More about wire wound power resistors:

The type of wire itself is much more resistive then say copper, and that is how one gets to build resistance. Say one wants to have 8 Ohms, and the resistive wire used is the type of 4 Ohm per foot. That would call for 2 feet length for 8 Ohms.

But, given that we would not wish to just have 2 feet of wire, it is often being wound around a cylinder made of an insulating material. The standard way to wing is one layer (single row), and that works great for some applications such as DC power, or 60Hz AC line and more.

The inductance of wire wound resistors in a single row depends on the a "form factor" (it has to do with ratio of length to diameter), the inductance is proportional to the diameter of the windings and most of all on the number of turns (proportional to number of turns square).

Say we have 1 inch diameter, and 5 inch winding length.

That would yield form factor F=.003.

Say we have 20 turns (windings), then the inductance will be around .003*1*30^2 = 2.7uH

That would yield .339 Ohms of reactance at 20KHz. Such reactance against say 8 Ohms resistive load resistance means a loss of nearly .01dB. So the example above shows a resistance construction that has little effect on audio.

But say we have 2 inch diameter and 5 inch length. That would yield a form factor of around .006.

Say we have 40 turns (windings), then the inductance will be around .006*2*40^2 = 19.2uH

That would yield 2.4 Ohms of reactance at 20KHz. Such reactance against say 8 Ohms resistive load resistance means a loss of nearly .38dB. The loss at 10KHz is around .1dB. That may be audible.

As a rule, the large single winding power resistor can work, but for lowering inductance, one needs to watch for a few factors:

1. Stay with single winding. Multiple winding may increase inductance exponentially.

2. Look for a wire wound resistor with fewer winding.

3. Go for a long resistor, with as small diameter as you can find

Also, consider paralleling wire wound resistors. Say you parallel 2 resistors of 16 Ohms each to get 8 Ohms. The inductance will be also cut by half. The purist may wish to mount the resistors in such a way that the winding directions of the 2 resistors is opposing each other. It should not matter here, but it is a good thing to keep in mind for higher frequency signal applications.

I am still not convinced that a series resistor to a speaker is a good idea. This post is a follow up of the inductance comment of wire wound resistor inductance.

Regards

Dan Lavry

http://www.lavryenginering.com