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Discussion Starter · #1 ·
Disclaimer: This is a newbie question.


It has always been my understanding that speaker with lower impendence are hardered to drive....i.e. it would take more amplification power to drive a 4 ohm speaker than an 8 ohm speaker (all other things being equal).


Now.....if this is the case, then when power ratings specs for a given amplifier are published, why is it that the amplifier can provide more power (watts) at a lower impendence? Logic would seem to say that the amplifier would have a harder time driving the 4 ohm speaker vs. the 8 ohm and hence, it would be able to provide lesser watts.


For example, below are the specs on my Yammy 4600 receiver.



Minimum RMS Output Power (8 ohms, 20Hz-20kHz, 0.04% THD)

Front Channels 130W +130W

Center Channels 130W

Surround Channels 130W +130W

Surround Back Channels 130W +130W

High Dynamic Power, Low Impedance Drive Capability Yes

Dynamic Power/Channel 8 ohms 165 W
6 ohms 205 W

4 ohms 260 W
2 ohms 340 W

Linear Damping Yes

Damping Factor (8 ohms, 20 Hz-20kHz) 140 (speaker A)


In this particular case, the receiver produces 205W at 6 ohms but 340W at 4 ohms. Am I missing something?
 

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I think you might be confusing speaker "sensitivity" and "impedance".


Speakers with lower "sensitivity" are harder to drive....in other words less efficient.


Read this about impedance.

http://www.audiovideo101.com/dictionary/impedance.asp


Bottom line..the lower the impedance...the more current draw from the amp. As a result...more wattage. However...more distortion comes into play along with heat and instability of the amp itself if driven with too low of an impedance.
 

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It does have a harder time. Doesn't mean that it can't do it. But look at the 2Ω power rating. 2Ω is 1/3 of 6Ω, so for the same input and gain. Audio amps are voltage amps, so the power output should be 3 times the 6Ω rating. (V=IR. V constant, decrease R and I increases proportionately.) Instead, it is 340W/205W, or only 1.66 times. Most likely is it current limited.


Some amps can't drive 2Ω reliably at all. They become unstable. Much of the "why" for impedance problems is circuit-dependent. Just think of it as decreasing impedance getting closer and closer to a dead short.


(Disclaimer: I'm using DC numbers for illustration. The real AC numbers aren't that simple. One missing factor is the increased reactance of many low-impedance loads.)
 

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Better answer this one right or TONYBDA will yell at you. or make some other Im an engineer you are not statement.



XOXO
 

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Discussion Starter · #5 ·
Quote:
Originally Posted by BMWM3Rod
I think you might be confusing speaker "sensitivity" and "impedance".


Speakers with lower "sensitivity" are harder to drive....in other words less efficient.


Read this about impedance.

http://www.audiovideo101.com/dictionary/impedance.asp


Bottom line..the lower the impedance...the more current draw from the amp. As a result...more wattage. However...more distortion comes into play along with heat and instability of the amp itself if driven with too low of an impedance.
Ok, so I read through the discussion on Impedence.....am I being thick or what??


"Impedance is similar to a dam in a river. If the river is a circuit (to be a true circuit the river would need to run in a complete circle) then the water is the electric current. The water flows freely when there are no obstructions to resist the flow. The dam in the river resists the flow of water. By fully opening the dam there is no resistance. The further closed the dam becomes, the more difficult it is for the water to flow. A dam of 1 ohm would have very little resistance (although a dam’s resistance to water is not measured in ohms, impedance is and so ohms will be used for this example). As the dam’s resistance to the water’s flow, it’s impedance, increases to 2 ohms it becomes more difficult for the water to flow. Similarly, as the dam’s impedance grows to 4 ohms and then 8 ohms and beyond it becomes more and more difficult for the water to flow. In an electric current, low impedance lets more current flow. "


TAKEAWAY: lower impedence = less resistance i.e. more current flow


"A well designed amplifier with a strong power supply will double the amount of power in watts that it sends to a speaker with each halving of impedance. For instance, an amplifier might send 100 watts to a speaker with an 8-ohm impedance. When the impedance is halved to 4 ohms that amplifier would send out 200 watts of power. In the real world, few amplifiers are actually able to double their power output as impedance halves due to limitations on their power supply and design."

"


TAKEAWAY: Good amp will provide more power as impedence lowers (without power supply constraints etc.).....makes sense....lesser resistance from speaker equals more watts.


CONFUSION:


"Additionally, few amplifiers are able to put out power for any significant length of time with impedances of 3 ohms or less (the amplifier must be able to supply huge amounts of power as the impedance decreases below 4 ohms – in fact, many receivers cannot operate below 6 or even 8 ohms). "

If lower impedence equals lower resistance, then why would an amp have difficulty putting out power with speaker with impedences LESS than 3 ohms.......... I thought lower impedence equals lower resistance which equals to more watts??


Am I being thick??
 

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Quote:
Originally Posted by Mark C Sherman
Better answer this one right or TONYBDA will yell at you. or make some other Im an engineer you are not statement.



XOXO
I only correct idiots who don't have a clue, yet post anyway....like yourself.
 

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Back to the water pipe, if you hooked up a 3 foot diameter pipe to your garden hose, there would be no flow restriction at all, but the water would come out in a trickle because your faucet couldnt keep up with the flow demand, and would stop the flow to all your other faucets in an attempt to keep up.

Similar to an amp, it will try to fill that pipe, and will over-drive itself and overheat.
 

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Watts law:


P=V^2/Z, where P = power in Watts, V = voltage, and Z = impedance in Ohms.


As Z approaches zero, power moves towards infinity...an impossibility.
 

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Quote:
Originally Posted by DLPKID
CONFUSION:


"Additionally, few amplifiers are able to put out power for any significant length of time with impedances of 3 ohms or less (the amplifier must be able to supply huge amounts of power as the impedance decreases below 4 ohms – in fact, many receivers cannot operate below 6 or even 8 ohms). "

If lower impedence equals lower resistance, then why would an amp have difficulty putting out power with speaker with impedences LESS than 3 ohms.......... I thought lower impedence equals lower resistance which equals to more watts??


Am I being thick??
No. :) This is the point where theory meets reality. The equations work for an ideal electrical source, but power amps are hardly implementations of the ideal. They all have limitations (largely because your credit card has limitations ;) ).


First, think of where that current comes from. It is drawn from power capacitors charged by a DC rectifier stage, and ultimately from the wall. Using approximate numbers, the wall outlet is capable of supplying 15A at 110V, or 1650 Watts. (Multiplying by about .65 for an RMS value yields a little over 1000 WRMS - I think.) So that's an upper limit right there. The limits of what the DC rectifier stage (mostly a transformer) can supply and what the capacitors can store further cramps the upper limit.


As input voltage increases (turning up the volume), current output also increases. At some point the power supply will run out of juice. The rectifier stage won't be able to convert enough AC to meet demand.


Keep the same input voltage (thus output voltage) and change the 2Ω speaker load to an 4Ω load. What happens? The output current drops by a factor of two. (4Ω/2Ω = (scalar)2) Now the power supply is well within it's abilities. The limiter is more likely to be the output transitors. Actual output of the amp is limited by different parts of the amp at different loads, which is why power output is non-linear with load.


Or think of it this way. The lower the load (speaker impedance) get, the more it looks like a dead short. At some point the current will be limited by the wires melting.
 

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Quote:
Using approximate numbers, the wall outlet is capable of supplying 15A at 110V, or 1650 Watts. (Multiplying by about .65 for an RMS value yields a little over 1000 WRMS - I think.)
110VAC is the RMS value, so the maximum power that can be supplied, at 110VAC on a 15 A circuit is (110V X 15A = )1650 Watts. RMS.
 

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Quote:
Originally Posted by DLPKID
If lower impedence equals lower resistance, then why would an amp have difficulty putting out power with speaker with impedences LESS than 3 ohms.......... I thought lower impedence equals lower resistance which equals to more watts??


Am I being thick??
Think of your river dam analogy. Pressure = voltage, while flow = current.


The dam has to keep a certain height of water to maintain a certain pressure. But with more flow coming out, something has to be working to push more water into the dam to keep that height steady.


A power amp is actually a voltage amp. A lower load impedance draws more current for a given output voltage, and if the impedance is too low and the output voltage the amp is trying to produce is too high, it won't be able to keep up, and the voltage may collapse and cause premature clipping, if the amp doesn't actually shut down.
 

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Thanks Tony, I wasn't sure about that.
 
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