sivan_and said:
Physically, it's too big. Your trying to use a minivan at the indy 500..sheesh.
Now THIS is what ya gotta do. This baby is 50 watts, air cooled, has an inductance of about 60 picohenries (although I must admit, I have only been able to confirm it is less than 250 pH due to test limitations).
The current flows in the same direction for all the resistors, and the current returns through the wire between each resistor, this pic is just prior to soldering all the leads to their common.
This baby has no external field.. and no internal field (well, ok, the external falls off as 1/R 19 and internally as 1/R 18. But , close enough.
btw, this is rev B...rev C will be designed for lower inductance.
Cheers, John
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At the risk of creating hard feelings there is no physical law that could apply to improve an audio amplifier's perfromance by installing parallel resistors backwards to each other. First, the inductive reactance of a spiral cut film resistor is so low that its own leads have more inductance than the resistive element itself. This has been amply studied using a VHF impedance bridge by someone highly qualified to do such a study who shall remain nameless here (not me, either). Therefore unless an audio amplifier is also intended to be an RF driver at TV frequencies resistor self indcutance is not a player. The circuit traces and wiring make up 99.99% of the parasitic inductance in an audio amplifier circuit. Second, turning a resistor end for end does not reverse the rotation of the spiral cut. Check out a screw if you want to understand the geometry better.
You fellows are looking at this from the wrong angle. The inductance of a resistor can be an issue at audio frequencies. Try measuring the power output of a loudspeaker amplifier using a big 8 Ohm wirewound resistor at 1kHz using V2/R and you'll get more power than you expect. Why? Because you measured the resistance of the dummy load at DC, but measured the AC voltage across it at 1kHz. And it can cause a significant error, not merely 1% - it can be 10%! (Been there, done that.)
The two important factors are the value of the resistor and the resistive material.
Carbon has much higher resistitivity than manganin (resistance wire) so a shorter path gives the same resistance. Since resistors are commonly wound as a helix, that means fewer turns and reduced inductance. That's why we use carbon resistors for valve and FET grid/gate stoppers.
The other issue is the actual value of the resistor. Who cares if a 1M resistor has a few uH of inductance? You would never notice it compared to 1M. But you would compared to 1 Ohm. It turns out that even wirewound resistors (which are more inductive because of the lower resistivity of their material) only begin to show significant inductance once their value falls below 1000 Ohms. For metal oxide resistors (with their higher resistivity), I'd expect to only see inductive effects below 100 Ohms.
A practical 100k resistor has no significant inductance. (Although it has measurable shunt capacitance.)
Low values of resistance can be made non-inductive by adding a Zobel network across them. (Non-inductive resistor of the same value as the original resistor, in series with an experimentally determined value of capacitance.)
The two important factors are the value of the resistor and the resistive material.
Carbon has much higher resistitivity than manganin (resistance wire) so a shorter path gives the same resistance. Since resistors are commonly wound as a helix, that means fewer turns and reduced inductance. That's why we use carbon resistors for valve and FET grid/gate stoppers.
The other issue is the actual value of the resistor. Who cares if a 1M resistor has a few uH of inductance? You would never notice it compared to 1M. But you would compared to 1 Ohm. It turns out that even wirewound resistors (which are more inductive because of the lower resistivity of their material) only begin to show significant inductance once their value falls below 1000 Ohms. For metal oxide resistors (with their higher resistivity), I'd expect to only see inductive effects below 100 Ohms.
A practical 100k resistor has no significant inductance. (Although it has measurable shunt capacitance.)
Low values of resistance can be made non-inductive by adding a Zobel network across them. (Non-inductive resistor of the same value as the original resistor, in series with an experimentally determined value of capacitance.)
No I'm not. 31.76 degrees, that is certainly the correct angle..😀EC8010 said:
EC8010 said:
I also have been there, done that. In fact, the resistor I pic'd is an output load resistor, designed specifically to pull current resistively from an amp output at sub nanosecond speeds. While I have no interest in actually making or measuring an audio amp at those speeds, that speed comes about as a side benefit of a different, far more notorious effect. That'd be B dot error.
The voltage created across a wire as a result of the collapse of the load's current. For audio, that can be significant, ESPECIALLY for low impedance circuits.. That resistor eliminates the field collapse error term. It is a pure resistive load with no error out to several hundred Mhz.
Yuck. I like my method better..😀EC8010 said:
....sorry if it's already been posted, but. non-inductive wire wounds are available. They're bifilar wound, to cancel their inductance. That's for power applications. We also used to use metal oxide from Victoreen. They weren't laser spiral trimmed. Good for high freqs.
jneutron said:
Ah, well, your method genuinely reduces inductance rather than trying to fudge it away.
Actually, it was because I used pretty resistors..notice the interplay of the slant fin copper with the baby blue body color, highlighted by the band colors..and don't forget the seagreen tap wire.. and all embraced by the tech looking angular repetitive aluminum, framing the structure, not unlike a pair of palm trees would a beach chair..with a corona in the middle..EC8010 said:
Cheers, John
Edit: verbage of course, consistent with the 10 days vacation I will embark upon tonight...
EC8010, you make a couple of unsupportable statements:
".......It turns out that even wirewound resistors (which are more inductive because of the lower resistivity of their material) only begin to show significant inductance once their value falls below 1000 Ohms. ..."
Just because your method of observation is incapable of detecting the inductance does not mean it is not there. You should correctly state the effect of the network on the circuit it is employed in. A 1 meg resistor in series with a 1uH choke certainly won't have much effect at 1kHz. Go to 10MHz and calculate the impedance to the two parts in series and see what you get. The inductance doesn't go away, it's alway there, even at DC. Now if you had said INDUCTIVE REACTANCE, you might have been closer to the truth.
"......A practical 100k resistor has no significant inductance. ..."
That's an opinion, not a physical fact.
"....Low values of resistance can be made non-inductive by adding a Zobel network across them. (Non-inductive resistor of the same value as the original resistor, in series with an experimentally determined value of capacitance.)..."
You must be referring to Zobel the Magician.
".......It turns out that even wirewound resistors (which are more inductive because of the lower resistivity of their material) only begin to show significant inductance once their value falls below 1000 Ohms. ..."
Just because your method of observation is incapable of detecting the inductance does not mean it is not there. You should correctly state the effect of the network on the circuit it is employed in. A 1 meg resistor in series with a 1uH choke certainly won't have much effect at 1kHz. Go to 10MHz and calculate the impedance to the two parts in series and see what you get. The inductance doesn't go away, it's alway there, even at DC. Now if you had said INDUCTIVE REACTANCE, you might have been closer to the truth.
"......A practical 100k resistor has no significant inductance. ..."
That's an opinion, not a physical fact.
"....Low values of resistance can be made non-inductive by adding a Zobel network across them. (Non-inductive resistor of the same value as the original resistor, in series with an experimentally determined value of capacitance.)..."
You must be referring to Zobel the Magician.
Audioservice, please bear in mind that I started my post by making it clear that I was talking about audio frequencies, not RF. I cannot imagine any RF application that would even use a 1M resistor, let alone put it in series with 1uH. Nevetheless, you are correct in that strictly I should really have said "inductive reactance".
No, my comment about 100k resistors was based on measurement. 2.5pF of shunt capacitance was needed to model a 100k wirewound resistor (tested, as it happens, up to 1MHz), but no series inductance.
I suggest you read up on Zobel networks. It's perfectly possible to correct an inductive resistor back to pure resistance using a Zobel network.
No, my comment about 100k resistors was based on measurement. 2.5pF of shunt capacitance was needed to model a 100k wirewound resistor (tested, as it happens, up to 1MHz), but no series inductance.
I suggest you read up on Zobel networks. It's perfectly possible to correct an inductive resistor back to pure resistance using a Zobel network.
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