Here's the schematic: http://www.audiodesignguide.com/my/new300bxls2004/schem2.jpg
Does anyone know why Andrea Ciuffoli suggested to parallel resistors that way?
Thanks for the replies 🙂
Does anyone know why Andrea Ciuffoli suggested to parallel resistors that way?
Thanks for the replies 🙂
The schematic says it is to reduce the inductance. This doesn't make any sense at audio frequencies.!! However, the power dissipation per resistor is reduced. This make some sense.
sawreyrw said:
that is true
but look at the way he suggested on how to parallel them. I think that's the heart of the my question.
he suggested that the bands (not sure of the term) must be in opposite directions.
Hi Jarthel
There are two components to this theory.
Firstly, resistors are trimmed to their final value by cutting a spiral groove into the surface. This gives rise to some inductance.
By placing two inductances in parallel, the total inductance is halved. Like wise four in parallel would quarter it.
Secondly, by reversing one resistor, I think he is trying to get the fields to cancel each other. However this would only work if the resistors were in close proximity (mutual coupling) and they were all manufactured so that the painted bands were always put on with the same orientation to the spiral cut. I don't know if that could be guaranteed.
But ,as the previous poster mentioned, I doubt it makes any appreciable difference at audio frequencies.
Cheers
Rob
There are two components to this theory.
Firstly, resistors are trimmed to their final value by cutting a spiral groove into the surface. This gives rise to some inductance.
By placing two inductances in parallel, the total inductance is halved. Like wise four in parallel would quarter it.
Secondly, by reversing one resistor, I think he is trying to get the fields to cancel each other. However this would only work if the resistors were in close proximity (mutual coupling) and they were all manufactured so that the painted bands were always put on with the same orientation to the spiral cut. I don't know if that could be guaranteed.
But ,as the previous poster mentioned, I doubt it makes any appreciable difference at audio frequencies.
Cheers
Rob
Hi,
paralleling will reduce inductance. It has absolutely nothing to do with the usable frequency.
The effect of reduce inductance will be influenced by the usable frequencies.
Wire wounds have inductance. There is a very simple way to wind a WW to reduce the self inductance to near zero.
Unfortunately putting two spiralled resistors side by side cannot use this inductance reducing property if both resistors have the spiral turns in the same direction. Batches of resistors will always have the spiral turns in the same direction.
However, some manufacturers may construct their resistors consistently from one handed spirals and other manufacturers may consistently spiral in the opposite hand. Here we have a solution that will reduce the inductance more than just the paralleling effect.
Select two equal value spiral formed resistors ( it matters not whether they are metal film, metal oxde or wire) made from opposite handed spirals. Placing them very close together and side by side will cause some mutual coupling of the fields produced by the spirals. The mutual coupling will reduce the overall inductance.
In a non inductive wire wound that is wound in spirals the first half of the resistor is wound in one hand and then halfway along the spiral the hand of the spiral is reversed until the end is reached. If the two halves are electrically and physically identical then the inductance from the two halves is almost exactly cancelled by the mutual inductance from the two end to end inductors. The remaining inductance in this type comes mostly from the lead out wires.
Who is going to reseach the spiral directions of the different manufacturers for us and put it into a Wiki?
paralleling will reduce inductance. It has absolutely nothing to do with the usable frequency.
The effect of reduce inductance will be influenced by the usable frequencies.
Wire wounds have inductance. There is a very simple way to wind a WW to reduce the self inductance to near zero.
Unfortunately putting two spiralled resistors side by side cannot use this inductance reducing property if both resistors have the spiral turns in the same direction. Batches of resistors will always have the spiral turns in the same direction.
However, some manufacturers may construct their resistors consistently from one handed spirals and other manufacturers may consistently spiral in the opposite hand. Here we have a solution that will reduce the inductance more than just the paralleling effect.
Select two equal value spiral formed resistors ( it matters not whether they are metal film, metal oxde or wire) made from opposite handed spirals. Placing them very close together and side by side will cause some mutual coupling of the fields produced by the spirals. The mutual coupling will reduce the overall inductance.
In a non inductive wire wound that is wound in spirals the first half of the resistor is wound in one hand and then halfway along the spiral the hand of the spiral is reversed until the end is reached. If the two halves are electrically and physically identical then the inductance from the two halves is almost exactly cancelled by the mutual inductance from the two end to end inductors. The remaining inductance in this type comes mostly from the lead out wires.
Who is going to reseach the spiral directions of the different manufacturers for us and put it into a Wiki?
From what I was told about resistor manufactur by a friend who worked in component distribution, there is no correlation between banding and spiral direction.
The resistor slugs are made, dumped into a big bin, then taken to the cutting machine, where the spirals are cut, then the slugs are dumped into another bin. Then they have the leads attached, and are then tested, coated, and marked but as you can see, they go will through that machine in a completely random manner. Of course, this doesn't meant to say you couldn't measure each resistor individually, but the markings are not indicative.
The resistor slugs are made, dumped into a big bin, then taken to the cutting machine, where the spirals are cut, then the slugs are dumped into another bin. Then they have the leads attached, and are then tested, coated, and marked but as you can see, they go will through that machine in a completely random manner. Of course, this doesn't meant to say you couldn't measure each resistor individually, but the markings are not indicative.
But is it a good recommendation that "paralleled resistors with the spirals opposite to each other" have good effects? (assuming a person is able to tell where the spiral is)
Hi Mac,
NO!!!
Read my post again.
turning a sprial cut/wound resistor end for end does not change the hand of the winding.
Go and have a think about it.
If that is too much, then get a bit of single core wire and wind a spiral for yourself and check what it looks like when you turn it end for end.
NO!!!
Read my post again.
turning a sprial cut/wound resistor end for end does not change the hand of the winding.
Go and have a think about it.
If that is too much, then get a bit of single core wire and wind a spiral for yourself and check what it looks like when you turn it end for end.
Andrew,
I understand that turning a spiral around has no effect. But look at what I have done. I didn't turn the resistor around, I turned the current around. The current flows in the opposite direction through two coils/spirals which are the same. That means the the magnetic fields are opposite to each other.
I understand that turning a spiral around has no effect. But look at what I have done. I didn't turn the resistor around, I turned the current around. The current flows in the opposite direction through two coils/spirals which are the same. That means the the magnetic fields are opposite to each other.
Hi Mac,
imagine two bar magnets side by side but with opposite poles adjacent to each other. They are still magnets (try pulling them apart if your not sure).
The magnetic lines of force leave one pole curve around to enter the opposite pole. A magnetic field circuit has been set up.
Now take your spiral resistors, they too create a magnetic field, albeit a weak one, and they too create magnetic poles. Placing two spirals of matching hand adjacent to each other but with each carrying current in opposite directions will create opposing magnetic fields but they will link at each end to form that magnetic circuit. They actually reinforce each other, rather than eliminate the field.
No, as far as I know, reversing the current has not reduced the inductance.
imagine two bar magnets side by side but with opposite poles adjacent to each other. They are still magnets (try pulling them apart if your not sure).
The magnetic lines of force leave one pole curve around to enter the opposite pole. A magnetic field circuit has been set up.
Now take your spiral resistors, they too create a magnetic field, albeit a weak one, and they too create magnetic poles. Placing two spirals of matching hand adjacent to each other but with each carrying current in opposite directions will create opposing magnetic fields but they will link at each end to form that magnetic circuit. They actually reinforce each other, rather than eliminate the field.
No, as far as I know, reversing the current has not reduced the inductance.
The band orientation will have no affect.jarthel said:
The act of splitting the current into two paths will, though. However, there are better ways.
Cheers, John
Yes, a magnetic field circuit has been set up. Exactly. By coupling the two opposite magnetic fields you reduce the inductance. It's true! If you put an equal but opposite current through to magnetically coupled inductors you will get zero inductance. Now, the two resistors are hardly perfectly magnetically coupled - far from it. But their magnetic fields will interact to some extent. So we will only get partial concellation of inductance. This is the same reason that you should twist your DC power supply wires (supply and ground/return) together; individually they each introduce inductance to your power supply lines (bad), but when coupled together, the inductances cancel nearly 100% because the currents in the two conductors are equal and opposite.
Try sticking your double-magnet thing to a plate of steel. It won't stick (not well anyway) because you've coupled the magnetic fields together and there is nothing left to have external influence. That's the point.
I realise now that I should not have said "cancelling magnetic fields" because that is not what happens. The magnetic fields couple and interact.
Seems to be an assumption here, there is not much coupling between the resistors in the first place.
In fact, the inductance is probably not worrying about in the first place.
Reversing the current in one of the resistors may very well increase the inductance because a magnetic loop is being created... much here would depend on the geometry of the whole deal.
Relax Jarthel... assuming the designer actually knows that a reduction in inductance is necessary at all, the arrangment won't be so critical.
Truth be told, if a parasitic inductance was actually a factor here, reducing it by a factor of only 2 seems pretty silly. Keep this in context with the author's assertion that flipping the resistor reverses the helix (it doesn't) and most of this is just absurd.
😉
In fact, the inductance is probably not worrying about in the first place.
Reversing the current in one of the resistors may very well increase the inductance because a magnetic loop is being created... much here would depend on the geometry of the whole deal.
Relax Jarthel... assuming the designer actually knows that a reduction in inductance is necessary at all, the arrangment won't be so critical.
Truth be told, if a parasitic inductance was actually a factor here, reducing it by a factor of only 2 seems pretty silly. Keep this in context with the author's assertion that flipping the resistor reverses the helix (it doesn't) and most of this is just absurd.
😉
I completely agree. It is absurd.
And I think that the parasitic inductance of the extra traces necessary to reverse-wire the second resistor will add more inductance than the weak cross-coupling will cancel out. Still, I wanted to make the point that two mutually-coupled coils with opposite currents effective cancel their inductances.
But maybe the author of the original schematic, actually put the resistors in opposing pairs like that because resistor sound is directional. Using two in opposite orientation restores symmetry! Oh wait, that is absurd too (but still believed amoung some of the hardest of the hard core).
oh yeah... you betcha.
Whenever I restore an old tube amp, I always mark the resistors I remove so I can put them back in the same way... don't want to reverse the current after they've broken in for so long!
😀
Whenever I restore an old tube amp, I always mark the resistors I remove so I can put them back in the same way... don't want to reverse the current after they've broken in for so long!
😀
poobah,
Ya, I agree. Sort of like putting 2 50 nH in parallel. The impedance of 1 uH is 6.28 ohm at 1 MHz, .628 ohms at 100KHZ. So what?
Rick
Ya, I agree. Sort of like putting 2 50 nH in parallel. The impedance of 1 uH is 6.28 ohm at 1 MHz, .628 ohms at 100KHZ. So what?
Rick
jnuetron is right (damn I hate it when that happens 🙂), paralleling two inductors cuts the inductance in half, ignore the spiral issue.
Whether there is any benefit to cut inductance this small (nano or pico henries?) in half is an altogether different question.
I personally don't think that deposited metal film resistors have the best sound.
Whether there is any benefit to cut inductance this small (nano or pico henries?) in half is an altogether different question.
I personally don't think that deposited metal film resistors have the best sound.
- Status
- Not open for further replies.