How Power is Drawn from an Amplifier

[Tasmaniak]

wow, I sometimes forget how boring things can be at component level...Whilst I understood it. My eyes just glazed over and my mind over to much more fruitful topics. Like Sex!

 

[Luke0011]

Throw which theory out the window?

Amplifiers have an a gain exactly for the reason you mentioned, because source units have different maximum RCA output voltages.

The impedence of a speaker determines what power it receives, so two 4 ohm speakers will receive about the same power if hooked up to the same system. Speakers do have an impedence curve thought which means the impedence changes at differring frequencies, so you can end up with slightly different power at different frequencies.

I assume you're referring to class-D amplifiers when you say "digital". They interact with speakers the same way as a normal amp. The only difference is how the transistors turn on and off to produce the sound wave.

The amplifiers that output the same power at diferent impedences have regulated outputs.

How does this relate to your original question though?

Sorry stonex miss read your quote as max gain as max volume.

Oh, bloody hell, here we go again....

OK. Typical MOSFET designs are current output devices - that is, they look, to the sub, like a Voltage Controlled Current Source. The controlling voltage is the input signal.

We can model this, roughly, as I(out) = g*V(in), where g is the gain of the amplifier. Note that for 99% of amplifiers, that's actually a fixed value, and what we consider the gain control is actually an attenuator on the input signal.

OK, let's take a hypothetical amplifier with g = 5. We apply a 1V RMS signal on the input. Since g = 5, the amplifier attempts to force 5A through the speaker.

If the speaker is, say, a 4 ohm sub, this results in a voltage of 20V across the output terminals, for a total output power of 100W.

OK, what if we swap in a 2 ohm sub? Now we see a voltage of 10V across the output, for a total output power of 50W.

So why can an amp deliver more power to a 2 ohm sub than a 4 ohm one? Well, that's down to the power supply. If the switchmode PSU in the amp runs rails of, say, +/- 50V, it can swing an output signal of (nearly) 100V peak to peak. That's an RMS voltage of about 35V. Since this is the largest voltage it's possible for this amp to swing, it has a peak theoretical output power of only about 300W into 4 ohms, but 600W into 2 ohms.

Upshot is, if your amp is rated to 1000 watts into 4 ohms, but with the "gain" at full you aren't swinging 178V peak to peak (63V RMS) across the outputs, your amp still has more left in it....you just need to crank up the signal source more.

Thanks for the technical response, i understand this partially so i have a few questions.
1. if the 2 ohm sub produces 50w is to be incorrect then 4 ohm at 100 watts would also be incorect also. what if you started with the 2 ohm example first and state the 4 ohm example as being incorrect???
Confusing.
2. Is it safe to assume that if the 2 ohm example is used the current increases and the voltage drops. As in the sub draws 20 amps thus producing 200 watts

Is it a similar principle to a car battery droppping voltage to increase current while cranking as is the input voltage in respect of the gain control setting aproxximating a set current and voltage differenciating output wattage.

 

[Tasmaniak]

can we get back to the topic at hand here please? .....sex.


-Edit-....I mean both hands

 

[theSeekerr]

Sorry stonex miss read your quote as max gain as max volume.
Thanks for the technical response, i understand this partially so i have a few questions.
1. if the 2 ohm sub produces 50w is to be incorrect then 4 ohm at 100 watts would also be incorect also. what if you started with the 2 ohm example first and state the 4 ohm example as being incorrect???

I didn't say that anything was incorrect in my original post....so now I"m the one who's confused.

2. Is it safe to assume that if the 2 ohm example is used the current increases and the voltage drops. As in the sub draws 20 amps thus producing 200 watts

Is it a similar principle to a car battery droppping voltage to increase current while cranking as is the input voltage in respect of the gain control setting aproxximating a set current and voltage differenciating output wattage.

No. Nothing like that at all, actually.

Sorry, my original examples were a bit poorly worded. In reality, your input signal would be an AC source which is continuously variable in amplitude (that is, voltage) and frequency. As a result, your output current is similarly variable in amplitude. The voltage is related by Ohms Law, that is, V = IR, where I is the output current and R is the speaker impedance, in this case.

Since Power = VI, you can substitute Ohms law and derive the expression Power = Current Squared * Impedance, which is also equiavalent to Power = Voltage Squared / Impedance

So yes, 200 watts into a 2 ohm sub takes more current and less voltage than 200 watts into an 4 ohm sub. However, that has nothing to do with the way a battery's terminal voltage sags when delivering high current, which is a purely chemical effect.

If you'd like anything more explained, just quote the relevant parts of my posts and ask away.