Help! 50w Class A or 150w Class AB debate

[Toobstheway]

Hi all,
I've got a newbie question that has been sparked up at work and maybe I can get a helpful answer. Well what actually started it is I just picked up a Forte Model 6 amp and from what I have been reading I should send it out to Jon Soderberg and have it converted over to run in Class A mode, which would bring it down to around 50 watts@8 ohms. Here's the delima, I have a set of NHT's that are rated at 86db and pretty much need a lot of power to run, and I feel that I should leave the amp alone and run it at it's original AB 150 watt configuration, but the guys at work are saying that the 50 watts in Class A mode is pretty much the same as 150 watts in AB mode, same load of course. I disagree. I am waiting on the speakers to arrive so I cant say how the amp will sound, but the overall consensus is that this amp sounds so-so until modified.

[bikehorn]

i don't know if being runin Class A would make it "louder" than 150 in AB, however, i'm pretty sure that 50 watts would be plenty of power to drive them...depends how loud you want to get i guess

[Apogee]

Do the conversion.

The soundstage will be much larger and the definition will be better.

The only place you really use power is the bass anyway. I would guess you'd find that rarely (if ever) will you be playing it loud enough to run out of gas. If it's running out of gas on a regular basis, you'll be buying new speakers anyway.

To me, I would much rather have it not get quite as loud and sound better all of the time rather than the other way around.

You won't be disappointed.

Don't worry, it will still get LOUD because 50w class A is still lots of power.

Just my $.02

Good luck,

[unclejed613]

it will only be class A up to the first 10 or 20 watts, and class AB after that unless you make drastic changes to the bias circuit and power supply. it will also run very hot, so you may want to add a fan or two. basically what you are diong by modifying the amp in class A is biasing the output transistors into heavy common mode conduction to eliminate the crossover switching between the positive and negative halves of the output stage. this means you will get 50 watts out in speaker drive and 50 or more watts in the form of heat. most class AB amps aren't designed with enough surface are on the heatsinks to handle this amount of heat on a continuous basis. also the driver circuitry was designed to be most linear in the range it operates in during class AB operation, and by shifting the operating point, you could send some of the driver transistors into nonlinear portions of their operating parameters, causing more distortion than you eliminate.

[AndrewT]

Hi,
until you can get corroboration of the effectiveness of this conversion, steer clear.

The difference of 50 to 150W is just 4db and should not make much difference in sound output/volume. Maximum volume will be about 103db vs 107db @ 1m from each, making about 2 or 3db less at the listening position from a pair.

The ClassA could sound superb but I would not risk my money on this until I was sure it would be good value. More research!

[Piercarlo]

Originally posted by AndrewT
The ClassA could sound superb but I would not risk my money on this until I was sure it would be good value. More research!

In my personal view, properly biased class AB amplifiers aren't really distinguishable from class A amplifiers, especially when the bias is the only difference between them and the rest of amplifiier circuit is just substantially the same (as in Self's design for example).

The real problem in class AB (with bipolars) is just mantain the "right" quiescent current to minimize crossovers distortion... an endlessy affair! :-(.
However in my practice i've found an interesting result: if crossover switching is confinated just to the output devices (i.e. the remaining of output stage - driver and eventual predriver is mantained in a continous non switching condition - that is not really a class A because the drivers, facing with the bases of output devices and hence with their HFE variations, works, from the current point of view, not very linearly), then crossover distortion, using output devices with good swicthing times and using a suitable amount of NFB, became effectively inaudible with components placed mainly in ultrasonic band, out of audio band.
In this case, if power devices are supplied from rails different from that dedicated to the remaining signal stage circuit (VAS and input stage), the crossover distortion became quite harmless (provided, obviously, of minimizing it with proper and *stable* bias).

In my own I'm not very favourable to solid state class A amplifier (class A *tube* amplifier is a totally different affair) because, in presence of consistent and continous current supplied by the PSU, the latter became of harsh design with great risk of having a "beatiful" class A amplifier which however sound not better than of a well designed class AB, due to a consistent increase of "garbages" generated by the PSU itself when solicitated by heavier than usual current demands (and garbages, with the usual simple rectifier + heavy capacitor scheme are prettly guaranteed). You may found yourself with a "warm" amplifier which is "warm" just for the power dissipated and not for sound accuracy.

All imho, of course.

Hi
Piercarlo

[carlmart]

Originally posted by Piercarlo
In my own I'm not very favourable to solid state class A amplifier (class A *tube* amplifier is a totally different affair) because, in presence of consistent and continous current supplied by the PSU, the latter became of harsh design with great risk of having a "beatiful" class A amplifier which however sound not better than of a well designed class AB, due to a consistent increase of "garbages" generated by the PSU itself when solicitated by heavier than usual current demands (and garbages, with the usual simple rectifier + heavy capacitor scheme are prettly guaranteed). You may found yourself with a "warm" amplifier which is "warm" just for the power dissipated and not for sound accuracy.

You raise very good points. But they seem based on thinking the PSU is bad. As long as the transformer is big enough, capacitance is plenty and everything is well filtered, which is the basis of a good Class-A design (or should be), I don't see why it will necessarily sound harsh.

One thing that seems to work is build an AB amp PSU as if it was class-A.

I don't know what this Jon Soderberg mod consists of. Perhaps what he does is increasing the transformer's capacity and lowering the voltage, keeping the same actual size, adding more capacitance and dissipation. Perhaps adding more output transistors, to split taking care of heavier currents.

[Piercarlo]

Originally posted by carlmart


You raise very good points. But they seem based on thinking the PSU is bad. As long as the transformer is big enough, capacitance is plenty and everything is well filtered, which is the basis of a good Class-A design (or should be), I don't see why it will necessarily sound harsh.

Indeed is not *necessarily* so. It may happen if (in my opinion):

1) Class A amplifier is thought simply as a "class AB with increased quiescent current" (in the Self's mood as I guess from his book) with PSRR totally based on the NFB.

2) If PSU are thought simply as an "increased" class AB PSU (i.e "raw" PSU with just capacitor and "bulkity" of transformer increased).

3) If not previewed that PSU garbage are directly related with increased intensity of recharging current for the capacitive reservoir.

Some time ago I've undertaken a small study for building a moderate class A power amplifier (50 watt 8 Ohm, as required by a friend). On the amplifier side was not a great turmoil: a part from increased quiescent current and dissipation exigences, criteria of designing class A and class AB are, from my point of view, identical because a class AB *must* be designed as it was a class A, with the same requirements of immunity to supply garbages and noise, especially at high power. In this case the only significative difference is that a class A work *constantly* at high power level while a class AB work only occasionally at full power (except for professional cases where however other rules apply).

But whence i undertaken the study of "other side" of problem (the PSU) i realize almost immediately that was not a simple question of increasing continous current capabilities and forget the question.
First of all became quickly evident that the usually optional separate supply of power devices respect to the signal part of circuit, in class A design became *mandatory*, otherwise were no chance for the PSRR usually obtained from excess NFB to overcome the problem of heavy intermodulation due to iniected garbages in rail supply. This was counterweighted by the absence of *autointermodulation* by means of rail supply, which is usually present in all class AB design (for which separate supply of output devices is again a good way to walk, although for different reasons).
A second tip what become evident is that Class A PSU must be conceived, from physical point of view, as *entirely separate unit*, with it's own screening problems to be solved. Indeed, if a reasonable ripple has to be guaranteed some lightly unusual means must apply in PSU design for class A. These means may be passive (inductive filtering) or active (active filtering or also true power supply stabilizing) but however require a change in mental approach, first of all in considering PSU not more an "appendices" of the main equipment but a full circuit by it's own, with related odds.

That amplifier was not realized. I don't know as the "bigs" of audio have solved these problem (if they have solved something.... Often a good way for solving harsh to solve problems is ignoring it until a good day came - If came! - with a new idea about it...) but, from my point of view, these are problems for *true* engineers, not for an hobbyist. And require a care that qualifies them definitely not straightforward as usually retained the concern about PSU. A class A amplifier PSU require near the same care required in designing SMPS, with similar problems, for example, about the capacitors to be employed in (figure out a capacitor subject to continue recharging current of tens of ampere and you realize what I've told).

I hope are now clear the reasons of my perplexity about "solid state class A power amplifier". From my point of view simply is not art for my hands.

Hi
Piercarlo

[carlmart]

Once again, more good points.

Originally posted by Piercarlo
[B] It may happen if (in my opinion):

1) Class A amplifier is thought simply as a "class AB with increased quiescent current" (in the Self's mood as I guess from his book) with PSRR totally based on the NFB.

Self's opinions and points of view on class A and regulators have already taken quite a beating. So we better let them be. A class A amp is not a class-AB with increased current.

PSRR has to be built-in in the amp design whatever the class you are working on.

2) If PSU are thought simply as an "increased" class AB PSU (i.e "raw" PSU with just capacitor and "bulkity" of transformer increased).

Certainly not, but for class A you have to count in the increased current demands both from transformer and total capacity.

PSRR of the first stages is paramount, because they will modulate the output. So separate secondaries and/or diodes and/or capacitors and/or regulators are in order for them.

3) If not previewed that PSU garbage are directly related with increased intensity of recharging current for the capacitive reservoir.

This I think are minor if compared to the speaker loads the amp will have to see, which is quite usually disregarded or not considered so important.

My view is that of a hobbyist too, even if I am quite critical on what I listen. Things like good regulation, good filtering, good quality parts, local NFB and good AC power can go a very long way, AB or A, if well implemented.


Carlos

[Piercarlo]

This I think are minor if compared to the speaker loads the amp will have to see, which is quite usually disregarded or not considered so important.

Carlos [/B]

Ok, i make a little addenda valid for class A or high power class AB amplifier (or for any amplifier which has to cope with heavy current on its outputs).

When I was buzzing around the project of building a class A amplifier I realize another little but essential thing: above a couple of Ampere of circulating current, don't exist any kind of ground on PCB which may be considered "quite ideal". That was added as a pair to another vital consideration for "sound" of high power (solid state) amplifier: don't exist any electrolytic capacitors able to really "close" rail to ground in AC. ESR and ESL are ever present in enough quantity to "sound" of their own despite of how the design may be good for itself. I though what effective (and audible!) quality sound of the amplifier is mainly related to the capacitor quality than circuit quality because, in AC, supply reservoirs are ever in series with speakers.

This state of matter had led me to a drastic conclusion: single ended power amplifiers (with one speakers end connected to a passive ground as usual) is not the better choice if system has to drive the load directly without additional remedies able to override the ESR-ESL term of the series capacitors+speakers.

These remedies may be, in my opinion, of two kind:

1) Lightly the implmentatio of remote sensing (possibly at speakers cabinet plug) for feedback; that imply what in the connection to speakers must be provided a sensing connection with additional wiring (screened possibly) which carry back just the feedback signal only "cleaned" by any other influences due to heavy current routing. If you forgive the yoke, I've called this "the leashed dogspeaker system" because the purpose is just make speaker sounding, not barking.... ;-)

2) More drastic, the systematic use of bridge (and eventually "true differential") connection. In this arrangement, speaker see only TWO "warm" connection and *never* see directly any component of the power supply (or, at least, it see the PSU mediated from the PSRR of the system at *both end* of connection and not only on the active side of cable as common with normal single ended amplifiers).
This conceptually appear as a "fencing" the speaker system in a close electrical environment in which amplifier not only "drive" them but also "protect" them from undue influences of PSU.

This obviously, for cost reasons, cannot be implemented in normal consumer equipment but I wonder about the so called "esoteric" equipment where the revenues may allow better technical solution than usual "biggering" of PSU which appear to me fairly inconsistent (and coarse) from technical point of view, especially if compared to their sales price...

However the world is running so... ;-)

Hi
Piercarlo