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Low inductance resistors for power amp - some measurements
Low inductance resistors for power amp - some measurements
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Old 9th March 2014, 10:48 PM   #21
HiFiNutNut is offline HiFiNutNut  Australia
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Originally Posted by Rundmaus View Post
Doesn't this give you the chance to remove the inductor from the output and use the inductance of the resistor (which is there anyway) instead?

Rundmaus
No it doesn't. We want an L with little R so that the impedance at low frequency remains low.

That is on top of what Keantoken has said (I have just read that).

Last edited by HiFiNutNut; 9th March 2014 at 11:03 PM.
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Old 9th March 2014, 10:54 PM   #22
DF96 is offline DF96  England
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Originally Posted by HiFiNutNut
I wouldn't mind if they were lossy inductors (working like ferrite?) but how do I know?
An inductor wound with resistance wire is almost guaranteed to be lossy.
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Old 9th March 2014, 11:02 PM   #23
HiFiNutNut is offline HiFiNutNut  Australia
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Originally Posted by DF96 View Post
An inductor wound with resistance wire is almost guaranteed to be lossy.
Interesting. I have these so called non-inductive ceramic wirewound 0.22R resistors I bought for the LMOSFET source resistors with 0.2uH indcutance. This is the lowest inductance I have measured with 5W+ WW resistors. Would you use it in your power amps? I am worried that 0.2uH is too high.
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Old 9th March 2014, 11:51 PM   #24
keantoken is offline keantoken  United States
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Originally Posted by peufeu View Post
Yes, but not for audio. It was the flat film type resistor, about 1cm2 in size. I epoxied it to the chassis for cooling. Works pretty well. As a source resistor, on second thought, maybe the capacitance could be a problem.
Across a 0.33R resistor, even something like 10nF might be okay. Remember these resistors are in series with the load; it's capacitances in parallel with the load which are the biggest problem for stability.
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Old 10th March 2014, 03:52 PM   #25
jneutron is offline jneutron  United States
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Originally Posted by HiFiNutNut View Post
I am having a hard time finding low inductance through hole resistors.

I have a cheap LCR meter that shows inductance as small as 0.1uH. When I measure, I always measure the value then deduct the value from the measurement of a short at the time of the measurement. I think the values I get should be accurate.

Bill
It is simple enough to make a resistor which is in the sub 100 picohenry or so range. The problem stems in how to measure without error as DF96 has pointed out, and how to apply it.

If the through holes are a distance apart, you can never obtain an inductance value below that of a wire bridging that distance with a single element. Paralleling multiple elements does increase the reluctance path, so the DC inductance will drop, but proximity effects will kill this as frequency rises. The current will distribute unevenly in it's attempt at following the path of least reactance.

I make resistors in the 100 pH range by interleaving many axial 1/4 watt metal films. However, I am unable to verify inductance below 250 pH due to physical limitations. I suspect that may be good enough for you. The value of resistance is easy to get from 100 milliohms out to tens of kOhms, the worst part is buying 40 or 80 one percenters from mouser or digikey.

It's easy enough to do, I guess I'll have to pop a sequence of pics into a new gallery, I don't have them where I type at the moment.

Let me know if you're interested.

jn
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Old 10th March 2014, 08:19 PM   #26
HiFiNutNut is offline HiFiNutNut  Australia
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Originally Posted by jneutron View Post
It is simple enough to make a resistor which is in the sub 100 picohenry or so range...Let me know if you're interested.

jn
That would be very interesting. I'd love to see your photos!

Last edited by HiFiNutNut; 10th March 2014 at 08:30 PM.
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Old 10th March 2014, 08:55 PM   #27
jneutron is offline jneutron  United States
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Originally Posted by HiFiNutNut View Post
That would be very interesting. I'd love to see your photos!
no prob. The first two are assembly of the resistor. It is comprised of three plates. The bottom plate all the resistors are soldered to. The middle plate has every other hole being soldered to, the rest pass through. The top plate has half the resistors soldered to it.
The first pic is a trial fit and photo op.

The second pic shows all the resistors soldered to the left side plate, and half of them soldered to the middle plate. The other half are passed through, and will be soldered to the right hand plate.

The current flows in the right hand plate, goes down half the resistors to the bottom plate, then turns around and goes through the other half of the resistors, but do so through the first set of resistors.

The 3rd and 4th pic show the completed assembly without then with leads.

If you tried to measure the inductance of this puppy, most of the reading will be the inductance from the edge of the assembly to the center. The current passes through itself in the resistive bulk, so that portion of the inductance is very low.

jn
Attached Images
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File Type: jpg DSCN1997.jpg (715.8 KB, 186 views)
File Type: jpg CVR1.jpg (164.1 KB, 181 views)
File Type: jpg CVR2.jpg (266.0 KB, 175 views)

Last edited by jneutron; 10th March 2014 at 08:57 PM.
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Old 10th March 2014, 08:57 PM   #28
infinia is offline infinia  United States
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who in the world
would use 40 1/4 W axial resistors and worry about sub nH?
jus asking
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Old 10th March 2014, 09:09 PM   #29
jneutron is offline jneutron  United States
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Originally Posted by infinia View Post
who in the world
would use 40 1/4 W axial resistors and worry about sub nH?
jus asking
Me.

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Sometimes I need to measure high rate of change currents with a very low resistor, and cannot allow the inductance of the resistor to get in the way. As the resistor value gets lower, the inductance will cause proportionally higher errors.


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Old 10th March 2014, 09:28 PM   #30
infinia is offline infinia  United States
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You must work @ Gov / MIL industrial complex LOL
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