# HIGH PASS FILTER in audio amplifier (HELP)

#### andrew132

Joined Feb 2, 2017
96
hello , i want to know if C2 and R1 are HIGH PASS FILter and the cutoff frequency is 100khz ? and what is the formula to calculate the cutoff frequency of R1C2, and what is the ROLE of C1 ?

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#### ericgibbs

Joined Jan 29, 2010
12,467
hi,
Have you calculated the RC cut off frequency as being 100kHz
Also note the output of the Amp is at Vsup/2 , so what do you think the role of C1 is.?
E
Is this a college assignment.?

#### andrew132

Joined Feb 2, 2017
96
hi,
Have you calculated the RC cut off frequency as being 100kHz
Also note the output of the Amp is at Vsup/2 , so what do you think the role of C1 is.?
E
Is this a college assignment.?
no not a college assignment , i just like to know what is the role of those elements

#### andrew132

Joined Feb 2, 2017
96
and is that low pass filter ?

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#### ericgibbs

Joined Jan 29, 2010
12,467
hi,
OK,
The output of the amp, with no sound coming thru, will be at half the battery voltage, so without the C1, there would be a DC current flowing into the speaker coil which would damage the speaker .
The R and C, filter out any high frequency 'hiss' on the output of the amp.
E

At higher frequencies the Impedance of the filter cap is low.
ie: the Equation is Zc = 1/ ( 2 * pi * F * C)

#### andrew132

Joined Feb 2, 2017
96
hi,
OK,
The output of the amp, with no sound coming thru, will be at half the battery voltage, so without the C1, there would be a DC current flowing into the speaker coil which would damage the speaker .
The R and C, filter out any high frequency 'hiss' on the output of the amp.
E

At higher frequencies the Impedance of the filter cap is low.
ie: the Equation is Zc = 1/ ( 2 * pi * F * C)
thanks

#### andrew132

Joined Feb 2, 2017
96
so the RC in serial are HIGH PASS FILTER
and the RC in parallel are LOW PASS FILTER
and Fc=1/(2*pi*r*c) is that right ?

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#### AnalogKid

Joined Aug 1, 2013
9,174
The R and C, filter out any high frequency 'hiss' on the output of the amp.
Ummm ... no. There is no perceivable hiss at 160 kHz and above.

While they do form a circuit that siphons off higher frequency energy relative to the audio band, the primary purpose of the R-C is to change the phase angle of the load impedance the output stage sees at high frequencies to prevent oscillation. This offsets the phase shift caused by the speaker's voice coil inductance. It is called a Zobel network, and is very common on National Semi datasheets and appnotes.

https://en.wikipedia.org/wiki/Zobel_network

ak

#### Audioguru

Joined Dec 20, 2007
11,249
Your R and C in parallel are NOT a lowpass filter.
A lowpass filter is a series resistor feeding a capacitor to ground. The resistor passes the low frequencies and the capacitor to ground forms a voltage divider to high frequencies.