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No change in wattage?

Discussion in 'Amps and Cabs [BG]' started by Jon R, Apr 18, 2002.

  1. OK... I ordered my new head and have been doing a little preliminary reading. In the specs it claims that the output is 300 watts @ 8 ohms AND 300 watts @ 4 ohms. This, I don't understand... it flies in the face of everything I learned in electronics 101. I was under the impression that if your amp pumped out 300 watts @ 8 ohms it would put out 600 @ 4 ohms, 1200 @ 2 ohms, 2400 @ 1 ohm, etc., etc., etc. The limiting factor would be the amps minimum required load... in my case; 4 ohms.

    What am I not understanding???? :confused:
  2. Is it an all-tube amp?

    If so, that's very likely your answer.
  3. It has a valve/solid state preamp and a mosfet poweramp... I believe.
  4. Lovin Bass

    Lovin Bass

    Feb 8, 2002
    Los Angeles
    Which brand and model is the amp?

    For a solid state poweramp (mosfet), there will be a decrease in power output as impedance increases.

    For a tube amp, there are two (or three) outputs: a set for 8(or4) ohm and a set for 4(or 2) ohm which is regulated by a transformer to adjust for each outputs intended impedance but still maintain the same wattage.

    You say that the poweramp section is solid state / mosfet which means that the specs do not appear to be correct. I am far from being a technician but I am reasonably sure that the solid state will not maintain the same power output at varying impedance loads.
  5. Steven Green

    Steven Green

    Jul 25, 2001
    Pacific NW
    Lovin Bass is right. What's the model of the amp in question?
  6. FunkySpoo

    FunkySpoo Supporting Member

    Feb 6, 2002
    While I've never heard of a mosfet amp having no wattage increase as the impedence goes down,not all amps double their wattage as you go from 8 to 4 ohms. My SM400 is 400 watts at 8 ohms and only 420 watts at 4 ohms. At least that's what the manual says
  7. Sorry... had to sleep. Anyway it's a Trace Elliot RAH300-12. The downloadable manual says it puts out 300watts... period. It also says that the pre-amps valve stage uses a double triode... OK, I'm not a Brit... do they mean two tubes? and if so which ones? Lastly, they say the speaker load can be either 8 or 4 ohms... nothing else... and nothing about if it would change your wattage!
  8. brianrost

    brianrost Gold Supporting Member

    Apr 26, 2000
    Boston, Taxachusetts
    This is true only if the amp can source the needed current (twice as much). In general, this is limited by the power supply and heat sinking.

    Check specs on some common amps:

    Peavey Bass MkIV [email protected] [email protected] [email protected]

    SWR Baby Blue [email protected] [email protected] [email protected]

    GK 400 RB [email protected] [email protected]

    Amplified Music Products BH-420 [email protected] [email protected]

    Notice how little power increases as impedance drops on the SWR! The Peavey and GK only gain about 50% (rather than the ideal 100%) as impedance is cut in half. The A.M.P. head comes closest to ideal, about an 85% increase.

    If you do the math on the SWR specs, the actual level change between 140 and 160 watts is only 1/2 a dB!
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