A bit confused about impedance

[Spellcaster]

I have a bass guitar cabinet with two K140's in parallel. The drivers are rated 8 ohms, but my Fluke meter shows 2.9 ohms when I measure the resistance.

I recognize that resistance and impedance aren't necessarily the same but it's got me wondering......I'm using a Fender Rumble 150 head designed for a 4 ohm speaker load. Right now, I'm mating the amp and cabinet with a Weber Z-Matcher, with the input side for the amp set to 4 ohms, and the output side for the cabinet set to 2.6 ohms. i don't hear any difference regardless of whether the cabinet is switched to a different impedance.

Is the Z-Matcher doing anything productive by being inline, or would it work just as well connecting head directly to cabinet?

63394

[Allanvh5150]

What you are measuring is the DC resistance of the speakers. Impedance is an AC measurement measured at various frequencies and averaged.

Allan.

[Horn Fanatic]

I have a bass guitar cabinet with two K140's in parallel. The drivers are rated 8 ohms, but my Fluke meter shows 2.9 ohms when I measure the resistance.

I recognize that resistance and impedance aren't necessarily the same but it's got me wondering......I'm using a Fender Rumble 150 head designed for a 4 ohm speaker load. Right now, I'm mating the amp and cabinet with a Weber Z-Matcher, with the input side for the amp set to 4 ohms, and the output side for the cabinet set to 2.6 ohms. i don't hear any difference regardless of whether the cabinet is switched to a different impedance.

Is the Z-Matcher doing anything productive by being inline, or would it work just as well connecting head directly to cabinet?

63394

Greetings Spellcaster -

The Impedance marked on a loud speaker is the nominal impedance, not the D.C. resistance of the voice coil, which will always be lower. Keep in mind, an impedance is a resistance, but a resistance is not necessarily an impedance. Impedance is frequency dependent, where as D.C. resistance is not. If you look at a nomograph for a typical loud speaker, you will notice a very sharp spike at free air resonance, then the impedance drops down rapidly followed by a gradual slope upward as the frequency increases. The electrical impedance of a typical loud speaker can rise up to 400 ohms as the frequency increases, then it will drop off gradually due to the limitations of the voice coil.

There are three basic elements to consider in loud speaker design, the DCR, the voice coil inductance and capacitance. Even magnet structures have an impedance. Within the parameters of a cone loud speaker other than the electrical, are the mechanical and acoustical elements, those being the acoustical and mechanical resistance, the reactance of the acoustical and mechanical mass, and the reactance of the acoustical and mechanical compliance of the suspension system.

For example: |Z| electrical will be the square root of the DCR squared + inductive reactance squared, at any given frequency. The same holds true for the acoustical and mechanical analogies as well.

Personally, I think folks are overly concerned about load impedance, considering it changes with every frequency that is presented to it. The only thing that should be of concern, is presenting a load impedance below what is recommended for an amplifier. That being said, it is not a good idea either to wire up an inordinate number of drivers as a speaker load even though the load impedance is within parameters. Excessive back EMF can damage an amplifier also.

Regards, H.F.

[grumpy]

Not to take away anything from the previous posts, but:

"Is the Z-Matcher doing anything productive by being inline, or would it work just as well connecting head directly to cabinet?"

No.

Yes, probably better.

[Spellcaster]

Thanks to all who replied. My background in hard-core electronics is sketchy, so I'm always interested in learning more theory. And, I appreciate the answer to my original question about using the Z-Matcher. I'm going to use the amp and box at band practice tonight for the first time, and I think I'll forego the matchbox this time and see how the amp and speakers work out wired directly together.