Question on Amp Classes

Discussion in 'AV Receivers' started by MikeEL, Feb 25, 2004.

  1. MikeEL

    MikeEL Auditioning

    Joined:
    Feb 25, 2004
    Messages:
    4
    Likes Received:
    0
    Trophy Points:
    0
    I recently "upgraded" my old JVC RX817 (105W) pro logic receiver to a refurb Pio DD/DTS VSX711 (100W). Fine so far, love the Pio for movies. But, compared to the JVC, it has no "balls" for music. Now, I know not all watts are created equal, but this is a serious difference. Thus my question:

    The JVC specs list it as a "Dynamic Super A" receiver. It also lists the power to 2 channels at 105W each at 0.007% THD. This seems like an awfully low THD when compared to other, newer, (and much more expensive) receivers. Does this have anything to do with the "dynamic super A" thingie or it that just jvcspeak.
    I also noted the JVC weighs about 15lbs MORE than the newer PIO - I mean this old thing is a tank!
    Hmmm...just wondering if this "dynamic super a" is some sort of switching class and if so, how does it work?

    Any thoughts?
     
  2. Brad_Harper

    Brad_Harper Stunt Coordinator

    Joined:
    Jul 5, 2001
    Messages:
    132
    Likes Received:
    0
    Trophy Points:
    0
    The "super A" probably refers to a Class A amp output stage design. A class A designed stage means that the output transistors of an amplifier are always biased to the "On" position. They do this to eliminate crossover distortion, which is the distortion that happens when a music waveform goes from a positive amplitude to a negative amplitude (or vice versa). Class A amps are only around 40% efficient but have the lowest THD ratings. This would account for the increased weight due to larger heatsinks.
     
  3. Kevin C Brown

    Kevin C Brown Producer

    Joined:
    Aug 3, 2000
    Messages:
    5,712
    Likes Received:
    0
    Trophy Points:
    0
    I have never come across a class A receiver. Not from a mass market Japanese manufacturer anyway...

    The JVC might be biased *further* into class A than the Pioneer though (runs in class A mode to higher power settings before switching over to class AB).

    Did the JVC run hotter or cooler than the Pioneer? Class A amps tend to run fairly hot, because of how inefficient they are. Did it have a fan?

    Also, there is a web site somewhere that compares receiver manufacturers' stated power rating vs what you really get in the real world. "100W" isn't necessarily 100W... Depends on how it is measured. [​IMG]
     
  4. Robert Cowan

    Robert Cowan Supporting Actor

    Joined:
    Nov 10, 2003
    Messages:
    504
    Likes Received:
    0
    Trophy Points:
    0
    a lot of amps have what they call a class A pre-driver circuit. if i understand it correctly, its the pre-amp thats operating in class a. this could be the case?

    but there is NO way for a receiver to be full class a. im building class a monoblocks, and they are bigger than any receiver for 1 channel of pure 100 watts class a, and cant be touched for more than a minute or so.
     
  5. JohnSer

    JohnSer Stunt Coordinator

    Joined:
    Dec 4, 2002
    Messages:
    198
    Likes Received:
    0
    Trophy Points:
    0
    "Dynamic Super A" is marketing speak. Either of what Kevin or Robert suggest could be true. But as we know, marketing can be very creative! The JVC is most likely a class A/B amp. The extra weight probably comes from a larger transformer. The larger transformer, probably accompanied by larger power supply caps, is what provides the umph behind it.

    If the Pioneer has pre-outs, a power amp might be a good upgrade path for you.
     
  6. LanceJ

    LanceJ Producer

    Joined:
    Oct 26, 2002
    Messages:
    3,168
    Likes Received:
    1
    Trophy Points:
    0
    I'm not exactly sure what amp design JVC uses, but keep in mind Matsushita, the parent company of Panasonic/Technics, also partly owns JVC.

    I'm not going to say they are 100% pure class A, but just about every Technics/Panasonic receiver made since the late 80s has had some sort of class A power amp section. This is why they run so damn hot when they are just sitting there idling with the volume turned all the way down (a sure sign a class A design is being used since the all of the amp's output transistors are running full blast to eliminate the switching distortion that class B amps have [i.e. one set of transistors handles the positive half of the music sine wave; the other set handles the negative half).

    These amps were usually called "New Class A", and then "Class H" when they started using a dual-rail (low/high) voltage system to help lower operating temperatures. The newest variation uses MOSFET transistors--these go into their clipping mode very smoothly, similar to what tubes do. And MOSFETs are hardier than "normal" transistors (they run hotter though).

    And at least back in the mid-90s when I sold their stuff, these receivers were the most reliable we sold, despite their heat generation (we had some customers that refused to buy one solely because of that one issue--though I think it just gives them personality!). And so far, my Technics SA-DA8 receiver that I bought in March 2001 & use for at least six hours every day since then is still puttering along with no problems whatsoever.

    Contrary to common belief, these amps don't run hot because Panasonic was too cheap to put in large enough heat sinks or they use lousy electronic components. They are just kind of a weird company (I mean that in a good way) and don't mind using unconventional designs. How many other companies use bamboo in their power capacitors or use a rechargeable battery in one of their top preamps to eliminate power supply variations?

    I've put this link here before but if you want to see what Panasonic used to build [​IMG] , check out this four year old review of a Technics power amp/preamp combo that sold for 3,000 pounds in the U.K. (not sure what that is in dollars): The Technics of yesteryear (while you're reading the review, keep in mind that British reviewers have traditionally been fans of warm-sounding components). But at least they still make decent entry-level turntables for vinyl collectors, their slick SL-1200 DJ table series & still include phono inputs on most of their receivers. Go here for more Technics propaganda [​IMG] at this cool website: Technics1210.com It has a lot of DJ-oriented info too. That "sound flyer" thingy is sort of cheesy sounding sometimes but it's fun to mess with.

    LJ
     
  7. JohnSer

    JohnSer Stunt Coordinator

    Joined:
    Dec 4, 2002
    Messages:
    198
    Likes Received:
    0
    Trophy Points:
    0
    Lance, I'll agree that Panasonic/Technics made some very good value equipment. I have had very good luck with their equipment holding up, over many years. Still have a 70's Technic receiver that is still in use.

    I don't agree that the JVC could be anywhere close to a pure class A design. At best, it is as Kevin suggests, it is a heavyly biased A/B amp. Todays receivers generate a good deal of heat, without including the output devices. DSPs, voltage regulators, and preamp stages, all generate heat.

    My HK520 (class A/B amp) runs very warm, without using any of its amp channels. Just running as prepro it will get much hotter than the Adcom 545II power amp.



    FWIW, here are some amp class definitions, from DIYAudio forum:

    I started this from memory, then decided to look it up. These definitions are from Audiolab's glossary

    Class H amplifier - Class H operation takes the class G design one step further and actually modulates the higher power supply voltage by the input signal. This allows the power supply to track the audio input and provide just enough voltage for optimum operation of the output devices. The efficiency of class H is comparable to class G designs.

    Class G amplifier - Class G operation involves changing the power supply voltage from a lower level to a higher level when larger output swings are required. There have been several ways to do this. The simplest involves a single class AB output stage that is connected to two power supply rails by a diode, or a transistor switch. The design is such that for most musical program material, the output stage is connected to the lower supply voltage, and automatically switches to the higher rails for large signal peaks. Another approach uses two class AB output stages, each connected to a different power supply voltage, with the magnitude of the input signal determining the signal path. Using two power supplies improves efficiency enough to allow significantly more power for a given size and weight. Class G is becoming common for pro audio designs.

    Class D amplifier - Class D operation is switching, hence the term switching power amplifier. Here the output devices are rapidly switched on and off at least twice for each cycle. Since the output devices are either completely on or completely off they do not theoretically dissipate any power. Consequently class D operation is theoretically 100% efficient, but this requires zero on-impedance switches with infinitely fast switching times -- a product we're still waiting for; meanwhile designs do exist with true efficiencies approaching 90%.

    Class AB plus B amplifier - Class AB plus B design involves two pairs of output devices: one pair operates class AB while the other (slave) pair operates class B.

    Class AB amplifier - 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.


    Class B amplifier - 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 A amplifier - 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.
     
  8. MikeEL

    MikeEL Auditioning

    Joined:
    Feb 25, 2004
    Messages:
    4
    Likes Received:
    0
    Trophy Points:
    0
    Whoa, lotsa good info here....
    Yes, the JVC runs very hot - I kept it on TOP of my rack so that air could circulate around/thru it. MUCH hotter than the PIO.
    The manual states the 105W @ 0.007% THD in stereo only (2 channel). Now, when driving in PL mode (all five channels) the THD shoots up to 0.7%. Interesting.
    Anyway, thanks alot for the answers and I think I will keep this tank around and build a modest two channel system out of it.[​IMG]
     
  9. JohnSer

    JohnSer Stunt Coordinator

    Joined:
    Dec 4, 2002
    Messages:
    198
    Likes Received:
    0
    Trophy Points:
    0
    Mike, I don't doubt that the JVC gets hot, most receivers these days, do. But if it was a pure class A - 2 channel, it would be drawing 1000W at idle. Most of which, would be converted to heat!!!

    PS: I have found that you can always find another room that needs a sound system. [​IMG]
     

Share This Page