The Boucherot Cell or popularly called Zobel Network was discussed in the last post, and most DIYer’s included our topic as part of the output stage Zobel network. However, , I will called it here as SWCK (Speaker Wire Capacitance Killer). Let’s see if you need this RL circuit or not. Let’s take a closer look of this circuit and what is the main purpose.
Theory Behind
In the real world, the Speaker is connected to the DIY Amplifier Build (or inot a Professional Amplifier) using a wire, as represented by the diagram below.
When the wire is long, I mean like 20 meters Long, that wire will introduce a capacitance into the speaker wire, hence it introduce a Capacitive reactance (Cx), and become a part of the total speaker load. Let’s represent this in electrical form, as shown below :
The worst thing to happen is to drive a capacitive reactance, the PF (power factory) become bigger and thereby reducing the real power delivered into the speaker.
Function
So, what it need to do is to cancel that effect of capacitance due to long wires by introducing an inductor LC that produce Inductive reactance to cancel the Capacitive reactance. So we have a coil.
But there is a problem with the additional of the Inductor (to Kill the capacitance), this may cause the total speaker circuit to be tuned, like high Q in RLC circuit at certain frequency, and what happen is that, the impedance will become zero, that means, the DIY Amplifier Build see the load as zero impedance at certain frequency, this is not good.
So, the solution is to add a Resistor RC in parallel to the Inductor to kill the high “Q” or from being tuned circuit. Is that makes sense now?
The Real World Function
It is really safe and makes us in peace of mind to add the RL circuit in our DIY Audio Amplifier Build used at Home, where the wire can be only 2 to 3 meters. Knowing the real function of it may give us more design idea to incorporate it or not in our next Amplifier Design.
In an active Speakers, where the Amplifier is inside the Speaker Baffle, there is no need for this RL circuit where the wire is just about 30cm to 60 cm. You may notice that in modern Class-D or Class-T Amplifier, this RL circuit has been omitted.
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Fellow DIYer’s, if you spotted error or wrong explanation, kindly let me know, welcome to comment below. If you feel the need of this post to your hobby or work, please share it with your friends – thank you for sharing.