More powerful amp for top or bottom?

Depends on crossover frequency how much power you need per amplifier above / below the crossover point.....

Al,
Educate me, please. I thought that many amps increase in output as a logarithmic function, not linear. As input power from the pre is "amplified" that each step in volume is not linear but a function of the amps total output power? ie turning a pre-amp up 1/2 way will not necessarily utilize 1/2 of the amplifier's output capability.
Even if we look at this as a linear function, 1/2 volume on the pre amp would be 1/2 of the amps output capability. Therefor, 1/2 volume for a 100 watt amp will be 50 watts and 1/2 volume on a 240 watt amp is 120 watts. That's about a 4-5 db imbalance?
Thanks (no I'm not an engineer and yes, that's part of my problem...)

Hi Elevick,

Those are good questions.

The volume control on a preamp or integrated amp is indeed not linear, and will approximate to some degree being logarithmic, which is necessary because our hearing mechanisms are to some approximation logarithmic.

Power amplifier gain, as you realize, is often expressed in terms of db, which is a logarithmic scale. However, what the amp is really doing is generating an output voltage that is directly proportional to the input voltage, meaning that the input voltage is simply multiplied by some numerical constant, and providing the capability of supplying the amount of current that the speaker's low impedance will draw at that output voltage.

In this case the W4S amps have a gain of 27db, which corresponds to a voltage gain of about 22.4 times, based on the 20(log(V1/V2)) relation between db and the ratio of two voltages. So what the amp is doing, within the limits of the maximum output voltage, current, and power that it can handle, is generating an output voltage that is 22.4 times greater than the input voltage, at any instant of time.

Viewed in that manner, I think you will see that if some other amplifier had a voltage gain of say 30 times, and we provide the input to it through a resistive divider which in conjunction with the amp's input impedance attenuates the signal by a factor of 22.4/30 = 0.75, then the two amps will be gain matched for any signal amplitude and preamp volume control setting that is within their capability.

Best regards,
-- Al

Al,
couldn't you change the rate at which the perception of increase or decrease changes simply by changing the potentiometer type? From Linear to Log or the other way?
Isn't there a third type? Can't 'member what it may be called?

Hi Magfan,

Sure. The potentiometer, or equivalent stepped attenuator, can have pretty much any characteristic the designer wants. Although a linear characteristic (or linear taper, as it is often referred to), would be unsuitable for audio applications because it could not provide the combination of range and resolution that our hearing characteristics require.

BTW I'll add a further thought to my previous post, concerning the reference to a volume control as usually being approximately logarithmic. That is somewhat misleading, because what is logarithmic is the relation between the amount of attenuation the volume control provides and the amount the knob is turned. But for any given position of the knob, there is nothing logarithmic about it. For a given setting, all a volume control does is to multiply the voltage going into it by some fixed fraction, meaning that all signal amplitudes are affected equally.

So in that sense, for any given setting a logarithmic (or any other) volume control affects the signal in a linear manner, just as a power amp does. Except that the control has a gain of less than 1 while the amp has a gain greater than 1 (as well as the ability to supply lots of current, provide a low output impedance, etc.).

Best regards,
-- Al