Do class A amps draw power even when turned off?

Zaiksman, thanks for that info. Yes it does make sense, even without the engine analogy.

Many class A amps stress to leave them on all of the time (except vacations). The fact they there are always running is less stress than powering on/off. Some will have a standby mode which is a nice comprimise.

some amps don't have an off switch. just a standby switch. this provides for instant on, keeps the circuits warm and prevents damage from power surge.

Timrhu: Exactly how I came to know that factoid is lost in the mists of time, but I'm guessing I probably first ran across it in an audio mag (I'm an *audiophile*, we're not *supposed* to be technically knowledgable! ;^)

But, after reading your comment I did a little web searching and thought this explanation was as good as any, authored by the emminent Randall Smith from his Mesa-Boogie site (which of course means it's written about tubes and guitar amps, but it could just as well be said about transistors and stereo amps), edited and cobbled together from a few different passages by yrs trly with my own little annotations in brackets:

"Class A amplifiers -- single-ended or push-pull -- suffer from maximum dissipation at idle. All the steady state DC power passing through the tubes goes up in heat. Only when that power fluctuates [i.e., is modulated by an input signal to create an output signal] does some of it convert to useful audio power to drive the speaker. Power that flows through the tubes and drives the speaker is power that is not dissipated as heat...The current fluctuations in a proper Class A amplifier are always centered around the mid-point, 50% of maximum, which is the same as the idle current. What this means is that there is no net increase in the current flow [averaged over time] like there is in Class B or AB, no matter how loudly you play...The circuit is “biased about the midpoint of its linear region”, and current fluctuates symmetrically between +50% and –50% for a total 100% of what’s available. Biasing at the midpoint means the tube is running hot and wasting most of the power except when driven to max...The other classes of operation, Class B and Class AB, are definitely not biased anywhere near that mid-point and that’s the key to their ability to run cool and produce more power [at the cost of higher distortion]...Dissipation is wasted power that the tube turns into heat. To use the car analogy: Class A idling is like having the engine running wide open with the brakes locked and the clutch slipping. All the engine’s power is being wasted and “dissipated” into heat. To get some useful output, you would have to let up on the brakes -- not completely but enough to get the car moving. Then there would be less total slippage (or dissipation), even though it’s now shared between the clutch and the brakes -- which are dragging but not locked. But now you’re getting some useful work from the engine’s power -- in the form of motion, instead of just waste heat. Engine power -- in any amount -- that’s used for moving the car, reduces the dissipation by that same amount because there is that much less total slippage at the clutch and brakes. At full speed ahead neither the clutch nor the brakes would be slipping. Nearly all of the power would finally be converted into motion (or sound) with virtually no dissipation through slippage friction. But just as you can’t drive your car at full speed all the time, the highly dynamic nature of music means that Class A amps spend most of their time dragging and slipping. There are only briefly dynamic peaks where most of the energy actually goes to the speaker...The dissipation rating of a tube [or heat-sinking in a SS amp] is what limits its power capability much more than the useable power that can pass through it. Like the clutch analogy, it’s the slippage converting energy into dissipated heat that kills it, not the power flowing through it when it’s fully engaged"

Thanks for everyone's help... I can always count on the members of Audiogon to pull thru for me!

Zaikesman, that's very interesting. I'm not questioning your statement but am wondering if you can site a source for it. I've noticed this phenomenon with an amp I have that claims to run class A for first fifteen watts. I always attributed the cooler amp after playing music to the fact that the amp is sometimes running A/B. Thanks.

But class-A amps throw off the most heat when they are not passing a signal, unlike class-AB amps (in other words, no significant amount of the constant electrical power being consumed gets dissipated as sound power by the speaker when silent, so more must get dissipated as waste heat).

.

Does this mean that it is drawing 300 watts even when I have it turned off

NO, it is consuming no power when it is turned off. The exception would be if it has a remote that is waiting for a command to turn on but that circuit would draw very little.

, or only when it is turned on regardless of whether or not it's playing music???

This is the correct answer. The power consumption does not go up when playing music. Class A amps consume the same amount of power whether they are playing music or not

Actually, if it's truely a Class A design (Pure Class A, the power usage (in this case 300W) will be constant, irregardless of whether or not there is a signal.

Only when turned on and not playing music (i.e., volume turned all the way down or source is stopped). Turn the volume up and the power usage goes up from 300W.