors.html
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Old 7th May 2017, 02:01 PM   #9
Conrad Hoffman is offline Conrad Hoffman  United States
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Good thread that I hadn't read before. I particularly like the use of the somewhat obscure "all-resonance" formula for doing a compensator, though I'm not sure it was actually implimented.

One of my reference resistors was a non-inductive type I wound myself. It was a bifilar winding of constantan done on a small diameter rod. Removed from the rod, the coil was laid in the bottom of a small glass jar so it could be oil cooled. The inductance was due entirely to the connecting lead length and similar to a 1/4 W metal film. No THD problem. I also used a bank of 40 ohm regular Dale resistors in parallel with no problems and minimal inductance.

IMO, I'd rather have the relatively minor inductance problem, than the THD problem presented by the device in question.

If anybody can suggest particular tests to figure out the root cause, I'm all ears. I think it's a magnetic saturation/curve issue, but what other frequency dependent non-linearities are there?
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Old 7th May 2017, 08:05 PM   #10
Elvee is offline Elvee  Belgium
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Magnetic non-linearity certainly is a possible culprit, but I'd rather think that it has to with the overlapping of the resisistive tapes: they contact through an oxide layer, which is probably non-linear as a semiconductor or a lossy dielectric.

A good test of the magnetic hypothesis would be to immerse the resistor in a powerful external DC field, generated by a a large neodymium magnet for example.
This will easily saturate, or at the very least heavily polarize the magnetic material, which should translate in a drastic reduction or change of the harmonics generated: most of the odds should vanish, leaving perhaps the even, if some parts are not thoroughly saturated.
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