Amplifier Power

C Dixon · From: Duluth, GA USA

In another thread there is a discussion of power regarding a Fender Twin. So as not to change the topic in that thread I thought I would define power from a purely scientific and engineers' standpoint using laymans; terms. Hopefully the following will take some of the mystery out of this very ellusive term when it comes to power amps.

Power in any electrical device is determined by multiplying the voltage times the current in any piece of that device. Example, if a given speaker is drawing 5 amps and the voltage that pushed those amps thru the speaker was 10 Volts, the power that speaker is delivering is exactly 50 watts of power.

Now that is the forumla. And it is a simple but accurate formula IF, we define a few things that can quickly muddy the waters. I digress. IF that current was direct (DC) and if the voltage was rock steady (often called DC voltage), then the above example is 100% true and there are NO variables.

But in musical instruments amps we are NOT talking about "DC" voltage resulting in DC current when it comes to speakers. We are talking about music AC voltage forcing a widely varying current. This is where the problem begins and has lived with us since day one when it comes to a given amp's power rating.

There are basically 6 ways that a manufacturer can determine power; and the following example will vividly show how ratings can often be very missleading.

1. True RMS power.

2. Peak power.

3. Peak to peak power.

4. RMS power using a fixed power source instead of the units power supply.

5. Peak music power.

6. Peak to Peak music power.

Add to this power, ratings that allow a certain amount of tolerable distortion. Note: this is NOT the distortion the rock and roll bunch started. Rather it is a fine line distortion that occurs right at the point when a tube or transistor is reaching its maximum power. In other words, it is being pushed ALMOST over the limit.

So lets put some figures to the above. IF an amp's true RMS power is 70 watts. the following will be the other rated wattages on the SAME amp:

1. True RMS power is 70.7 watts.

2. Peak power is 100 Watts.

3. Peak to peak power is 200 watts.

4. RMS power using a fixed power supply other than the amps power supply. This can vary widely all over the place, but assuming the amp's power supply was 90% efficient, it would be approx 81 watts.

5. Peak music power 110 Watts. (allowable 10% distortion).

6. Peak to peak music power would be 220 Watts. (allowable 10% distortion).

Because of this ability to rate amps over such a wide range, manufacturers played games for many years. Even legitimate manufacturers would sometimes be guilty of playing games with the figures.

So in the late 60's The federal government got into the act and put some rules on it; and for a while it was rigidly enforced. They determined that the ONLY true power rating was True RMS Power using a given amps own power supply.

However, because of pressure and marketing forced rationale (psuedo engineering) from manufacturer's; they agreed to permit Peak music power IF, it was clearly stated. If it was not stated "MUSIC power" they were not allowed to use it.

But under no circumstances was Peak to Peak power to be used again. Since it was virutally meaningless; even though some manufacturers vehemently argued to the contrary. Fortunately for honest folk, they lost that battle.

So power ratings is still a somewhat nebuluous thing. And since enforcement tends to fade once the hoopla dies down, some companies are once again skirting and/or flaunting the rules.

Hopefully the above helps in understanding this most confusing subject. In a word, unless it says true RMS power it is as phoney as a 3 dollar bill. But more importantly, when comparing two different amps, it is totally foolish to speak of power, UNLESS both are rated using the same method.

carl

David Doggett · Posted 18 Sep 2003 5:40 pm

Carl, a while ago in a thread about tube versus solid state sound, some people said tube amps rated at lower watts could sound as loud as ss amps rated at higher watts, that "tube amp watts and solid state watts are not the same." That was news to me. I always thought a watt is a watt. If a tube amp and ss amp were both rated at say 50 watts true RMS, wouldn't that mean they have the same loudness?

C Dixon · From: Duluth, GA USA

Absolutely David.

However we must keep in mind how the human ear works. Since it is NOT a linear device, the brain does NOT perceive things the same that can be measured precisely.

Let me explain. Very few musicians and sadly electronic techs, know the difference between power and gain. These two things which are very much a part of ANY amplifier are two different animals.

NOW when we add in a third element (dynamic range) it can cause our ears to perceive this device is louder than that device; EVEN though both are putting out precisely the same wattage.

Dynamic range in tubes (because their electrons are virtuall uninhibited in its inherrent vacuum) is for all practical purposes instantaneous. With transistors this is NOT the case, because their electrons are inhibited since they are traveling in a solid material. Engineers rate it as "transient time".

So there is a slowing down in transistor amps and dynamic range suffers.

In fact, when transistor amps first came onto the scene, a 500 watt true RMS power amp was perceived to be less loud than a 55 watt true RMS tube amp!! This due to their horribly poor dynamic range.

Now you ask, what is dynamic range? Ok it is simply how fast ANY amplifying device can go from a low level signal to a high level signal (and vice versa). To put it in musical terms. Compare the difference in a 22 rifle shot (crack) to a continuous note from a bassoon.

If an amplifier cannot go from no level to a high level instantly, the human ear will perceive that it has low or no power. Thus the difference in transistor amps and tube amps.

While transistors will probably NEVER have the dynamic range of vacuum tubes, they have made great strides over the last 50 yrs. And modern transistor amps are coming quite close.

But a trained ear will almost always perceive that a tube amp driven to 55 amps is much louder than a transistor amp driven to 55 amps. EVEN in triple blind tests. And EVEN though both are putting out exactly 55 watts!!

Hope the above helps,

carl

Earnest Bovine · From: Los Angeles CA USA

7. Carvin power is 1000 watts.

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