### 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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