Impedence question from teh n00b

[evilkat]

So I'm thinking of buying my first ever home theater setup (currently using a craptastic 2.1 computer speaker setup!), but I have a question about buying the correct Speaker/receiver.

For the fronts I want to use the Axiom M80s @ 4ohms. My center will be VP100 @ 8ohms and the M2 @ 8ohms for the rear.

I've been looking at the Yamaha rx-v659's manual and it only goes down to a min of 6ohms (although the manual says to use this setting for 4ohm speakers as well).

Not knowing too much about this, and having heard that you can easily destory speakers if you don't match them up correctly, I'm very worried about how this could affect my speakers.

Can on of you audio-gods pls explain to me what the risks are and if I should go for a setup like this?

Additionally, it would be incredibly useful to me if you had any particular recommendations for receivers that would work well with the Axioms.

Thanks!

 

[Seth=L]

The M80's will demand more of your amplifier, they take more wattage to acheive the same volume. Many mid-level receivers can't produce the same enough wattage to handle a 4 ohm load. Here is the good news, the Yamaha you are looking at has preouts. This means you could buy a seperate amp, just a two channel, to power your front speakers. This reduce the demands on the receiver's amplifier, and most certainly boost your stereo listening performance.

I would start another thread about a seperate 2 channel amp, and explain the situation. Many will likely suggest one of the Behringer amplifiers which would serve well as a low impedance driving amplifier, and they are cheap. I cannot personally say how good they are with home speakers as I have only heard them with PA speakers, but nonetheless many members use them and claim they are just as good as other amplifiers costing many times more. If it were me, I would find an Adcom or Carver on Ebay for less that $200 and just use that for the amplifier.

 

[evilkat]

Seth thanks for the info. But unfortunately, I have more questions now

1.
I can't seem to wrap my head around the fact that a LOWER impedence means you need MORE power. Since impedence is measured in Ohms, I'm assuming this has to do with resistance or some sort of internal resistance of the speaker. Since from high school I remember P=I^2R how is it that a LOWER resistance requires MORE power to get the same I through the speaker???

2.
According to Monsieur DellaSalla himself, he recommends using the 8ohm setting on the receiver when driving 4ohm speakers! As godly as he is in his knowledge of all things electronic, I've heard that it's a Bad Thing(tm) to have a mis-match between Amps and Speakers.

Can someone reconcile all this for me?

 

[MDS]

1. I can't seem to wrap my head around the fact that a LOWER impedence means you need MORE power. Since impedence is measured in Ohms, I'm assuming this has to do with resistance or some sort of internal resistance of the speaker. Since from high school I remember P=I^2R how is it that a LOWER resistance requires MORE power to get the same I through the speaker???

A lower impedance means there is less resistance and that means the receiver/amp will have to supply more CURRENT. V=IR. When the R goes down, the I must go up to keep the V the same. Now speakers aren't exactly a pure Resistance (impedance varies with frequency) but close enough to understand the concept. The POWER will go up too because of the equation you gave above.

When the receiver is designed for 8 Ohms, running 4 Ohm speakers makes it work much harder and it may not be able to supply the current necessary. Most well built receivers can handle 4 Ohm speakers at low volumes but crank it up and it may get too hot and activate its internal protection circuit and shut off.

2. According to Monsieur DellaSalla himself, he recommends using the 8ohm setting on the receiver when driving 4ohm speakers! As godly as he is in his knowledge of all things electronic, I've heard that it's a Bad Thing(tm) to have a mis-match between Amps and Speakers.

He recommends that because all that switch does is lower the rail voltage (see my explanation of that in the 'These go to eleven' thread). That gets the unit UL certification but robs you of potential power. If you examine the advice closely, he says that you should use the 8 Ohm setting but watch the volume and monitor the receiver to see if it is getting too hot.

Now the solution to your problem is to get the M60 instead of the M80. No more worries about overtaxing the receiver.

 

[evilkat]

MDS, thanks for that enlightening post! I'll have to go read the other post you mentioned. In the meantime, I have to admit, I'm surprised that no-one here has told me to go get a better receiver, instead of toning down the speakers

I'm also wondering about Seth-L's earlier post. He mentioned that I might consider getting a two-channel Amp...Now this is probably going to be a stupid question, but I have to ask, do I not need amping for all my channels? Or do I need a 2 channel amp only because it's the M80s that may need the extra juice?

Any recommendations for a mid-level receiver that goes down to 4Ohms? I'm triyng to keep the receiver <=$500

 

[MichaelJHuman]

The formula to know is:

E = IR

Volts = Amps * Resistance

Change the resistance, assume your rail voltage (your amp's equivalent of blood pressure) holds constant and the current drain must go up according to physicists and electricians.

Imagine shorting a battery. It gets hot. Because with no resistance and a constant 1.5 volts, you are drawing its maximum current. It can only produce so much current (say 200 miliamps or so.) This is what happens to an overloaded amp. You are asking it to draw more current than it's able to and it will clip.

 

[markw]

In the meantime, I have to admit, I'm surprised that no-one here has told me to go get a better receiver, instead of toning down the speaker.

That's because finding a receiver that can handle four ohm speakers at volume are hard to find and tend to be more expensive. A more logical solution is getting a pair of comparable speakers that can be driven by virtually any receiver.

It's not like you already own the M80's already. You're still in the planning stage and it's up to you whether you select to heed the sage advice being offered. Of course, there are always some that would rather try to lower the river than raise bridge.

 

[evilkat]

Thanks Mike...I had thought it was more of a power thing than voltage thing, but now that I look at your explanation and the one put forth by MDS, it makes a lot more sense!

Mark, I was just surprised that everyone actually made a sensible suggestion I mean this is an Audiophile forum and the last thing I expected was a cost-saving sensible suggestion It always felt like (at least from the reviews on the main page) the motto around here was "Thou shalt not sacrifice audio quality to save a few dollars!"

So that begs the following question; is there is an appreciable difference between the Axiom M60s and the M80s? I hardly consider myself an audiophile, but if I'm gonna buy some speakers I wanna get the right ones. I do live in a 1 bed apartment so I can't have the volume cranked up very high, so I'm not too worried about total volume.

So the question is, will lthe M80's extra tweeter and Midrange woofer give me more appreciable audio quality over the M60? Sorry, I just don't know too much about these things (On the other hand, ask me about computers )

I'm trying to decide if I should yet again put off buying my setup until I have enough saved up for a high-end receiver or if I should just get the M60s now.

 

[jcPanny]

Axiom Speakers

Kat,
Consider using the Axiom M60s for the mains. They are 8 ohms and an easy load for any receiver. With the money you saved, upgrade to the VP150 center and maybe the QS8s for surrounds.

If you are set on the M80s for mains, look for a refurbished Yamaha RX-V2500 (about $400). It has a better amp and will do better with the low impdance.

 

[evilkat]

Thanks for all the help guys.

In the end I decided to go with the Yamaha RX-V2700

I figure the receiver will be a long term upgrade that will last me a while so I didn't mind sinking in a bit of money on this. I also decided to go with the M60s until the need for something bigger/better comes along

 

[Nuglets]

If this is your first real setup, you will undoubtetly be happy with it once you get it properly set up. I've never heard those speakers before but judging by everyone else's opinion of Axiom, they seem to be a great choice. Good luck, don't get frustrated learning your new equpment. It will take time to get it just the way you want it, but it's worth it once you have it.

 

[evilkat]

It is indeed my first 'real' system...and it's quite a bump too! I can honestly say that I have not had a TV since the middle of Junior year in college! I've been making do with my lovely Dell 20" flat panel and Altec Lansing 2.1 system, saving up for a killer rig, all the while dying with envy when I see what my friends have!

Based on what everyone has been telling me, I have a feeling that this is going to be something I'm going to REALLY enjoy! I was browsing through the Yamaha PDF manual and I have to say it does look intimidating! Hopefully I won't blow anything out while I mess with it! Maybe I'll even have a system I can finally add to my sig