Tone Control

(Filters)

What is a filter?

It is sometimes desirable to have circuits capable of selectively filtering one frequency or range of frequencies out of a mix of different frequencies in a circuit. A circuit designed to perform this frequency selection is called a filter circuit, or simply a filter. A common need for filter circuits is in high-performance stereo systems, where certain ranges of audio frequencies need to be amplified or suppressed for best sound quality and power efficiency.

Passive Filters

This type of filter is used when no amplification of a signal is required. The circuit diagram below is used to cut out high frequencies from a frequency spectrum. The resistor and capacitor will have the same current flowing through them (with an AC supply) and V out is the same as dividing off a voltage between two resistors. As the frequency of the voltage signal increases then the impedance of the capacitor increases and a greater voltage will be divided off at V out.

RVin Vout

0V

Passive TREBLE cut filter

Passive Filters

R

Vin Vout

0V

Passive BASS cut filter

This time the capacitor and resistor have swapped places

Break Frequency and Frequency Response Diagrams

There will be a point where the frequency coming into the previous two circuits will cause the impedance of the capacitor to increase past or decrease below the resistance of the resistor. This will be the break frequency of the filter i.e. where the bass or treble frequency ranges end. Remember that the gain of this type circuit can never be higher than a factor of 1 because there is no amplification.

Voltage gain

Frequency (kHz)

General shape of Bass cut filter

TheoreticalActual

1

0.1 1 10

Break (cut off) Frequency

Active BOOST and Active CUT filters

In ET4 we learnt about Frequency dependent resistors I.e. capacitors. They could be used to tune a circuit to a particular frequency and cause a resonating effect between the inductor and capacitor (when their impedance was equal). By including a capacitor in a circuit with an Op-Amp we can get it to produce the following :-

•Active Bass Boost•Active Bass Cut•Active Treble Boost•Active Treble Cut

Active Bass Boost

r

R

Vin Vout

0V

When in series the impedance and resistor of the capacitor and R are added together. Thus when the frequency of the input voltage increases, the resistance of the RC circuit increases and so does the gain factor of the Op Amp.

Active Bass Cut

R

r

Vin Vout

0V

As previously the impedance and resistor of the capacitor and R are added together. This time when the combined resistance of the RC network increases the gain factor of the Op Amp decreases

Active Treble Boost

As with the previous Active Bass Boost circuit the capacitor and R are used in parallel with each other. This enables us to be able to lower the value of the resistance of the RC network and hence raise the gain of the Op Amp.

R

r

Vin Vout

0V

Active Treble Cut

The circuit below is reliant on the frequency of the input signal for its gain factor. If the frequency increases then the capacitor’s resistance increases and the combined resistance of the parallel section becomes that of R2. If the frequency decreases then the resistance of the capacitor also decreases and the combined resistance becomes that of the capacitor. If this happens then the Op Amp will amplify less than previously.

r

R

Vin Vout

0V

Calculating the Break Frequency

To work out the break frequency for each of the diagrams we have seen use the formula below.

1

2RC

This is the value of the resistor in the RC circuit

fco =

Cut off or BREAK Frequency