ProSoundWeb Community

Sound Reinforcement - Forums for Live Sound Professionals - Your Displayed Name Must Be Your Real Full Name To Post In The Live Sound Forums => Pro AV Forum => Topic started by: Robert Quigley on June 01, 2020, 02:30:15 pm

Title: Impedance question
Post by: Robert Quigley on June 01, 2020, 02:30:15 pm
New guy here. This is probably a dumb question and I think I already know the answer but I want to double check.

I recently got a pair of passive cabs rated for 8 ohms. The amp powering them is 8 ohms. The horns in the cabs are 4 ohms and the woofers are 8 ohms.

One of the woofers is gone. I'm looking to replace it with an oem woofer but they don't make them anymore. So, I took to the internet and found a few that are 4 ohms.

My question is, could I put a 4 ohm woofer to replace the 8 ohm woofer and still power it with an 8 ohm amp?
Title: Re: Impedance question
Post by: John Roberts {JR} on June 01, 2020, 02:56:45 pm
you could but you shouldn't

JR
Title: Re: Impedance question
Post by: Chris Grimshaw on June 02, 2020, 06:56:34 am

My question is, could I put a 4 ohm woofer to replace the 8 ohm woofer and still power it with an 8 ohm amp?

The answer, like so many others, is that it depends.

Chances are your amp isn't exclusively capable of driving 8ohm. So, a 4ohm woofer might be okay.

However, if there's a passive crossover in there, then the chances are that it's designed to work with the original 8ohm woofer. Crossovers rely on the speaker being a particular impedance to function correctly, so changing the woofer impedance will mean the crossover isn't aligned well and the results will be unpredictable.

I'd say that more information would be useful. Some questions to start you off:
- Which speakers?
- What did the previous woofer die of?
- Which amplifier are you using? Is there any EQ or limiting involved in the signal chain?

Chris
Title: Re: Impedance question
Post by: Brian Jojade on June 02, 2020, 07:18:12 pm
+1 on Chris's response.

Yes, you probably could use the 4 ohm speaker, but it will change the behavior of the cabinet.  If you have a pair of cabinets and want to go that route, replace BOTH of them, or they won't match.
Title: Re: Impedance question
Post by: Luke Geis on June 03, 2020, 03:03:08 am
The passive crossover unit depends on the load it is driving to work right. So you need to replace it with an 8-ohm speaker. Can you put a 4 ohm in there and have it work? Yes, but will it work correctly? No.
Title: Re: Impedance question
Post by: Mike Caldwell on June 03, 2020, 08:25:35 pm
In general going from an 8 ohm woofer to a 4 ohm woofer and not changing anything else will cut the low pass frequency to the woofer in half.
Title: Re: Impedance question
Post by: Chris Grimshaw on June 04, 2020, 07:59:43 am
For a first-order filter, yes.
For a 2nd or higher filter, it'll stagger the knee and add some weirdness. XSim is free - have a play around.

Chris