paralleling resistors to reduce inductance but...

[jarthel]

Here's the schematic: http://www.audiodesignguide.com/my/n...004/schem2.jpg

Does anyone know why Andrea Ciuffoli suggested to parallel resistors that way?

Thanks for the replies

[sawreyrw]

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.

[jarthel]

Originally posted by sawreyrw
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.

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.

[rpapps]

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

[AndrewT]

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?

[pinkmouse]

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.

[jarthel]

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)

[macboy]

Rather than trying to find some mystical reverse-wound resistor which is otherwise identical, wouldn't it be easier to just connect two identical resistors so that the current flows in opposite directions and therefore creates cancelling magnetic fields?

[AndrewT]

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.

[macboy]

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.