?Sabe usted por qué los cables de entrada de amplificadores de audio, preamplificadores y otros circuitos necesitan ser blindadas? ¿De dónde vienen los ronquidos o chasquidos que se producen cuando ocurre una conexión sin blindaje a con blindaje deficiente?

El ronquido que oímos cuando colocamos el dedo en la entrada de un amplificador u otro circuito de amplificación de audio, tiene una frecuencia de 50 Hz que corresponden justamente a la frecuencia de la tensión de la red de alimentación.

Y es precisamente la red de alimentación la responsable por la presencia de esta señal, presente casi en todas partes y eventualmente por su entrada nuestros circuitos que no hayan sido blindados contra esto.

Lo que ocurre es que la circulación de una corriente alterna por una red eléctrica, como la domiciliaria, crea en torno de cada conductor un campo electromagnético que induce, en cualquier cuerpo conductor colocado en las proximidades, corrientes de igual frecuencia.

Estas corrientes son amplificadas por los circuitos de audio y aparecen como sonidos (ronquido) en los parlantes y audífonos (figura 1).

 

Figura 1
Figura 1

 

Para evitar que esta señal (presente en todas partes en que exista una red de tensión alterna) entre en los amplificadores, es necesario que sus puntos más críticos sean blindados.

Para esto usamos diversos métodos.

El primero aparece en la figura 2 y es el más conocido consiste en el uso de cables blindados para todas las entradas de señales donde la inducción de "ronquidos" pueda aparecer.

 

Figura 2
Figura 2

 

El blindaje debe ser, obligatoriamente, conectado a la masa del equipo, para que las corrientes inducidas tengan un recorrido y sean, por lo tanto. desviadas del circuito de entrada a masa

El segundo aparece en la figura 3, y consiste en la utilización de un punto común para la conexión a masa de todos los puntos de un equipo.

 

Figura 3
Figura 3

 

Un cable grueso, conectado al chasis, si fuera de metal, se usa como aparece en la figura 4, con el uso de capacitores, ya que los transformadores, por “barra ómnibus" de baja resistencia, desviando todas las señales inducidas por la red, que pueden provocar ronquidos en la salida.

 

Figura 4
Figura 4

 

Tenemos finalmente, el propio desacoplamiento de la fuente, que aparece componentes "inductivos", pueden impedir que las señales inducidas lleguen a tierra por si solas.

Los capacitores usados deben ser cerámicos o de poliéster con altas tensiones de trabajo y valores comprendidos entre los 50 nF y 500 nF.

 

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