4 ohm stability - a big deal?

[Paul Ebert]

It seems that most of the inexpensive gear only lists power output into 8 ohms. I'm planning to build my speakers, so do I really need to keep the impedances high?

I've noticed that Harman Kardon calls their designs "high current". Will they drive a 4 ohm load?

[Glen B]

The inexpensive gear is often rated only for 8 ohms because of cost. The ability to drive lower impedances requires larger power supplies and more robust output stages, which translates into greater cost. Regarding Harman-Kardon, I don't know about the "high current" part, IMO its an overused term. Looking at the specs of the current crop of HK electronics, the difference between their 8 ohm and 4 ohm ratings is only about 0.3dB, hardly what one would call "high current." Nevertheless, HK components have always tended to be true to their specs and will safely drive 4 ohm loads with no problem. As you build your speakers, keep in mind what electronics you intend to drive them with now and in the longterm.

[Lex]

what you also have to keep in mind is it's still an average impedence. So, it takes pretty good gear to be rated at 4 ohms, because sometimes, it even dips below that. It can take it for short periods of time, but an extended spike below 4 could burn up output stages in cheap gear that doesn't have the capacity to handle the load.

Running at 4 ohms will create more heat, and probably lessen output device's lives for all but quality high end gear. Harman is a true audio company with roots that go deep to the audiophile side, so their their receivers are better than some companies. In the end, there's really no real gains for most speakers to be designed at 4 ohm loads though, it's a design consideration. So, if you want to insure the greatest compatibility it's simple, choose a 8 ohm design.

[gp4Jesus]

I agree with Lex & glenn. HK makes robust equipment. However "High current" does not promise "highly stable" at lower impedences. 2 ohms is nearly a short circuit! Perreaux made an excellent sounding amp stable to 1 ohm w/an incredible current rating!!

I have 80 W/ch amps*, 30 amp peak, rated to 4 ohms; I used one* to drive** a pair "4 ohm" subs. Their 3.2 ohm low point in their impedence plot caused that amp, even at low volume, to run quite warm, "great heater" @ moderate levels. Same amp into a very stable 8 ohms load at deafening levels, ran slightly warm to the touch.
* have 2 now; they have 6 transistors per ch ** 4th order LP @ 60 or 100hz depending on system at the time

IMHO ask yourself the following:
Does your HK have a 4* ohm rating? a 2 ohm rating
will your intended speaker design's impedence** plot stay above[I] 4 ohms below[B] say, 250 hz?
Dip below 4 ohms between 250 hz & 5K hz for a narrow freq range?
** inductive/capacitive
* I've seen pro gear "drive 2 ohms all night long under tough conditions" have modest current ratings

If the answer is yes to most of the above, your HK is likely safe; consider forced/exhaust venting for your shelving.

good luck and happy building

[Glen B]

IMHO ask yourself the following:
Does your HK have a 4* ohm rating? a 2 ohm rating
will your intended speaker design's impedence** plot stay above[i] 4 ohms below[B] say, 250 hz?
Dip below 4 ohms between 250 hz & 5K hz for a narrow freq range?
** inductive/capacitive
* I've seen pro gear "drive 2 ohms all night long under tough conditions" have modest current ratings

If the answer is yes to most of the above, your HK is likely safe; consider forced/exhaust venting for your shelving.

good luck and happy building

Great point. Also throw severe phase angle into the mix. Unfortunately, not everyone will have an impedance plot available unless the product has been reviewed by one of the A/V magazines, and they've taken detailed lab measurements and posted the results.

[gp4Jesus]

Glen:
Thanks for phase angle comment. While some may not understand it, its worth saying because it is a factor.

Paul:
Its unlikely you'll have problems armed w/the info in this thread.
One other test you could try, (listening optional!) play to some bass heavy material though full range speakers* rated @ 8 ohms . Set the volume just above what your anticipated movie level would be. Let it go for 15-30 minutes. Touch your HK firmly for several seconds; make a note where you set the knob of how warm the HK. If its barely warm to the touch thats good.
* let the speakers and HK amp section handle it all; no subs here

Then connect a 4 ohm 5 watt resistor to each channel; turn down volume to about half, and repeat the test, checking* the HK's temp every few minutes*. @ the end of 30 minutes if your HK's temps is the same or only slightly warmer, repeat the "4 ohm" test at the "8 ohm" volume level. If its slighly warmer still, you're likely safe although a resistor is easy to "drive." Zero back EMF and the other things mentioned in other replies.
* if you find it get warm or hot fast stop the test!

If you're comfortable w/the "resistor" test results, repeat the test w/your home growns. start @ 1/3, then 2/3s. If it doesn't get "hot," You're closer to being sure of enjoying the satifaction of YOUR home theatre w/YOUR speakers!

It may seem like a lot of work but it beats toasting your HK.
If it make you feel any better, several years ago I was "hot" on the same idea for all channels* 4 ohms and all. I had a killer twin sub setup already; separate amps for each**. Unfortunately with now 5 kids no time to build much less test. So I went with some Polk RTi A series speaker all around with which I'm pleased.
*I have the same amps; I had planned to then as I'm doing now: the LCR channels bi-amped with Hafler XL280s with another XL280 full range to the rears! I don't sweat low impedences, they're stable to 2 ohms.
** see previous post

[chrispy35]

I think you'd burn up the resistor before you got very far. 5 W is not much and the amp should have no issue with that (1.12 amps).

Chris P.

[Ntruder]

Is this really a big issue to worry about if your listening levels are low to moderate?

I rarely get my volume levels above -40db on my HK AVR 2600. At lower listening levels, should I even worry about having 4 ohm speakers in front and center?

[ClosetSciFiGeek]

Just be done with it and buy an Emotiva UPA-2 for $299 and run the fronts you build from it(this opens you up to all the nice 4ohm DIY stuff). Your receiver will thank you and its weak power supply will be able to dole out more current to the other three(5.1) or five(7.1) channels.

http://emotiva.com/upa2.shtm

[Ntruder]

That's not a bad idea at all...

[gp4Jesus]

Paul:
If it make you feel any better, several years ago I was "hot" on the same idea for all channels 4 ohms and all. I had a killer twin sub setup already; separate amps for each.

Several years ago I stumbled across this site along w/ several speaker DIY sites.

http://sound.westhost.com

[whoaru99]

It's not that big of a deal...Don't stuff your amp/receiver into a closed cabinet and allow it plenty of breathing room. It'll be fine...

[gp4Jesus]

It's not that big of a deal...Don't stuff your amp/receiver into a closed cabinet and allow it plenty of breathing room. It'll be fine...

If you think it may run too hot, try elevating 2-4 inches or close off that shelf/compartment and force ventilate*. I recall my Belles Series 1** sitting in the open, on carpet, driving my 3 ohm subs got warm to the touch @ low volumes, rather hot*** at near-club*** levels***. On a milk crate, heat was far less of an issue.
* I'll send you a link to a $20 solution of which I plan to use 4 in my new rack
** no 2 ohm rating
** into a very stable 8 ohms, barely warm

Either way keeping things cool ensures a long, reliable life. hope this helps. cheers, tony

[gp4Jesus]

Ok so this may $ up to $30-way less than replacing your AVR

http://www.coolerguys.com/ac120vfans.html