For DIY Audio Amplifier Builder, the Boucherot Cell or commonly known as Zobel Network is composed of Series Resistor-Capacitor (RC network), Capacitor CA and Resistor RA at the back-end or at the output stage of a discrete power amplifier design, as in the figure below, it is often misunderstood its function and simply accept it as copy and paste.
Some engineers called it “Snubber”, because it will “snub” the effect of the circuit that is dependent on frequencies. It seems to “me” that the circuit was adapted from generation to generation dated back to those days of early amplifier design.
Function
The main function with respect to the DIY discrete Audio Amplifier design, is to stabilized the amplifier when the load as seen by the amplifier goes high impedance, this is happening when the frequency of interest goes high, and the DIY Amplifier build will tend to oscillate.
What is it? …. Again? The Speaker (the driver or Load ) – is composed of a Resistor RB and Inductor LB (winding of coil) looking at the speaker construction.
The Resistor does not change the resistance when frequency change, but the Inductor does, such that the Inductive reactance is expressed with the formula
Lx= 2 x Pi x H x F
where “F” is the frequency dependent that, when it goes high, the Impedance goes high as the Inductive reactance is directly proportional to the frequency. Therefore the Impedance seen by the Amplifier will be :
Total Impedance (Z) = Resistance + Inductive Reactance
If the amplifier sees this high impedance load (as if the Speaker load is not connected), the DIY Power Amplifier Build may oscillate (I say, it may …… but maybe not depending on how good is your audio power amplifier) .
This is not good if the DIY Amplifier Build Oscillate. The Zobel Network (design and published by Otto Zobel of Bell Labs who published a reference paper on image filter in 1923) , will going to stabilized the effect of the Inductor by cancelling the Inductive reactance created due to the high frequencies
The Zobel netwotk then is place to “Snub” the effect of the internal inductor of the Driver (speaker). When the frequencies goes high, the Capacitor CA goes nearly zero ohms due as seen in the formula :
Cx = 1 / (2 x Pi x C x F)
So, what will happen at high frequencies is that, the Impedance of the Zobel network will be dominated by RA, which is like being shorted to the Ground due to the effect of Capacitive reactance CA.
We can see then that RA is equivalent to 10 Ohms (which is very close to 8 ohms speaker) or 4.7 Ohms (which is very close to 4 Ohms speakers) – we use these values as there is no standard 8 ohms or 4 ohms resistor in 2~5 Watts range.
Make sense now? If the Amplifier then is stable enough at high frequencies or when the speaker is not connected, there is no need for the Zobel Network. It will be simply a decoration and may take-up PCB space and component if the circuit is not needed. However, the next good question is, how to know if the amplifier is stable? This question may be investigated and understand well, then, charge that to the net post.
Let’s have a deeper know how on this subject matter : Boucherot Cell – – – a my research with Zobel Network, I encountered Boucherot Cell electronic filter used in Analog Audio Amplifier to damp high frequency oscillation that may occur in the absence of a load at high frequencies. this was in name and honor of Paul Boucherot – a French Engineer who was a pioneer of AC electric power distribution and design induction motors.
Fellow DIYer’s, if you spotted error or wrong explanation, kindly let me know or please…… 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.