Class A bias in amplifiers

[@12voltvids]

A true class A amplifier is a single ended output. 1 transistor, or 1 tube. The full waveform is reproduced by a single tube, or output transistor. Class B is a push pull design, where one output transistor or tube amplify the positive portion of the waveform and another for the negative half of the waveform. Class B are very poor in the distortion department due to the crossover distortion that occurs at every reversal of polarity in the AC audio signal. Class AB overcomes this by applying a small amount of bias to keep the transistors or tubes from going into cut off at idle. Depending on how much bias is added will determine how much of a waveform can be produced before the amplifier is driven back into a push pull design. Close enough explination?

[@marianneoelund2940]

No, you can build class A with a complementary output stage. It just requires a bias that is high enough that neither side ever turns completely off.

[@salvadorrodenas3071]

Keeping class A area is harder when more voltage, output voltage is set. So counterintuitively, a class A/B 100 watts amp will have more watts in class A that a 300 watts class-A/B. It is impractical to design a 300 watts pure class A due to the current and heat wasted. For the other hand, avoiding more than 100 watts of class A amplifiers keep it reasonable. Another thing to take into account is that the class A envelope is halved when loudspeaker impedance is halved too.

[@andershammer9307]

It used to be that low powered solid state amps sounded pleasant and sweet but high powered amps sounded harsh. Which isn't so true today. But in my experience the hotter an amp gets the warmer it sounds.

[@SamiyuruSenarathna]

Thank you. We enjoy technical explanations!

[@marianneoelund2940]

Doesn't qualify as a technical explanation, when lacking numerical examples.

[@reginaldmorton2162]

Why do amps that produce the same wattage (200watts) contain more transistors and are larger than cheaper versions that produce 200 watts? Does more transistors equate to more quality and better design?

[@user-tk3uy7jl9x]

Not necessarily... Some designs have a lot of auxiliary circuitry with transistors, that doesn't affect the signal directly. Also you can have the signal be produced by more than one transistor at the same time, to distribute the load and reduce what each transistor handles. Basically an amplifier with more transistors might mean that the circuit has more features, but not necessarily that it "sounds" better.

[@malmsneve2415]

it also seems to suggest a lot more control and consideration of parts if you go the discreet route. when you pick and choose every transistor you can be more certain the signal is being tonally effected the way you choose it to be.

[@gotham61]

Naim Audio amps are class B, as was David Berning’s old EA230 tube amp.

[@andershammer9307]

The EA230 was in interesting design but it sounded weird. Like the louder sounds were being inverted.

[@dornauge1995]

Cant be Class A Bias "guessed" by looking at the idle power consumption?

[@Geerladenlad]

How much output impedance do the BHK amps have?

[@j-man72b72]

Output Impedance 50Hz, 2.8VRMS

[@InsideOfMyOwnMind]

Ah go ahead and confuse us. Throw in some sliding bias and sliding rails and you're well on the way as there are so many ways to do it.

[@roimark358]

WHY YOUR DESIGN ENGINEERS CHOOSE CLASS AB AND NOT CLASS A? AND WHY YOU PUT HIGH BIAS CURRENT WHEN YOUR GETTING ONLY 1 WATT CLASS A? WHAT IS THE REAL DIFFERENCE BETWEEN CLASS A AND CLASS AB IN SOUND PERFORMANCE? I HOPE THIS CHANNEL ANSWERS TO QUESTIONS?

[@markw7303]

sooo what exactly is the class a bias in watts in bhk amp

[@marianneoelund2940]

A mono amp that can drive 300W into an 8 ohm load, with a quiescent power consumption of 70W as Paul mentioned, probably has an output stage bias current of 400mA or just under. That means it could stay in "class A range" when delivering peak currents of up to +/- 800mA. With an 8 ohm load, that means up to 2.5W. With a 4 ohm load, it's only half of that. But most amplifiers only have bias current in the 75-150mA range, which means the "class A range" is only around 1/10 W to 1/3 W into 8 ohms.

[@markw7303]

@@marianneoelund2940 iv heard otherwise but i dont argure the math. Only others i can find are avm and lux and acuphase of course

[@markw7303]

@@marianneoelund2940 my existing coincident 300b int amp pushing 18w of a Id likely go the current luxman that suposidely does 30 in a and 90 in ab

[@marianneoelund2940]

@@markw7303 Are those the manufacturers' claims, or measured by a third-party lab? You can push the "class A" number up by setting an unreasonably low limit for the off-side cutoff current. But in the end, it's all about avoiding the crossover notch, and bias current isn't the only factor; the range of "class A" operation won't necessarily tell you which amp sounds cleaner.

[@markw7303]

@@marianneoelund2940 i forgot coda which will bias any of 3 ways. I just havent been able to demo And really the best allround examle and the only manufacturer i belive numbers from. They are and oem for others such as sanders and legacy and will bias from 8 to 20 watts of class a

[@MarcelOoms]

If an AB amp is in class A, are both transistors of a pair conducting at the same time? If so, is that not a problem?

[@kevinroosa1315]

Yes, they are both conducting. In the transistor pair for the output, the NPN transistor is biased positively, whereas the PNP transistor is biased negatively. Both draw a small amount of current at all times, thus the range between the positive and negative bias for the transistor pair is considered a "Class A region." Once the input signal exceeds the bias voltage in the positive section of the wave, the NPN transistor begins to conduct from collector to emitter. In the negative section of the wave, the PNP transistor conducts from the emitter to collector. These two transistors operating in tandem is what creates the current swing necessary to drive a low-impedance load, like a loudspeaker.

[@kevinroosa1315]

Class A amplifiers generally only use NPN or n-Channel MOSFET output stages, and operate in a fundamentally different manner. The transistor(s) in this arrangement are typically biased in a way such that the entirety of the input signal is shifted into the positive operating region, and a large capacitor is used to block the resulting positive DC voltage from entering the load (loudspeaker).

[@MarcelOoms]

OK, thanks

[@christianholmstedt8770]

The design challenge is to get a smooth transition in the crossover between the transistors. A bad design will have large crossover distortion..

[@marianneoelund2940]

Kevin Roosa wrote, "Class A amplifiers generally only use NPN or n-Channel MOSFET output stages" Only preamplifier or driver stages are built single-ended. Practical power amplifier output stages, even class A, always have active devices for both output current polarities (push-pull), in order to provide acceptable efficiency. This will be true even if you have only one power supply polarity (unipolar supply) and a series DC blocking capacitor at the output, as you still need to drive signal current in two directions to the speaker. Granted, you could if you wanted to, build a class A power amp using single-ended output transistors, but this would require a low-resistance high-power resistor internally, to provide the other direction of current, and the efficiency would be abysmal. For example, if you made a class A amp like that with a unipolar supply, that can output 30W into an 8 ohm load, the power supply would need to be at least 90V and the optimum internal pull-down resistor is 8 ohms 300W with an output stage idle current of 5.6A. Quiescent power consumption is over 500W per channel, so you would probably want to set it in your fireplace and use it as a fire substitute to heat the entire first floor of your home. Using complementary output devices instead (still class A), the internal 8 ohm resistor goes away, the required power supply voltage is only 50V (still unipolar), output stage idle current is 1.375A and the quiescent power consumption is under 70W per channel. That's how commercial class A amps are designed. You can also use +/- 25V bipolar supplies instead, to get the same efficiency figure and toss out the DC blocking capacitor.

[@Bluephrog]

Sorry but I find your explanation very confusing. 3 simple drawings can explain these 3 classes.

[@gman76utube]

It’s relatively simple. Neither of the output transistors ever go into cutoff under any condition in the output waveform. Or the other way to state it is this, the output transistors are biased at a current that is higher than the load current. For example, if you have a maximum output voltage of 10v and a 10 ohm load, the max output current is 1A. A Class A output stage will be biased at just over 1A. Even so, an output stage that is biased at 500mA is still considered Class A if the output current is never greater than 500mA. And once the output current exceeds the steady state (idle) bias, this goes to the term “leaving class A”. Firstwatt has some articles on the topic.

[@marianneoelund2940]

@gman76ut Practical Class A amplifiers are more efficient than that. The idle current only needs to be half of the peak output current because the latter is the difference between the two output transistor currents. In your 10V 10 ohm example, output stage idle current only needs to be 500mA; as output voltage swings away from 0V, current in one side of the output stage rises, while current in the other side decreases by the same amount. At peak output, one side conducts 1A and the other side drops to near 0A, thus sending 1A to the load. A class AB amplifier with a 100mA idle current can provide up to +/- 200mA of current to the load in its class A region. (But that does not mean it will achieve the same low-distortion performance as a true class A amp does at the same power level.)