Class A amps without vacuum tubes?

Not knowing anything about power amps or electronics in general I am wondering why a Class A power amplifier can’t be made without vacuum tubes. Is it that solid state components can’t take the heat? I see by definition that Class A consumes 200% of its rated power output at idle so maybe that’s the reason?

I understand that they are in full power mode all of the time so there is no delay when asked to produce output power up to the rated maximum. I just dont know why it cant be done without vacuum tubes.

Thanks in advance for any enlightenment

From our friends at Google:


Does Class A amplifier sound better?

Class A design is the least efficient but has the highest sound fidelity . Class B design is a little more efficient, but full of distortion. Class AB design offers power efficiency and good sound. Class D design has the highest efficiency but isn’t quite as high-fidelity.


What does class A in an amplifier?

Class A. A Class-A amplifier is one in which both output stages of the device are constantly on at full power . Because both stages are constantly on, Class A is considered to be the least efficient of power amplifier designs, with an average efficiency of about 20% (50% at best, theoretically).


What is pure class A amplifier?

The “purity” of Class A designs has been at issue in the last few years, with “pure” Class A being loosely defined as an idling heat dissipation of more than twice the maximum amplifier output . For a 100 watt amplifier, this would be 200 watts out of the wall on a constant basis.


What is the main advantage of Class A amplifier?

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Advantages of Class A Amplifier

It has high fidelity because of the output exact replica of an input signal . It has improved high-frequency response because the active device is ON full time, i.e. no time is required to turn on the device.


Class A amplifiers

A class A amplifier is biassed so that it conducts over the whole of the cycle of the waveform. It conducts all of the time, even for very small signals, or when no signal is present.

The Class A amplifier is inherently the most linear form of amplifier, and it is typically biassed to ensure that the output from the device itself, before it is passed through a coupling capacitor or transformer, sits at half the rail voltage, enabling voltage excursions equally either side of this central point. This means that the largest signal can be accommodated before it hits either the top or bottom voltage rail.

Normally a class A amplifier will start to become non-linear as the signal approaches either voltage rail, so operation is normally kept away from this situation.

For the amplifier to operate correctly in its class A condition, the no signal current in the output stage must be equal to or greater than the maximum load current for the peak of any signal.

As the output device is always conducting this current represents a loss of power in the amplifier. In fact the maximum theoretical efficiency that a class A amp can achieve is 50% efficiency with inductive output coupling or just 25% with capacitive coupling. In practice the actual figures obtained are much less than this for a variety of reasons including circuit losses and the fact that waveforms do not normally remain at their maximum values, where the maximum efficiency levels are achieved.

Accordingly, the Class A amplifier provides a linear output with the lowest distortion, but it also has the lowest efficiency level.

Class B amplifiers

A class B amplifier is biassed so that it conducts over half the waveform. By using two amplifiers, each conducting our half the waveform, the complete signal can be covered.

To achieve this, two active devices are used and input waveform is split so that one active device conducts during half of an input cycle, the other during the other half. The two halves are summed at the amplifier output to reconstruct the complete waveform.

At times, class B amplifiers called “push-pull,” because the outputs of the active devices have a 180° phase relationship. However this term is used less widely these days - it tended to be very common when vacuum tubes / thermionic valves were used and in recent years the term has fallen into disuse.

The efficiency is much higher, but the class B amplifier suffers from what is termed cross-over distortion, where one half of the amplifier turns off and the other comes into play. This results from non-linearities which occur close to the changeover point where one device is turning on and the other is turning off. This point is notoriously non-linear, and the distortion is particularly noticeable for low level signals where the non-linear section of the curve represents a much larger portion of the overall signal.

Although the maximum theoretical efficiency of a class B amplifier is 78.5%, typical efficiency levels are much lower.

Class AB amplifiers

As might be expected a Class AB amplifier falls between Class A and Class B. It seeks to overcome the cross-over distortion by slightly turning on the transistors so that they conduct for slightly more than half the cycle and the two devices overlap by a small amount during the switch-on / switch-off phase, thereby overcoming the crossover distortion.

This approach means that the amplifier sacrifices a certain amount of potential efficiency for better linearity - there is a much smoother transition at the crossover point of the output signal. In this way, Class AB amplifiers sacrifice some of the efficiency for lower distortion. Accordingly class AB is a much better option where a compromise between efficiency and linearity is needed.

Classes AB1 & AB2
Thermionic valves or vacuum tubes were widely used for high power audio and RF linear amplifiers. To save cost, weight and power consumption, amplifiers were run in class AB, and two amplifier sub-classes were often mentioned: Class AB1 and AB2. These sub-classes are applicable to only thermionic or vacuum tube technology as they refer to the way in which the grid was biassed:

  • Class AB1: Class AB1 is where the grid is more negatively biased than it is in class A. In Class AB1, the valve is biassed so that no grid current flows. This class of amplifier also gives lower distortion than one running in class AB2.
  • Class AB2: Class AB2 is where the grid is often more negatively biased than in AB1, also the size of the input signal is often larger. In this class grid current flows during part of the positive input half-cycle. It is normal practice for the Class AB2 grid bias point to be closer to cut-off than occurs in Class AB1, and Class AB2 gives a greater power output.

Amplifier Classes D to T

There is a variety of different amplifier classes which tend to be based upon switching techniques rather than using analogue approaches.

  • Class D Amplifier: A Class D audio amplifier utilizes switching technology within the amplifier. As the output devices are either on or off, Class-D amplifiers can theoretically reach efficiency levels of 100%. In reality the actual levels attained are less, but nevertheless the efficiency levels achieved are very much higher than the other analogue classes.

One of the first class D amplifiers for audio use was introduced by Sinclair in the UK around 1964. Although the concept was good in theory, the amplifier did not work particularly well, and when it did, the amplifier tended to cause large amounts of interference to local radio and television sets as EMC precautions were not normally applied to equipment at this time.

  • Class G Amplifier: Class G is a form of amplifier that uses multiple power supplies rather than just a single supply. For low level signals a low voltage supply is used, but as the signal level increases, so a high voltage supply is utilized. This is gradually brought in to action up to full rated power output as required. This gives a very efficient design as additional power is only used when it is actually required. The change both e higher voltage supply can be achieved without detriment to the output signal fidelity. In this way, the amplifier is able to provide both low levels of distortion, whilst also providing high levels of efficiency. This approach can be complex to design from scratch, but if engineered correctly, it can work well. Fortunately the difficulty of design can be reduced if one of the many audio ICs that use Class G is used.

There are very many more amplifier operational classes available to the designer these days. Modern silicon technology has opened up many more doors, but despite this, the basic three amplifier classes of class, Class B, and Class C, with the derivative Class AB which is a cross between Class A and B are still the most widely used.



As this has appeared on April 2nd, I’ll assume your question is a serious one.
There are SS Class A amps everywhere.
For example, see Pass Labs.


As Ron mentioned, all the Pass Labs XA series are SS class A. Gryphon also makes well known SS class A amps. They do have a distinct sound IMO (in part due to the harmonics) and for me it’s the easiest to hear a holographic image with depth.


Hey Baldy. Not sure where you heard that Class A amps can’t be made with sold state. That is most certainly not the case. Look at Nelson Pass’s Aleph series or even my own design of the Genesis Stealth. Class A and solid state.

The way to look at the power consumption is like this. We’ll use a mono block example to keep it simple. A 100 watt class A amplifier is constantly drawing 200 watts from the wall socket (ignoring the losses of the power supply etc. which makes that number greater) at idle.

That 200 watts must be dissipated in the heat sinks because it’s at this point a big heater.

If we were then to deliver the amp’s full 100 watts of potential into the speaker, the heat of the amplifier would be reduced in half! This is because 100 watts is heating the speaker, the constant class A bias of the amp is heating the amplifier in its always on state.

So, at full power the class A amp has its lowest heat.


Well that didn’t take too long to resolve.
Thanks to all for the help in understanding.

It was nice to see the actual differences between A, The different AB types along with the rest. The nuts and bolts end of the gear is what I understand the most. The mechanicals are easy for me to understand but the circuitry not so much. The last time I did any electronics stuff was in the '70’s putting some Heathkits together.

OK, now why aren’t SS class A amps more popular at the top of the heap?


Inefficiency and heat IMO.

Seeing as heat and inefficiency are basic to the design but the Class A fidelity is so sought after that a solid state Class A amp could be made to the highest standards and have the Class A sound without having to deal with 100 year old vacuum tube designs and reliability issues.

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@Baldy I personally use a Class H SS amplifier from the 80’s. The great thing about Class H (which is a lot like a Class G, having 2 voltage rails, one high and one low) is it can also be driven in Class A for much of the power requirements for your speaker. (the first 30 or so watts, which is as much as 90% of the music content) The great part about this, is one can choose to allow it to remain in Class AB which greatly improves the efficiency, or select the much less efficient Class A but it creates a lot more heat. The best part of this all is that this selection also brings with it well over 3dB of headroom. (that’s more than twice the available rated power)

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Interesting. I saw the other classes mentioned but didn’t read much about them. I did see the Class D amps are constantly switching on and off as power is requested and that seems to be the sticking point with high fidelity and Class D.
The Class G and H also sounds interesting. These are good subjects to read up on during the early morning hours.

@Baldy The real Achilles heel to Class D operation is the speed in which MOSFET transistors can switch on and off. (too slow) This causes large amounts of TIM (transient intermodulation distortion) which is best described as harshness. Borrowing from the RF space the GaN-FET transistor transient speeds can now be increased significantly, removing the TIM issue allowing Class D to rival Class A performance outputs. But as with everything, GaN-FET is expensive and MOSFET is cheap. (as well as GaN-FET’s availability)

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Thanks for the explanation.
So it appears as though the newest high speed GaN-FET transistors and some high surface area radiators / heat sinks could be used to make a “pure class A” transistorized amplifier?

I see the GaN-FET class D amps are also getting high praise at the moment.

@Baldy The best part of Class D w/GaN-FET is that the efficiency is so high, large heat sinks are not required in contrast to say, Class AB. GaN-FET opens the window for designers to create a very high efficiency, exceedingly high quality output amp with a very small package. You don’t need Class A, Class D w/GaN-FET can bring performance that rivals the benefits of audio quality of a Class A. Think of it like this, Class A is push/pull like analog, Class D is like DSD with very fast on/off

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Nice. Thanks again for the great explanations.

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Just get yourself a JC5 or a pear :grin: of JC1+s
JC5 is a distillation of 2 JC1+ in 1 chassis for a
ton of $$$$ less $5999.00 vs 8999.00 x 2 and
still be grinning ear to ear… :grinning:

Best wishes Baldy


The JC1+ is on my very short list of possible power amps to try before I die.
The other is a Pass XA160.8 which I didn’t realize was transistorized Class A until this discussion today. :grin:

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I see that the GaN is very close to ideal. Even a mechanic like me can understand that graph.

Boulder amplifiers are also (mostly) Class A and transistor. I believe their integrated amp is AB.

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Thanks. The industry even at the very top end is a lot bigger than I think it could be and still survive.
I’m glad to see that all of these different high end manufacturers can offer such a varied product line and still be profitable. It bodes well for new product R&D.

Well Baldy…aka “Looney Tunes” your avatar…
Both Audio Advisor and Music Direct offer a trade
up-in program…

The trade up from my former Parasound A21 to
JC5 made acquiring my JC5 very doable…
The trade in value for my A21 was better than
I thought possible.

You could go JC5 then move up to JC1+…

JC5 is one very powerful yet sweet nuanced
musical amp. Delivering 1st 12 watts out of 400
in Class A…With my music levels peaking 80db ish
seldom would my JC5 move out of Class A with my
Focal Aria 948s sensitivity at 92.5dbs …great music
all the while.

The asking price for the JC5 is all to sweet to ignore
for such a great amp.

Best wishes

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