JAMO C 80 manual Motional Feedback MFB, Active Impedance Control AIC, Boundary Gain Compensation

Jamo C 80  page 0

Jamo Tech platforms

Motional Feedback (MFB)

C 80 SUB features a Motional Feedback circuit. When a large woofer has been abruptly driven by a strong sonic impulse such as an explosion or a timpani, the woofer continues moving after the impulse stops. The MFB circuit com- pares the impulse to be reproduced with the actual movement of the woofer. If the woofer moves unnaturally in relation to the input signal, the MFB circuit corrects the error before it becomes audible. Furthermore, the MFB makes it possible to equalize the subwoofer to go deeper without losing any precision. The result is incredibly accurate, deep bass, full of punch and dynamics.

Motional Feedback (MFB)

Active Impedance Control (AIC)

All C 80 mid/woofers feature Active Impedance Control (AIC), an innovative extension of existing techniques to reduce distortion in electrodynamic loud- speaker drivers.

The AIC makes the impedance more linear in the mid and high frequency range. AIC also increases the sensitivity and total SPL in the upper range of the driver.

AIC - Active Impedance Control:

Impedance [Ohm]

Impulse response [V]

0

	Time - msec.
Without MFB	With MFB

30

25

20

15

10

5

0

  driver with AIC      driver without AIC

Boundary Gain Compensation

The Boundary Gain Compensation filter in C 80 SUB compensates for the room influence at low frequencies, as any room amplifies the lowest frequen- cies. The magnitude of the room’s amplification depends on the room size and where the sub is positioned in the room – normally it increases the closer the sub get to a wall. This can result in too high a bass level from approx. 60Hz and increasing downwards – in the range between 20 - 30Hz it peaks up to 5 - 10dB in an average room!

At first this could sound really impressive, but the sound quickly begins to sound ‘boomy’, especially when listening to music, and rather annoying. So the Boundary Gain Compensation ensures a very linear in-room bass response and gives you more flexibility when it comes to placement and choosing your favourite listening position.

Boundary Gain Compensation

SPL/Volts [0.33 oct]
Off		On
10.0		100.0

Frequenzy ]Hz]

200500 1k 2k2k 10k 20k Frequenzy ]Hz]

The effect of AIC is a clearer midrange with a markedly reduced ‘roughness’, es- pecially noticeable on vocal reproduction. In figures, this amounts to a reduc- tion of third-order harmonic distortion of around 6dB (ie a 50% reduction).

Harmonic distortion

[dB] -35

-40 -45

-50

-55

-60
-65		driver with AIC		driver without AIC
200	500		1k		2k
					Frequenzy ]Hz]

When a voice coil is moving in and out of a magnet gap, it changes induc- tion. When the coil moves inward, the induction increases and vice versa. This causes the current from the amplifier to be modulated, creating very audible distortion.

Furthermore the current flowing in the voice coil modulates the magnetiza- tion of the magnet assembly which also causes distortion of the current.

By using an extra coil, placed inside the voice coil with opposite polarity to the voice-coil winding, the inductance variation and the flux modulation can be significantly counteracted.

Compared to the conventional conductive rings in a magnet system AIC has the advantage that it is significantly better at higher frequencies i.e. in the midrange (on voices) where distortion is most audible to the human ear.