Give me your opinion on this impedance

[Jeff B.]

I have noticed in several speaker reviews recently the impedance of some commerical speakers dropping below 3 ohms for a limited region. And, it appears that these speakers weren't really causing any problems, or at least not enough that the manufacturer was concerned about it.

So, I decided to experiment a little, Below is the impedance of a vented speaker that uses three identical woofers wired in parallel. These are crossing over to a single midrange at about 350Hz using first order slopes in the crossover region. The plot is for woofers and midrange only with their crossovers (the tweeter is not connected here).

With crossover elements in place, the impedance runs about 2.9 ohms for a DC resistance and has a minimum impedance of 2.41 Ohms at 180 Hz. However, as is the case with paralleled drivers, the phase angle is very mild in this range and typically runs in single digits whenever the impedance is below 3 ohms.

So, my question is - Do you think this is still a fairly benign load since the lower impedance is limited in bandwidth (the region with the mid and tweeter will probably run above 8 ohms), or do you fell that 2.4 ohms minimum is pushing it, even with the low phase angle?

Give me your thoughts.

[[email protected]]

Re: Give me your opinion on this impedance

granted, my experience is limited, but i would most definetly raise the overall impedence.

[Cinemadesigner X]

Re: Give me your opinion on this impedance

This impedance is much like the old PSB Stratus Gold which required significant current to control. So people have to buy a serious amplifier to drive this speaker and not a reciever.

IS the quality of the speaker worth a $1000 amplifier?

[Paul W]

Re: Give me your opinion on this impedance

I've also seen a few speaker reviews with surprisingly low impedance. IIRC, those speakers were in a price category not likely to be teamed with wimpy amps. So, if the target audience is the Krell crowd, should be okay. Sony receivers, maybe not so good.

[r-carpenter]

Re: Give me your opinion on this impedance

what's the efficiency of the speaker?

[Zero the Hero]

Re: Give me your opinion on this impedance

I would try it. Doesn't look like too hard of a load with that phase, as you said.

[ctbowman]

Re: Give me your opinion on this impedance

I think that the region where you have the dip is right where you see a lot of music energy and that it could cause problems if you didn't have a robust amp. Unless your listening at pretty moderate levels. Going from memory, but, I think that the general rule of thumb is that half of the music energy a speaker see's is between 20hz and 350hz. And I know there is a good amount of energy spent in the 80hz to 350hz range. But what is your gut feeling? Any experience in this area?

[fastbike1]

Re: Give me your opinion on this impedance

What's the advantage? Does it, in your opinion, outweigh potential problems?

[OlderMongrel]

Re: Give me your opinion on this impedance

Jeff, you could purposely go with cheap, higher resistance inductors on the woofer section, to bring Z up a bit, but of course only if the slightly higher Q and lower sensitivity would not cause problems. What's the inductor resistance of the crossover you're using?

John

[Jeff B.]

Re: Give me your opinion on this impedance

Answers to some of the questions -

I liked the look of the three-woofer tower and it allows me to extend this frequency range from near the floor to a height that easily crosses to the midrange and reduces floor bounce effects as well. This can, for the most part, eliminate baffle step lossess, which can result in higher sensitivity and cleaner bass, since you don't have to artificially pull up all of the low end to compensate.

The three parallel woofers result in pretty high sensitivity. Even with crossover series losses it is 96dB/2.83V sensitivity.

The series resistance before the woofers was about .8 Ohms. I thought of raising it a little, but I don't want to impact woofer damping much. In its current state it can do low 30Hz before roll-off in different, moderately larger box.

The target group is me. Most of these drivers are extremely hard to get anyway, so no one will be copying this speaker. The amp is not a receiver, but a thirty year old Carver power amp that appears to still work perfectly. I have been driving the mock-up and have not noticed any issues, but I wanted some opinions.

[lhwidget]

Re: Give me your opinion on this impedance

Hi Jeff,
The system looks like one which will stress an amp at moderate to high levels, yet work fine at moderate levels. The moderate phase angle in the low range will definitely help with power dissipation problems in the amp, yet the impedance is low.

I doubt you'll listen to a lot of crazy loud rock, do you have a large collection of pipe organ music?

I wouldn't hesitate to try it, but I'd monitor the amp, both its output voltage to calculate the current its typically producing, and the temperature it's operating at. Depending on what protection circuits it has, you may get a polite heads-up from the amp if it runs hot, or it may stress it hard enough to end its 30 year life if you push the system some on a regular basis...

[ocdSCHACK]

Re: Give me your opinion on this impedance

I always give it a simple test, partly because my knowledge is simple, but it works. Play it loud for a while and keep your eye on the amp. None of my amps get stressed with low impedance until the volume gets turned up. I do worry your 30 year old amp might not have some sort of protection mode for this method.

[jim85iroc]

Re: Give me your opinion on this impedance

If the amp that you're powering them with isn't complaining, I can't see a reason not to use them as-is. Those old Carvers are beasts.

[Paul Carmody]

Re: Give me your opinion on this impedance

You're asking us for our opinion on impedance? I thought it was supposed to be the other way around.

I have noticed in several speaker reviews recently the impedance of some commerical speakers dropping below 3 ohms for a limited region. And, it appears that these speakers weren't really causing any problems, or at least not enough that the manufacturer was concerned about it.

Yeah, I've noticed this a lot. Maybe we worry too much about system impedance For example, here is the Impedance of the Polk RTi A1, which I cite in my article on impedances...


This is not some amp-hungry speaker. It's just a puny bookshelf. $340 a pair.

[charlielaub]

Re: Give me your opinion on this impedance

I would try it. Doesn't look like too hard of a load with that phase, as you said.

It's not the low-ish magnitude of the driver impedance that kills you, it is the combination of that plus phase angle near 45 deg. that is supposedly really hard on amps because of this increased current demand.

Here's an article on this sort of topic:
http://sound.westhost.com/patd.htm

I can't quite tell the exact phase from your graph because of the scale, but you might want to do some quick calculations along the lines of what Rod presents in the linked article.


-Charlie