Quick question about OHMS

[TaaDow]

Ok so I'm fairly new (and clueless) about all this stuff but I've been reading about OHMS, Impedance and whatnot. Here's my question: If I have a set of speakers that are rated as having an impedance of 6 ohms, they require more current from the amp to drive them than a set of 8 ohms speakers correct? I have also read that if (for example) a pair of speakers are 4 ohms, it requires twice the current of speakers that are 8 ohms. I have a Yamaha amplifer that is compatible with 6 ohms speakers but puts out 85 watts per channel @ 8 ohms. So does this mean that the amp will actually put out more watts @ 6 ohms than if the speakers were 8 ohms?

 

[MDS]

Ok so I'm fairly new (and clueless) about all this stuff but I've been reading about OHMS, Impedance and whatnot. Here's my question: If I have a set of speakers that are rated as having an impedance of 6 ohms, they require more current from the amp to drive them than a set of 8 ohms speakers correct?

Correct.

I have also read that if (for example) a pair of speakers are 4 ohms, it requires twice the current of speakers that are 8 ohms. I have a Yamaha amplifer that is compatible with 6 ohms speakers but puts out 85 watts per channel @ 8 ohms. So does this mean that the amp will actually put out more watts @ 6 ohms than if the speakers were 8 ohms?

Yes again, but with a caveat. The lower impedance will draw more current, thus taxing the power supply. Because it is rated for 6 Ohm speakers, it can likely handle them easily. You could get away with even lower impedance speakers, such as 4 Ohm, but the increased current demand will tax the power supply and if you turn it up high, the receiver may get very warm or even shut itself off.

V (voltage) = I (current) x R (resistance). The power supply can only supply a given rail voltage so you can consider voltage constant. So if the R goes down by a factor of two, the I is going to up by a factor of two (assuming the power supply can actually get there, as stated). Speakers don't present a constant resistance either, the impedance (loosely considered R, though it really isn't) varies with frequency.

 

[TaaDow]

Ok so that being said, would it be accurate to say this - if an amp puts out 100 watts @ 8 ohms, that it would put out 200 watts @ 4 ohms (now I know that this would overload most home theatre-type amplifiers, but for argument's sake...). So if that were true then my Yamaha amp which puts out 85 watts @ 8 ohms would produce roughly 125 watts @ 6 ohms? I am just curious to put a number to the actual number of watts produced so I can know how much I'm putting my speakers up against.
Thanks

 

[Polkfan]

Ok so that being said, would it be accurate to say this - if an amp puts out 100 watts @ 8 ohms, that it would put out 200 watts @ 4 ohms (now I know that this would overload most home theatre-type amplifiers, but for argument's sake...). So if that were true then my Yamaha amp which puts out 85 watts @ 8 ohms would produce roughly 125 watts @ 6 ohms? I am just curious to put a number to the actual number of watts produced so I can know how much I'm putting my speakers up against.
Thanks

Theoretically yes, assuming the amplifier is perfect. No amp is, but some come close.

 

[Polkfan]

Although it would be more like 113 watts at 6 ohms for your amp.

 

[Mr.T]

Correct.


Yes again, but with a caveat. The lower impedance will draw more current, thus taxing the power supply. Because it is rated for 6 Ohm speakers, it can likely handle them easily. You could get away with even lower impedance speakers, such as 4 Ohm, but the increased current demand will tax the power supply and if you turn it up high, the receiver may get very warm or even shut itself off.

V (voltage) = I (current) x R (resistance). The power supply can only supply a given rail voltage so you can consider voltage constant. So if the R goes down by a factor of two, the I is going to up by a factor of two (assuming the power supply can actually get there, as stated). Speakers don't present a constant resistance either, the impedance (loosely considered R, though it really isn't) varies with frequency.

Actually, what mostly gets taxed here is the sound output transformer and MOSFET(s)(Power output Integrated circuit "IC").
Power supply circuits are indirectly involved with the sound output stages, because of the DC voltage supplied to the sound output IC and MOSFET(s). The output stages get the toll drawing too much current causing components overheating which could cause break down in a long run.

 

[annunaki]

Actually, most current home theater amplifiers (receivers) will give you little gain in power, if any, by using a lower impedance. Most amplifiers (receivers) have what would basically amount to an impedance matching circuit built into the output. So whether you run 8, 6, or 4 ohms one stills sees about the same amount of power. This is not the case with all amplifiers but is seeming to become quite popular. Some high quality amplifiers (not necessarily expensive) will double (or be withing 80% of doubling) their power output each time the impedance if halved down to a certain point. Sometimes down to 1 ohm, but usually to 4ohms. For instance if an amplifier did 150 watts at 8 ohms, it would do 300 at 4ohms, 600 at 2 ohms, and 1200 at 1 ohm. This would be a difficult and expensive amplifier to build, but talk about having dynamic capability!!

 

[MDS]

Some high quality amplifiers (not necessarily expensive) will double (or be withing 80% of doubling) their power output each time the impedance if halved down to a certain point. Sometimes down to 1 ohm, but usually to 4ohms. For instance if an amplifier did 150 watts at 8 ohms, it would do 300 at 4ohms, 600 at 2 ohms, and 1200 at 1 ohm. This would be a difficult and expensive amplifier to build, but talk about having dynamic capability!!

You know that true doubling down is nearly impossible to achieve and that what many amp manufacturers do is rate an amplifier as capable of 150 watts at 8 Ohms when it can really do 200 watts at 8 Ohms. That way it appears to double down to 300 watts at 4 Ohms. Its the same game that receiver manufacturers are blamed for, but let's not get into the power discussion again.

The original poster wanted the basic theory behind it all and is aware that things aren't so perfect in the real world.

 

[annunaki]

That is why I stated "or be within 80% of doubling". If an amplifier can do 150 watts at 8 ohms and 200 watts at 8 ohms it would depend upon the distortion present when the measurment was taken. For instance the amplifier, as you are suggesting would do 150 watts at 8 ohms with .03% thd + noise. When it would do 200 watts at 8 ohms, it would have say, .1% thd + noise. The thd + noise would have top be stated each time the power level increased and impedance decreased.

 

[JoeE SP9]

There are amplifiers that do double their output as the load in ohms is decreased. A good example is Krell. Krell's big class-A amplifiers double their power as the output impedence is lowered. ie 100 watts into 8 ohms, 200 watts into 4 ohms, 400 watts into 2 ohms, 800 watts into 1 ohm. Of course they are rather expensive and large.

 

[PENG]

Even with expensive regulated power supply in an amplifier, there is always going to be some voltage drop somewhere in the system. This voltage drop will increase as current increases (V=IZ, Z being the impedance). It is possible to approach "double down" but as someone mentioned before, those who claims "double down" likely simply understate their output at higher impedances.

 

[20to20K]

How about 4 ohms and 8 ohms together?

I just had the following problem today...I tried to connect my Denon 3803 to the following:

Polk LSi 15 - front (4 ohm)
Polk LSi C - center (4 ohm)
Polk Fxi 30 - rears (8 ohm)

After about 20 minutes playing in 5 channel mode the reciever cuts off. I'm not playing it
loud. I call Tweeter. They claim AVRs tend to have a problem playing speakers with mixed
impedences and may shutdown in this configuration. The Denon has no impedence selector. This sounds fishy to me and if it's true sounds lame on Denon's part.

If it is true (I will continue to research) it appears I may need to buy a seperate amp to
drive the 8 ohm rears. This is still cheaper than upgrading the rears to the $900 LSi FX's(also 4 ohms).

Does this make sense?

 

[Votrax]

It has nothing to do with the mixed impedance or the 8 ohm speakers. It's the 4 ohm speakers which are activating the speaker protection circuit.

Here's the spec's from the 5803 manual which are identical for all the models below it as well.