4 vs 8 ohm drivers, crossovers and amps?

[markusA]

The other day I was offered a sweet deal on a pair of 4 ohm drivers that would be perfect for my project. Still I'm a little concearned that they are rated 4 ohm.

I've ran some initial basic models for the passive crossover and it would seem that when all is said and done the impedance of the speaker might be down to 2 ohm.

I'd say this is pretty close to the point where they become difficult no matter what amp you're running and forget about using a tube amp.
Are the 4 ohm drivers for active set-ups only or what?


What's your take on using 4 ohm woofer drivers in a hifi-build?

Oh, did I mentione that I'll be pairing it with a 8 ohm CD in a waveguide.
The obvious upside would be higher sensitivity voltage wise which would mean less damping on the tweeter which is probably a good thing.
As long as the amp is up for it...

 

[weltersys]

I've ran some initial basic models for the passive crossover and it would seem that when all is said and done the impedance of the speaker might be down to 2 ohm.

I'd say this is pretty close to the point where they become difficult no matter what amp you're running and forget about using a tube amp.
Are the 4 ohm drivers for active set-ups only or what?

What's your take on using 4 ohm woofer drivers in a hifi-build?

A 4 ohm nominal speaker with an impedance of 2 ohms is not labeled correctly, the impedance minima should only drop to 80% of the nominal rating.

A tube amp with a four ohm transformer tap will work fine with a four ohm speaker.

Low impedance speakers are usually targeted for auto use, but if your amp is rated for low impedance use, no problem.

I routinely run home stereo amps below 4 ohms (3 or 4 pairs of speakers) at a low output level, the amps don't even get warm as the increased efficiency of multiple speakers means only a few watts (at most) are needed for the level I listen at.

 

[markusA]

Well, the driver itself doesn't dip like that but once you have the passive crossover in there things tend to look a little different.
The driver might be marked 4 ohm but that doesn't mean the Re is 4 ohm, it's just the caracteristic impedance of the driver.

The series coil will aid the situation some and raise the impedance but due to the complex nature of the impedance the effect will be minor.
The parallell cap otoh will lower the impedance in a significant way and the end result will be a dip below the drivers internal impedance.
In my sims the end result would be close to 2.7 ohm.
2.7 ohm seems low to me if I want to toy with tube amps?

Naturally adding resistors would change everything but that would really be wasting good power.

If I'm wrong I'm happy to learn the right way but I need to hear the logic behind it. I won't just accept something because it's the common way of doing things.

 

[xjr100]

Are the 4 ohm drivers for active set-ups only or what?

It depends on the driver.

for example, this driver used in theese speakers:
Audes 155, 156: AUDES (Z is 3.4Ohm @85Hz)

Excellence 5 (1st version with passive radiator):
AUDES

 

[markusA]

I'm sorry to be a pest but listing commercial offerings and saying it depends doesn't really say anything. Just because you can buy it doesn't mean it's any good.

I'm sure a Z=2 ohm can sound wonderful with the right amp but I'm not sure a tube amp will be such a good match. That's why I'm enquiring about it.

I'm looking for some information about the pro's and con's of 4 ohm vs 8 ohm drivers.

 

[head_unit]

As a speaker engineer, my opinion is that at moderate volumes any amp should be fine. But cranked to clipping, if the speaker is dipping to 2 ohms, that is indeed hard for any typical amp. Tube amps in particular, with their high output impedance, won't be optimum.

And no, you are correct to not want resistors in series with a woofer. Even series connecting woofers is not good since their impedance peaks don't coincide so the power split can be weird.

Finally, don't spend a lot of time building a project that makes you uncomfortable! Find an alternative.

 

[markusA]

My current amps are pretty stout and can deliver close to 1kW into 2 ohms so it really is a non issue but I like to know what I'm getting myself into before spending the money.

Considering the sensitivity I'll be having ear-splitting levels with little effort, a 10W amp will probably be enough. Lol.
I imagine the sensitivity will be something like 100dB/2.83V*1m?