Can I run my 4ohm speakers through an 8 ohm stereo receiver safely?

So i am assembling a 2nd stereo system for my other home away from home in tucson and I bought a pair of Kef Reference 104/2's, pretty decent speakers. they run up to 200W per channel at 4 ohnms. Ok i got that

120 w
the receiver I am thinking about getting is a used Harman Kardon 3490 which says it runs 120 watts per channel driven into 8 ohms.

my dumb question is will this still work together or do I need to get only a 4ohm receiver/integrated amp?
I am electronically challenged, to say the least!

 

It looks like the 3490 can handle two pairs of 8 ohm speakers, which from the perspective of resistance is 4 ohms (nominal - I know). So it should be able to handle one set of 4 ohm speakers. Just don't try to use a second pair (4 or 8 ohms) at the same time.

 

would it be generally true that a receiver that says it operates at 8 ohms will accept my speakers without incident?

 

The specs show it is rated to drive 4 ohm loads, although the 30 wpc increase indicates a weak power supply, so I wouldn't push it driving those speakers. Ideally, the 4 ohm wpc rating will be double the 8 ohm wpc rating.

 

No.

 

I wouldn't do it, personally.

 

interesting. i didnt think any of this mattered but thought i should ask. there arent a lot of used 4Ohm integrated amps in my area (tucson) and they all start at 8 ohms....all similar to the one I described above. I wonder what I should do then?

 

Your HK is a receiver, not an integrated amp. All receivers, AVR's, integrated amps and power amps are rated at their 8 ohm spec. Those that can handle lower ohm loads, such as 4 ohms will indicate so in their specs.

 

And there would not be many that could.

 

so are you guys saying that a company like Kef made 4 ohm speakers to be nearly impossible to match on purpose? i thought this was gonna be a simple case of finding used gear and throwing it together....now I have nothing I can pair it with?

Ive frankly never heard of that type of scarcity in units that would work together....nor do I have any experience, obviously from my stupid questions.

so those 200-500$ receivers will never work for my speakers then?-

 

Most mass market receivers under a certain price range aren't built to handle 4 Ohm speakers. Most use separates to drive them.

 

Shine, I didn't know you were so close! Hah! Well, what I would do is look up the specs on the internet and see what the manufacturer for the amp says.

 

Many speaker companies make loudspeakers with a nominal impedance of 4 Ohms. Kef are just one of many.

As you know, impedance varies across the frequency range - hence the word "nominal". It will go below 4 Ohms and above 4 Ohms. Furthermore, impedance is a complex function with a resistive component and a reactive component, and yet we are assigning a unit that is typically used to measure resistance. Often in his measurements, John Atkinson (Stereophile) will point out when a speaker presents a particularly punishing load by referring to not just the low Ohms value, but to the high phase angle, which gives you an idea of the reactive component.

There is a lot more going on than you may think, and the amplifier has to deal with the load - and many cannot.

 

Look at it this way, you bought better than consumer grade speakers, so you will have to drive them with better than consumer grade power. There are plenty of integrated amps and separates that will drive them with no problem.

 

I always thought the general rule of thumb for an amplifier being able to safely drive a 4 ohm load was that it should deliver 2x the 8 ohm load power rating in other words in this case 240 watts. It's more of an issue of the amp not being sufficient

 

In my experience you have to get pretty far into the high end to see actual doubling. My $1,500 Cambridge Audio 840 integrated can't quite do it. *

I have an old Sony multi-channel STR DB-840 receiver (which is probably better made than most of their stuff today). It is rated at 90 WPC for both 4 and 8 ohms (IIRC), although you have to flip a switch on the back for 4 ohms. For many years I've used it with 4 ohm speakers without a problem. Admittedly I don't blast it

When I went to look, I couldn't find the HK specs for 4 ohm. However the manual specifically states that you can run two pairs of speakers as long as they are both 8 ohm (which means the impedance seen by the receiver is 4 ohms).

*Digital amps are another matter.

Edit - you may find this thread worth reading

http://www.audiokarma.org/forums/archive/index.php/t-358442.html

 

Your receiver is rated for 4 ohms. You should be fine, Stick to the KEF speakers only. Don't use a second pair of speakers at the same time off of your Harman-Kardon when using your KEF speakers.

 

For what it's worth:

The harman/kardon HK 3490 power reserves were quite impressive for a receiver of its price and size. It came very close to running both channels at continuous full rated power (120watts) and held up well for the 1kHz 8-ohm and 4-ohm power vs. distortion tests. In fact, the amp section in this receiver was so stable that I ran all of the power tests with both channels driven and the HK 3490 never complained or broke a sweat during my torture tests. Into 4-ohm loads, the HK 3490 exceeded its 150wpc power rating by putting out 167wpc x 2 @ 0.1% THD + N and a whopping 227wpc of dynamic power with both channels driven.

Source: http://www.audioholics.com/av-receiver-reviews/hk-3490/hk-3490-measurements

I don't think you'd ruin your speakers, if you don't push it too loud, but that being said, you could try to get a better amp for what you bought. The real impedance of your speakers could go much lower than 4 ohms at certain frequencies. My speakers are rated 8 ohm but they dip as low as 4 ohm.

 

So long as manufacturer's guidelines with respect to impedance are followed, there should not be cause for concern under normal operating conditions. I would suggest monitoring the amplifier when run for long durations at high output, however.

In any case, most modern equipment has built in thermal protection should heat become excessive.

 

Switching to the 4 ohm setting limits current to save it from cooking.

 

I run an As-500 with 1 pair of 4 ohm speakers with impedance selector on high for months without issue. I rarely go over about 10-11 o'clock on the dial, however for hours and it barely even gets warm at all. In fact I can rarely feel even any warmth whatsoever anywhere on top of the amp so I figure it is ok. Depending on who you believe that HK supposedly is even better with one pair of 4 ohm speakers.

Here's the specs on the AS-500

Minimum RMS Output Power (8 ohms, 20 Hz - 20 kHz) 85 W + 85 W (0.019% THD)
Maximum Power (8 ohms, 1 kHz, 10% THD) 105 W + 105 W
High Dynamic Power/Channel (8/6/4/2 ohms) 130 / 150 / 185 / 220 W

 

I think the receiver would be OK driving them at normal volume. If you play AC/DC at jet engine levels, it'll probably blow up.

The other concern (more of a concern IMO) is the condition of the KEF woofers, mainly IIUC the surrounds, that are internal to the cabinets. The speakers are >20 years old and have 'known' issues with this (as many older speakers do as well) - Google it to get more information.