I have a vp-150 which is a six ohm speaker, but the manual that came with it reads to set my reciever to 8 ohm.Is this right ?
Thanks, steve

Yes, that is correct. The switches don't seem to "match your receiver to the load", they just reduce the power that can go to the speakers so it's harder to play music with the volume up all day and fry the receiver. I forget the details but apparently there was some kind of liability thing and so lots of audio mfgs added those switches.

This is from Audioholics,

Impedance Selector Switches

"This so called feature, used by some manufacturers, is designed to prevent overheating of the receiver or damage to its output transistors because of excessive current flow. The manufacturer accomplishes this in one of 2 ways: 1) Stepping down rail voltage supplied to the power amp or 2) feeding half the signal strength to a voltage divider of power resistors. Both of these methods severely limit dynamics and current capability of the power amp. This results in an audible decrease in bass capability and dynamics transient sound because the 4 ohm setting effectively increases the receiver's output impedance. Unfortunately many manufacturers put these features on their products to ease customer concerns with driving low impedance loads and for safety reasons when getting UL approvals. Note: In order to meet UL requirements, a receiver cannot be rated down to 4 ohms without having this switch onboard. Receivers without this switch are usually rated down to 6 ohms. In most cases, well designed receivers can easily handle 4 ohm loads safely and efficiently. It is highly recommend to keep the impedance switch set to 8 ohms regardless of your speakers impedance and make sure your receiver has plenty of ventilation."

>>Note: In order to meet UL requirements, a receiver cannot be rated down to 4 ohms without having this switch onboard.

Ahh, THAT'S why HK and Denon receivers aren't rated for 4 ohms even though they can drive M80s happily...

Wonder how it works for big honkin' power amps. I don't remember seeing switches on those...

The Rotel 1080 has no such switch,I don't see a UL listing on it either.Maybe they just don't do the Underwriters Laboratories deal.

So would this also apply when i get the 80's and a seperate amp should i run it at 8 ohms.If i do run it at 8 ohms am i losing 200w since the amp is rated at 200w @ 8ohms and 400w @4 ohms.Also if i get the outlaw 770 7.1 can i run it at 4 ohms if the vp-150 and the qs8 are 6 ohm or do i run it at 8ohm?Or should i get a seprate amp for the 80's and run everything else off my reciever?

Sorry about all the questions but i,m new to seperates.
thanks ,steve

What receiver do you have now ? You could get an 2 channel amp for the M80s and run the rest of your system with your receiver,providing it has preouts to connect an external amp.

It is not not the amp that dictates the ohm rating it is the speakers.If you get a speaker that is rated @ 8 ohm like the M60s then the 200 watt amp will produce the 200 @ 8.The same amp running the M80s @ 4 ohm will then produce the 400 watts at 4.I haven't seen a amp that has a impedence selector switch on it.They normally have a good enough power supply as to not have to worry about a lower ohm speaker.

There is a pretty good thread here discussing the same thing.

thanks for the reply,
I have the yamaha 1400 it has pre-outs.One other question though if I have my fronts at 400w and the rest at 110 wont my fronts over power the rest of the system?

Steve

No,once you calibrate all the speakers they will play at the same db level.

At the risk of sounding dumb.If all the speakers will play at the same level where's the benefit of having a 400w amp would i then need a 400w amp for all speakers to hear the benefits?

Your patience is appreciated like i said i,ve never used seperates.in the past i just bought a reciever and speakers, hooke them up and bam sound.

Steve

You don't need to get a 400 watt amp to drive the M80s.You very well could get one with a lower rating.I would say the benifit of a seperate amp would be so not to be concerned about the amp running out of power on demanding loads.

That's a bit misleading, actually. Calibrating speakers doesn't mean they'll all play at the same level. After all, your surround speakers are rarely playing at the same volume as your mains. All calibrating does is ensure that if each of the speakers are fed a signal of the same amplitude, they will each produce sound of the same level.

In other words, calibrated speaker will sound as loud or soft as they're supposed sound with the material they are asked to reproduce. These levels are all relative to whatever your master volume happens to be set to, of course.

You certainly put into better words than I did.You are correct.

>>If all the speakers will play at the same level where's the benefit of having a 400w amp would i then need a 400w amp for all speakers to hear the benefits?

All good questions. There are a few reasons why having a bigger amp for the mains is a Good Thing :

1. When the DVD soundtracks are being mixed, the mains get most of the signal, followed by center, then surrounds. The center gets most of the dialog, however. What this means is that in a properly calibrated system you will be putting more sound out of the mains and will need relatively more power for those channels.

2. If you go with M80 mains you will have two channels which can't be driven by all receivers & amps. By putting those on a dedicated power amp designed to run all day at 4 ohms you ensure no problems.

3. Most receivers don't have enough power supply capacity to put out full rated power into all channels simultaneously. If you run two channels through a separate power amp that frees up more power supply capacity for the remaining channels.

Steve, regardless of what sort of receiver or separate amp you might use, setting it at a certain impedance(if it has such a setting)doesn't mean that you're "running" it at that impedance. As previous replies have said, the only effect of the lower impedance setting is to reduce the maximum voltage and therefore the maximum power available. You're not "losing" power by leaving the setting at 8 ohms; you're keeping the possibility of outputting maximum power, as long as the receiver/amp can handle it without shutting down. As to whether you actually need the maximum power which the receiver is capable of, that depends on factors such as your distance from the speakers, the size of the listening room, the dynamic range of the recording and your average listening level. It's entirely possible that you're not using all the power that you have now and that a slightly more powerful separate amp would be of no real benefit.

That's right. Best not to think of it as the "4 ohm" switch, but the "stupid government says we have to put this stupid switch on if you're going to run at 4 ohm" switch

So let me see if i have this right. Ohms are determined by the speaker not the reciever.The lower the ohm rating the more voltage required to drive it which equals more heat.So if i have a 4ohm speaker and my reciever is set to 8 ohm the speaker will try to draw the needed voltage which will produce more heat and possible damage my reciever.If i switch my reciever to 6 ohm setting telling it i have more difficult speakers to drive it will reduce the available voltage allowed to the speaker so it wont overheat.So its a saftey feature not a performance feature.kind of like a computer processor that will run at a lower mhz if it gets to hot so it does'nt fry.

I think i get it now i wonder how many people switch it thinking they'll get more power though.

So if i have a 20x25x8 room do you think one seperate amp 400wpc @4ohms for the mains with my yamaha 1400 110wpc@8 ohms will be loud enough? or would i be better served with more powerful amp for all channels?

Steve

Very close. You have the concept right but not all the words.

The lower the ohm rating the more current the speaker will draw for a given volume control setting... but it will play louder as a result. For a given loudness, a bit more power will be wasted in the amplifier with a 4 ohm speaker so it will run a bit hotter.

It is a safety feature not a performance feature. Exactly. Another way to get the same safety is not to play so freakin' loud that the amp is running wide open all the time

You definitely don't need a more powerful amp for all channels with that room size and speakers. A 400 wpc amp for the mains will take you into the "hot damn !!" range. The reality is that you would probably get away without ANY power amp as long as weren't playing loud music all the time -- the receiver will shut down if it starts to overheat -- but a good power amp on the mains will give you an extraordinarily nice system.

Have to go. Return of the Giant Hogweeds just came up on the CD player

Steve, as John said, the possible problem would be that the 4 ohm speaker would draw more current from the amplifier, the amp might start to overheat, and then its protective circuit would shut it off. Getting a little fancy, Ohm' Law states that current equals voltage divided by resistance(I=E/R), so if the voltage stayed the same but the resistance was 4 ohms instead of 8 ohms, theoretically twice as much current would flow, possibly creating an overheating problem. So, the lower impedance switch or setting reduces the maximum voltage(E)so that the lower R doesn't result in as much increased current(I). My suggestion would be to not make any assumptions about the adequacy of your 1400, which is a quite powerful receiver. Keep in mind that speakers of the sort that you'll be using draw about 1 watt at a comfortably loud average listening level. Split-second peaks might draw up to 100 watts or more, depending on several factors. See how things work out before you assume that you need more power.