What are the Ohm ratings? newbie

Newbie question!

I have Spica speakers which are rated at 4 Ohms. My HK AVR300 says it has an 8 Ohm output. The system sounds OK at moderate volumes but I have to crank it up to really make it sound good.

1. What does the Ohm ratings mean?
2. Would a receiver that is rated with a 4 Ohm output be able to drive the Spica's at lower volumes?
3. What are the performance issues with matching / mismatching speaker Ohm to receiver Ohms.


The 4 Ohms of your speaker is the effective resistance of the cross-over ckt (within it) as seen from the speaker binding post. The higher this eff. #, the easier it is for the amplifier to drive the speaker. The reason is that the amp. puts out a current that develops a voltage across this eff. resistance. If this eff. resistance is low, then the amp develops a lower voltage compared to if it had been 8 Ohms. In this case, the amp develops HALF the voltage it would have! If the developed voltage is low, then the xover ckt passes along a smaller current into the speaker voice coils. Thus, the cone drivers hardly move developing a low volume sound. What you will hear is that most/all of your bass will be missing! You will have to crank up the volume forcing the amp to put out more current so that the cone driver excursion can be increased, which will increase the SPL level in the room. Also, since your amp is putting out more current, it'll get hotter quickly!
The HK seems to be rated @ 8 Ohms which means that, although it will drive the 4 Ohms speaker, it will do the job with mediocre quality, it'll get hot & over the long term you'll electrically fatigue the electronics within the amp esp. the output devices, which will be running very hot temp-wise. Electrical fatigue will cause the bias points for the output devices to drift from the factory setting, it'll cause stress to silicon material of which the output BJT or MOSFET is made. All this will create steadily increasing distortion in the output sound. Your ears will adjust to this gradually distorting sound as you will be living with the system & will be accustomed to hearing its sonic signature. However, your friends, who visit, will immediately be able to tell the distorted sound.
HK & many other consumer oriented manuf. do not put large enough power supplies in their amps 'cuz it costs too much to get a large xformer + the heat-sinking material that is reqd. is also expensive + they need a diff. paradigm for removing in-chassis heat while the amp is operating. All this adds too much cost for them.
When you read specs for real beefy power supply amps, you'll see something like: 100W @ 8 Ohms, 200W @ 4 Ohms, 400W @ 2 Ohms, etc. I.E. as the speaker resistance halves, the power doubles. But for cheesy power supply amps you'll read: 100W @ 8 Ohms, 125W @ 4 Ohms. & there will be *no* mention of power out @ 2 Ohms! I.E. a modest increase in power as speaker resistance halves (here I RANDOMLY picked 25% increase only to illustrate a point).
Power supplies is one major factor that distinguishes a great amp from just any Joe-blow amp.
Thanks for all the detailed information!

I do have an external Kinergetics sub/amp on the system which xover at 100 Hz, relieving some of the power requirements of the Spicas. From what I understand, the lower resistance (ohm ratings) (and more demanding voltages) occur at the lower frequencies. Thus if I'm taking these away 100Hz and below from the Spica, I'm not requiring as much from the receiver's amp.

Here's an old Stereophile quote about the Spica's impedience curve and power tolerance (http://www.stereophile.com//loudspeakerreviews/446/index.html):


"The woofer is acoustic suspension and the impedance varies from a low of 3.7 ohms to a high of 15 ohms. The impedance curve is smooth in the midrange, but the dip to under 4 ohms rules out poorly designed receivers or integrated amps anything not rated for 4 ohms. It is with relatively inexpensive receivers, by the way, that most TC-50s are sold, according to the manufacturer. Unlike the Dayton Wright, power handling is comparatively limited. Stick with 100 watts or less, and a really good 40-70W amplifier will be far better than a less transparent high-powered unit.

I remember when I originally got the HK, one of the things I was thinking about was not to get TOO powerful of a receiver because you can overload the Spica's.

On the other side of things, the HK will also "shutdown" if it is trying to run too much current. I've never had this happen even at very loud volumes. I've never noticed any distortion at the high volumes either. It appears that the HK is handling the Spicas OK. Maybe because the Spica's average higher than 4 ohms for most of the spectrum.

I like what I've read about the NAD T762. It's rated at 100W continuous 4/8 ohm or 230W dynamic at 4 ohm. (not sure what the difference is... 6 vs 2 channel?!).

1. Do you think the NAD would provide more "umph" at low volumes (my main objective)?

2. Would the NAD possible be TOO much for the Spicas?

Thanks again for any input!

I don't have any experience AT ALL with Spica speakers - let me make that very clear to you! Also my experience with NAD is very limited.

My brother used to own NAD gear some time back & what I did notice was that despite the small chassis the amp could put out quite high levels of current. They seem to have some proprietary voltage doubling circuits that allows the amp output stage to increase wattage & current.

So, from that experience & reading the specs it APPEARS that the NAD is better suited than the HK for low impedance/hard-to-drive speakers. It seems therefore it would output more current at a lower volume setting than would the HK.

However, if you have a sub & you can control the xover freq to something relatively high such as 100Hz or 120Hz then you can make the sub do all the low freq. work & still keep the HK. Why not do this?
(P.S. I did read your post re. problems with your sub. No experience there but *maybe* you need a new & better sub?)

I have a HK AVI200 in my HT setup & I know about the auto-shut off - I have played that 5-ch amp loud enough to invoke the auto shut-off! My amp also has a "loudness" button which emphasizes bass by 3dB at lower volume settings (testimony to the fact that the AVI200 power supply is pretty cheesy). When this loudness button is engaged I get bloated bass for 2-ch listening but for movies it's just fine as I'm looking for quantity more than quality. If your HK has such a loudness button, maybe you want to play with that? BTW, I engage the "direct" button on my HK so that it bypasses all the tone control circuitry (bass & treble knobs), which has a tendency to screw up sonics.
Whoa--don't confuse continuous power with "dynamic" power. They are very different things--and neither is terribly meaningful, frankly, the first because music isn't continuous and the second because there's no standard definition for it.

I'd say the NAD would likely give you more oomph. On the other hand, if the HK is working for you, you don't notice any distortion (and neither do your friends), and it doesn't seem to be running too hot, it's probably up to the job you're asking it to do. Basic rule for amps: If it sounds ok, it probably is.
No offense meant here, but, using an amp that isn't designed for lower impedances may offer one the opportunity to enjoy the sound while shortening the amps life, and maybe the speakers too. I've suggested this before, but the Spicas deserve a better amp. IMHO, a Forte 4 (a) would be worthy of consideration.
More about what the first guy said. He absolutely correct in all. But what needed to be mentioned is than audio signals introduces different impedance (ohms)right across the audio spectrum, simultaneously. If your amp isnt up to the job it will certainly be identifiable with speakers rated at 4ohms, since the current needed to maintain a signal and keep your amp stable is increased. A decent power supply will have no problem with 4 or 8 ohm speakers. a lousy power supply limits the bandwith and creates higher harmonic distortion (escpecially in the higher order 4th or 5th). This is where your power starts to go south. In short - if your are noticing this, than your in the market for a new amp.