Class A/B net current when push and pull are on

[crunchynut]

I think that I should apologise to crunchynut for hijacking his discussion. I have seen other people do it and I hadn't realised how easy it is to wander off topic.

Sorry.

I reckon it's ok for subjects to wander once the initial query has been answered, and I think Crunchnut had got what they were after.

I did get my question answered, thank you. But also thanks @jimmy b for considering OP

[James Perrett]

Most but not all Wonks and AFAIK no one has every stuck one on an AP rig?

I'm sure that they are routinely measured on an AP rig or similar - indeed you'll see the AP logo on many of the published performance figures from Hypex who make good class D amplifiers. Here are some independent measurements on a Hypex NC500 kit which is very similar to several commercial amplifiers (you'll have to dig around on the website to find the test methodology though).

https://www.audiosciencereview.com/foru ... ave%20both.

I'm sure that you'll find similar information for Icepower and the other good amplifier manufacturers.

[Jimmy B]

A most interesting review. It appears that, tested alone, this class-D amp is more than "good enough". I wonder if the problem of radiation to other connected devices and cables has been solved. I can imagine that it could be, as SMPSs no longer seem to be a major problem. Perhaps meeting FCC or EU regulations for EMC means that we don't need to worry about this?

[Jimmy B]

There are two types of class B (Selfs) output stages. The emitter follower and the Complimentary Feedback Pair (CFP) The former needs a bias current of around 100 to 150mA which means a 50W per ch amp would still dissipate some 10W per channel. The CFP requires a tenth that current and so are much more efficient.

That's interesting. I never thought to worry about this before, when electricity was "free".

[Nazard]

Wonks wrote: ↑Thu May 25, 2023 7:57 pm
However, just about all active monitors use class D amps these days.

Most but not all Wonks and AFAIK no one has every stuck one on an AP rig?

All ATC active monitors, HiFi and Pro, have 'grounded source A/B Mosfet amplifiers, including their subwoofers. Which makes sense, considering they have developed a range of excellent Mosfet A/B power amps for domestic and professional use over the years, plus other electronics.

[ef37a]

Wonks wrote: ↑Thu May 25, 2023 7:57 pm
However, just about all active monitors use class D amps these days.

Most but not all Wonks and AFAIK no one has every stuck one on an AP rig?

All ATC active monitors, HiFi and Pro, have 'grounded source A/B Mosfet amplifiers, including their subwoofers. Which makes sense, considering they have developed a range of excellent Mosfet A/B power amps for domestic and professional use over the years, plus other electronics.

Different strokes Naz' Engineers find different solutions to the same problem and there will be benefits and disadvantages involved.

MOSFETs have the advantage of a partial negative temperature coefficient (they CAN suffer thermal runaway, just less likely than for bipolars) They are also immune to secondary area breakdown but of course Joe Pub' can destroy anything! According to Self their lower gm (current to voltage ratio, a term borrowed from valves) makes them less linear and a bipolar design can always achieve ultimately lower distortion than MOSFETS. Their hugely wide bandwidth makes them more prone to parasitic oscillation. They are also more expensive at any given amplifier power rating.

Do not also discount the influence of the marketing dept! At one time, to proudly splash "New High Power MOSFET OutputStage" had a certain cache' with parts of the audio world. Mr Self is one of only few 'sane' voices in that world.

Dave.

[Hugh Robjohns]

...a bipolar design can always achieve ultimately lower distortion than MOSFETS.

Maybe, but how low is low enough? The 300W ATC grounded-source mosfet amp measures 0.002% THD. I'd be very happy with that. The Bryston 4B SST2 Bipolar amp I'm listening to now is spec'd at 0.005% THD.

At one time, to proudly splash "New High Power MOSFET OutputStage" had a certain cache' with parts of the audio world.

That would be the time when BJTs weren't really up to the job, then.

[ef37a]

"That would be the time when BJTs weren't really up to the job, then."

Not sure what that means Hugh? I am not saying the two amplifiers you mentioned are not very fine indeed and one can argue the need for lower and lower distortion but 0.005% is not that startling, when you look in Doug's book. One of his designs does not even reach 0.005% even at 20kHz.

Self also points out that his amplifiers give their performance with stock parts. Fine though Brystons are they are expensive and I understand the output stage needs selected transistors?

Dave.

[Hugh Robjohns]

Not sure what that means Hugh?

Just that BJT amps couldn't deliver the 500W and 1000W power levels PAs wanted cost effectively that MOSFETS could do in the 70s and early 80s. BJTs have improved since then...

Fine though Brystons are they are expensive and I understand the output stage needs selected transistors?

Expensive, yes, but built like outhouses, rated very conservatively, and warrantied for 20 years.

Who cares if they use selected transistors — I've no idea if they do or not because none of mine have ever failed.

My first 4B NRB amp was traded in at 22 years of age for a 4B SST2 about 5 years ago, and AFAIK that amp is still in daily use at PMCs service department, never having had its lid taken off!

The good Mr Self is a perplexing mix of common sense, production pragmatism, and bonkers perfectionism-just-because-he-can. I cite his 24 op-amp balanced stereo line input stage.

[ef37a]

Well yes, that line amp is bit of an overkill but, marketing again, if you want to sell a device with balanced line inputs with lower noise than a pretty simple unbalanced input, that's what you have to do!
I also found it a fascinating design process. Other's MMV! As you say, a product achieved by throwing a lot of money at it, just like Bryston?

High power 80's bipolar amps? Weren't Amcron, then Crown using BJTs?

We could go on Hugh but all I am saying is that with the going of guys like Hugh Ford, B.J.King etc and magazines with the mandate to open up active monitors and test the amps..We shall never know!

Dave (aka "Grumpy old ^%%r!)

[Hugh Robjohns]

High power 80's bipolar amps? Weren't Amcron, then Crown using BJTs?

Yes, the famous Amcron DC300A.... pushing out 155 of the finest American Watts per channel...on a good day...

[ef37a]

High power 80's bipolar amps? Weren't Amcron, then Crown using BJTs?

Yes, the famous Amcron DC300A.... pushing out 155 of the finest American Watts per channel...on a good day...

They did do bigger ones.

Dave.

[Hugh Robjohns]

In the 70s ?

[Wonks]

Amcron was simply the European trade name for Crown PA products to avoid confusion with an existing European budget hi-fi maker (as Crown also made hi-fi products).

The classic DC-300. Made from 1967-2007. 9 versions in all.

[merlyn]

... According to Self their lower gm (current to voltage ratio, a term borrowed from valves) makes them less linear and a bipolar design can always achieve ultimately lower distortion than MOSFETS. ...

gm is transconductance? That's the gain of a voltage controlled current source. Volts in, Amps out, I/V = 1/R. 1/R is conductance, more correctly admittance for AC.

I think the point about MOSFETS was they were twice as expensive, but not twice as good.

[ef37a]

... According to Self their lower gm (current to voltage ratio, a term borrowed from valves) makes them less linear and a bipolar design can always achieve ultimately lower distortion than MOSFETS. ...

gm is transconductance? That's the gain of a voltage controlled current source. Volts in, Amps out, I/V = 1/R. 1/R is conductance, more correctly admittance for AC.

I think the point about MOSFETS was they were twice as expensive, but not twice as good.

Just an old valve jockey Merl' going by "mA per volt". Self goes to great pains to make the point that bipolar transistors are VOLTAGE controlled devices and base current is just an unfortunate by product of a less than perfect device!

Have to admit to being caught up in the hype myself. I still have a 150W* Maplin kit MOSFET amp that did sterling duty as a bass guitar amp and it still sits on a shelf in my living room connected to a gash 8" speaker for bass G when the mood takes me.

*Maybe into 4 Ohms, more like 80 into 8R.

Dave.

[Nazard]

The Benchmark AHB2 is a fascinating design, some details in this link:

https://benchmarkmedia.com/blogs/applic ... lification

And goes some way to addressing the OP's query about push-pull crossover transitions.

And specs for continuous average output power are: < 0.0003 % THD+N at full rated power, 20 Hz to 20 kHz

One that is certainly on my wish list.............

[merlyn]

So if gm is constant it doesn't affect linearity. If gm varies with the amplitude of the signal, then that would introduce distortion.

For the record I don't think there is anything wrong with MOSFETs. The kind of linearity Mr. Self is after ... probably not necessary for a bass guitar amp.

[ef37a]

So if gm is constant it doesn't affect linearity. If gm varies with the amplitude of the signal, then that would introduce distortion.

For the record I don't think there is anything wrong with MOSFETs. The kind of linearity Mr. Self is after ... probably not necessary for a bass guitar amp.

Ha! Did not build it for it's hi fi qualities (though it is pretty good. Very low noise) but for the claimed indestructibility.

Thing about gm, the output devices are essentially emitter/source followers so there is more 'gain' in the feedback loop = lower distortion. In the front end Self uses emitter resistors to linearize that stage as well.

I am pretty sure gm varies with drain/source current? Certainly does in BJTs but that has improved greatly since the days of the 2N3055!

Dave.