| AudioREVIEW's Forum Archives - Amplifier-Preamplifier |
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|   More Class A, Class B questions | jbangelfish
Nov 13, 2003 9:05 AM | | I have read through the descriptions of A and B and as I said, I have a very basic understanding. My Parasound amps claim to be class A A/B. When I read through the manual, it states that they are pure class A up to >6 watts and AB thereafter. Sounds like they are overstating something to make them look better. I also mistakenly stated earlier that their distortion rose to .5 at full power but the book says .05 at full power, better than I thought and .009 at typical levels. So the question is: is there class A A/B or is it pretty much bs or at least irrelevant?
I also am curious where the old Crown DC300A fits into all of this. It is quite heavy, uses large heat sinks and runs warm but this alone doesn't tell what it is. Someone mentioned amps which used a DC approach and this is apparently what they did but I won't claim to understand how. I understand the difference in AC and DC and DC seems to make sense. Some phono preamps run off of DC current with the use of batteries and they are supposed to be very good. It also makes sense that most all preamps are class A and fairly irrelevant but how would the use of direct current affect this?
As for the old Crown amp, it is called a direct current amplifier, hence the DC300 designation. Can anyone explain the effect that this would have on it's performance or what class it would fit into? I'm guessing that it could still be class A, AB or B or even some other class as DC alone may not determine the classification.
I believe that Crown joined up with Harmon Kardon and no longer makes DC amps unless the Studio Reference model is DC. They seem to have gotten away from home stereo and are more into PA equipment. The DC300A was often used this way but was not it's intended use. Would like to hear from anyone who has a better understanding of these things (which could be almost anyone) than myself. Thanks,
Bill |
| Did you read my posting about A and AB amplfiers? | skeptic
Nov 13, 2003 10:55 AM | | I gave a description of an analogy of A, B, and AB amplifiers some time ago. In it I said that a class A amplifier was like a man with a hand saw trying to cut down a tree by himself. A class B amplifier was like two men with a saw twice as long with a handle at each end, working together but that for each stroke in one direction, only one of the men did the work and the other took over in the opposite direction. I said that there was a problem when they changed directions because they would hit a snag but in an analogy to class AB, the one who was about to stop, exerts just a little bit of extra work in the opposite direction to make the transition smooth.
Your Parasound amplifier apparantly operates as both a class A and Class AB amplifier having the advantages of both. It has no crossover notch distortion because at low power in the small signal region, it uses just one device which doesn't have to switch which is the advantage of class A amplifiers. When power output reaches 6 watts, it switches to class AB (on evey half cycle) using a pair or multiple pairs of other devices as in a class AB amplifier. The trick of course is to make the tranisition smooth. It uses this class A amplifier as a substitute for what most other amplifiers try to do through clever biasing.
The Crown DC300 is a class AB amplifier. It is called DC because it is DC coupled presumably from input to output. This means that there are no de-coupling devices in the signal path from one stage to the next or from the output to the speaker. It does this by clever choice of circuit topology and bias voltages. Theoretically it can have a flat frequency response down to zero hertz where other amplifiers which are ac coupled would at some point have a rolloff and would not respond at all to a dc input. |
| Yes, I did | jbangelfish
Nov 13, 2003 3:11 PM | | And while I understand the basic concept, I don't claim to fully understand it. Having no engineering background whatsoever, schematics and technical circuit explanations are rather difficult for me to grasp. I'm working on it. Your analagy of the push pull theory was fine but I guess I understood it to that point anyway. Class A works all in one direction and this makes sense to me as a clean and uninterrupted source of power.
The Parasound appears to be a fairly well made and well designed amp. I'll compare it to pure Class A sometime just for kicks. Specs are comparable to the Crown but is more powerful. They do things in different ways but the outcome seems reasonably close.
Glad to hear what the Crown is regarding A and B, it is what I suspected. The DC approach still seems to be a good one and I wonder how many other amps have done this. It remains to be one of the cleanest amps that I have ever heard.
Bill |
| Yes, I did | skeptic
Nov 13, 2003 5:43 PM | | Crown equipment was originally designed for professional use. They made some of the finest 2 and 4 track tape recorders ever. They were used in radio stations, recording studios, and on location. They were exceptionally rugged and performed beautifully. They were at consumer audio shows but were always in the millionaire category of high end equipment. I never met anyone who actually owned one. They caught the attention of the consumer world at the 1968 NY consumer audio show by introducing the DC300. It was an eye opener, by far the most powerful amplifier on the market. And solid state. And completely reliable, and no distortion, and flat response from DC to who knows where. That was 35 years ago and the rugged design, outstanding manufacture, and excellent performance still give most of today's best equipment a run for their money. Talk about value.
The DC300 is clearly a class AB (push pull) amplifier. Parasound's amp which you described sounds just like a class A amplifier up to six watts at which point it apparantly switches somehow to becoming a class AB amplifier. I can only guess not having seen the schematic. This is a very esoteric subject and has to do with the way the tubes or transistors in the output stage are biased (powered) when there is no input signal present. It is very arcane for someone who is not technical and it totally unnecessary to understand. Just be aware that most (but not all) class A amplifiers are low powered and tubes, not transistors. BTW, except for the output stage and sometimes the driver (previous) stage, all amplifier stages in power amps and preamps are class A.
Either amplifier will do a fine job powering the AR9s. Let us know if you hear any difference and which one you like better for the job. |
| Yes, I did | RGA
Nov 13, 2003 6:28 PM | | Many amps, including my Sugden are DC coupled class A/B designs. My amp is not frequency limitied as many home audio amps have been over the years so if you have the speakers you are not going to want for bass.
The preamp section of any amplifier "sound" is far more important than the power amp section anyway assuming the power amp is competant. All preamps are class A and yet don't all sound alike.
The issue is not the A or A A/B or A/B it's how well it's implemented.
The new Marantz amp is a 25 watt class A amp but also has a switch you can flip to turn it into a class A/B amplifier with more power. A nice feature in a way because it will depend on your speaker's demands. The VAST MAJORITY of speakers made over the last 14 years will never ever require more than half of the 25 watts to play at moderate to loud levels. However some speakers do and if you want to crank them to defening levels then the added power is nice. Unfortunately, it has been my experience that if an amp "NEED" more than 25 watts to sound good then it probably needs close to 200watts per channel for a marked improvement. And the Marantz does not have that much on tap But then it's pretty inexpensive so what do you expect. Supposed to sound very good to boot.
Most integrated amplifiers come with a jack to add a power amp. So I like the idea of a high quality sounding pure class A or very good class A/B amp. Then if you want power because you buy a beast of a speaker to drive - you have the facility to add on the power amp without losing the sound quality.
So if you have an 8 ohm 90db speaker and a 25 watt amp and you sell your speakers for 79db speaker 1ohm speaker then you can always add your 2kilowatt monoblocks to the integrated. I assume you would also need a step-up trenasformer to your home to get 4000 watts from the wall socket and still run the rest of your house.
Personally, I'd rather add two subwoofers(assuming very good ones) - and hey they come with the optimal power amp included. |
| Thanks guys | jbangelfish
Nov 15, 2003 7:48 AM | | I'll have some interesting tests to play with after picking up the AR9's. I will probably sell one of the Parasound amps as they are rated at 385wpc in 4 ohm. Having several amplifier and preamplifier choices, sorting out all the differences might take me awhile. Eventually, I'm sure I'll just settle for the Parasound amp and preamp but comparing them to the old Crown and Accuphase can't hurt. I'm getting old and lazy and I do like the remote operated preamps out of convenience. The Parasound pre seems to be pretty well made anyway. The PLD 2000 appears to have been their top of the line.
I have to admit that I'm intrigued by class A amps. A friend of mine who also has AR9's runs his with a Son of Ampzilla amp. He says that it is rated at around 80wpc of class A power. From what I've read, this should equate to about 300wpc or more of AB power. A comparison of his to my Parasound would be interesting. Will post our test findings. |
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