just plain confused about ohms

[Jason M Hancock]

I'm trying to educate myself about near-field monitors and amps and I'd like to purchase something within the next month. I'm trying to mix and match monitors and amps to come up with a good price on new gear.

I understand that the ohm rating of the speakers must match or exceed the ohm rating of the amp. Problem is, the amps I'm looking at have two ratings (4 ohms and 8 ohms) and most of the monitors I'm considering are rated at 6 ohms. To make it even more confusing, some of the amps say they'll have one wattage at 4 ohms and a different wattage at 8 ohms. ?! I'm looking at 6!

Do these amps have some switch to choose 4 or 8 ohms? Or do they just automatically adjust to any ohm rating within the 4 to 8 range?

And what the hay is "nominal" when it refers to a monitor's ohm rating?

Please help. I've listed below the units I'm considering. Will I have to call the fire department the minute I plug these in or will any combo thereof work?

Monitors
Tannoy Proto J: 6 ohms (nom. impedance) 50w (100w program)
Hafler M5: 6 ohms nominal 200w
Yorkville YSM-1: 6 ohms nominal 70w program
Alesis Mark One Mk2: 4 ohms 120w program, 200w peak

Amps
Hafler TA1100: 50w per channel into 4 ohms
40 w per channel into 8 ohms

Alesis RA100: 100w per channel into 4 ohms
75 w per channel into 8 ohms

Alesis RA150: 75w per channel into 4 ohms
45w per channel into 8 ohms


Thanks!!!!

 

[skippy]

This does come up from time to time. Power amp ratings are lots of fun: basically, they are telling you how much oof the amp can drive into a load. Generally speaking, you want to keep your speaker impedance higher than the minimum rating point for a transistor amp, as you know.

When multiple ratings are listed, you can usually get an idea of what an amp can safely deliver into a load within that range by just interpolating the data points (call it averaging). According to your figures, then, the Hafler should do 45W into 6 ohms, the RA100 should do 87.5W, and the RA150 should do 60W. The amps essentially "automatically adjust": fact is, they just don't care, as long as the load is within a given range. So far, no fire alarms!

The Alesis Mark Ones are rated at a nominal 4 ohms. This is right at the bottom of the load range for all the amps you mention: those speakers will provide a fairly hefty load. But there are still no fire alarms in sight. Any combo will work, although I personally would prefer somewhat higher powered amps for monitoring (you definitely want the speakers, and your ears, to run out of steam well before the amp itself does).

Speakers are rated with a "nominal" impedance, because their impedance varies all over the place with respect to the frequency of the signal: for a 4-ohm nominal, the impedance may be as low as 2 or 3 ohms at the woofer/cabinet resonant frequency, say 40Hz, and as high as 100 ohms at some point in the tweeter's range, maybe 8kHz. If you look at a plot of the impedance-versus-frequency curve, it will have peaks and valleys all over it. However, end-users need a rough figure of merit so that they can figure out if their speakers will melt their amps. So smanufacturers essentially pull a number out of the air that describes a nominal, average, or close-enough approximation.

This is one of the reasons that certain speaker designs are referred to as "difficult loads". Either their impedance curves vary more widely than the average bear, or their electromechanical design stores up a lot of energy in certain bands and causes havoc with the amplifier's feedback loop, resulting in time-smeared signals and unexpectedly high distortion levels.

Too much information. Anyway, when you see "nominal" in this application, just think "Close enough...".

 

[sonusman]

As long as your amp is rated for an ohm loud that is the same as your speakers, or lower, you need not worry.

Less resistance (lower ohmage) means the amp can deliver more power to the speaker. The amp HAS to have SOME resistance. Don't ask why, it doesn't matter. So, the amp will deliver more watts with a lower resistance. Thus, the dual ratings.

Usually on newer amps, you don't have to select on the amp between 4 and 8 ohm loads. Older amps sometimes had different jacks or a switch to select ohm load.

So, if you are considering a 6 ohm speaker, you will need a 4 ohm capable amp. Most will handle 4 ohms well enough. Your power output at 6 ohms will be somewhere between the 4 ohm and 8 ohm ratings. A good rule of thumb is considering an amp is to get an amp that will supply 150% of the power rating your speakers can handle. Trust me on this one, I have done massive live sound systems for years and this is the rule that has saved many of drivers on these systems. So, if your speakers are rated for 100 watts at say 4 ohms, you will want an amp that supplies 150 watts at 4 ohms.....

Nominal ohm ratings are nothing to worry about. That is just a way for you to match stuff up. A speakers ohm rating will go lower and higher depending upon the intensity and frequency of the stuff being put into it.

Good luck.

Ed

 

[darrin_h2000]

The lower the ac resistance the more heat the amp does put off,so getting the 8 ohm speakers will help preserve that amp but if you want to monitor at higher volumes 4 ohms is good for that (most amps are designed to do this ) .

Some of the tannoy speakers are 6ohm and are a happy medium between the two but require more math to figure out the power requirements.

If this is confusing they have active monitors that take the guesswork out of it.