Somewhat surprisingly, the TL071 (or TL072 dual opamp) is quite a bit faster than many people realise, and it is quite sufficient for most audio applications. The opamp can be any common device for low frequencies, but at high frequencies (above about 2kHz) a high speed device is required for best performance. Ceramic capacitors are recommended for the most effective high frequency bypass. C4 is a supply bypass capacitor, and should be used at each IC package, with the supply rails both decoupled to ground with 100nF to 10♟ caps. If offset is not a problem for you (and there is absolutely no reason to use DC coupling), simply connect the non inverting input to the earth (GND) rail as shown. Use JFET input opamps and the problem goes away, because their input bias current is so low. Offset will normally only ever be an issue with bipolar input opamps and high values for R3, because the input bias current flows through the resistor and causes the offset. A 100nF capacitor (Cx) in parallel is highly recommended to bypass the non inverting input to earth for AC - this helps to reduce noise. However, if used, the resistor from +in to earth (ground) should be the same value as R3 to obtain minimum DC offset from the opamp output. Maintaining DC accuracy serves no purpose and is actually impossible. In general, this is completely unnecessary because the filters are usually not DC coupled anyway. The opamp shown in the schematic is a single device, but most commonly dual or quad opamps will be used for this kind of application.Ī resistance (Rx) from the +ve input of the opamp is optional to minimise the DC offset voltage. The two caps (C1 and C2) are always the same value. Component values are calculated from the formulae below, or by using the calculator program (see below for details). The resistor and capacitor values shown give a Q of 4, with a centre frequency of 159Hz. The input impedance is (roughly) the value of R1.įigure 1 - Multiple Feedback Bandpass Filter If a high impedance is used, it adds to the total input resistance, causing unpredictable centre frequency and response. The source impedance must be low with respect to the input resistance, and normally these filters are driven from an opamp buffer. This is the basis of several projects that use this filter type, and I have included a small calculator programme to make it easier to determine the component values for different filter parameters.Ī schematic for the filter is shown in Figure 1. The 'speech' from a good vocoder is quite intelligible, and is 'ear candy' of the very best kind for experimental musicians. For those who have not heard of the vocoder, it is a device that takes a music source as one input and vocals as the other, allowing a guitar, keyboard or complete ensemble to be made to speak or sing. These filters are useful for equalisation, analysis and other tasks such as the Sound to Light converter (Project 62) or even a fully functional Vocoder. The multiple feedback bandpass filter is a simple looking design, but it is difficult to calculate the values for a given set of parameters.
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