Stereo Toner Control Circuit

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STEREO TONERCONTROL CIRCUITOBJECTIVE: Bass, Treble and MIDI control: Passive Approach with

Bass: 300; MIDI: 300-3000; Treble: 3000-20000; with adjustable gain

of 18dB.

2011

SYED MASHHOOD HAFEEZ

1/20/2011


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INTRODUCTION:Filtering one frequency or range of frequencies out of the mixers of different frequenciesis sometimes desirable. To have this filtering, we need circuits capable of selectivelyfiltering one frequency in a circuit. A circuit designed to perform this frequency selectionis called a filter circuit, or simply a filter. A common need for filter circuits is in high-performance stereo systems, like we have used in our project, where certain ranges of

audio frequencies need to be amplified or suppressed for best sound quality and powerefficiency. We all are familiar with equalizers, which allow the amplitudes of severalfrequency ranges to be adjusted to suit the listener's taste and acoustic properties of thelistening area. Here, in our project we use bass and treble adjustment for the same purpose.Like for trance lover of music, bass must be high and treble must be low to enjoy the drumbeats. We are also familiar with crossover networks, which block certain ranges offrequencies from reaching speakers. A tweeter or in other words a high-frequency speakeris inefficient at reproducing low-frequency signals such as drum beats, so a crossovercircuit is connected between the tweeter and the stereo's output terminals to block low-frequency signals, only passing high-frequency signals to the speaker's connectionterminals. This gives better audio system efficiency and thus better performance. Bothequalizers and crossover networks are examples of filters, designed to accomplish filteringof certain frequencies. Although we havent use tweeter model in our project, but the basicconcept of tweeter is to block low frequency signal and give us better sound. These lowand high pass filter will be discussed later in this report.

In short,

A filter is an AC circuit that separates some frequencies from others within mixed-frequencysignals.

Now according to the objective of our project, our filter circuits are composed of passiveelements. Which is further explained on a theoretical part.


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PASSIVE APPROACH FOR FILTER:A passive filter is made only from passive elements, since most filters are linear, in most cases; passivefilters are composed of just the three basic linear elements; resistors, capacitors and inductors. Accordingto the basic concept Inductors block high-frequency signals and conduct low-frequency signals, whilecapacitors do the reverse. But A filter in which the signal passes through a resistor, in which a capacitorprovides a path to ground, ( i.e. RC filter) presents less attenuation to low-frequency signals than high-

frequency signals and is alow-pass filter. Capacitive high-pass filters insert a capacitor in series with theload; If the signal passes through a capacitor, and has a path to ground through a resistor, then the filterpresents less attenuation to high-frequency signals than low-frequency signals and is a high-pass filter.Resistors on their own have no frequency-selective properties, but are added to inductors and capacitorsto determine the time-constants of the circuit, and therefore the frequencies to which it responds.

A passive filter has several advantages over an active filter:

Guaranteed stability. Passive filters scale better to large signals (tens of amperes, hundreds of volts), where active

devices are often impractical No power consumption, but the desired signal is invariably attenuated. If no resistors are used,

the amount of signal loss is directly related to the quality (and the price) of the components used. Inexpensive (unless large coils are required) In contrast to an active filter, it does not require an external power source (beyond the signal).

PROJECT EXPLANATION:Our projects is basically a tone control circuit used to modify an audio signal before it is fed to speakers,headphones by way of an amplifier.

The circuits consist of a network of high-pass, low-pass, and band-pass filters. In their most basic form,they attenuate the high or low frequencies of the signal. Active tone controls may also amplify or boostcertain frequencies. This is called treble or bass boost or cut, respectively. Attenuate means to reduce ordiminish in amplitude. When we turn down the volume control on our stereo, we basically areattenuating the signal being sent to the speakers.

BASS BOOST OR LOW-PASS FILTER:A low-pass filter is a filter that passes low-frequency signals but attenuates(reduces the amplitude of) signals with frequencies higher than the cutoff

frequency. The actual amount of attenuation for each frequency varies fromfilter to filter. It is sometimes called a high-cut filter, or treble cut filter whenused in audio applications. A low-pass filter is the opposite of a high-pass filter

By definition, a low-pass filter is a circuit offering easy passage to low-frequency signals and difficult passage to high-frequency signals All low-pass

filters are rated at a certain cutoff frequency. That is, the frequency above whichthe output voltage falls below 70.7% of the input voltage. This cutoffpercentage of 70.7 is not really arbitrary, all though it may seem so at firstglance. In a simple capacitive/resistive low-pass filter, it is the frequency at which capacitive reactancein ohms equals resistance in ohms. In a simple capacitive low-pass filter (one resistor, one capacitor),the cutoff frequency is given as:

Our Bass or Low pass circuit
http://en.wikipedia.org/wiki/Capacitorhttp://en.wikipedia.org/wiki/Low-pass_filterhttp://en.wikipedia.org/wiki/Low-pass_filterhttp://en.wikipedia.org/wiki/Low-pass_filterhttp://en.wikipedia.org/wiki/High-pass_filterhttp://en.wikipedia.org/wiki/High-pass_filterhttp://en.wikipedia.org/wiki/Resistorhttp://en.wikipedia.org/wiki/Active_filterhttp://en.wikipedia.org/wiki/Sound_recording_and_reproductionhttp://en.wikipedia.org/wiki/Signal_%28electrical_engineering%29http://en.wikipedia.org/wiki/Loudspeakerhttp://en.wikipedia.org/wiki/Headphoneshttp://en.wikipedia.org/wiki/Electronic_amplifierhttp://en.wikipedia.org/wiki/Electronic_filtershttp://en.wikipedia.org/wiki/Attenuatehttp://en.wikipedia.org/wiki/Frequencieshttp://en.wikipedia.org/wiki/Treblehttp://en.wikipedia.org/wiki/Bass_%28sound%29http://en.wikipedia.org/wiki/Filter_%28signal_processing%29http://en.wikipedia.org/wiki/Frequencyhttp://en.wikipedia.org/wiki/Signal_%28electrical_engineering%29http://en.wikipedia.org/wiki/Attenuatehttp://en.wikipedia.org/wiki/Amplitudehttp://en.wikipedia.org/wiki/Cutoff_frequencyhttp://en.wikipedia.org/wiki/Cutoff_frequencyhttp://en.wikipedia.org/wiki/High-pass_filterhttp://en.wikipedia.org/wiki/High-pass_filterhttp://en.wikipedia.org/wiki/Cutoff_frequencyhttp://en.wikipedia.org/wiki/Cutoff_frequencyhttp://en.wikipedia.org/wiki/Amplitudehttp://en.wikipedia.org/wiki/Attenuatehttp://en.wikipedia.org/wiki/Signal_%28electrical_engineering%29http://en.wikipedia.org/wiki/Frequencyhttp://en.wikipedia.org/wiki/Filter_%28signal_processing%29http://en.wikipedia.org/wiki/Bass_%28sound%29http://en.wikipedia.org/wiki/Treblehttp://en.wikipedia.org/wiki/Frequencieshttp://en.wikipedia.org/wiki/Attenuatehttp://en.wikipedia.org/wiki/Electronic_filtershttp://en.wikipedia.org/wiki/Electronic_amplifierhttp://en.wikipedia.org/wiki/Headphoneshttp://en.wikipedia.org/wiki/Loudspeakerhttp://en.wikipedia.org/wiki/Signal_%28electrical_engineering%29http://en.wikipedia.org/wiki/Sound_recording_and_reproductionhttp://en.wikipedia.org/wiki/Active_filterhttp://en.wikipedia.org/wiki/Resistorhttp://en.wikipedia.org/wiki/High-pass_filterhttp://en.wikipedia.org/wiki/Low-pass_filterhttp://en.wikipedia.org/wiki/Capacitor


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The transfer function for Low-pass filter of our circuit is :

TREBLE BOOST OR HIGH PASS FILTER:A high-pass filter's task is just the opposite of a low-pass filter: to offer easypassage of a high-frequency signal and difficult passage to a low-frequencysignal. A high-pass filter, passes high frequencies well but attenuates (i.e.,reduces the amplitude of) frequencies lower than the filter's cutoff frequency.In our project the cut off frequency is at 3000*2. It is sometimes called a low-cut filter or bass-cut filter.

The simple first-order high-pass filter shown in Figure is implemented by

placing an input voltage across the series combination of a capacitor and aresistor and using the voltage across the resistor as an output. The cutofffrequencyfc, at which the output power is half the input power, is,

The transfer function for High-pass filter of our circuit is:

MIDI BOOST OR BAND PASS FILTER:A band-pass filter works to screen out frequencies that are too low or too high, giving easy passage onlyto frequencies within a certain range. Band-pass filters can be made by stacking a low-pass filter on the end ofa high-pass filter, or vice versa. There are applications where a particular band, or spread, or frequencies need tobe filtered from a wider range of mixed signals. Filter circuits can be designed to accomplish this task bycombining the properties of low-pass and high-pass into a single filter. The result is called a band-pass filter.Creating band-pass filter from a low-pass and high-pass filter can be illustrated using our projects MIDI diagram:

Our Treble or High pass circuit

T(S) for low pass filter

T(S) for High pass filter

T(S) for MIDI circuit OR Band pass
http://en.wikipedia.org/wiki/Frequencyhttp://en.wikipedia.org/wiki/Attenuationhttp://en.wikipedia.org/wiki/Cutoff_frequencyhttp://en.wikipedia.org/wiki/Capacitorhttp://en.wikipedia.org/wiki/Resistorhttp://en.wikipedia.org/wiki/Resistorhttp://en.wikipedia.org/wiki/Capacitorhttp://en.wikipedia.org/wiki/Cutoff_frequencyhttp://en.wikipedia.org/wiki/Attenuationhttp://en.wikipedia.org/wiki/Frequency


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What emerges from the series combination of these two filter circuits is a circuit that will only allowpassage of those frequencies that are neither too high nor too low.

GAIN ADJUSTMENT (18DB):Gain controls on an amplifier or equalizers here, arebasically just small potentiometers or volume controls

that allow us to adjust the incoming signal to theamplifier or like here in our project it is used to adjustthe gains of different equalizers, so the amplifier workswell with our choice of bass treble etc. or simply say, tomatch the level of other amplifiers in your system. Herein our circuit, after each of the filters, we used such orderof the combination of op amp and potentiometer so thatwe can get the adjustable gain of 18dbB. The circuitdefines the design of such adjustment.

OPERATIONAL AMPLIFIER:We have used an active device such as an op amp to set out the gain i.e. 18dB. The operationalamplifier is arguably the most useful single device in analog electronic circuitry. With only a handful ofexternal components, it can be made to perform a wide variety of analog signal processing tasks. Tofacilitate true AC output from an amplifier, we can use what is called a split or dual power supply, withtwo DC voltage sources connected in series with the middle point grounded, giving a positive voltage toground (+V) and a negative voltage to ground (-V). Split power supplies like this are frequently used indifferential amplifier circuits. Most amplifiers have one input and one output. Differential amplifiers have

two inputs and one output, the output signal being proportional to the difference in signals between thetwo inputs. The voltage output of a differential amplifier is determined by the following equation: V out= AV(Vnoninv - Vinv)dback, which constitutes the foundation of almost all automatic control processes.Op-amps typically have very high input impedances and fairly low output impedances.

According to our project, the op-amp used with characteristics given below:

Connecting the output of an op-amp to its inverting (-) input is called negative feedback. Thisterm can be broadly applied to any dynamic system where the output signal is "fed back" to theinput somehow so as to reach a point of equilibrium (balance).

When the output of an op-amp is directly connected to its inverting (-) input, a voltage followerwill be created, usually known as buffer. Whatever signal voltage is impressed upon thenoninverting (+) input will be seen on the output. A voltage buffer amplifier is used to transfer avoltage from a first circuit, having a high output impedance level, to a second circuit with a lowinput impedance level.

An op-amp with negative feedback will try to drive its output voltage to whatever levelnecessary so that the differential voltage between the two inputs is practically zero. The higher

the op-amp differential gain, the closer that differential voltage will be to zero.

A summer circuit is one that sums, or adds, multiple analog voltage signals together. There are twobasic varieties of op-amp summer circuits: noninverting and inverting. We have used the

inverting one.

Our Gain adjustment circuit
http://en.wikipedia.org/wiki/Output_impedancehttp://en.wikipedia.org/wiki/Input_impedancehttp://en.wikipedia.org/wiki/Input_impedancehttp://en.wikipedia.org/wiki/Output_impedance


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STEPS INVOLVED IN DESIGNING OUR CIRCUIT: We selected the values of passive elements (resistors and capacitors) according to the

requirement of the filter which we have to design. The transfer functions of low, high and band

passes respectively are given below:

We used an active element, i.e. op-amp as a voltage buffer at the input so that it isolates theimpedance of the circuit and the input signal.

Also for MIDI, we cascaded the bass and treble (i.e. high and low pass) circuits to make a band-pass filter. Again for cascading e used buffer.

After each filter we used a voltage buffer for the same purpose as mentioned above. Then we use an op-amp (i.e. an active element) with inverting configuration to set the gain

according to the objective of our project i.e. 18dB. For that we use variable resistors for each of

the filters to vary the gain from -18dB to +18dB.

Then all the three output of the toner is summed through a summer amplifier, and thenconnected the output of the summer to the power amplifier.

We have also built the power amplifier, a class AB power amplifier.

The circuit diagram below contains all the practical information and all the values of elements we have

selected.

T(S) for Band pass filterT(S) for high pass filterT(S) for low pass filter


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PCB:


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Stereo Toner Control Circuit

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