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Old 4th August 2006, 01:07 AM   #1
preiter is offline preiter  United States
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Default Noob amplifier design question

A question about the input of amplifier designs.

Most amp circuits I see on this board have on the input: a parallel resistor to ground, a series capacitor and a series resistor in some configuration.

The series capacitor is there to block DC.
The parallel resistor, as I understand it, allows a small current to flow from the signal source, which some output stages require to work properly.

I have never been able to figure out the purpose of the series resistor. Anyone care to enlighten me?
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Old 4th August 2006, 02:11 AM   #2
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Default Re: Noob amplifier design question

Quote:
Originally posted by preiter
A question about the input of amplifier designs.

Most amp circuits I see on this board have on the input: a parallel resistor to ground, a series capacitor and a series resistor in some configuration.

The series capacitor is there to block DC.
The parallel resistor, as I understand it, allows a small current to flow from the signal source, which some output stages require to work properly.

I have never been able to figure out the purpose of the series resistor. Anyone care to enlighten me?

as far as i know :

The parallel resistor stops high gain oscillation when you unplug the source.

The Series resistors increases impedence a little bit

and the series/parallel configeration as a whole creates a devider.

the cap across the input stops rf getting into the input from cables etc

the in series cap blocks dc.
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Old 4th August 2006, 07:51 PM   #3
AndrewT is offline AndrewT  Scotland
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Hi,
there are commonly two filters on the input to an amp.

A low pass and a high pass.

The high pass usually comes first. series capacitor followed by cap to ground. The -3db frequency of this filter is F=1 / [ 2 Pi R C ]
it is usually set between 1Hz and 10Hz.

The low pass filter usually follows, it is a series resistor followed by a capacitor to ground. F-3 is usually set between 40kHz and 300kHz

There is somtimes a resistor to ground from the input terminal to reference the input end of the capacitor to ground and help prevent a DC voltage building up on it if it is left unconnected. This resistor is usually about 10 times the input impedance of the amplifier.
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