4 ohm mid/woofer, 8 ohm tweeter, single capacitor for tweeter HPF

[RWL2024]

Hi,

This is not for a hi-fi or music listening application, but rather for a "toy".

I understand that if I had an 8 ohm mid/woofer and an 8 ohm tweeter, and I didn't want a real crossover - just a single capacitor for the tweeter HPF - if I used a 3.3uF cap, the HPF would be at about 6KHz.

If I instead used a 4 ohm mid/woofer with the 8 ohm tweeter and the 3.3uF cap, would the HPF frequency change, or would I just get my desired effect of the 8 ohm tweeter not being as loud relative to the 4 ohm woofer as it would (in my theory) be, relative to the 8 ohm woofer?

Thanks.

 

[rayma]

In a parallel crossover (the usual kind), the tweeter and woofer don't interact at all, regardless of their impedances.
You can always attenuate the tweeter with a series resistor.

 

[RWL2024]

Thanks, so just to confirm, a 4 ohm mid/woofer with an 8 ohm tweeter and a 3.3uF cap on the tweeter would still give me about a 6KHz tweeter HPF, just like it would if the mid/woofer and tweeter were both 8 ohm drivers?

 

[rayma]

Exactly, in a parallel crossover, as you are using.
Then you may have to match the tweeter level to the woofer by adding a series resistor, which then may require a smaller capacitor value.
Unless you use an L-pad attenuator, which also has a shunt resistor in parallel with the tweeter.

 

[RWL2024]

Thanks 😊

 

[Michael Bean]

Understand that the "eight ohm" rating of that tweeter is a nominal value, an average...but the actual impedance in any particular frequency range may be much different. If you have an impedance curve to look at for that tweeter, see what the actual impedance is around the frequency range of choice, and figure the proper cap value from that...and if you use a series resistor to pad the output level downward, add that resistance to your calculations.

Mike

 

[rayma]

Yes, with a crossover frequency at 6kHz, you may want to add a Zobel network (which is a series RC network) across the tweeter,
to flatten its impedance in that range.

 

[AllenB]

the 8 ohm tweeter not being as loud relative to the 4 ohm woofer as it would (in my theory) be, relative to the 8 ohm woofer?

This is not the case. Sensitivity relates to a number of other factors.

 

[steveu]

Tweeters as insensitive as a mid or woofer, are very rare. Tweeters almost always need to be attenuated to bring them down to the mid-bass level. So the use of a high impedance tweeter is a step in the right direction, but probably not enough. You probably still need to attenuate it. A woofer will not be damaged by exposing it to high frequencies, but it will do bad things with those frequencies that you probably want to avoid. A good cross-over is a collection of carefully measured hacks tailored to the drivers you are using. A cross-over designed for resistor loads is not very useful.