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Let's say that a Class A/B amp, rated at 200 watts into 8 ohms, provides 4 watts of Class A power.
If the same amp is rated at 400 watts into 4 ohms, will the available class A power also increase, when using 4 ohm speakers?
If so, will the Class A power also double into 4 ohms?
Follow Ups:
The best solution is to actually listen to the stuff. It is supposed to be a hobby, give it a try.
If your Speakers are Sooo inefficient that you need 200 let alone 400 watts... then it's unlikely you will hear much if Any benefit from a Class A type.
Horses for courses.. Applies imo.
You ask a technical question and you get 2 plausible answers - but with totally contrary answers. One says you get more watts into 4 ohms, the other says less.
Which to believe? I'd suggest the sensible way is to simply ask the manufacturer. Please let us know.
On another "power output" point, many amplifiers boat double the power output into 4 ohms compared with 8 ohms, but other amps say much the same. Is this down to amplifier design, or the power supply built into the amp? I don't know.
My 30 watt Class A amp is quoted at 30 watts into 8 ohms, 60 into 4 ohms, 120 into 2 ohms and 150 into 1 ohm. Not many manufacturers dare to give 2 or 1 ohm outputs. This only indicates to me that this 30 watt amp is likely to be better with difficult (electrostatic) speaker loads than many nominally much more powerful (into 8 ohms) amps.
The question piqued my interest and a dove straight in without noticing John's answer. He is saying that at the output level to give 4W into 8ohms if you half the load to 4 ohms the power will double as it is a classA/B amplifier that can increase output current on demand, within the limitations of its power supply. But if it is was a 4W into 8 ohm class A amplifier the maximum output current is fixed and as the load decreases the amp cannot supply more current to maintain the voltage swing and you would get current clipping. The most power it could supply into 4ohms before clipping would be 2W. So the classA/B will double the power into 4ohms but now it leave the classA region at 2W instead of 4W.
Regards,
13DoW
That makes sense. Therefore, I think you're right and I'm wrong. However, the amplifier will still maintain 5.657-volts at its speaker terminals when the load changes from 8-ohms to 4-ohms and this raises a question in my mind. If you have a solid state Class A amplifier, does that mean it will no longer provide constant voltage at its speaker terminals under varying load? How does that work with a pure Class A amplifier like a big Krell or a Pass Labs?
Thanks,
John Elison
"However, the amplifier will still maintain 5.657-volts at its speaker terminals when the load changes from 8-ohms to 4-ohms "
Yes, it will, but if the transistors in the output stage are biased to be always on (class A) for 4W into 8 ohms, as you lower the impedance, you will leave class A as this current demand is exceeded. Transistors are biased for a standing current, not in terms of voltage.
Here is that behavior demonstrated in Aleph 3 measurements, with the lines being 8, 4, and 2 ohms starting at the bottom. The first deviation on these curves is at about 20W, 12W, and 6W respectively. While the curve for the 8 ohm load turns up to vertical almost immediately afterwards (running out of rail voltage before cutoff), the 2 and 4 ohm curves show a likely transition into a small region of class B operation at those higher powers with those lower impedances.
Hi John,
I think things get muddy with solid-state class-A power amps as I am sure almost all have push-pull outputs that means they can operate beyond their class A region as class AB to maintain output voltage into decreasing loads (depending on the capabilities of the power supply).As an example the PASS XA200 is a mono-block rated as 200W into 8 ohms and 400W into 4 ohms. The standing power consumption is specified as 760W. For 200W onto 8 ohms the voltage rails must be +/- 60V. If the standing power dissipation is 760W and we assume it is all in the output stage then the output bias current is 6.3A that would correspond to an output current swing of 4.5Arms and a class A output power of 2*60*4.5 = 160Wrms into 8 ohms.
The Stereophile measurements show that it starts to clip above 300W into 8 ohms from which we can assume it actually has +/- 70V voltage rails. That would mean the bias current is 5.4A dc and a class A output current of 3.8Arms leading to a maximum class A power of 118W into 8 ohms.
The XA200 is able to maintain its maximum output voltage into 4 ohms to provide 400W but because we know the bias current is 3.8Arms 'only' the first 3.8*3.8*4 = 58W is class A power, the rest is class AB.Regards,
13DoW
Edits: 03/10/19 03/10/19 03/10/19
In other words, by your description even the big push-pull amplifiers that claim to be Class A power amplifiers are actually Class AB amplifiers under some loading conditions. That's very interesting! I appreciate your explanation.
Thanks,
John Elison
John,
I was a little premature in my last reply. I thought the Pass XA200.8 a push-pull amplifier but it is not (though it sort of is)
The Pass Aleph series was marketed as single-ended class A except the designs had a neat twist to dynamically adjust the output stage bias current so the amplifier output current swing could be 2x the DC bias current (rather than 1x the DC bias current for a pure single-ended class A design). The subsequent Pass X series are balanced push-pull amplifiers but I forgot that the XA series is a combination of X and Aleph, so the XA200.8 does not have the push-pull output of the X series. Instead it will have an Aleph-like class A bias current that could provide a dynamic output current twice the bias. Arguably that extra range is equivalent to class AB but if we re-visit my previous calculations with twice the maximum output current:
200W into 8 ohms sets the voltage rails as +/- 60V
If the DC power is 760W then the bias current could be as much as 760/120 = 6.3A. If the Aleph-like output can supply double the DC bias that is 9Arms (2*6.3/SQRT(2)) which is more than enough to provide 200W/8ohms in class A and provide 324W/4ohms in class A. Stereophile's measurement shows that the XA200.8 can supply 400W/4ohms so there is still a deficit in my calculations but this second round is much closer to matching the product description.
Regards,
13DoW
Regardless, I still think you're right about the initial Class A power being a constant in a Class AB amplifier even though the voltage at the speaker terminals remains the same with varying load.
Thanks,
John Elison
Power amplifiers are really voltage amplifiers and, ideally, will keep a constant output voltage swing by sourcing/sinking more and more current as the load resistance reduces to zero.
Real-world amplifiers have finite output current capability. They might be able to double output current from 8 ohm to 4 ohm but less likely to double again into 2 ohm. At some load resistance level the output current will saturate and the amplifier will no longer maintain the desired output voltage.
A true class A amplifier has a fixed maximum output current. It will try to maintain a constant output voltage as the load resistance reduces but once it has reached its current maximum the voltage will fall.
A class AB amplifier will try to maintain the output voltage using its class A current but when that is no longer enough it moves into class B to provide the extra current to maintain the output voltage. The output power level at which it moves from class A to class B is inversely proportional to load resistance.
(All real-world amplifiers have finite output resistance, neglected above, that will reduce output voltage linearly with output current. For solid-state amplifiers this can be very small, especially if feedback is used.)
13DoW
Thanks for the various explanations -- perhaps this illuminates why the manufacturers' ratings of the Class A power capabilities of their amps are sometimes vague and divergent, with some claiming 20 watts Class A in an A/B amp.
In another thread here, several years ago, John Curl said that 10 amps is about all that can be achieved in Class A, within an A/B amp, and even that is very difficult to do.
My question was prompted by the introduction of the New Halo A21+ amp, which seems to be a tweaked version of the A21, which I own.
In a review of the A21+, which has a 20% higher power rating than the A21, the reviewer quotes Richard Schram as saying that the A21+ delivers 6 watts of class A power.
In several reviews of the original A21, the class A power is stated variously as 7.5, 8, and "about 10" watts.
If any of those are correct regarding the A21, I wondered why the more powerful (and pretty much more everything, according the specs) A21+ would produce a lower Class A rating.
Which led to my curiosity about the 4 vs 8 ohms output.
With a class B or class A/B the power rating sets the maximum voltage swing. For your example of 200W into 8 ohms that is an output voltage of 40Vrms (as John pointed out), or 56V peak. Let's assume the amplifier voltage rails are +/-60V. Now, if it is also to have 4W of class A into 8ohms we know the DC current in the output stage must be at least 1A. So that is a standing power dissipation of 2*60V*1A = 120W. That is a fair amount of heat to get rid of and heat sinks are expensive. If the A21+ is 20% more powerful the voltage rails must be 20% higher and the standing dissipation would be 20% higher too. If Parasound are using same metalwork (likely) then they must decrease the output stage DC bias by 20% to keep the standing heat dissipation the same. That's my guess.
Regards,
13DoW
The Duke is correct. Class A has constant current flowing through the amplifying devices. Power is IxIxR, so if speaker impedance is halved, power is halved.
The class A/B power is determined by V^2/R so when R halves the power doubles. The class A region will be limited by the DC bias current in the output stage. As P= I^2*R, as R halves the class A power in the load also halves and you will go into class AB at half the voltage swing you did in the 8 ohm case.i.e. For 4Wrms into 8 ohms that is an rms current of 0.707A and a peak current of +/- 1A. So the amp output stage must be biased at 1A, at least. When the load is 4 ohms the voltage that +/- 0.707Arms can generate across the load is +/- 2.82 Vrms.
P = V^2/R so the class A power is 2.82*2.82/4 = 2Wrms
Or we can arrived at the same destination using P = I^2*R = 0.707*0.707*4 = 2W.Regards
13DoW
Edits: 03/09/19
nt
Solid state power amplifiers are actually voltage amplifiers because the voltage at their speaker terminals remains constant regardless of speaker load. Therefore, if an amplifier's voltage remains constant under full power with either a 4-ohm or 8-ohm speaker, its voltage for any lower power output will also remain constant.
Power = (voltage) 2 / resistance
Therefore:
200 = 40 2 / 8
and
400 = 40 2 / 4
and
4 = 5.657 2 / 8
and
8 = 5.657 2 / 4
Yes, Class A power will also double when resistance is halved.
Best regards,
John Elison
"Yes, Class A power will also double when resistance is halved."
He's right - but there's another case to consider. The amp will produce a maximum of 4 watts into eight ohms before moving away from class A operation, but in some cases, such as dynamic or sliding bias arrangements that some amps use, the 4 ohm class A to class B switchover could be at a higher - or lower - voltage / power level.AND since most of use don't use audio amplifiers to drive resistors, and prefer to drive speakers - the impedance of the speaker will matter. Impedance is a vector quantity, not a scalar like resistance. That is to say, a scalar quantity like resistance has only one characteristic value- the magnitude of resistance. A vector quantity like impedance has both magnitude AND phase angle (and they vary with frequency.) So you'd need to know both of these values at the frequency of interest to predict the Ohm's law current/voltage/power interaction with the amplifier. Makes it more complex, and makes answering the "if 4 watts at 8 ohms how many watts at 4 ohms" a bit more complex, because in addition to 8 or 4 ohms we need to know phase angle to calculate the power.
For those to whom vector quantities are not immediately intuitive, here's an analogy from weather. Temperature - when it comes to a weather report - is a scalar value. It's 35°f in Chicago right now. Just a value for the temperature magnitude is all you really need to know here.
But WIND VELOCITY- velocity is a vector quantity it has both MAGNITUDE and DIRECTION. To say that wind is 10 mph is not sufficient to characterize it. You need to also specify direction. Right now wind here is 8 MPH from due north. So 8 mph from 0°. Impedance is like that, too. 4.7 ohms at 182° is the correct way of specifying impedance at a given frequency. The impedance phase angle is a measure of how the current waveform either leads or lags the voltage waveform, and this has an impact on the current that will be drawn from the amplifier at a given voltage, and hence the power level.
Now add some kind of dynamic bias scheme into the amplifier topology and you have quite a complex situation to evaluate when you ask the question "if my modern Krell amplifier puts out 32 class A watts into my (~8 ohm) Joseph Audio Pulsar speakers, how much would it put out into my (4 ohm) Wilson Audio Sophia speakers? -
BUT a rule of thumb is - generally safe to say- twice as much power, or near twice as much.
Edits: 03/10/19 03/10/19
Thanks! I see where I was wrong about the Class A power rating doubling when resistance on the speaker terminals is halved. I realize that speakers provide a complex impedance rather than a simple resistive load to a power amplifer, but I don't think it's necessary to delve into that for the purposes of this discussion.
Thanks again,
John Elison
SS devices are actually current switching devices
Vacuum tubes are voltage amplifiers.
Happy Listening
No, SS amps are voltage amplifiers. A given voltage in gives a given voltage out, depending on the gain of the amplifier. Power (current) varies depending on the load.
I think I just proved that solid state amplifiers are voltage amplifiers by the fact that their voltage remains constant under a varying load.Tube amplifier, on the other hand, are power amplifiers because their power remains relatively constant under a varying load.
Better luck next time,
John Elison
Edits: 03/12/19
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