Designing Analog Filter for Audio Processing

[nyk]

HI all!!

I need to design a ANALOG filter for processing input audio. I intend to make a low pass filter with cutoff around 10kHz.
The input to filter is from a line-in or microphone after passing through amplifier.
Can anyone with experience in audio tell me how to proceed as to choice of quality factor, response, order of filter.
The idea is to separate the noise and other high frequency components. The filtered output is to be fed to a ADC(which is later)
and then processed digitally.
Plz guide as to how to approach.

 

[BradtheRad]

Re: Designing Analog Filterfor Audio Processing

1.

The simplest method is a high-frequency diverter. Install a capacitor to ground. To select the value it depends on surrounding impedances.

First order. Low Q. Gradual rolloff curve. May be unsatisfactory.

2.

A Pi design gives a steeper rolloff curve. Two capacitors with a resistor in between.

3.

A coil in series. To select the value depends on surrounding impedances. Low Q. Gradual rolloff curve.

For a steeper rolloff curve you can use two coils in series (Pi configuration). Between them put a resistor to ground.

4.

A combination of 2 coils and a capacitor, arranged in similar manner as above filters.

Or a combination of 2 capacitors and a coil.

5.

An op amp in configuration with any of the above. Sallen Key topology. Allows capacitors and resistors to yield a steeper rolloff curve than they would without the op amp.

Second order or higher is known as Multiple feedback topology. This may be easiest to construct since capacitors are easy to switch (whereas coils require much more work).

6.

You may hope you can simultaneously do two jobs with one op amp: both to amplify a mic level signal, and to filter it with high Q. This may not be possible with one op amp.

For steepest rolloff curve, consider the arrangement using 3 op amps. Biquad topology (also related to Tow-Thomas).

 

[LvW]

Re: Designing Analog Filterfor Audio Processing

5.
An op amp in configuration with any of the above. Sallen Key topology. Allows capacitors and resistors to yield a steeper rolloff curve than they would without the op amp.
Second order or higher is known as Multiple feedback topology. This may be easiest to construct since capacitors are easy to switch (whereas coils require much more work).

6.
You may hope you can simultaneously do two jobs with one op amp: both to amplify a mic level signal, and to filter it with high Q. This may not be possible with one op amp.
For steepest rolloff curve, consider the arrangement using 3 op amps. Biquad topology (also related to Tow-Thomas).

Hi BradtheRad,
may I correct you in two points?

* A 2nd oder filter always has a roll-off of 40 dB/dec - independend on configuration and technolgy: passive or active, RC or RLC. Also the classical "multifeedback topology" is of 2nd order only. The only remarkable difference is: A passive RC filter can provide maximum pole Q values of 0.5 only; however, the roll-off is 40 dB/dec.

* Also the 3-opamp topology (Tow-Thomas, or Kerwin-Huelsman-Newcomb, or Fleischer-Tow) is of 2nd order only and, thus, has not "the steepest roll-off".

 

[BradtheRad]

Re: Designing Analog Filterfor Audio Processing

Hi BradtheRad,
may I correct you in two points?

* A 2nd oder filter always has a roll-off of 40 dB/dec - independend on configuration and technolgy: passive or active, RC or RLC. Also the classical "multifeedback topology" is of 2nd order only. The only remarkable difference is: A passive RC filter can provide maximum pole Q values of 0.5 only; however, the roll-off is 40 dB/dec.

* Also the 3-opamp topology (Tow-Thomas, or Kerwin-Huelsman-Newcomb, or Fleischer-Tow) is of 2nd order only and, thus, has not "the steepest roll-off".

Yes I needed correcting. Got a refresher by checking my 'Active Filter Cookbook.' That's what I get for relying on my memory.

It is possible to achieve a third order low-pass filter with one op amp and three capacitors.

The number and configuration of capacitors (and/or coils) is the more relevant factor in determining the order no. and steepness of rolloff curve.

Multiple op amps are not a guarantee of improved Q or order number, but they are useful to add more types/stages of filters.

 

[LvW]

Re: Designing Analog Filterfor Audio Processing

............
It is possible to achieve a third order low-pass filter with one op amp and three capacitors.
The number and configuration of capacitors (and/or coils) is the more relevant factor in determining the order no. and steepness of rolloff curve.
Multiple op amps are not a guarantee of improved Q or order number, but they are useful to add more types/stages of filters.
.............

Yes, agreed.
For my opinion, selection of a filter for a specific application is a rather challenging process. You have to find a trade-off beween several - partly conflicting - requirements.:
active vs. passive, topology, filter order, lowpass approximation (ripple, with/without zeros), impedance level, amplifier type (opamp, OTA, CC,...),.....