amp power (1 Viewer)

[Yee-Ming]

my question to the EE's out there is, does an amp draw the same power regardless of volume level, or does higher volume level draw more power?
I'm wondering because it seems to this non-EE (I did high school physics but that's about it) that volume would be regulated by a variable resistor, so with lower volumes resistance would be turned up to limit actual output to the speakers, but this current would be "lost" as heat in the resistance. for higher volumes, the resistance would be turned down to let more current through to the speakers to do "actual work" in driving speakers to higher SPLs.
that being the case, the amp would then draw the same amount of current regardless of what volume it's playing back at, and low volumes would mean more current lost as heat.
OR:-
do amps somehow draw less current at low volumes, which seems to make sense since the amp is doing less work in driving the speakers? if so, how exactly does a volume knob reduce the amount of power drawn?
perhaps my earlier simplistic model of variable resistors is only used in simple devices (e.g. portable transistor radios) and sophisticated hifi amps and receivers use some sort of logic circuits?
oh well. could someone with the requisite electrical/ electronics knowledge humour me please with an answer?

 

[Craig_Kg]

Yes
A class A amp will draw the same power regardless of load.
A class B amp draws power according to load.
Almost all amps these days are class A/B. With these the small amount of distortion that occurs crossing from one amplifier rail to the other is avoided. Some use a large bias so that for small outputs (usually < 5W) the operation is pure class A - these run hotter than a low bias A/B as a result.

 

[Yee-Ming]

thanks Craig!
now I'm really going to stick my neck out, could someone explain the differences between Class A & B, or point me to something that already does? I vaguely recall that Class A amps draw "full power" in that both positive and negative amplitude of the AC current sine wave are drawn, whereas class B only the "top half" is used. or something like that. is this correct? and how does that translate into how the amp then deals with higher or lower volumes?
or am I getting in way over my head here?

 

[Saurav]

OK, I'll give it a stab:
The volume control usually comes before a fixed-gain stage. So, you're right, as you turn the volume down, some part of the signal is lost in the resistor. However, this isn't the amplified signal, this is the line level signal (around 2V, which is the output of your CD player or other source). The output of the volume control (which is still a line level signal) is then fed to the amplification stage, which amplifies it by a fixed amount. This takes the signal up to the level needed to drive speakers, which is several volts (depending on power and speaker impedance). So, you don't run a high power signal through a resistor to control volume, because like you said, that would result in a lot of wasted energy.
So, the amp gets an input signal that varies in strength (as controlled by the volume resistor), and it amplifies that input signal by a fixed amount. This is where class A and class B differ. Loosely speaking: A class A amp is biased so that the amplification device (tube or transistor) is conducting for the entire signal cycle. A class B amp conducts for only one half of the cycle (i.e. the sine wave). So, class B amps have to be "push-pull". This is where you feed the signal to one device (the top half) and feed the inverted signal to another device (the bottom half), this way when one half cuts off, the other half takes over. This naturally results in distortion, because the handoff between the two halves is never perfect. Hence the class A/B design, which is somewhere in between - the second half takes over a little before the first half cuts off, so the distortion is reduced.
This might help:
http://www.duncanamps.com/technical/ampclasses.html
Try a search on Google for "amplifier classes of operation" or something like that, and you should be able to find more articles easily.

 

[JohnVB]

Ok,

So how does one find out what type of amplification (Class A, AB or B) does an amplifier use?

- bones

 

[Saurav]

Some (many?) amps tell you in the product literature. I would guess that the more consumer-oriented amps might not have this information, since the marketing department might decide that their buyers aren't interested in it, while the more enthusiast-oriented amps may have this information, because those companies think that their buyers might want to know? That's speculation, of course. Other than that, it is very hard to tell just by looking at a circuit, one would have to perform fairly detailed analysic and/or measurements in order to tell what class an amp is operating in.

 

[JohnVB]

Anyone know what class amps the Denon 3802 uses?

- bones

 

[Craig_Kg]

Just about every amp and receiver is a low bias class A/B (inc the Denon 3802).

A high bias class A/B amp will have "class A operation up to x Watts" in the specs - it's a bragging right. A class A amp will have it blaring out in its specs (plus the heat sinks will always be warm regardless of volume).

 

[John Royster]

Anyone know what class amps the Denon 3802 uses?

don't know for certain but most likey class B.

 

[Yee-Ming]

whoa. (a la Keanu in Matrix)
thanks Saurav. most enlightening. I guess I've now just taken my first steps towards a wider world...

 

[Saurav]

whoa. (a la Keanu in Matrix)

LOL. He was certainly a master of eloquence.

 

[Larry B]

Saurav:

As along as we're back on this topic: I am a bit unclear as to why class A runs hotter. In class A, the tube (or transistor) is running all the time, while in a class B 2 tubes are each running half the time (let's call it half to make it simple). So, it seems the total heat output should be the same. Is the difference then that two tubes have greater surface area for heat dissipation?

Larry

 

[Saurav]

Not quite. In class B, each tube/transistor is reproducing half the signal. So, if there is no signal, both devices are turned off, and there's no current flowing through them. In contrast, active devices in a class A amp are always biased to be in the middle of their operating region, so there's always 50% of the peak current flowing through them, even if there is no input signal present.

Extrapolate that to real music (instead of a signal on/off scenario), and now you're dealing with what percentage of the signal is closer to zero power, and what percentage is closer to full power. Most real music spends more time close to zero than close to peak (I think a rough rule-of-thumb is around a 20dB difference between average and peak power). So, an amp spends a lot of its time reproducing a relatively small signal, i.e. it spends a lot of time close to the idle operating point. When close to idle, a class B amp uses very little power, a class A amp uses 50%.

And of course, I could be completely wrong.

 

[Ted Kim]

A class A amp will draw the same power regardless of load.

I have a true class A amp and it draws the same power regardless of whether at idle or full power.

 

[Saurav]

Of course. I was thinking of a single-ended class A amp, but there's no point in comparing that to a class B amp, which is always push-pull. So you're right, a class A push-pull amp always draws the same power regardless of the signal being amplified. As the signal changes, one tube/transistor draws more and the other draws less, but the combined power output is constant.
Is that a little more correct? I know I don't have a very good grasp of all this.

 

[Ted Kim]

Of course. I was thinking of a single-ended class A amp, but there's no point in comparing that to a class B amp, which is always push-pull. So you're right, a class A push-pull amp always draws the same power regardless of the signal being amplified. As the signal changes, one tube/transistor draws more and the other draws less, but the combined power output is constant

Suarav, you are right about class A push-pull but a single ended class A amp will still draw the same power. The output devices are always on and hence must amplify the signal or dissipate the heat from being on. Hope that clarifies things a bit.

 

[Rommel_L]

I am a bit unclear as to why class A runs hotter...

This where the efficiency of the amps come into play. In a perfect amp, the output (power) is 100% electrical energy (which is what we want). But in a real world, some of the output is given off as heat (or heat dissipation).

In the case of a class A amp, which is 25% efficient, 75% of the output power is given off as heat.

fyi...

-RL-

 

[Rommel_L]

And yes, most amps/receivers use class AB amps (which by the way a class AB amp is comprised of 2 class B amps, one amplifies the upper half of the signal, the other the lower half) because they require less power (more efficient) to amplify the WHOLE SIGNAL compared to a class A amp and it runs cooler.

The reason we use vacuum tubes for class A amps (used in brands like Conrad Johnson and McIntosh) is that they can handle the high heat stress, semiconductors can't.

fyi...

-RL

 

[Chu Gai]

The 3802 is a class B device.

 

[Saurav]

Suarav, you are right about class A push-pull but a single ended class A amp will still draw the same power.

OK, you're probably right.