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Old 7th January 2013, 09:30 AM   #11
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Not noticable even at the cutoff frequency of the resistors.

Low values are inductive by length alone. An extreme example is attempting to use a length of copper wire as a shunt resistor: an inch of arbitrary sized wire might have 100 microhms resistance, but 10nH inductance, thus an L/R time constant of 100us, or a cutoff frequency of only 1.6kHz. The only audio applications that might notice such resistance are heater power, which is 50/60Hz, so this is irrelevant.

Power resistors, wound with many turns of resistance wire, are noticably more inductive, the larger values usually having a cutoff frequency of a few MHz (again, irrelevant to audio).

Most film resistors are built with a helix on a ceramic core. The component of the inductance due to its being a helix instead of a straight line, is smaller than the inductance of a straight line in the same place. It is safe to disregard film resistors as inductive.

Standard body 1/4W carbon or metal film resistors, mounted on a PCB over a ground plane, are more like 3nH, and have enough capacitance that they approximate a transmission line. Values from, say, 10 to 300 ohms, look like their rated value up to the low GHz. That could be extended to a few kohms if the resistor is lifted off the ground plane (more inductance, but less capacitance), or a hole cut in it.

Higher value resistors, even in free space, have enough capacitance between terminals (on the order of 0.1pF?) that they become capacitive at such frequencies. Something you'd expect to find in a tube amp, like a 100k resistor, is all capacitor at 100MHz. This capacitance lumps together with surrounding (wiring, plate, Miller, etc.) capacitances, so you will literally never notice it.

Finally, highly inductive resistors would be very beneficial to most tube amps, allowing better HF performance by reducing loading.

Oh, and one final note: everyone makes their resistors the same way, so buy the cheapest brand possible for the tolerance and power rating. As with any other product in a competitive commodity market, brand loyalty is futile.

Tim
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Last edited by Sch3mat1c; 7th January 2013 at 09:32 AM.
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Old 7th January 2013, 09:34 AM   #12
Zen Mod is offline Zen Mod  Serbia
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tnx to God , I'm both blind and deaf ........ so I don't need to care about super positioning of resistors

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Old 7th January 2013, 10:14 AM   #13
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I have to remind to everyone that the measures are not all in the audio

and of course, the parallel is useless for carbon resistance.
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Old 7th January 2013, 10:27 AM   #14
palmas is offline palmas  Portugal
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using 2 parallel resistors to reduce inductance is a mistake:

to use a parallel you need double resistance resistors. If they are helical winded coils, for double resistance chances are you have double the number of turns. As inductance is proportional to the square of the turns, it is possible to have more than double the inductance, meaning the paralell will have more inductance than a single resistor.
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Old 7th January 2013, 10:31 AM   #15
Zen Mod is offline Zen Mod  Serbia
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luckily , I don't need to care even about measurements
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Old 7th January 2013, 10:43 AM   #16
SY is offline SY  United States
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Quote:
Originally Posted by Zen Mod View Post
tnx to God , I'm both blind and deaf ........ so I don't need to care about super positioning of resistors
And tnx to FSM, I've actually run a factory making resistors so I know that the assertions about direction are sheer fantasy.
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Old 7th January 2013, 10:47 AM   #17
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And tnx to FSM, I've actually run a factory making resistors so I know that the assertions about direction are sheer fantasy.
You mean Flying Spaghetti Monster?
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Old 7th January 2013, 12:01 PM   #18
DF96 is offline DF96  England
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Quote:
Originally Posted by Sch3mat1c
Most film resistors are built with a helix on a ceramic core. The component of the inductance due to its being a helix instead of a straight line, is smaller than the inductance of a straight line in the same place. It is safe to disregard film resistors as inductive.
? Have I misunderstood you?

Quote:
Originally Posted by palmas
using 2 parallel resistors to reduce inductance is a mistake:

to use a parallel you need double resistance resistors. If they are helical winded coils, for double resistance chances are you have double the number of turns. As inductance is proportional to the square of the turns, it is possible to have more than double the inductance, meaning the paralell will have more inductance than a single resistor.
It is just as likely that a double value resistor has a higher resistance material and less turns from the laser cutter, so smaller inductance. However, people worrying about inductance in most audio applications of ordinary resistors need to get out more often. I have designed (DIY) VHF equipment without too much worrying about such things; only very small and very high values have problems from parasitic L and C respectively.
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Old 8th January 2013, 12:22 PM   #19
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The aspect ratio of the helix is very poor -- a nice beefy solenoid, with diameter equal to length, has lots of inductance (relative to its largest dimension and number of turns). If you reduce the diameter, it gets thinner and thinner, until at D = 0, it's a straight wire again, and thus has the inductance of a straight wire, which is, obviously, the least inductance you can have between two points. At D = d (wire diameter), it's a helical... wiggle, hardly a coil. For such thin coils, the amount of inductance gained by "wiggling" the wire in a helix is very small, and the inductance is only a little above that of the straight line component. In other words, if you use a piece of wire where you put the resistor, it will have some inductance; the excess inductance due to inserting the resistor is small relative to the inductance that's already there.

Now, if you get something shaped more like those megaohm wirewound resistors of old, yeah, the wire is so fine and there's so damn many turns that the inductance will be much more significant than the lead and body length. Again, assuming you could measure it on top of the resistance (I've pulled such wirewounds from Tektronix plugins, in the attenuator section, so they can't be too bad).

Now, on film resistors, if the film thickness is equal across values (I would guess they thin it out for large values, but I don't know), then the only way to increase resistance is -- yes, to make a tighter helix with more turns, but, the conductor width drops proportionally, while the conductor length rises proportionally, so the total resistance goes as the square as well. So the cutoff frequency remains substantially constant. Which makes sense -- the L/R time constant can be defined by bulk properties rather than circuit properties, depending on permeability of space, bulk resistivity, and... apparently, an area, possibly the conductor cross sectional area?

If the film thickness is reduced as well, resistance will go up even faster, so that the inductance becomes infinitesimal very quickly.

My Tek 475 scope has 200MHz bandwidth, and has to have low phase shift near cutoff (a Bessel type response). That puts filter poles up into the GHz! You can't use crappy components everywhere if every single one is going to bite into your bandwidth within a decade of cutoff. And heck, 1/4W resistors, of reasonable values, working on a ground plane, were all they needed (well, and a copious amount of magic, which may not be as easy to find for the amateur).

Oh, and regarding parallel resistors: having established that the inductance is already small and changes little with value, it is true, then, that resistors in parallel have lower inductance. The best way to imagine this is NOT as if each resistor is an abstract inductance, but that the necessary physical length of the component has inductance, whether it's a resistor or a nichrome helix or a hunk of copper wire. Putting multiple paths in parallel reduces inductance, because the current path becomes wider. Aspect ratio is an important part of inductance; litz wire (a bundle of many current paths), ribbon and sheet (wide conductors, little room for magnetic fields to get through) are widely used at RF for this reason.

Note, finally, that resistors which *are* inductive at the frequency of interest have the same cutoff frequency no matter how many are placed in parallel (assuming no coupling between them). The inductance goes down by providing more current paths, but so does the resistance.

Tim
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Last edited by Sch3mat1c; 8th January 2013 at 12:28 PM.
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Old 8th January 2013, 01:20 PM   #20
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OK, you appeared to say that the inductance of a film resistor is less than a piece of wire, which puzzled me. Now you are saying that it is not much more than a piece of wire. I agree with the latter.
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