How does a crossover effect ohm rating?

[Myth_103]

If I were to build a cabinet with a 15 crossed over to a 6.5" like fEarful's designs, what would my overall ohm rating be if the 15 and 6.5 were both 8 ohms?

 

[David1234]

In theory, the high frequencies will see only an 8 ohm tweeter, and the lows will see only an 8 ohm woofer.

So it will still be an 8 ohm cab.

 

[spode master]

Most Xovers have a complex impedance curve that if designed right should never drop to far bellow Rated Z but will increase in the extremities.

You will have two impedance spikes that surround port resonance in a ported box, and an increase in impedance as the Tweeter inductance increases.

There will always be a small amount of insertion loss when using passive xovers (small increase in impedance) but if quality components are used it should be negligible.

A simple two way xover HF and LF are normally run in parallel (I've seen some interesting exceptions).

The LF stage never drops bellow DCR has its peaks around resonance then impedance increases rapidly to high frequencies at Xover. The HF stage Increases impedance rapidly to bass frequencies often xover point is close to where the impedance curves overlap. There is an impedance bump at xover point. Then the impedance increases in the high frequencies due to HF voice coil inductance.

Thats all before we start tinkering with L-Pads, impedance networks, eq networks etc.

Big fun.

 

[rcarraher]

Most Xovers have a complex impedance curve that if designed right should never drop to far bellow Rated Z but will increase in the extremities.

You will have two impedance spikes that surround port resonance in a ported box, and an increase in impedance as the Tweeter inductance increases.

There will always be a small amount of insertion loss when using passive xovers (small increase in impedance) but if quality components are used it should be negligible.

A simple two way xover HF and LF are normally run in parallel (I've seen some interesting exceptions).

The LF stage never drops bellow DCR has its peaks around resonance then impedance increases rapidly to high frequencies at Xover. The HF stage Increases impedance rapidly to bass frequencies often xover point is close to where the impedance curves overlap. There is an impedance bump at xover point. Then the impedance increases in the high frequencies due to HF voice coil inductance.

Thats all before we start tinkering with L-Pads, impedance networks, eq networks etc.

Big fun.

Ditto on the exceptions, I've seen some wild cross over circuits,but a simple answer here is that the two speakers would usually be in parallel for a cross over to even work, and therefore the cabinet would have a "nominal impedance" of 4 ohms. Effectively, the frequencies would "see" one speaker or the other, but the over all impedance would still be 4 ohms.

 

[MNAirHead]

I asked a speaker only store - they basically said tweeters were "free ohms"

We went exactly why this happens - you'd be bored with the banter - you get a free tweet

 

[billfitzmaurice]

If I were to build a cabinet with a 15 crossed over to a 6.5" like fEarful's designs, what would my overall ohm rating be if the 15 and 6.5 were both 8 ohms?

8 ohms. http://www.bcae1.com/xoorder.htm

 

[spode master]

8 ohms. http://www.bcae1.com/xoorder.htm

Hey Bill,

Like the Xover link, raises a question that I've had about impedance networks.

In your opinion is there ever a good reason to use a impedance flattening network for flattening the woofers impedance at tuning?

The component values get so ridiculously large and expensive, would there ever be a reason that using such a network would be warranted?

 

[billfitzmaurice]

In your opinion is there ever a good reason to use a impedance flattening network for flattening the woofers impedance at tuning?

No. A well engineered crossover uses the actual impedance at the crossover frequency for component value calculation, while a very well engineered crossover is tested and tweaked until the desired result is obtained. When you do so a zobel is an unnecessary waste of parts.

 

[DukeLeJeune]

Hey Bill,

Like the Xover link, raises a question that I've had about impedance networks.

In your opinion is there ever a good reason to use a impedance flattening network for flattening the woofers impedance at tuning?

The component values get so ridiculously large and expensive, would there ever be a reason that using such a network would be warranted?

If you're talking about flattening the woofer's impedance peaks around the box tuning frequency, nope it's not worth it. However, in a two-way system, in many cases it does make sense to address the high frequency driver's resonant peak.

In my opinion it sometimes makes sense to smoothe out the impedance curve, but usually it's not necessary. For the home stereo market I manufacture speakers intended to work well with low-powered specialty tube amps, and for that application a smooth impedance curve is a high priority. Prosound amps in general are vastly more tolerant of loudspeaker impedance swings.