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Why does a pure Class A amp run so hot ?

 

New member
Username: Nader

Post Number: 1
Registered: Oct-04
Hi,
I just bought on ebay a Marantz PM94. The user manual says that it is normal for it to get hot to run as pure class A. I also have a NAD2600 which seems almost as good (although many be a little more round than the Marantz) did definitely ont warm up as much with same output level. Another point is that the MArantz has a 490W power supply while the NAD a 970Watt, again I am surprised why the NAD has to heat up so much.

Any info or advice is welcome !
Thank you
 

New member
Username: Nader

Post Number: 2
Registered: Oct-04
Oh I forgot one info,

My speakers are the infamous Infinity Kappa 9A amp-killers ! Am I at risk with the MArantz PM94 ?

Thanks!
 

J. Vigne
Unregistered guest


There are different "classes" of operation in ampifiers. What a particular design operates in is determined by the way the signal is treated as it goes from the positive side of the waveform (~) to the negative side. It is typical for most audio amplifiers to run in Class B or AB. (There are subdivisions of these classes also that represent a combination of qualities of operation.) Class AB is typical in the amplifier section of a product such as NAD. In this class the amplifier is operating as a push pull design. There is a amplifying device (in this case a transistor) that handles the positive side of the waveform (a PNP) and a similar device for the negative side (a NPN). When the waveform is in the positive side of its swing the negative device is turned off. When the wave is negative, the positive device is turned off. This is Class B. The switch happens at the point where the wave crosses from positive to negative. Because the switching is not perfect, this type of operation creates what is known as notch distortion. It is efficient because the output devices only operate for half the signal. More power is usually the advantage of Class B.
In Class A operation the output device is never switched off. By staying on all the time there is no point where the signal crosses from one output device to another so the notch distorton is eliminated. (There may be more than one output device even though they are not switching.) Class A operation is inherently what is termed Single Ended. Since the output devices are constantly on they generate more heat. Without the distortion product Class A sounds cleaner and, for most people, more natural. That of course depends on the quality of the amplifier.
The NAD tends to run hot because the outputs are biased "hotter" meaning more voltage is running through them. This is another way to push the amplifier into a more powerful mode of operation.
Virtually all audio quality pre amps run in Class A due to the small voltage levels present. Most power amps run in Class A for the first few watts and then switch to Class AB or Class B operation for efficiency.



 

J. Vigne
Unregistered guest

For more on amplifiers go here:

https://www.ecoustics.com/electronics/forum/home-audio/6.html


 

New member
Username: Nader

Post Number: 3
Registered: Oct-04
Many thanks for the clarification and the link !
 

mauimusicman
Unregistered guest
J.Vigne.......remember the JVC and Technics "fake" class "A" amps? Those were a class a/b amp with a small amount of bias current sent to keep the transistors from switching off. Japanese audio.....things that make ya wanna scream!
 

New member
Username: Dallaghan

Post Number: 1
Registered: Nov-04
My Old Technics Amp proudly displays "New Class A" on the front!! I take it this is the kind of fake class A that you speak of? (As it was a fairly cheap amp back in the early 90's!)

:-)
 

J. Vigne
Unregistered guest



What the Japanese manufacturers weren't telling you, and probably neither did the salesperson, is that all Class AB amplifiers run in Class A for a small portion of the wattage (typically 1 to 5 watts) before switching to Class AB for the sake of efficiency ( and less heat). By the justifications given by some manufacturers any analog amplifier could creatively be labeled Class A since it does pass through this operation on its way to worse sound.



 

mauimusicman
Unregistered guest
Because the transistors are always on, unlike a class a/b amp where they are switched on and off.
 

mauimusicman
Unregistered guest
Martin.......yeah it is. However, that doesn't make it a bad amp. Just not the class A it claims to be. Some of those old Technics amps from the 80's were, and still are great sounding amps. I tell you, if you were to find the right old Technics amp and hook it up to a pair of Green Mountain Europa's at $1K/pair, with a decent TT or CDP, you could have, for under $1500.00 total, a system that would make your audiophile bud's green with envey. And thats no joke.
 

vector
Unregistered guest
I have an old JVC Super A pumping out 75 watts RMS with an audio control EQ, Yamaha analog tuner,Pioneer PL 550 turntable. Technics cd player, and a pair of JBL 4311's all bought back in the late 70's. The system is at the cottage now but still gives out great sound.
 

J. Vigne
Unregistered guest

Let me guess. An Audio Technica cartridge.


 

Anonymous
 
I have a Technics Amp, Tuner, Cassette Deck, Turntable, EQ, and speaker's I bought back in 1983 and let me tell you the Amp runs HOT at abought 3/4 volume.
I had to give it it own shelf in my audio stand because it was creating a heat issue with the other components.
I believe the components are probably far better than anything available today. I Operate a Audio store here in Vancouver British Columbia Canada, and I use this system as a benchmark for audio clarity and quality.
The Technic's components in sync with my 1000W American Studio Monitor's send's shiver's up and down paying client's backs when they here the system.
They always ask, Is the system for sale"? I tell them "No".
It is worth everything in promoting audiophile sound quality and workmanship.
Thanks Technics.
 

New member
Username: Kenshi

Post Number: 1
Registered: Feb-05
Hello everybody!

I have recently inherited an old amp from my cousin which is really, really great!! Can anybody help me giving me its specs? I've been searching around a lot but haven't found anything yet.. Well, it is a marvelous TECHNICS SU-V4A (new class A). It is grey, it has about 17-20 years and, it gets hot and it has REALLY nice sound!!
Does anybody know its specs!??? Help me please!

Thanks a lot!
 

New member
Username: Thebeholder

Post Number: 2
Registered: Feb-05
I've had Technics SU-V7 for many years (bought it in 1981, if I'm not mistaken). I liked it a lot - I believe that it sounded better than my current NAD c372. It may well have been a Class A device because it's power consumption was more than 5 times its output (the theoretical efficiency limit for Class A devices is 25%).

As far as it concerns the specs, if I remember correctly, Technics had THD 0.003% at -3dB at 1dB, 0.007% at the rated power at 1kHz and 0.01% at the rated power in 20Hz-20kHz. It was rated, I think, at 2x60W into 8ohms. I belive, SU-V4 is different only by a bit lower output (2x50W or something like this). By the way, this very low level of THD would be consistent with operating in Class A regime.
 

Unregistered guest
Any comment about class D amps?

Also.. anyone have a link to good info on class c amps ( rf )

thanks

Soundguy
 

Silver Member
Username: Cheapskate

Post Number: 132
Registered: Mar-04
don't forget class H. true class H amps are supposed to be the efficiency champs at around 90% efficiency and don't require heatsinks.
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