a few terms...
Home > Home Recording Forum > Gear Gab > a few terms...
Posted on May 10, 2005 12:08 pm
whosyourdaddy00
Hold 'Em Czar
Member Since: Dec 30, 2004
ok does anyone know what exactly "Class A" means? best i can figure is it's transformerless. but i know next to nothing about electronics. but i see this one often...also "rectify" has something to do with electricity, but again, i'm clueless....anyone know anything about these terms?
[ Back to Top ]
TadpuiI am not a crook's headMember
Since: Mar 14, 2003
May 10, 2005 06:29 pm I hear these terms a lot, especially associated with tube amps. From a bit of Googling, I see that to rectify is to convert from AC to DC power.
As for Class A, there seem to be as many definitions as there are people to write them...all of them are a bit over my head, but they all seem to relate to the efficiency of the way the amp uses the power supply. Class A is the least efficient. That's about all I can gather...
zekthedeadcowEat Spam before it eats YOU!!!Member
Since: May 11, 2002
May 10, 2005 09:25 pm According to Sun Micro:
"In the United States, the Federal Communications Commission (FCC) governs the levels of electromagnetic emissions from a digital device. Electromagnetic emissions can interfere with radio and television transmission. To reduce the risk of harmful interference, the FCC has established requirements for manufacturers of digital devices.
A manufacturer of a digital device must test and label the product to inform an end-user of the maximum emission level from the product when used in accordance with its instructions. The FCC has established two classes of levels, Class A and Class B. A system that meets the FCC Class A requirements may be marketed for use in an industrial or a commercial area. A system that meets the more stringent FCC Class B requirements may be marketed for use in a residential area in addition to use in an industrial or a commercial area. "
Electromagnetically Class A actually is worse then Class B and according to the FCC should not be used in a residential area...
oh the joy of marketing spin :) nice to know that building a home studio is technically illegal as it violates zoning laws :)
TadpuiI am not a crook's headMember
Since: Mar 14, 2003
May 10, 2005 09:35 pm I think that's a different "Class A" than I was reading about. Here's a snippet of the things that I was reading:
Amplifier classes:
Audio power amplifiers are classified primarily by the design of the output stage. Classification is based on the amount of time the output devices operate during each cycle of signal swing. Also defined in terms of output bias current, (the amount of current flowing in the output devices with no signal).
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.
pjkPrince CZAR-mingMember
Since: Apr 08, 2004
May 10, 2005 10:00 pm rectify is indeed turning AC power into DC power. The power transformer (in a tube amp) will turn 120V AC power into much higher AC, say 250 to maybe up to 750 Volts AC. The tube amp uses DC power to activate the tubes, etc, while passing the AC signal (which comes from your sound source) out to the speakers.
Diodes can be used to rectify AC to DC, in several different ways, and tubes can be used to rectify AC to DC. Tube rectifiers will give a voltage drop as the current draw goes up, referred to as 'sag' which can be sought after in amp builders.
Class A amplifiers transmit both sides of the sine wave all the time. It can be thought of like a lumberjack's saw. If there's only one person, then he/she =) will have to create big force from little force all the time, both on the plus (+) side of the stroke, as well as on the negative (-) side of the stroke. This is how single output tube tube amps work. They're creating amplified signal all the time, for the positive part of the sine wave, as well as the negative side of the sine wave. (class A tube amps can have two or more output tubes as well, they're both doing the exact same thing). These are the least efficient, as mentioned, but some characteristics can be favorable.
Class AB is where one lumberjack is doing most of the pushing when he can, and a little pulling, while the other person is pushing most when he can, plus doing a little pulling.
Class AB, is where there's a phase splitter tube that divides the positive swing of the sine wave to one (or more) output tubes, and send the negative swing of the sine wave to the other tube(s). This isn't totally one or the other (like in class B) but is kinda like a fade from one track to another: as the positive part of the wave comes closer to unity, the other tube starts kicking in, then the wave transferes to the negative part of the swing, the other tubes take on all the amplifying task. This way there's much more efficiency, but much less of the crossover distortion (which is very undesirable)
transformerless is not indicitave of class A, as they can exist together or not, they don't depend on or not of each other.
transformerless usually refers to the AC being rectified from the wall power. One would turn Ac to DC directly from the 120 VAC, and not using a transformer to 'transform' it to a higher voltage. This is a dangerous idea, as the power transformer also isolates the user from the wall power and possible shocks. I won't go into the full detail (on which I'm somewhat sketchy) but safe to say it's to be avoided unless you're very knowledgeable, or foolhearty.
Even in a system where you're rectifying 120 vac, it's way safer to use a 1:1 isolation transformer to give safe AC out of the other side of the iso transformer.
hope that helps
May 11, 2005 05:00 pm wow guys EXCELLENT REPLIES!!!
so a "tripple rectifier" amp, goes from AC to DC back and fourth a few times...wierd.
thanks all
TadpuiI am not a crook's headMember
Since: Mar 14, 2003
May 11, 2005 05:25 pm Actually, I think that a triple or dual rectifier has a rectifier for each channel. The Mesa Boogie Triple Rectifier has 3 channels, and 1 rectifier per channel.
May 11, 2005 05:27 pm ahhhhhhhh!!! now THAT makes more sense...
cheers
chris
jmailjimmie neutronMember
Since: Feb 14, 2005
May 11, 2005 07:43 pm You guys need to talk to Walter...
TadpuiI am not a crook's headMember
Since: Mar 14, 2003
May 12, 2005 12:23 am Bwahahahahahaha
