"Class A" nonsense

[Steve Conner]

Mooreamps: What you said about bias points is true enough. But a single-ended Class-B amp is unusable for audio, so I stand by what I said.

Bob: I think you have the operation of your beloved '64 AC30 wrong. This is what's supposed to happen in a push-pull Class-A amp:

1: The tubes must be set up such that their idle current is exactly half of the maximum current they can supply with the grids pushed as far positive as the driver can take them. This is achieved by adjusting load impedance, screen voltage, screen resistance and so on.

2: The push-pull drive now increases the plate current of one tube as it decreases the other. If our two tubes were idling at 48mA, then at the positive peak of the sine wave, one of them might conduct 40mA and the other 56mA. At the negative peak, the roles would reverse: 56mA and 40mA.

3: When the amp is producing its maximum output, then at waveform peaks one tube is passing 96mA and the other is exactly at the point of cutoff, passing 0. Any more drive results in clipping, because the tubes are fully on and fully off.

Note that at all times the sum of plate currents is 96mA. This is one way of recognizing a Class-A amp: the supply current doesn't change from idle to full output.

If we set the tubes up differently, such that their idle current was less than half of the maximum possible current, then we're in Class-AB. When one tube is cut off, the other one can still go further on.

For instance, a 50-watt Marshall (or a Vox AC50 ) idles at 34mA per tube, for 68mA total idle current. But it'll draw over 300mA when producing its rated 50 watts. At waveform peaks, one tube is fully on and passing about 450mA, while the other one is completely off.

The above discussion concerns unclipped power output only. All tube amps self-bias into a different class if you overdrive them hard enough.

The TPS61165 is for a white LED backlight driver board I'm building. I got a demo board for the chip, hooked the power up backwards, and blew it. I figured since it was already blown, I had nothing to lose by trying to change the chip out.

[Merlinb]

To decide if an amp is class A, you need only ask a simple question:
If a device goes into cut-off, does the output power continue to increase?

If the answer is no then we have class A. This is why single-ended amps can only be class A (or C, which doesn't really apply to us). As soon as it goes into cut-off (or grid-current limiting) the output signal stops increasing, we cannot squeeze any more out of it.

If the answer is yes then we have class AB or B. One device may be in cut-off, but another device continues to provide output power. This is why Class AB or B amp always need at least two devices.

[balijukka]

this requires that the output is not clipped, won't it.

jukka

[Rhodesplyr]

Here's a jewel from Rick Campbell :

A couple more definitions for the list:

"True Class A" -- Same as "Class A" but costs twice as much.

"Pure Class A" -- Same as "True Class A" but costs ten times as much and contains components made of Unobtanium.

jukka

I appreciate all the responses, but I think I like this one the best.

In my opinion, few end users are ever really going to know what "Class A" operation of a circuit is, much less why they should want it. If it weren't mentioned, they wouldn't notice. What irritates me is seeing the term turned into a marketing device. It certainly makes far more difference what kind of output transformer is used or what kind of speaker is hooked to it.

It reminds me of the THD figure wars among 1970s Japanese receivers or the Spinal Tap joke about the amp with a volume knob that goes up to 11.

Most of the classic Hi-Fi amps like Marantz 8Bs are Class AB1, and I think McIntosh amps approach Class B. It never stopped them from sounding good.

David

[Steve Conner]

I guess I agree. I recently rebuilt this old hi-fi amp, and it's very much AB1: it operates its EL34s in ultralinear mode with 400V B+ and 40mA idle current, for a measured power output of 30W/channel.

But to me it sounds just great. If I wanted low THD, I'd have used a solid-state amp instead.

[jmaf]

2 - In a true "class A" amp the current swing is almost non-existent ( current through the output tube(s) is almost constant ), while in an AC30 the current swing through the output tubes is significant and very visible.

Robert, I think your entire post was right, up until that point. Voltage swing is a consequence of tube current flowing through the output transformer. If you have zero current swing then you have a silent amp, no?

[hasserl]

...ah, not really true, because almost ALL musical instrument amplifiers are rated with around 5% THD because of the "nature of the (rock-n-roll) beast" and the fact that 5% THD is the amount that is "...barely perceptible..." to most listeners.

...so, the wording "...maximum undistored power output..." cannot really apply to musical amplifiers, which is our subject. They (manufacturers) know there's 5% THD in the mix, which lets them: (a) claim slightly more output power and (b) account for the manner in which most (but certainly not all) amplifiers are actually used and operated.

...(tongue-in-cheek observation: ah, but that is a very good kind of 5% THD in the hands of most blues players!)

But just because manufacturers fudge on the power rating doesn't justify changing the description.

And by that undistorted standard, there would be plenty of guitar amps that would have to have a zero watt rating, as there is no level at which the signal is undistorted.

I'm not aware of any, even the mighty 5150 has some level of undistorted output. It may generate distortion in the preamp fairly early, but in that case use the clean channel.

But that's a good point, how do you set the power rating of an amp that distorts the signal purposely in the preamp? Should it be the max undistorted power output as measured at the power amp only?

[Rhodesplyr]

But that's a good point, how do you set the power rating of an amp that distorts the signal purposely in the preamp? Should it be the max undistorted power output as measured at the power amp only?

I'm not sure even maximum undistorted power output works for me, though I do think the definition should be generally confined to the output stage.

Given something like two output tubes in a standard push-pull arrangement, the circuit would be rated for a certain amount of power output at the point where both tubes were still conducting, but one was very close to cutoff, while the other was drawing almost 100% of the B+. The reason I say that is because I think one of the advantages of Class A is that it makes minimal instantaneous current demands on the power supply. However, when one tube starts drawing more current than both tubes at idle, the operating points begin to shift, and demands are made on the energy storage of the power supply.

Take, for example, the Leslie 32H amp. Stock, it has four 5881s in parallel push-pull, and all share one 75 Ohm cathode resistor--with no cathode bypass capacitor. Up to a certain point, there is little/no AC on the cathodes because when one side's going positive, the other side is going negative. The sum is zero--assuming matched tubes....

However, when it crosses into AB1, suddenly, the presence or absence of a cathode bypass capacitor becomes significant because it tends to stabilize the cathode voltage and reduce the appearance of negative feedback across the cathode resistor. In fact, I sometimes add a cathode bypass capacitor to this amp, and by choosing the value carefully, I can tune how it behaves at that point. By making the cathode bypass cap (& the B+ storage) larger, I can extend the clean power out of the amp--to a point.

A "Class A" amp, to my way of thinking, shouldn't cross that line where a cathode bypass capacitor is needed (in push-pull), but as others have pointed out, it would be tricky to impose some kind of brick wall to prevent a circuit designed for Class A operation being driven beyond that.

I think understanding what happens when an amp moves from Class A to AB1 is important for techs and designers, but not to players out to buy an amp. If the amp is a head, I'd rather see that it has multiple output taps for different speaker impedances than the claim that it's "pure Class A."

[Robert M. Martinelli]

@ Chuck

Some 25 W RMS clean ( undistorted ), some 34 W when fully "cranked".

@ Steve

I have re-read my posts and can't find anything wrong, ( with the exception of my English, still in need to be improved ) the way I defined the class and the operating conditions of my AC30 seems OK to me, the output tubes are on ( signal-wise ) for more than 180 degrees ( Pi ) but appreciably less than 2*Pi, and there is a significant current swing ( unlike in class A amps ) both these facts place the AC30 in "AB1 territory" IMHO.

BTW, congratulations on your sight/soldering ability! - At the moment I have problems in properly soldering 2N3055s, let alone something smaller....

Cheers

Bob

[Robert M. Martinelli]

Robert, I think your entire post was right, up until that point. Voltage swing is a consequence of tube current flowing through the output transformer. If you have zero current swing then you have a silent amp, no?

Hi jmaf,
Maybe due to my bad English, I expressed myself in a wrong manner....I meant that in a class A amp the current draw does not change ( only ideally, it changes a bit in reality ) between "idle" and "working" conditions.

In class "AB1" the current swing is way more significant and always very evident.

My apologies, I'm working hard to improve my English, but it seems I'm miserably failing.....

Cheers

Bob

[Merlinb]

this requires that the output is not clipped, won't it.

jukka

Yep, and that's the only universally logical way to define class of operation.

[balijukka]

Yep, and that's the only universally logical way to define class of operation.

That's the way my teachers taught me, and how I thought for many years.
If it clips, it's a clipper, not an amplifier.

But since they (damned buggers) dug up that IRE thing, and then I found the same in Reich's book, and then I even found this one from one site. :

Electrical Engineering Glossary Definition for Class A
Glossary Term: Class A

Definition

The simplest type of amplifier, class A amplifiers are those in which the output transistors conduct (i.e. do not fully turn off) irrespective of the output signal waveform. This type of amplifier is typically associated with high linearity but low efficiency.


I don't mind anymore if someone uses hazy terminology as long as I understand what they mean. And if I don't, I just ask clarification.

IMHO Marketing people use whatever they like if it sells. The dumb ones are the ones that buy. Unfortunately geetarr players are very closely related to the bass players who are famous for their rapier like intelligence.

jukka

[Merlinb]

Electrical Engineering Glossary Definition for Class A
Glossary Term: Class A

Definition

The simplest type of amplifier, class A amplifiers are those in which the output transistors conduct (i.e. do not fully turn off) irrespective of the output signal waveform. This type of amplifier is typically associated with high linearity but low efficiency.

That definition has not been thought through, and makes little sense. Just about every class A amp ever made would have to be reclassified as not-class A on the grounds that they could potentially be driven to cut-off. For audio purposes only sine-wave exitation is a universally valid parameter, due to its relationship with power (rms) output.

[hasserl]

Here's a couple of thoughts from Aiken on the topic:

A class A amplifier is defined as one which is biased to a point where plate current in all the output devices flows for the entire 360 degrees of an input cycle, at the full, unclipped output of the amplifier.

(emphasis mine)

The people who say a class AB amp is "class A at lower volumes" are technically wrong, but for the right reasons. If you were to define class A as being only conduction for a full 360 degree phase angle, you would be correct. However, there is more to the definition of amplifier classes than that.

The defining factor in a determining whether or not an amplifier is class A, class AB, or class B *has* to be made at the full output before clipping, otherwise, the class definitions have no meaning whatsoever. It is indeed, a very black and white thing

As you can see, there is indeed such a thing as a "true class AB" amplifier, just as there is a "true class A" amplifier, and the class definitions are not at all ambiguous, except to those who don't understand them, or choose to ignore them for marketing advantage.

One more thing: What if you push the class A or class AB amplifier into clipping? Does it then become a class AB/ B, C, or D amplifier? No, of course not. It is simply the same class amplifier it was to begin with, but driven into clipping. A class A amplifier driven to clipping is still a class A amplifier by definition. This is why amplifier classes are defined the way they are. Otherwise, the class designations would have no meaning. Any amplifier can be driven beyond it's limits into a fully-clipped square wave output (unless it is limited), but that doesn't make it a class D switching amplifier, now does it?

[tekhed7]

To decide if an amp is class A, you need only ask a simple question:
If a device goes into cut-off, does the output power continue to increase?

If the answer is no then we have class A. This is why single-ended amps can only be class A (or C, which doesn't really apply to us). As soon as it goes into cut-off (or grid-current limiting) the output signal stops increasing, we cannot squeeze any more out of it.

If the answer is yes then we have class AB or B. One device may be in cut-off, but another device continues to provide output power. This is why Class AB or B amp always need at least two devices.

I think many here seem to forget that it's more that an amp OUTPUT STAGE can be "capable" of operating in a certain "Class" rather than BEING a certain class amp.
Any Fender Champ 6V6 will operate in Class A mode when the guitar is mildly strummed, but if you were to push the output STAGE so hard that it's cut off almost 1/2 the time, then it's running nearly Class B.
But since tube Output Stages, unlike transistor ones, don't really have a clean cutoff point, in terms of plate current vs. grid voltage, the boundries get fuzzy.
The point of running Class A vs., say, AB2 is you won't have any ["cross-over"] distortion due to the handoff of the signal between the side that's pushin' & the one that's pullin'. But that gets tough when your often pushing the power supplies to their limit as you push the guitar amp into clipping.
The only way to totally avoid "cross-over"... as I see... it is to go buy an Ovation...
or an old Martin (acoustic).