6 OHM verses 8 OHM ...what are the differences?

[HiDeffjeff]

why are some receivers rated at 6 OHMs and some at 8 OHMS?...Which is better and do the speakers have to match?

[akirby]

The difference is obviously 2 ohms. Why do you ask?

It's the impedance. You have to match the impedance of the speakers to what the receiver will handle. Since:

Volts = Amps X Ohms

and Volts is a constant, then lower OHMs allow higher wattages. You can see this in amp ratings:

200W into 8 ohms
400W into 4 ohms
800W into 2 ohms
1600W into 1 ohm

8 ohms is pretty standard. You normally have to get into the high end stuff to see anything less than 6. At least that's been my limited experience. Cable size increases as the OHMS decrease.
You don't want to drive 4 or 6 ohm speakers with a receiver that's only rated at 8 ohms - you'll eventually overload it. An overloaded receiver will get very hot.

Hope that helps.

[akirby]

And to answer your question, just make sure the receiver's ohm rating is less than or equal to the speaker's ohm rating. Lower is better as long as you're not overloading the receiver or the speaker.

[Richard]

Overall the gist of Allens response is correct.

While the formula is correct, for this application we would use the power formula instead:

watts = current X voltage

and Volts is a constant, then lower OHMs allow higher wattages. You can see this in amp ratings:

the correct statement is:

IF volts can remain constant then the amp will double the power output as you divide the impedance in half.

200W into 8 ohms
400W into 4 ohms
800W into 2 ohms
1600W into 1 ohm

This can happen if the amp is built correctly but this is a rare breed for sure! Finding a stereo amp that will double in power from 8 to 4 ohms with both channels driven is quite an accomplishment.

The key to low fi and mid fi receivers is using speakers that have a high efficiency rating as measured in DB and is called sensitivity. Around 90DB or better is very nice for these applications. Below 88DB and you will be getting into current or voltage hungry speakers.

How much power you need is directly related to the size of the room, how loud you want it and speaker sensitivity. In general, from about 60 watts on down you are looking at a small room with 225 square foot or less. For larger rooms I suggest something more in the 120 watt range. I guarantee this simplistic formula is bound to vary! For the best response using a dedicated multichannel amp or multiple stereo amps is recommended over a receiver.

To achieve a meaningful difference between one amp and another you need to double the power.

So what brought on this question Jeff? Are you upgrading a system or putting together a new one?

Thanks

[HiDeffjeff]

Richard, I'm thinking about getting the Pioneer Elite VSX-59txi which is rated at 160 w x 7 at 6 OHMS, unfortunately, my speakers are 8 OHM although they are rated for up to 200 watts. I was just wondering if it would be a bad idea to match up a 6 OHM amp to 8 OHM speakers

[akirby]

This can happen if the amp is built correctly but this is a rare breed for sure! Finding a stereo amp that will double in power from 8 to 4 ohms with both channels driven is quite an accomplishment.

Well, I used this one to verify it:
http://www.krellonline.com/html/m_Class ... 700cx.html

700W into 8 ohms
1400W into 4 ohms
2800W into 2 ohms

Then again, nobody ever accused Krell of being mediocre.

[akirby]

Richard, I'm thinking about getting the Pioneer Elite VSX-59txi which is rated at 160 w x 7 at 6 OHMS, unfortunately, my speakers are 8 OHM although they are rated for up to 200 watts. I was just wondering if it would be a bad idea to match up a 6 OHM amp to 8 OHM speakers

Nope - that's fine. Just don't try it the other way around.

You'll lose some power using 8 ohm speakers so you'll be running around 120 watts per channel, but that's still plenty of power. And as Richard mentioned, the sensitivity of the speakers will also make a big difference.

Richard's formula is definitely more correct - I was just trying to point out the relationship between Ohms and watts and why decreasing one increased the other, so I did the simple version.

[Richard]

You'll lose some power using 8 ohm speakers

Only if they are innefficient which is not common for 8 ohmers but there are a few out there.

6-8 is not a huge deal unless the speaker has a low impedance hump which is never reflected in the simplistic impedance rating provided by manufacturers. That kind of info typically only comes with a full review using test equipment; unfortunately rare these days. Stereophile still does these tests and more.

I was just trying to point out the relationship between Ohms and watts and why decreasing one increased the other

They definitely have a proportional relationship but...

Well, I used this one to verify it:
http://www.krellonline.com/html/m_Class ... 700cx.html

700W into 8 ohms
1400W into 4 ohms
2800W into 2 ohms

LOL..........ROTFL......

K......

I thought we were talking about common everyday stuff here, not the Ferrari of audio...

So like how about telling us the price tag on that beast while your at it...

[akirby]

Umm......looks like about $14K MSRP. Or $4K more than my entire home theater including the leather recliners and authentic theater seats!

I assumed that if the receiver was pushing 160 watts into 6 ohms that it would not be able to put out that much power into 8 ohm speakers. This is based simply on the multiple ratings published by the amp/receiver mfrs.

e.g. my Denon 2803 is rated at 125 Watts/channel into 6 ohms but only 90 into 8 ohms. So I was assuming that his would be putting out closer to 120 watts if connected to 8 ohm speakers.

I guess the question really is what's the difference between 125 watts into 6 ohms versus 90 watts into 8 ohms? Is it really more power to the speakers or is it the same power delivered differently?

[Richard]

or is it the same power delivered differently?

Exactly.

In your case lets say your Denon was like that Krell. At 90W into 8 it really should do 180 into 4 and it won