Amp Differences? Class A/B vs. Class G

[Saturn5]

I'm looking at some Outlaw amps for my home theater and am considering either one of their 5 or 7 channel balanced 200W/ch amps or going with their 200W monoblocks. The multi-channel amps are Class A/B, and the monoblocks are a hybrid design that is Class A/B up to about 80W, then switches to a Class G design. They say the swich is inaudible - so I'll just take that as fact for now.

I'd like to get some opinions on the differences and advantages of one class over the other. Going with the monoblocks will let me add channels and speakers incrementally, which is nice, but would I be better off biting the bullet and going with the single big amp to start with?

My system currently:

Outlaw 970 pre/pro

RF-7 mains powered with a QSC PLX-2402

RC-7 center - currently not used until I get more amplifiers

I haven't added surrounds yet, but I'm looking at RS-62, RB-75, RS-7, even RF-5 or similar. Something that would match as close as possible to my Reference 7 stage. If I go with monoblocks, I can just add 1 for the center for now and then 2 more when I add surrounds, and 2 more later for rear surround. I also haven't decided if I'll keep the RF-7's on the QSC. I may move them to the Outlaws if I like how they sound.

I'm posting here because I know a number of us here have Outlaw equipment, but I didn't want to post on the Outlaw forum since everyone there will be pro-Outlaw. I'm not looking for a sales pitch - just some technical advise. []

[RockOn4Klipsch]

Borrowd for "Ray Garrison"

First, note that while class A, AB, B, C, and (I think) D are
recognized as defined, standard classifications and mean the same thing
no matter which manufacturer is using the term, other classes (H, G, K,
EIEIO) are not so universally defined, and someone's class "G" amp
might not be operating in the same "class" as someone else's class "G"
amp.

Having said that...

Class
A operation is where both devices conduct continuously for the entire
cycle of signal swing, or the bias current flows in the output devices
at all times. The key ingredient of class A operation is that both
devices are always on. There is no condition where one or the other is
turned off. Because of this, class A amplifiers are single-ended
designs with only one type polarityoutput devices. Class A is the most
inefficient of all power amplifier designs, averaging only around 20%.
Because of this, class A amplifiers are large, heavy and run very hot.
All this is due to the amplifier constantly operating at full power.The
positive effect of all this is that class A designs are inherently the
most linear, with the least amount of distortion.

Class B
operation is the opposite of class A. Both output devices are never
allowed to be on at the same time, or the bias is set so that current
flow in a specific output device is zero when not stimulated with an
input signal, i.e., the current in a specific output flows for one half
cycle. Thus each output device is on for exactly one half of a complete
sinusoidal signal cycle. Due to this operation, class B designs show
high efficiency but poor linearity around the crossover region. This is
due to the time it takes to turn one device off and the other device
on, which translates into extreme crossover distortion. Thus
restricting class B designs to power consumption critical applications,
e.g., battery operated equipment, such as 2-way radio and other
communications audio.

Class AB operation allows both devices
to be on at the same time (like in class A), but just barely. The
output bias is set so that current flows in a specific output device
appreciably more than a half cycle but less than the entire cycle. That
is, only a small amount of current is allowed to flow through both
devices, unlike the complete load current of class A designs, but
enough to keep each device operating so they respond instantly to input
voltage demands. Thus the inherent non-linearity of class B designs is
eliminated, without the gross inefficiencies of the class A design. It
is this combination of good efficiency (around 50%) with excellent
linearity that makes class AB the most popular audio amplifier design.

Class
AB plus B design involves two pairs of output devices: one pair
operates class AB while the other (slave) pair operates class B.

Class
D operation is switching, hence the term switching power amplifier.
Here the output devices are rapidly switched on and off at least twice
for each cycle. Since the output devices are either completely on or
completely off they do not theoretically dissipate any power.
Consequently class D operation is theoretically 100% efficient, but
this requires zero on-impedance switches with infinitely fast switching
times -- a product we're still waiting for; meanwhile designs do exist
with true efficiencies approaching 90%.

Class G operation
involves changing the power supply voltage from a lower level to a
higher level when larger output swings are required. There have been
several ways to do this. The simplest involves a single class AB output
stage that is connected to two power supply rails by a diode, or a
transistor switch. The design is such that for most musical program
material, the output stage is connected to the lower supply voltage,
and automatically switches to the higher rails for large signal peaks.
Another approach uses two class AB output stages, each connected to a
different power supply voltage, with the magnitude of the input signal
determining the signal path. Using two power supplies improves
efficiency enough to allow significantly more power for a given size
and weight. Class G is becoming common for pro audio designs.

Class
H operation takes the class G design one step further and actually
modulates the higher power supply voltage by the input signal. This
allows the power supply to track the audio input and provide just
enough voltage for optimum operation of the output devices. The
efficiency of class H is comparable to class G designs.

Music is art Audio is engineering

[JohnA]

If you are running RF-7, the diffrences are irrelevent. You will rarely exceed 2 watts and, if you want to keep your hearing, you will never exceed 80 watts.

Concentrate on amps that sounf good at low power. I like amps that have a high threshold between Class A and Class B, since most listening is normally below 1 watt.

[RockOn4Klipsch]

John is very much right, if you do the math in order to reach max at reference level of about 114 dB which is rare in movies you only need 16 watts on a speaker that had a 102dB effciency.

The switching that happens in class G in an A/B to G amp happens at a higher output which the A/B usually covers. So seldom would it actually switch to the G side. All amps are different but I think you usually get between 20-40 watts of class A power before it switches.

[DrWho]

John is very much right, if you do the math in order to reach max at reference level of about 114 dB which is rare in movies you only need 16 watts on a speaker that had a 102dB effciency.

That is only true when listening to a single frequency.

All source material (except test tones or some wierd new age) are going to consist of a whole heck of a lot of frequencies all at once. This shows up in the crest factor of the source material.

Just as a frame of reference, I could get a 50W amplifier to clip into a single Chorus II, yet measure about 90dBA from 1 meter when playing Blue Man Group. It's been a while so I don't remember the specific numbers, but the point is still valid.

[Saturn5]

Thanks for the info. I guess all that really matters is how that particular monoblock compares in sound to the multi-channel amp. I have found a couple reviews of Outlaw amps on the Outlaw forum, so I'll check them out as well. I may end up buying both so I can do an A-B comparison and keep the one I like better.

After all, that's the fun part, right? []

[russ69]

I have a personal preference for mono blocks but buying seven at a time can pound the wallet pretty hard. I think they can make good amps in any configuration and they can also make bad ones. Find an amp/amps in your price range that sounds good to you and you'll be OK. You might want to look at some Parasounds, they would sound very nice with your RF-7's and they can drive any loudspeaker load. My brother has JC-1's and they are very nice.

Thanx, Russ

[Ricci]

John is very much right, if you do the math in order to reach max at reference level of about 114 dB which is rare in movies you only need 16 watts on a speaker that had a 102dB effciency.

In addition to what DrWho said ...

Don't forget that the sensitivity measurement is usually in the midrange somewhere and that it is referenced to 1 meter from the speaker usually. Every time you double the distance from the speaker you generally lose 6db of SPL, so that if you sit 4 meters away which is not unusual you lose approximately 12db from that 102db sensitivity. Which puts you down in the 90db range. Still quite good when you consider that some inefficient bookshelf speakers will be as low as 74db at that point, but you can see where having a few extra watts for peaks is not a bad idea.

[RockOn4Klipsch]

Agree with DrWho and John, I didn't know about the distance equation as far as the loss of 6dB of SPL per 1 meter. Thanks, always learning new stuff, and usually enjoying the trials and errors.