Onzow MFB20

Introduction

　Here we are going to make a motional feedback (MFB) system.
20 cm's full range unit with a detecting coil (Onzow MFB 20) is
used for this purpose.

　　Closed box is employed after all the discussions.

　　The voltage detected by this detection coil corresponding to
the movement of the voice coil is negatively fed back to the input
side of the circuit. Accordingly the movement of the voice coil is
similar to the input signal and reproduction of bass different from
that of non MFB system will come true.

　



　　Impedance (unit without baffle)





　　output of detection coil



　1/5 scale



　　This is very similar to the impedance curve.
　　

　　fo=60Hz

　　Front baffle(spruce) is residue of my house under construction.



　　Two pieces of wood are combined like a violin. Hinoki wood(Japanese cypress)
is used for other　parts.

　　Speaker base is made of residue of the column of the house. It's heavy
and strong.
　

　　Minimal equipment is required. But when I build back-loaded horn
syaytem, I had to use nails.




　　As is seen slight prominence of the frame is not beautiful. In this
case I make a groove with trimmer and the Hotate.

　
Speakers are finished.　

　　Impedance


　　foc is about 100Hz. A resonance occurs at this frequency. In the
traditional method of design low frequency reproduction characteristics
are mainly determined by the foc. But in MFB system foc is independently
designed.

　　Sound pressure level


　　Without MFB this system sounds pretty good. You should remember
this sound of resonance and you'll be a sound master.

　　How to make MFB



　MFB is on with an adapter like this. Amplifier used is Kenwood R-SG7
.At 600 ohm position of VR MFB is spelled on. Low boost NBON2 is
applied.

TEST

　　Ron Carter, Jaco Pastrius, etc. any base player sounds good. Is sounds
definitely like trombone.


　　Frequency response(MFB ON)


　　　Look at a dip at 100Hz without input signal. Fortunately no speakers
are broken by oscillation by this time.


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