4ohm drivers with 8ohm crossover?

[nonsuchpro]

Hey all,
What happens when you used a 3way 8ohm crossover with 4ohm drivers? Do the crossover points shift up or down?

Thanks!
Rob

[MarcelvdG]

With a straightforward first-order 2-way crossover, the low-pass corner frequency would shift down and the high-pass corner frequency would shift up, creating a dip in the middle. With more complicated cross-overs it is hard to say without knowing the crossover schematic, but generally you will get dips or peaks around the crossover frequencies.

[nonsuchpro]

Originally Posted by MarcelvdG

With a straightforward first-order 2-way crossover, the low-pass corner frequency would shift down and the high-pass corner frequency would shift up, creating a dip in the middle. With more complicated cross-overs it is hard to say without knowing the crossover schematic, but generally you will get dips or peaks around the crossover frequencies.

Thanks!
I'm not tying to do anything, I was just curious.

Rob

[AllenB]

Although halving the inductances and doubling the capacitances and halving resistances sets you up for 4 ohms in theory, the fact you're using a whole different driver with different needs might be an even bigger issue than that.

[nonsuchpro]

Originally Posted by AllenB

Although halving the inductances and doubling the capacitances and halving resistances sets you up for 4 ohms in theory, the fact you're using a whole different driver with different needs might be an even bigger issue than that.

I'm sure... I'd never use it that way, I would either build it correctly or buy at least the right ohm.

[AllenB]

FWIW, I don't actually see anything plainly wrong with it.

[benb]

If I had these components and I "had" to make them work "right" and quickly, I'd just add a 4 ohm resistor of appropriate power rating in series with each driver. This still does at least one "bad" thing, making the woofer see a damping factor of 1 (or slightly less) instead of whatever the amplifier and wiring offer.

I've read enough about designing crossovers (for baffle step, different driver efficiencies, other things I don't remember offhand or maybe haven't even heard of) to conclude that for a Good Speaker System a crossover has to be designed specifically for the exact models of drivers to be used, and even after that final tweaking may be done based on listening tests and measurements.

Thus I second AllenB's post #4.

[AllenB]

Originally Posted by benb

...for a Good Speaker System a crossover has to be designed specifically for the exact models of drivers to be used, and even after that final tweaking may be done based on listening tests and measurements.

Yes the exact models, but more specifically, your own pair...and mounted in their enclosure system. Further to this, the more you measure plus the more you know just what and how to measure, also considering the intended application of the system, the less you will need to tweak (up to a point though because some minor changes for the better seem to defy logic...but this is just my experience and therefore my opinion.)

I also agree with your point that the rated impedance is a smaller issue in the whole scheme of things. For example, here's a theoretical audiophile argument that might illustrate how it gets into too fine a detail to make any good assumptions from.

(Hypothetically).....Start with a 4 ohm driver that is accepted as sounding good enough to go in a home hifi system. Say that most amps will be OK with it (as most probably are). That could be the end of it but some might say the differing loads on the amp will expose a differing distortion signature from the amp for each driver. Some may then say that any normal speaker has such a wildly varying impedance that it is a moot point. Others will say just compensate the impedance and be done with it and yet others will say that impedance compensation is not good for sound quality.

In the end, some will say that this is always a problem and it can be reduced by an order of magnitude if you use efficient speakers. Some will go on to build horns. They may also claim that it can be reduced by using class A amplification and some will do just that.

Then you'll get those that wont notice an appreciable difference.

Bottom line, horns and class A amps are either difficult to build or expensive....and the potential benefits are not huge. Totally worth it in my humble opinion, but a conventional speaker can be made to sound very nice all the same when done right.

[nonsuchpro]

I'm not noticing a difference between an 800hz 4ohm 12db Low pass and an 800hz 8ohm 12db Low pass... why is that? No other speakers are hooked up just 1 driver when I'm A/Bn

[AllenB]

The difference is not massive. Changes in the order of a few dB are only just becoming reliably perceptible. Especially in the case of listening to a woofer on it's own like that. Who knows what's really going on. Maybe the driver distortion components which the crossover is not reducing, are confusing the issue. Maybe the lack of harmonics (tweeter) to highlight a peak. Maybe you're crossing near cone breakup whose signature is confounding what you're hearing. Maybe impedance concerns are giving unanticipated results. It might be interesting to listen to the two cases again with an appropriately crossed tweeter in place.