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Old 20th June 2011, 05:21 PM   #1
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Default what is R and C in series to ground?

In an application note of a chip im using it has a 1 nF cap in series with a 20ohm resistor on between the speaker wires and ground.

What is this? an RC low pass wouldn't need the resitor in series to ground. :S
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Old 20th June 2011, 05:36 PM   #2
kevinkr is offline kevinkr  United States
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Google "Zobel Network"
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Old 21st June 2011, 08:51 AM   #3
Mooly is offline Mooly  United Kingdom
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The values look a bit strange

Post a circuit showing what you mean.
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Old 23rd June 2011, 02:02 AM   #4
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that is for inductive loading compensation. most complementary-symmatry amps that are in IC forms requre this especially if it is bootstrapped type. this should be located near the amp IC/speaker terminals.

Last edited by DavesNotHere; 23rd June 2011 at 02:05 AM.
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Old 23rd June 2011, 02:07 AM   #5
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As frequency increases then the speaker inductor goes up in impedance.
The zobel network counteracts this by decreasing impedance as frequency goes up.

This gives a more equal load to the amplifier over the audio frequency band.

Zobels are also put across speakers after the crossover again to give a constant load to the crossover.
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Old 25th June 2011, 02:12 PM   #6
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Quote:
Originally Posted by chico1st View Post
In an application note of a chip im using it has a 1 nF cap in series with a 20ohm resistor on between the speaker wires and ground.

What is this? an RC low pass wouldn't need the resitor in series to ground. :S
It is a snubber circuit. From National application note:

DETERMINING SNUBBER CIRCUIT COMPONENT VALUES
The Snubber circuit is composed of RSN and CSN. The common
value of 0.1μF is chosen for CSN and then a starting value
of 10Ω is used for RSN. The pole location created by the circuit
is found using Equation 4 below.
f-3dB = 1/(2πRSNCSN) (Hz) If small, high frequency oscillation is observed on the output then the snubber values need to be adjusted. Leave the value
of RSN set to 10Ω and increase the value of CSN until the oscillation
ceases.

Tom
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Old 27th June 2011, 03:24 PM   #7
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yeah neat, it makes sense as a way to flatten out the response.
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Old 27th June 2011, 03:33 PM   #8
DF96 is offline DF96  England
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If specified correctly the audio response is not affected, but potential ultrasonic instability is suppressed.
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