ZOBEL NETWORK FOR LOUDSPEAKER

fig1: LOUD SPEAKER INTERNAL VIEWfig2: VOICE COIL DIAGRAM Before we go in to our discussion of" zobel network compensation for loudspeaker", we shall have a brief look over internal structure of loudspeaker and what a voice coil is...

A loudspeaker is an electroacoustic transducer that converts an electricalsignal in to sound ,The voice coil moves in accordance with the variations of an electrical signal and causes sound waves to propagate through a medium.fig1 shows the internal structure of speaker and fig2 a voice coil!!

WORKING :

1.voice coil is a copper winding which becomes an electromagnet when current is passed through it.

2.if a time varying current(a.c signal) is passed it magnetises and demagnatises according to current variation.

3.the voice coil is concentrically placed in a permanent electromagnet.

4.when the coil is electromagnetised by current,then coil tends outward due to repulsive force of permanent magnet and voice coil.

5.when coil is demagnetised it rests in original position,likewise the coil moves producing sound that is varying according to the applied signal and hence it reproduces the voice or music.

now we enter in to the present article of"ZOBEL NETWORK"!!!

Zobel networks are a type of filter section based on the imageimpedance design principle. They are named after "Ottozobel" of Bell labs who published a much referenced paper on image filters in 1923.

as voice coil is a copper wound coil this behaves as an inductor which offers impedance at high frequencies.we know that at high frequencies the inductor blocks ac,in a similar way the loudspeaker blocks ac components and exhibit high impedance.so that the impedance characteristics are nonuniform at all frequencies. THE LOUDSPEAKER IMPEDANCE IS INDUCTIVE AT HIGH FREQUENCIES(unusual from being resistive).

DESIGNING ZOBEL NETWORK: we can design,If the voice coil inductance is known, then a suitable value of capacitance may be calculated quite readily. The first thing to determine is that frequency where the inductive reactance is equal to the DC resistance of the voice coil ...

f = Rvc / ( 2 p Lvc )

Where ...

f = frequency
Rvc = Resistance of voice coil
Lvc = Inductance of voice coil

Once this figure is found, it is a simple matter to calculate the capacitance for the Zobel network ...

C = 1 / ( 2 p f Rvc )

Using the simulated speaker above as an example, we already know that Rvc is 6.2 ohms, so ...

f = 6.2 / ( 2 * p * 1.5^-3 ) = 658Hz
C = 1 / ( 2 * p * 658 * 6.2 ) = 39^-6F = 39uF

we can safely assume that the formula works, and is easy enough to use. The resistance will nearly

always be approximately equal to the voice coil resistance - in some cases it may be found that a small variation is needed, but this is unlikely to be significant.

capacitor can be a bipolar electrolytic could be used, the main problem with bipolars is that they are not stable over time. I recommend that polyester, polypropylene or oil filled paper cap be used.

DISADVANTAGE:The Zobel network will flatten the impedance of the speaker, but at the cost of power dissipation, and a slightly lower than expected overall impedance. Naturally, the power dissipated by the resistor is turned into heat, not sound, reducing effective efficiency. The lower impedance may cause some stress to certain amplifiers, but most should be able to cope with the slight extra loading!!

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