Active filters/ Tone controls

[Amagic]

Hi does anybody know how to read the circuit attached, I'm trying to find out how to work out the max and min Gain for each circuit,

the part that is confusing me the most is the capacitor, what is the capacitors job, I'm trying to find out how the circuits reacts to various frequencies, and what are the formulas used

If anybody can help i would be very grateful

[Angelo Skouras]

[IMG]D:\z0Aggelos\filterdiagrams.jpg[/IMG]Dear friend

Here comes an audio engineer from far Athens (Greece) to explain your question.

The basic part of both circuits is an inverting amplifier made of OPAMP (see fig. 1). The gain of this stage is A=R2/R1

The capacitor, is actualy a resistor whose value changes with frequency. The bigger the frequency the smaller the resistor of the capacitor (and the oposite hapens with an inductor)

If we put a capacitor in paralel with R1, (see figure 2a) at big frequencies the C has a smaller resistance, and the OPAMP, that cannot distinguish between the true resistor and the "resistor" of a C and amplifies more. The bigger the frequency the bigger the amlification, so an R1/10 comes in series with capacitor to avoid infinite gain, for stability reasons (see figure 3a).

Now if we put the C in paralel with R2, by the same mechanism, we take an amp that has a gain that decays with frequency (see figures 2b,3b).

The two diagrams that you have in your thread combine the 3a,b diagrams with a variable resistor giveing 3a at one edge and 3b at the other, and intermedate circuits with intermedate sounds when the wiper of the variable resistor is between.

Their diference is that one gives 3a when the wiper is left, and the other when it is right, and the bigger C makes the hole event happen at lower frequencies for the bass filter.

If you want, I can send you a desing of a 3band equaliser for guitar. It is passive and I have designed on my own.

I hope I helped you, and I hope you see the draws....

[Angelo Skouras]

Sorry I try to send the diagrams....