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Marantz PM7200 Class A

 

John Ph
Unregistered guest
I've just bought a Marantz PM7200 integrated amplifier which boasts to have class A operation at 25W. However, I've tried to listen over and over again, with different sorts of music, but cannot see any difference between the normal class AB and class A.

Does anyone have experience about this amp? Maybe I need to wait till it breaks in for some time...
 

nout
Unregistered guest
I own this amp too. Untill now I've never used class a (and I probaply never will) simply because I don't hear the difference.
Maybe, just maybe when listening through headphones the treble appears to be smoother than it already is.

Who am I kidding?

But be assured: Marantz PM 7200 is an excellent amp with a very smooth and at the same time a very revealing sound.
Who needs class a when the sound is velvet-like, warm and deep already?
 

John Ph
Unregistered guest
Thanks for sharing. If someone says there is a clear distinction between AB and A in this amp then I'll train my ear more or change the room setting or change other device to achieve it.

I heard that in class A, you will hear more accurate sound especially for instrumentals or vocals.
 

Sean Reid
Unregistered guest
Marantz is such a liar when they claim class A in this amp. Marketing gimmicks only!
 

nout
Unregistered guest
To me it's a gimmick indeed, but there are plenty people who hear the differences between the two modes. I wouldn't call them liars.
 

J. Vigne
Unregistered guest

The audible difference in a Class A amp and one running in Class AB will vary with many other things invloved in the reproduction of music. Most importantly is the way the class distinction is managed. A Class A amplifier has no cut off point as the signal moves from a positive signal to a negative signal flow. This leaves the output devices on at all times. In a textbook example of a Class AB amplifier, the outputs are switched on and off as the signal is in the positive or negative swing through its signal path. Ideally this is done at the exact instant the signal passes from +1, through 0, to -1. If you look at that text book description of the switch from on to off, or + to -, you will see what is termed notch distortion, a glitch in the waveform that respresents the output device not being able to switch on fast enough, or soon enough, to accurately follow the signal's true path as a continuous flow. When this glitch is pronounced in its variation from accuracy, you will hear it as a distinct type of distortion that is, due to the complexity of the waveform that is music, more a fatiguing quality than a distinct sound. In its worst cast the music will become lifeless, harsh and flat, very much like the effect of too much negative feedback, which exists in most AB amps and is virtually absent from Class A designs. The benefits of Class A operation are a less fatiguing sound that opens up the music to sound less electronic, that whole "illuminating" the music idea. It is very much like what you hear as the sound of a single ended, as opposed to a push-pull, amplifier. Obviously a single ended design, with no positive and negative devices (the push-pull parts), can only be a Class A design.

The text books, however, do not represemt what you are going to find in real world amplifiers. The class of operation depends on the bias that is applied to the output devices. The more the output is biased to remain on, the more it operates in Class A. At the first few watts of most modern amplifers (that would be used for home audio purposes), the outputs are on constantly. This gives a smoother sound to those first few, important watts. As the amplifier is asked to produce more power, the outputs are biased so they run more and more into a Class AB operation. There are distinctions made between Class AB1 and Class AB2 that represent how long the outputs remain in Class A operation until they switch to a more efficient Class AB. This switch from one class to another should be imperceptible if the amplifier is well designed. So, in point of fact, everyone is likely to own a Class A amplifier if they never run more than those first few watts where the outputs are left on at all times. Certainly all small signal devices such as pre amps and output circuits in CD's and so forth are always run in Class A. Any advertising that claims Class A operation of a pre amp as a virtue not shared by competitors is mostly dishonest.

There are too many considerations in how a circuit is designed to simply make Class A a necessarily better sounding amplifier. Whether you can hear the difference between the same amplifier running in Class A vs. Class AB can say as much about how the amplifier handles Class AB as whether its claims of Class A operation hold water. My understanding of the Marantz amp is it runs hot at all times which is often the case for a Class AB amp that is heavily biased towards Class A operation even though it does have a switch point in the signal's traverse. This could be one reason you hear little, if any, change in the sound of the amplifier. It's overall sound and ability to deliver its output power are what should be most important.



 

Bronze Member
Username: Asimo

Post Number: 44
Registered: Apr-04
While testing amplifiers to upgrade my NAD C350 I discovered the Marantz PM 7200 claimed to be class A amplifier at a reasonable price.
I took the Marantz PM 7200 to my house for comparing tests for a weekend. I did not find any difference between normal and class A mode.
I also found my NAD C350 amplifier to be at least equal or even better than the Marantz PM7200.
I must join the members who think that Marantz PM 7200 class A is rather a gimmick
 

nout
Unregistered guest
You'll get a free NAD poster!

 

J. Vigne
Unregistered guest

What does Asimo have to do to get a T-shirt?


 

nout
Unregistered guest
He must finish this sentence:
"I hate Marantz because..." and send it to members@marantzgimmicks.com

The most original entry will be awarded with a "NAD rules, Marantz sucks" T-shirt
 

New member
Username: Ca_convert

CardiffUK

Post Number: 1
Registered: Jan-05
J - thanks for the interesting explaqnation on class A vs Class B operation. Can you recommend any good sites for an idiots guide to audio amp design?

 

J. Vigne
Unregistered guest

Tube or solid state?

Very good, nout.


 

New member
Username: Ca_convert

CardiffUK

Post Number: 2
Registered: Jan-05
Solid state. I didnt realise that valve amps were serious hi-fi contenders until recently when i started reading about it again. I have always associated EL84's and their like with Marshall et al. A valve guitar amp is so much sweeter sounding when overdriven than than a tranny one,

Nout, very good!
 

Bronze Member
Username: Nuck

Parkhill, Ontario Canada

Post Number: 15
Registered: Dec-04
J, thanks for the rundown, always like to listen to a pro.
However, my h/k avr65 is supposed to be switching between a/ab in auto matic, and I can hear that especially in live(like James Taylor).
The transiant is good, and the mid response lifts considerably.Which is nice for this show, but at higher volumes.(wife factor).
Any opinions on this factor? I am considering popping for the avr85 to replace my little one.

Cheers
 

J. Vigne
Unregistered guest

Sorry, I don't understand:

"my h/k avr65 is supposed to be switching between a/ab in auto matic"

Maybe I was unclear about the conversion from Class A to Class AB in the above description. It was meant as a primer and not an entire explanation of the class of operation. At low wattage the outputs are on constantly which puts them in Class A mode. As the voltage demands are increased the outputs slide into a Class AB mode.
Most amps run in Class A for the first few watts, some as high as 15-25 watts, but, more typically, the first 2-3 watts only are in Class A. Class AB is mostly a matter of efficiency of power. By switching the outputs off for half the cylce they can run cooler, and, therefore, produce more power per period of on time with less heat sinking.

If you are using fairly efficient speakers you can reach a comfortable listening level where the majority of what you hear is reproduced in Class A and the transient peaks above two or three watts are being reproduced in Class AB. In a situation such as this the peaks are relatively short term and it would be difficult to imagine you can actually hear the transition point. If you are running less efficient speakers or playing at higher volume the amp is likely to be running as a Class AB amp at virtually all times.
I don't understand what it is you're hearing that would be the switch point from one mode to the other. Nor do I understand the "automatic" mode you suggest. Please explain.


 

J. Vigne
Unregistered guest

I don't know if this is going to be helpful or not. Only you know at what level of expertise you can begin reading. Probably the best advice is to put something on the order of "audio amplifier design" or "amplifier DIY" into a search engine or two and see what satisfies your knowledge level.

http://www.depalma.pair.com/Analog/analog.html

http://www.passdiy.com/

Most of my reading is on tubes. Here's a site that has lots of links that you might find helpful.

http://www.worldtubeaudio.com/


 

J. Vigne
Unregistered guest

Here's a little more on bias point. It is meant for tubes but the information holds true for most solid state devices also.

http://www.tubedepot.com/whisbipo.html


 

J. Vigne
Unregistered guest

http://www.angelfire.com/ab3/mjramp/designing.html

 

New member
Username: Ca_convert

CardiffUK

Post Number: 7
Registered: Jan-05
Thanks for the links. I found them interesting, though I still struggle to fully understand how a FET works at conceptual level.
 

J. Vigne
Unregistered guest

I'm afraid I can't help much there. I read the lit on J-FETs and MOS-FETSs and all I come away with is they are transistors that aren't. Voltage and current driven seem to be the most important part of what I needed to know.

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