Input Sensitivity of 'Power' amplifiers, the 306/606 case

The recent discussion on the forum about the input sensitivity of a Quad 306 and 606 in a Bi Amping application is the reason for this post.

First let me get things straight:

A ‘power’ amplifier is a voltage amplifier! The delivered power is the result of the applied voltage to a load, most of the time a loudspeaker.

A loudspeaker is not a power transducer, but a voltage transducer! The absorbed power is a result of the voltage output of the amplifier applied to the impedance of the loudspeaker.

Why is everybody (including me) talking about power? Well, this is successful brainwashing marketing, like the Horsepower for cars. The HP is only a meaningful value if you now the revs and the torque, so is power in amplifiers and speakers only a meaningful value if you now the impedance and the frequency concerned.

Input sensitivity of amplifiers is defined as follows (regrettable): the voltage at the input that is needed for delivering maximum power in a 8 Ohm resistance at 1kHz and  with a total harmonic distortion of 0.1%.

Sometimes amplifier manufacturers use different standards, but the impedance, distortion and frequency should be specified.

This ‘Power’ way of thinking leads sometimes to wrong conclusions and assumptions. An example; in some HiFi magazines the output power is measured in other impedances than 8 ohm. In some cases the output power is halved, which seams low fi. But the output voltage is only 3dB lower, so within limits that are excepted by the HiFi community.

The 306/606 case:

The 306 has a sensitivity of 375mV for delivering 50W in 8 Ohm.

Power is the squared voltage divided by the impedance. P=U x U / R

So in the 306 case the output voltage is 20V

The 606 has a sensitivity of 500mV for 140W in 8 Ohm

The output voltage is 33,47V

In a Bi Amping setup the voltage delivered to the tweeter and woofer section should be the same given a value of the input voltage, otherwise the loudness will not be even spread amongst the total spectrum.

So we are interested in the voltage gain of the amplifier, this the ratio of the output voltage and the input voltage (U out / U in).

For the 306 this is 53,33, for a 606 this is 66,94 this looks far apart, but in decibel terms this is 1.97 dB, within the limit of 3dB. (dB is 20 x log Gain 606 / Gain 306). In practice the bass is a little bit louder, if the 606 is used to drive the woofer section. Which is logical, because of the rule of thumb of Stefaan; 90% of the power is delivered below 1000Hz.

If you want the voltage gains exactly right, the sensitivity of the 606 should be lowered, how much? The voltage gain of the 606 should be 53,33, so the input voltage should be 33,47 divided by 53,33 is 628 mV. This can be done by replacing R11 from 7.5 Ohm to 9.4 Ohm. This is not a practical value, so we choose 10 Ohm.

The voltage gain of the 606 is (33.47 / 0.5) x (7.5 / 10) is 50,21.

The difference in db is now: 20 log 53,33 / 50,21 is 0.52 dB. Which is as close as we can get with standard components (or use 18 Ohm and 20 Ohm in parallel as a replacement for R11).

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