View Full Version : Another Ohms Post...

Morpheus77

11-20-2006, 08:49 AM

Ok, I'm still a little confused about the whole Ohms thing. I have read back several reviews, and its all jiberish to me, like a foreign lanuage. I know its simply the amout of resistance the speaker has to the current coming in? At least that's what I gathered. My question is this....Is it easier to push a four ohm speaker, or harder? In other words do I need more watts to move a four ohm speaker or less? Thanks again for all the input in advance.

Tommy

GMichael

11-20-2006, 09:31 AM

Ok, I'm still a little confused about the whole Ohms thing. I have read back several reviews, and its all jiberish to me, like a foreign lanuage. I know its simply the amout of resistance the speaker has to the current coming in? At least that's what I gathered. My question is this....Is it easier to push a four ohm speaker, or harder? In other words do I need more watts to move a four ohm speaker or less? Thanks again for all the input in advance.

Tommy

I'm not the best at this but I'll give it a shot until someone else comes along.

It's not the wattage that's in question for lower ohms. As the resistance goes down, the need for more Amps go up. This can burn up a receiver if turned up too loud. Most people here seem to think that you are OK with 4 ohm speakers if you are only driving 2 and not driving them too loud. I would think that amps made for 4 ohm speakers would be, and sound better.

What speakers and amps/receivers are you looking to mate?

kexodusc

11-20-2006, 10:13 AM

Is it easier to push a four ohm speaker, or harder? In other words do I need more watts to move a four ohm speaker or less? Thanks again for all the input in advance.

Tommy

Not really easier or harder to "push" (we use the word "drive" conventionally when talking speakers, not sure it's any better) a 4 ohm speaker than an 8 ohm speaker, but there are some considerations.

I suppose in a literal sense, it's easier to drive a speaker of lower impedance (ohms) than higher. But as GM mentioned, it trades one problem for another - low impedance leads to higher current demand. This is fine until you reach the current limits of the amplifier (which can happen quite fast depending on which speakers/amps you're using). A 4 ohm load will reach that limit twice as fast as an 8 ohm load. That's the danger when driving lower ohm speakers. More wattage is better here usually, because in most solid state amplifiers that means the power supply has a higher current handling ability. So more watts gives you more headroom. But you really need to think in multiples of 2 X or more when adding watts if you're worried about power handling. CV's are pretty high sensitvity speakers usually, and need fewer watts to play as loud as many other speakers, so that helps balance out the low ohms thing for you....

nightflier

11-20-2006, 10:49 AM

Not to add any more confusion, but speakers rated at let's say 4 ohms don't necessarily stay at 4 ohms. They can easily dip down to 2 ohms (Martin Logans for example). An inexpensive receiver will easily clip with a hard to drive 4 ohm speaker.

Dusty Chalk

11-20-2006, 11:22 AM

It's harder to drive a four ohm speaker than an 8 ohm speaker. Same voltage drop leads to higher current demands requires more wattage.

kexodusc

11-20-2006, 11:54 AM

It's harder to drive a four ohm speaker than an 8 ohm speaker. Same voltage drop leads to higher current demands requires more wattage.

Yeah, bad voltage drops would, and do, suck. All depends on what factors you wanna manipulate in your scenario. You increase the DC resistance of a drivers voice coil, one of the largest contributors to the impedance profile and you'll decrease sensitivity.

A lot of 4 ohm speakers (especially Cerwin Vega boomers) are of higher sensitivity than a lot of 8 ohms speakers, which compensates for the current demands. A 96 dB sensitive 4 ohmer would handle voltage drops far better than an 91 dB sensitive 8 ohmer, if maintaining the same SPL was the goal, etc. There's also a pragmatic side to this...if you listen to your music at 90 dB or less in 15 x 15 room, even most 50 watt amps ( at 8 ohms) or HT receivers will be fine driving a 4 ohm speakers to all but obscene levels.

When in doubt, more watts are better.

hermanv

11-20-2006, 12:47 PM

I have not run into a modern amp that cared very much whether it was driving 4 Ohm or 8 Ohm speakers. Tube amps generally have transformer taps for both so those don't care at all.

Watch out if you parallel more than one pair of speakers. This is still true even if your amp has A and B speaker outputs, they are just paralleled inside the amp when you switch to "both" or "A & B". Paralleling 4 Ohm speakers can get you into trouble with smaller amps.

(nightflier; I'm pretty sure my M-L ReQuests manual says 1 Ohm at 20 KHz).

Smokey

11-20-2006, 01:51 PM

Good reading guys, and here is more :)

Most good amplifiers tend to keep their output voltage the same. So usually speakers have

about 20 volts cross their input when connected to amplifier output.

And since speakers ohms is an “average” numbers (speaker ohms vary from 2 to 20 ohms), the voltage is the same. The only thing that change is current. So if we put an 8 ohm speakers cross the amplifier, the current would be around 20v/8 ohm= 2.5 amp.

But if we put a 4 ohm load cross the amp, the current would be 20v/4 ohm= 5 amp. So as one can see, the lower the load, the higher the current would be. And harder on the amp.

hermanv

11-20-2006, 04:41 PM

Smokey;

Although you are right about an amplifier being a constant voltage source, using your example the speakers are also 3 dB louder (5A x 20V = 100W instead of 2.5A x 20V = 50W), so if you turn the volume knob down to get the same volume (wattage) now the current is only up to 3.54 Amps. It's still more amps, but switching from 8 to 4 Ohms, the current doesn't double for the same SPL

Smokey

11-20-2006, 05:44 PM

Although you are right about an amplifier being a constant voltage source, using your example the speakers are also 3 dB louder (5A x 20V = 100W instead of 2.5A x 20V = 50W), so if you turn the volume knob down to get the same volume (wattage) now the current is only up to 3.54 Amps. It's still more amps, but switching from 8 to 4 Ohms, the current doesn't double for the same SPL

Hermanv

Could you explain your post in more detail (may be some calculations). And how did you get 3.54 amps?

Thanks.

hermanv

11-20-2006, 09:03 PM

Watts = amps squared times Ohms.

So 50 Watts divided by 4 Ohms = 12.5

Square root of 12.5 = 3.5355339

3.54 Amps x 3.54 Amps x 4 Ohms = 50.126 Watts

So to get 50 Watts with a 4 Ohm load you need 3.54 amps.

By the way it takes 14.6V instead of 20V, this where turning the volume knob down results in less current.

Morpheus77

11-21-2006, 12:52 AM

Wow! Thanks for all the responses. To answer you question GM, This is what I'm looking to do. I'm wanting to hook up four CV fifteens in the living room several months from now. Right now, I have to wait until my lease is up here at my apartment, before I can find us a house. I got transferred in my job, and didn't have time to find a house, so I had to get an apartment. Anyway, I already own one pair of CV's which are rated at 8 Ohms. The other day, I found a guy on Ebay who was selling an older pair of CV's, and I gave him an offer, and he told me he would end the auction for so much, so I gave him what he wanted, and fortunantly he lived a few miles away. To make a long story short, I did my research on them before I made the offer. According to the CV website, the speakers that I just made the offer on, were "compatible with receivers rated at 8 Ohms or less." I figured, ok, shouldn't be a problem. I get them, only to find out that they are four Ohms. Now my question is, can I run a four speaker set up with two different Ohms ratings on my Onkyo receiver? Now I do know that my receiver will run a four Ohm speaker, but will it be wise to run both. Now, just for the record, I do plan to get a four channel amp for this particular set-up. What are my options? Thanks again to everyone in advance.

Tommy

hermanv

11-21-2006, 08:56 AM

A 4 Ohm speaker paralleled with an 8 Ohm speaker will load the amplifier with 2.666 Ohms which is probably OK, but nearing the limit on heat dissapation. If you are a listener who likes his sound at ear splitting levels, this would be unwise for any amplifier not designed with nice big heat sinks and power reserves (read most receivers).

There is another problem, by paralleling these two speakers the 4 Ohm one will be 3 dB louder than the other. You also need to find the SPL ratings for these two speakers to determine if this will work. Make sure the SPL is listed as dB per 1 Watt. A different popular SPL rating scheme is dB per 2.83VRMS (that equals 1 Watt at 8 Ohms, but 2 Watts at 4 Ohms).

The ideal answer would be that he 8 Ohm model is 3 dB more efficient than the 4 Ohm model. That would result in equal volume for the two speakers hooked in parallel. For example if the 4 Ohm model was 86dB SPL and the 8 Ohm model was 89dB SPL. Conversely if the rating is in SPL per 2.83VRMS then we want the SPL ratings on the two models to be the same.

I hope this clear, if you can find the ratings and rating method, I can help, if you can tolerate the response delay which might be a full day.

These problems may dissapear if you buy a suround receiver with seperate gain adjustments for every channel, you will probably need a SPL meter (Radio Shack has a mediocre, but servicable one for around $55).

http://www.radioshack.com/product/index.jsp?productId=2103667&cp=&origkw=SPL+meter&kw=spl+meter&parentPage=search

Smokey

11-21-2006, 02:35 PM

So to get 50 Watts with a 4 Ohm load you need 3.54 amps.

By the way it takes 14.6V instead of 20V, this where turning the volume knob down results in less current.

Thanks hermanv

I see what you are saying now. Since relationship between watts and current is non linear, it does throw a curve in the way of thinking :)

Now my question is, can I run a four speaker set up with two different Ohms ratings on my Onkyo receiver?

If the Onkyo is only two channel, (as hermanv said) it is not a good idea. Not only be hard on the amp, you will also loss all of bass responce from speakers.

Dusty Chalk

11-21-2006, 04:05 PM

Watts = amps squared times Ohms.

So 50 Watts divided by 4 Ohms = 12.5

Square root of 12.5 = 3.5355339

3.54 Amps x 3.54 Amps x 4 Ohms = 50.126 WattsSo you divided by four, took the square root, squared them, then multiplied by four again?

I think I actually followed that.

hermanv

11-21-2006, 04:33 PM

The last line was meant as a confirmation of the arithmetic, if your answer is right you get the same answer working backwards as where you started. :)

Dusty Chalk

11-21-2006, 08:08 PM

Oh, yeah. I vaguely remember my math professors in school doing that (I, on the other hand, was always too sloppy...).

Coilgap

12-26-2006, 08:22 PM

The impedence means the resistance to a AC signal and music is an AC signal. Also Impedence will change with frequency. I have a pair of Cerwin Vega the woofer is marked 8 ohms so is the tweeter horn and the 2 midranges. Now dc resistance at the speaker terminals reads around 6 ohms. But some time while I am playing some tunes the impedence will be around 4 ohms maybe even lower. Some modern day receivers don't get along well with such speakers even though they do get rather loud with mderated input power. The face plate reads 102db with 1 watt. WOW! I have drive them with an Adcom 555II they get very loud but the woofer I believe can't take what that thing can dish out. I have heard it pop and the the air in the room actually shakes. So what you need is a high current amp section be it a seperated unit or a reciever with a high current amp built in.

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