posted
I was wondering, what defines the class of an amplifier? I have seen quite a bit about class D amps lately, and as of a month ago I had never so much as heard of them. I was wondering what the benefits are of a D class amp vs. an A/B or other class amp. Just kinda curious!
--RougeDude-- "If it's not loud enough, your probably already broke!"
[This message has been edited by RougeDude (edited 01-04-2000).]
posted
If I remember the way Tech explained it, the different classes are determined by the was the amp operates, produces power, and how it runs
------------------ spydermann
Don't get caught in the WEB Gimmie some bass baby keep you RF and others, MTX for me Is that me in the top 10 now?? Do you smell what the BASS is cooking...
Posts: 6799 | From: cincinnati, oh, USA | Registered: Jun 1999
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posted
Amplifier class is determined, i'm pretty sure, by the interaction between the signal section and the power supply, and the way the powersupply transistors operate.. For detailed info, wait until the unavoidable occurs (IE, Eli47 sinks his teeth into this post :-) )
ShadowStar
Posts: 2578 | From: Somewhere In the Northeast | Registered: May 1999
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posted
I am neither Tech nor Eli but I answered a similar post acouple months ago and didn't get corrected too much so here goes
Class A: The output transistors are always on. All of the output devices reproduce the whole output waveform. The efficiency is bad (<25%) since the outputs are always on
Class B: There is transistor for the positive part of the output waveform, and one for the negative ouput. This is more efficient since half of the output devices are off at any given time. The problem with Class B is that it takes 0.7 Volts to turn on a transistor so with very small inputs neither the positive or negative ouputs will turn on. Which brings us to
Class AB: Same as class B except the output transistrs are biased in such a way that for small inputs both the positive and negative ouputs are both on. However when there is a mismatch between the pos and neg ouputs there will be what is called crossover distortion. Some Class AB amps have more than others. At very small output levels (less than a couple watts) this type will operate like a class A. The efficency is around 60%.
Class D: This is completely different than the previous types. With class A and AB the outputs just amplify the input signal. With class D the input signal is sent to a Pulse Width Modulation chip. This PWM chip outputs asquare wave with pulse widths varying depending on the input voltage. The PWM square wave is sent to the output devices. This turns the outputs on and off with varying widths. This output is then sent through a filter made of capacitors and inductors that returns the audio signal. This filter is what determines how well the output resembles the input. When the PWM signal is high the outputs are fully on (no voltage across output), when the PWM signal is low the outputs are fully off(no current through ouput). Since power is voltage times current theorectically there is no power dissipated by the output devices. Most Class D amps are for sub frequencies only and have efficiencies above 75%. Class D can be used for full range audio frequencies but the PWM frequency has to be raised and the filter must be designed properly. I have heard that Infinity had porblems with RFI in their full range Class D's a couple years back.
In the cases above the amplifier will have a power supply with a rail voltage of say +/- 30 Volts. In some cases the power supply will have two voltages, say +/- 30 Volts and +/- 20 Volts. The amp will use the lower rail voltage for lower power operation, and use the upper rail for high power operation. This will increase efficiency since it is lowering the voltage across the output device, and thus lowering the power dissipated by the output device.
A twist on this type is the BASH circuitry used by Blade, The power supply is basically a Class D amp whose ouput is always a couple volts more than the output needed. The output devices are a conventional class AB configuration, but dissipate much less power since there is only a couple extra volts across them.
A new kid on the block is Class T, I haven't done much research but I think it is like a Class D with a variable PWM frequency. It should have great efficiency with much better SQ than conventional Class D amps. I think Techman has looked into these chips and can shed more light on the subject than me.
Sorry that this is so long, and if there are any questions or corrections let me know. Any spelling errors must be tolerated, that took me a long time and I don't want to spellcheck myself.
posted
Well put JC2. ShadowStar and Audiophyle, I would have thought that with your vast knowledgebase, and the experience youv'e both had on this forum, that at least one of you could have come up with some information. I find JC2's answer very well put, nothing I can add or intend to correct, but like ALL members I can read and learn too.
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E.C. Wuz here
Posts: 1057 | From: Vancouver, British Columbia, Canada 'eh | Registered: May 1999
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posted
Eli- The problem is, I've answered this question a couple times before, and I'm lazy- Actually, I've been pretty busy lately doing some car stuff, school stuff and catching up on a semester of sleep and junk food, So I dropped by to make sure there weren't any problems but couldn't really do much answerin' :-)
ShadowStar
Posts: 2578 | From: Somewhere In the Northeast | Registered: May 1999
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posted
Well, I have to say that I too am too lazy to type that out... I have it now in a document for easy call up in the future.. I will also be updating my webpage soon with faq's like this, I need a few more though...
------------------ STILL waiting to hear a GOOD pair of HLCD's!!
If it dont fit, force it... if it breaks, it needed to be replaced anyway...
quote:Originally posted by RougeDude: I was wondering, what defines the class of an amplifier? I have seen quite a bit about class D amps lately, and as of a month ago I had never so much as heard of them. I was wondering what the benefits are of a D class amp vs. an A/B or other class amp. Just kinda curious!
--RougeDude-- "If it's not loud enough, your probably already broke!"
[This message has been edited by RougeDude (edited 01-04-2000).]
Well, RougeDude, does that answer it? Or is more info needed? I have a definition of Amplifier Classes on my web page, your'e welcome to check it out here; http://eli47.tripod.com/page2.html .
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E.C. Wuz here
Posts: 1057 | From: Vancouver, British Columbia, Canada 'eh | Registered: May 1999
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posted
Most car amps have a PWM power supply that operates in a manner very similar to a Class D output. They take the 12 volt input and switch MosFETs on and off at around 50kHz and send that through a transformer to get a higher voltage, that output voltage can be regulated by varying the pulse widths sent to the MosFETs. I haven't ever taken a good look at a large Class D but I assume they use the same kind of power supply.
Posts: 2575 | From: GA | Registered: May 1999
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posted
First, good job jc2. I think this is the second time you caught this before me... hehe Second, shermbot... Do you remember more of this you were told... From what I've seen the power supply is still the same old regulated or unregulated (manu- facturers choice) dc to dc converters. Just ask'in
'Where there's a will there's a wall' 'Would you like a little SQ with your SPL?'
Posts: 926 | From: Sugarland, Tx | Registered: May 1999
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