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Replace resistor by low-inductance resistor - Is it audible?

ctrl

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#1
Since it is often claimed here in the forum that replacing cheap sand-cast resistors with low-inductance resistors would lead to clearly audible improvements, and since it's no use pointing out that the sound change (unless it's completely imagined) is very likely due to slight deviations in the resistance values, I'm now so annoyed that I want to get to the bottom of it ;)

UPDATE: Also check out "Capacitor upgrade in crossover - Is it audible?".

Considered are a 20W sand-cast resistor with about 3.28 Ohm and a low induction 4W metal oxide resistor with about 3.29 Ohm - - I could not find a better pairing among my resistors.
1584804264085.png
1584804276771.png

The resistance measurement results:
1584804909181.png

As expected the measurement curves of the two resistors are practically identical.

UPDATE: The 20W sand-cast resistor (instead of e.g. a 5W) was chosen for comparison to create a worst-case scenario (big "bad" sand-cast resistor against "short" metal oxide resistor) and because only for this resistor a metal oxide resistor with exactly the right impedance was available.


Is the MOX resistor really low induction?

First of all it should be checked if the MOX resistor is really low inductance compared to the sand-cast resistor.

For this purpose we first look at the impedance at 5kHz for both resistors.

Arta-Limp reports for the sand cast resistor that the measured impedance has a resistive part of 3.28 ohm and that the imaginary part is inductive with a value of 675 nH.
1584805753799.png



For the low-inductance MOX resistor Arta-Limp provides for the impedance as resistive part 3.29 ohms and for the imaginary part 183nH - so much less than for the sand-cast resistance.
1584805723369.png



Next we look at the impedance/induction of the two resistors against the entire audible frequency range:
1584806221872.png

The manufacturer's data are correct, the MOX resistor is indeed low inductance compared to the sand-cast resistor.


Is this difference audible when playing music over loudspeakers?
A test will provide information on this.


Test Setup
The test setup is very simple. Since the inductive component is noticeable at high frequencies, we measure the effects on the frequency response of a tweeter.
The tweeter is measured in the near field. Every resistor is connected in series to the tweeter.
We use the SB21RDC tweeter from SBAcoustics and a calibrated iSEMcon EMX-7150 as measuring microphone.
1584807611949.png



Measurement results

The measured frequency response of the tweeter with the respective resistors looks as follows:
1584808424761.png


Of course, you're kidding! That's just twice the same measurement with different colours - isn't it?
Unfortunately no, that's really the effect when exchanging a cheap sand-cast resistor for a low-inductance MOX resistor.

But it must be possible to detect an effect, otherwise your test is simply faulty.
Let's have a look at the frequency response measurement for both resistors with a finer scale:
1584808870563.png


There are very small differences. To be able to show it better, we'll normalize to the frequency response of the MOX resistor and look at the deviation of the sand-cast resistor. For this purpose, we resolve the scaling to 0.01dB:
1584809043894.png


Now we see the effect of the higher inductive part of the sand-cast resistor, because at high frequencies the frequency response drops slightly compared to the low-inductance resistor - up to 20kHz by as much as 0.02 dB!

Even if the effects should increase at higher resistance values, even 5 times the value is inaudible.

The effects of replacing a sand-cast resistor with a low-inductance resistor are practically zero.
 
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solderdude

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#2
The effects of replacing a sand-cast resistor with a low-inductance resistor are practically zero.
The effects caused by placebo however is considerable with a lot of speaker build enthusiasts. :D
I also never found any issues with resistors in speakers.
In feedback paths of high bandwidth amplifiers it might be a different story though.

Next test.... capacitors (needs distortion tests in the electrical plane) in the LS path
 
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JeffS7444

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#5
What about the use of inductive vs non-inductive resistors when used as grid/gate stoppers in amplifiers: Is it reasonable to expect negligible differences here too when working at audio frequencies?
 

SIY

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#6
What about the use of inductive vs non-inductive resistors when used as grid/gate stoppers in amplifiers: Is it reasonable to expect negligible differences here too when working at audio frequencies?
No, because the gate or grid stoppers are working at RF. Sometimes there's no difference between (say) carbon film, metal film, metal oxide, sometimes there is. It depends on the circuit.
 

pjh

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#8
This is so great! Now we need to find out if metal oxide resistors really are quieter in a preamp circuit than carbon composition resistors.
 

Selah Audio

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#9
The greatest difference is the one you didn't measure - profitability. ;)

I normally use the non-inductive resistors in tweeter circuits but more for a tighter tolerance rating than the slight difference in inductance. The encapsulation can affect the cost as some resistors are made for more demanding environments; however, this isn't an issue / advantage in a home audio speaker.
 

SIY

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#13
This is so great! Now we need to find out if metal oxide resistors really are quieter in a preamp circuit than carbon composition resistors.
Depends on the circuit and the position in the circuit. Generally, the limit in an engineered circuit is the active device.
 
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#14
Good work, but I'd suggest doing some time domain tests as well, as you've only covered frequency domain.
 

SIY

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#17
Yes I am, a square wave and an impulse test on both would've shown more of a difference in inductance rather than solely frequency domain.
No, inductance is inductance, whether it's calculated by t or f. It doesn't magically change when transformed between representations.
 
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C

ctrl

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Thread Starter #18
What about the use of inductive vs non-inductive resistors when used as grid/gate stoppers in amplifiers: Is it reasonable to expect negligible differences here too when working at audio frequencies?
No, because the gate or grid stoppers are working at RF. Sometimes there's no difference between (say) carbon film, metal film, metal oxide, sometimes there is. It depends on the circuit.
Unfortunately I am not familiar with electronic circuits. Clear however is, with increasing frequency the effect of inductive part increases considerably. I can extend the measurements up to the measurement limit of 40kHz.

Impedance measurement up to 40kHz with resistive and imaginary part:
1584825005180.png



The frequency response measurements up to 40 kHz:
1584825042195.png




I normally use the non-inductive resistors in tweeter circuits but more for a tighter tolerance rating than the slight difference in inductance.
In tweeter circuits I use MOX too.
Agree with you, the reason for using low inductive resistors is also the lower tolerance. But that's also the only reason for me; maybe the smaller dimensions compared to sand-cast resistors.
 

SIY

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#19
Z = R + iωL. So the second term increases with increasing frequency, but that's the impedance increasing, not the inductance (L).
 
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#20
No, inductance is inductance, whether it's calculate by t or f. It doesn't magically change when transformed between representations.
Correct, but both representations give different viewpoints from the other, otherwise we would only have one representation if they were the same.
 
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