Tech note: Volume setting does not equal power

Discussion in 'Amps and Cabs [BG]' started by fdeck, Oct 19, 2005.

  1. fdeck

    fdeck Supporting Member Commercial User

    Mar 20, 2004
    Madison WI
    HPF Technology LLC
    I have seen it too many times: "The amp sounded loud, even when only turned up to 2, but the other amp had to be cranked up to 6." Or something like that.

    The volume control is not a power knob. Volume and gain settings are not a way to compare power levels, because these controls are designed to react differently in different amplifiers.

    Like it or not, unless you believe the specs (itself the subject of endless threads), if you want to compare power levels, you can only do it by cranking each amp to the point where the power amp and not the preamp is distorting at a level that begins to sound bad. You have to hold the test conditions constant, e.g., by using identical speakers in the same kind of room setup.

    How you do this without going deaf, when amps are routinely being made with 600 Watts or more, is a total mystery to me. But fortunately I don't have a dog in that fight.
  2. Petebass


    Dec 22, 2002
    QLD Australia
    This reminds me of interview with Mr. (Firstname?) Peavy himself about the early days of guitar amps. He originally built them with electronics that would see the volume knob deliver linear power. By that I mean that with the volume knob at 50%, the amp would produce 50% of it's available power and so on.

    The problem was that Fender were doing something completely different. Their amps were set up so that 90% of power would be delivered with the volume knob at 40% or thereabouts, and turning the knob up further did virtually nothing. So when guitarists of the time compared amps, the Fender with the volume knob on 3 was as loud as the Peavy with the volume knob on 6. So people would assume that the Fender was therefore twice as powerful.

    Would this have been the case if both amps were compared at max power while ignoring the position of the volume knob. Mr Peavey claimed his amps were louder, but he had a lot of trouble educating the masses. I the end he had to revert to as non-linear volume control like that used in the Fenders.
  3. Tim__x


    Aug 13, 2002
    Alberta, Canada
    He can't have done that, his amps may have had lower input sensitivity, but without resorting to an AGC (something one would never use on an audio signal), it's impossible to make volume control settings directly control power output.

    To put it in simple math.


    Where Vo=the amplifiers output voltage, Vin= the input voltage, Ac= the amplifiers closed loop gain, and Ps the volume pots setting (a value between 1 and 0).