Ohm question

[dan_wilkinson]

Hi all, im fairly new to audioholics, and i have been reading some of the audio principle guides (amongst reviews etc. aswell), and i have not succeeded in finding anything from which i have been able to extrapolate how the number of ohms relates to impedance (meaning does a speaker [for example] with 4 ohms resistance have a higher or lower impedance than a speaker with 8 ohms resistance?). My speakers seem to be of a rare breed (6 Ohms) and i was curious to know what this means performance/quality wise in comparison with 8 ohm speakers. I am running these speakers off a Yamaha RX-360 RS Stereo Reciever. it says on the outputs that it has a 6 Ohms min, so i think i can safely say that the amp has the same resistance... which brings me to another question - on the back of my reciever (which i got off my brother for nothing and consequently no manual with it) it says 160Watts output. i assume this is RMS because PMPO fingures always seem to be in the thoutsands... o and there is A + B speaker outputs, so would that be per channel, or total power? btw i only use A output.

thanks in advance for your advice,
Dan

 

[markw]

You should lurk a little longer before making these assumptions.

1) The "impedance" of a speaker is calculated and expressed in ohms. The higher this number, the higher the resistance. For speakers, this is not an easily measurable figure you can get by sticking a VOM on the speaker.

This impedance figure will vary, sometimes considerably, over the audio spectrum. Going higher (more ohms) is not bad but sometimes it can dip (less ohms) to a very low level. Some speakers dip very, very low and amplifiers generally don't like low impedances and react poorly by going into the protect mode or worse. Some amps are more forgiving than others on this. Generally, you get what you pay for.

This number (specified in ohms) is calculated as the average "resistance" over the entire frequency range and takes into consideration the results of all the drivers, the crossover and the effects of the speaker enclousre.

2) Actually, "ohms" on the speaker terminals on the receiver/amp have no bearing on whatever ohms are in the receiver. This merely indicates what impedance speakers you should use. If you could (and trust me, you can't safely) measure the output of the receiver, you would probably find it's much, much less than an ohm.

3) That power figure on the back ofthe receiver has little bearing on it's power output. That just indicates how much current it draws from the wall socket when it's running. ...not unlike the watt rating on a lite bullb. If that's the case, your electronic clock would be putting out a few watts of power and obviously it isn't.


enjoy...

 

[av_phile]

Hi all, im fairly new to audioholics, and i have been reading some of the audio principle guides (amongst reviews etc. aswell), and i have not succeeded in finding anything from which i have been able to extrapolate how the number of ohms relates to impedance (meaning does a speaker [for example] with 4 ohms resistance have a higher or lower impedance than a speaker with 8 ohms resistance?). My speakers seem to be of a rare breed (6 Ohms) and i was curious to know what this means performance/quality wise in comparison with 8 ohm speakers. I am running these speakers off a Yamaha RX-360 RS Stereo Reciever. it says on the outputs that it has a 6 Ohms min, so i think i can safely say that the amp has the same resistance... which brings me to another question - on the back of my reciever (which i got off my brother for nothing and consequently no manual with it) it says 160Watts output. i assume this is RMS because PMPO fingures always seem to be in the thoutsands... o and there is A + B speaker outputs, so would that be per channel, or total power? btw i only use A output.

thanks in advance for your advice,
Dan

Impedance is a function of frequency that appears across the speaker load. Personally, I'd prefer to look at the impedance-frequency plot of a speaker which is more meaningful for me. Some speakers rated at 8-ohms can dip to 2 ohms at certain frequencies, usually in the bass region and rise to as much as 40 ohms in the mids or highs with dips and peaks in between.

Using a VOM or VTVM yields a much lower resistance reading than the stated nomical impedance value. What they measure is the VOICE COIL resistance which is the DC resistance. Many 8-ohm speakers can measure between 3.2 ohms to 6 ohms as their DC resistance. Bear in mind that Impedance is DC resistance (ohms) PLUS reactive impedance (capacitive and inductive ohms) that come about in response of frequency signals. So the DC resistance is the minumum load that the amp sees when no signal is being amplified. But once the signal comes in, reactances to the freqeuncies come in to add to the load.

NOMINAL values that you see at the speaker's back depend on how speaker manufacturers rate their products. Some are based on average across the audible spectrum or what is common in most frequencies. Others use as standard a few percentage points above the minimum impedance. I've seen speakers with a minimum of 3 ohms at 45hz added to the DC resistance of 5 ohms to arrive at a nominal value of 8 ohms.

With regards the relationship of speaker impedance with amps, it's safe to match the two. In solid state amps, when in doubt, it's safer to use higher impedance speakers. Ideal amps will double its power as the load is halved. But only if the design of the amp and its power supply allows it to do so. That means, if the amp behaves as a constant voltage sources, it can deliver more current as the load is decreased. That is usually the trait of many so called "high current" receivers and amps. They eschew a MINIMUM Load speaker of 4 ohms at the back. Some are silent, preferring that you refer to the manual which, true to form, gives a continuous power rating down to 2 ohms.

But most commerical receivers and amps out there, mostly Japanese, behave as constant current sources that cannot deliver more current as the load is decreased. They usually have this caution at the back that says MINIMUM 8-16 ohms. So it's best to use the speakers that match the amp's minumum requirements for safe operation.

With regards the power indicated at the back, I believe this is Electric Power Consumption. Not the audio output power of the receiver.

 

[Unregistered]

I just purchased a set of Athena AS-B2 speakers. They are 8ohm rated speakers. My receiver however is rated at 6ohms. Is it ok to drive my speakers with this receiver?

 

[av_phile]

No problem. I presume your amplifer is solid state. In solid state devices, when in doubt, always go for the higher impedance, not lower. So your 8-ohm speakers can be safely driven by your 6-ohm receiver. I bet the back of your receiver has a speaker impedance caution: not lower than 6 ohms. So you're quite safe.