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Making LP – LowPass – filter of the low frequencies – for our subwoofer.

In our subwoofer is used the low-frequency dynamic loudspeaker, which can reproduce only specific part of the spectrum of signal – its low-frequency part – for that it and it is necessary. Therefore, if we will feed to it entire spectrum of signal by pillar, it will spit and grunt, therefore as unable all this to reproduce. Thus, we will obtain the mountain of distortions instead of the decent sound. In order of them to get rid, we must cut off that part of the spectrum of signal, which [sab] reproduce cannot and it must. For this there is the filter of low frequencies. Well, it is more precise, it will be.

We will not fumble on the sites in the Internet and commonplace take already finished diagrams – we everything will calculate and will solder ourselves. Especially because this is not so already it is complicated, as it seems at first glance.

After conferring with tomcat, we decided not to throw out here the heap of the formulas, from which in any normal person the hair begin to stir simply everywhere, but to use special soft for calculating any different filters. Programm is called Filter Of wiz Of pro. We start.

Filter of wiz of pro

This is its main window, from where everything begins. You do see buttons with the mysterious combinations of beeches and the dark-blue strips? Here is it to us and it is must.
Thus, programm can manage the following filters:

HP – HighPass –  filter of the high frequencies
BP – BandPass – the band-pass filter
BS – BandStop – the band rejection filter
USR – the user type of filter – that you want, that also count.
At the given moment us interests the first button – LP. We press.


To the left – form for the filling of initial data for the filter. To the right – AChKh (you be healthy!) filter. Thanks. This I did not sneeze, this tomcat does not know what is of AFC (you be healthy!). Thanks.  Aplitudу – frequency characteristic makes possible for us clearly see which will be the signal level at one or other frequency or another. It is very convenient, very.
It is good, we start the initial parameters.

Passband Of attenuation ( weakening signal in the passband ) we leave 0,5 dB, it will choke.
Stopband Of attenuation – weakening signal at the cutoff frequency.
Passband Of frequency – this is frequency, is more accurate the frequency band, which our filter must pass; weakening signal will begin from this frequency. Usually this frequency composes from 100 to 300Hz, depending on subwoofer, accomodations, the acoustics, with which this subwoofer will be used. Well, let us select for example 150Hz.
Stopband Of frequency – cutoff frequency – the frequency, at which realizes the maximum weakening of signal. We select 500Hz. I will explain below why.
Overall Of filter Of gain ( the overall gain of filter ) we leave 0 dB – our filter it will not be either strengthen or weaken signal.

Now a little theory, even not theory, but simply it is desirable to describe from where they grow foot in all these cutoff frequencies and weakenings of signal.

Tomcat with the beech

There is this concept in the filters as the slope of the decrease. Actually, this piece designates as rapidly will be weakened signal in the selected frequency band. And there is one additional concept for the filters – this is the order of filter. Order not in the sense “wash after itself dishes, pig”, but in the sense of a quantity of elementary it is sectional in the filter (to each component usually it is fallen one capacitor).
We look at the graph, where AFC of the filters of different orders are disfigured.


Now, I do hope, it is understandable what is the slope of the decrease is? The steeper “the hill” on the graph departs downward, the greater this slope itself. And as is evident, it healthily depends on the order of filter. Our task lies in the fact that obtaining filter with a maximally possible slope of the decrease. However, with an increase the ordinal number of filter, it increases and the complexity of its circuit solution; simply by speaking – it is necessary to the fig of any pieces of iron. Therefore a compromise between the filter discrimination and its complexity in practice finds.

However, let us continue.
We harvest the button Of calculate.
Clap – appeared one additional bookmark – Calculation of results. We leave there.

Calculation of results

Thus, we have already table with some results. Is above – different types of filters – Battervort’s, Chebyshev‘s, inverted of Chebyshev’sElliptical and Bessel’s. In the column to the left us interests three parameters – Order – order, Circuit of stages – a quantity of units of diagram and Passband of ripple – nonuniformity in the passband. We will gather the filter Of  battervort’s – these are the most extended filter, in it the best linearity and the simple circuit solutions.

It is evident according to assigned by us initial data that is obtained the filter of the 4th order. In principle, if on what that to the reasons for this little or vice versa much, changing cutoff frequency and/or weakening at the cutoff frequency it is possible to change the order of filter. By the way, it is possible to immediately assign the order of filter, after placing the mark Of force Of filter Of order and after indicating the necessary order of filter.
On the whole, we harvest Next.


On this page to us they propose to admire to calculated AChKh of the selected filter.
We go further.


It is now necessary to select the schemotechnics solution for the future filter for two units of diagram – Stage1 and Stage2. Are possible 13 versions – to any taste and size. Without philosophizing we slyly select very first – filter Of  selen and Ci. We harvest the button Of select and go further.

Circuits w of comp

Here almost and everything. Program calculated for us the ratings of the necessary components of the schematic of our filter. We can leave everything as is, and it is possible to a little change the ratings of elements and to look as this will reflect on resulting AChKh, which can be looked on the following page.

Preparedly almost

Now it remained to compose the overall diagram of our filter and to solder it.
Diagram will appear as follows:

Galina [Blanka]

In the filter is used [OU] [K]1401[UD]2[B] – this is the quadrupled operational amplifier of the general purpose. Very convenient piece – microcircuit one, and operational amplifiers immediately four. It is possible so to use [K]1401[UD]2[A] or [K]1401[UD]1. In principle, it is possible to use other operational amplifiers, but on the pay there will in that case be no longer one housing, but two or three.
On the element DA1.1 is assembled the buffer stage. DA1.2 and DA1.3 – directly very filter of the 4th order. The unusualness of the ratings of resistors R2-R5 explains by the fact that in me there were no near at hand resistors of precisely those ratings, which were obtained according to the calculation and it was necessary to compose them from the series-connected resistors with the value of 47 kilohms. The nourishment of filter must be stabilized.
Uff… well like everything thus far.
In the following, conclusion we will quickly examine amplifier for entire this disgrace and let us connect all this together – filter, amplifier and box. Well it is understood let us look how to us to be with the nourishment – to eat that everyone wants.

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