Impedance change effect on crossover?

[Soulman]

A friend of mine recently replaced some 4 ohm speakers in his PA subs, with more powerful speakers, but the replacements were 8 ohms. He has also changed to using tops rated at 8 ohms. He has plenty amp power, so that is not a problem.

My question is, will the crossover built into the subs be affected by the impedance changes, and if so,....how? Will it still work at the same frequency separation? I am obviously not a teckie on speaker electrics, but I have seen some crossover PCBs with an impedance value stamped on them. I didn't want to worry my friend unnecessarily.

[Rob_Beech]

I'm not entirely sure on this one, but I believe 2 way crossovers have some form of input impedance which may or may not be related to the impedance put across the outputs (both low and high). It's a while since I studied such things, but alot of 2 way crossovers will see an 8ohm load if an 8ohm sub and an 8ohm top are connected. This I believe is often where power ratings are mistaken.

 

Someone will be along in the morning to correct me I'm sure. What I can tell you is that obviously the crossover should be under less stress due to the decrease in power, but then again will the loss in power cause you to need to drive the amps harder. My guess is no, and amp that is 100% efficient in terms of power to impedance will output half what it does at 8ohms as it will at 4ohm. 3dB less for a theoretical load. That is if the driver specs are identical all but impedance.

 

 

Rob

[jamesperrett]

I'm not entirely sure on this one, but I believe 2 way crossovers have some form of input impedance which may or may not be related to the impedance put across the outputs (both low and high). It's a while since I studied such things, but alot of 2 way crossovers will see an 8ohm load if an 8ohm sub and an 8ohm top are connected. This I believe is often where power ratings are mistaken.

 

The biggest effect will be that the filter frequencies will change. With a standard passive crossover you will find that the speakers will overlap too much at the crossover frequency - giving an amplitude peak in the midrange (with a corresponding impedance dip).

 

Cheers

 

James

[Soulman]

I'm not entirely sure on this one, but I believe 2 way crossovers have some form of input impedance which may or may not be related to the impedance put across the outputs (both low and high). It's a while since I studied such things, but alot of 2 way crossovers will see an 8ohm load if an 8ohm sub and an 8ohm top are connected. This I believe is often where power ratings are mistaken.

 

The biggest effect will be that the filter frequencies will change. With a standard passive crossover you will find that the speakers will overlap too much at the crossover frequency - giving an amplitude peak in the midrange (with a corresponding impedance dip).

 

Cheers

 

James

Thanks for this guys. James,...does this mean that changing from 4 to 8 ohm drivers raises or lowers the crossover frequency, or that the nominal separation frequency stays the same , but that the cut off slope (dB/octave) flattens, thereby allowing more bass to the tops and more hi to the subs - in essence making the separation less effective?

If so does this mean that the (values of the ) components on a crossover PCB are determined by the impedance of the drivers to be used?,....or is there a general point here that the component values determine crossover frequency, but cut off slope is steeper (better?) for lower impedance drivers?

...and one last point, 'amplitude peak in the midrange'. I assumed that the sub crossover is around 125 to 150 Hz. Even with a flatter separation , I was surprised that overlap might extend to midrange, or have I misinterpreted this.

Many thanks (in advance!) for your patience.

[Rob_Beech]

I assume he means mid range, being the middle of the range, ie around the point of the crossover. Rather than mid range in terms of the middle of the entire useable frequency range of the system.

[jamesperrett]

I assume he means mid range, being the middle of the range, ie around the point of the crossover. Rather than mid range in terms of the middle of the entire useable frequency range of the system.

 

I missed the bit where subs were mentioned and assumed that we were talking about the crossover between the HF and the bass/midrange driver. The same applies to the sub/top crossover. If the impedance is raised the turnover frequency at the sub output is increased while the turnover frequency of the top output decreases so you will end up with both drivers working in parallel. This may give a pleasing upper bass hump or, if the manufacturers have reversed the phase of one of the drivers to account for the phase characteristics of the crossover, you may find that the output of the two drivers cancels out.

 

Matching the crossover to the driver is an important process. In one speaker system that I use, there are taps on one of the coils to account for variations in bass driver characteristics and all the speakers are tested to see which tap should be used before the speaker leaves the factory - despite the fact that they use good quality drivers to start with. Speaker impedance is critical when thinking about crossover design.

 

Cheers

 

James.

[Soulman]

I assume he means mid range, being the middle of the range, ie around the point of the crossover. Rather than mid range in terms of the middle of the entire useable frequency range of the system.

 

I missed the bit where subs were mentioned and assumed that we were talking about the crossover between the HF and the bass/midrange driver. The same applies to the sub/top crossover. If the impedance is raised the turnover frequency at the sub output is increased while the turnover frequency of the top output decreases so you will end up with both drivers working in parallel. This may give a pleasing upper bass hump or, if the manufacturers have reversed the phase of one of the drivers to account for the phase characteristics of the crossover, you may find that the output of the two drivers cancels out.

 

Matching the crossover to the driver is an important process. In one speaker system that I use, there are taps on one of the coils to account for variations in bass driver characteristics and all the speakers are tested to see which tap should be used before the speaker leaves the factory - despite the fact that they use good quality drivers to start with. Speaker impedance is critical when thinking about crossover design.

 

Cheers

 

James.

 

Thanks for this James,...some very interesting stuff here, and it kind of confirms the suspicions I had originally. One last point,..is what you say about speaker impedance being critical in (passsive) crossover design,...a good argument for preferring active electronic crossovers,...or is that another can of worms? Thanks again.

[Simon Lewis]

is what you say about speaker impedance being critical in (passsive) crossover design,...a good argument for preferring active electronic crossovers,...

 

Yes. In an active system, the driver can be driven directly from the amplifier, removing impedance related crossover problems and improving damping factor.