Amplification: watts, current, and ohms

[Xenogear]

Hi,

I'm curious about what makes amplifiers with the same watts of output differ from one another. Take two amplifiers with the same watts rating. Does that mean that they are equal in terms of power? Let's say the watts rating is accurate. Why would one be able to drive low impedance loads better than the other?

Back when I was in college, I wanted a Carver amp. I remember being fascinated by the fact that Carvers are smaller and lighter than your typical amp of the same watt rating. But, from what I have read, Carvers are supposedly better able to drive lower impedance loads.

What determines how well a particular amp will be able to drive a low impedance, less-sensitive speaker system?

 

[jcPanny]

Amp power

Most outboard amps are stable into 4 ohm loads and some as low as 2 ohms. If they have an output power spec for 4 ohms then that is a good indication that they are stable into that impedence. In theory, power into 4 ohms should be double that into 8 ohms and higher quality amps will be closer to double the power into 4 ohms.

Your best bet is to pick the speakers and then select a receiver and/or amp that will drive them.

 

[bandit]

What makes an amplifier capable of driving low impedences is its ability to provide more current (amps) than one that doesn't. Electrically speaking - Power = Current X Voltage. This is why often when you see an amp promoated as capable of driving low impedences the 4 ohm (or less) rating usually carries a higher power rating at 4 ohms than at 8 ohms. More current is flowing.

In a perfect world - if you havled the impedence from 8 ohms to 4 ohms you would exactly double your output power. This is not usually the case though. The amps power supply most likely can't deliver that kind of current.

In designing an amplifer that is capable of driving low impedences probably the most important single consideration is the power supply. Also, you'd need to ensure adequete cooling for the amp itself. The quality of the final output devices will also have to be high quality.

Bandit

 

[annunaki]

To be able to put two amplifiers' power ratings head to head, they must be taken in the same manner. For example:

Amplifier A: 100 watts continuous x 2 (two channels driven as per FTC) at 8 ohms, 20hz-20khz +/- 0.5db, with < 0.07% thd

Amplifier B: 100 watts continuous x 2 (two channels driven as per FTC) at 8 ohms, 20hz-20khz +/- 0.5db, with < 0.08% thd

These amplifiers would essentially be identical in terms of performance

Now if the two amplifiers looked like this:

Amplifier A: 100 watts continuous x 2 (two channels driven as per FTC) at 8 ohms, 20hz-20khz +/- 0.5db, with < 0.07% thd

125 watts continuous x 2 (two channels driven as per FTC) at 4 ohms, 20hz-20khz +/- 0.5db, with < 0.1% thd



Amplifier B: 100 watts continuous x 2 (two channels driven as per FTC) at 8 ohms, 20hz-20khz +/- 0.5db, with < 0.08% thd

175 watts continuous x 2 (two channels driven as per FTC) at 4 ohms, 20hz-20khz +/- 0.5db, with < 0.1% thd

We are easily able to tell here that one amplifier (B) more capable at running lower impedance, or more reactive loads. We can also tell that it has more substantial reserve power for transients, i.e. dynamic power.

It is very difficult to judge performance of two amplifiers unless they are rated in the same manner. One can get an idea of an amplifier's performance from less telling specs, but it is difficult to say the least. Hopefully this helps you some.