choke RFI filter

Thread Starter


Joined Feb 29, 2004
I want to make a preamp for a mic or line. I have squematics that show RFI filters after the mic/line switch, before the instrumental preamp. In both signal paths of the balanced input appears an inductor with a 220pF capacitor to ground, but the squematics don't specify the value of the inductor!
I don't have much experience in real-life circuits. Maybe someone of you know the values of a LC RFI input filter.
Please help :(


Joined Nov 14, 2003
My guess is that you have to pick the value of the inductor such that the resonant frequency of the L and C is equal to the frequency of the signal you want to use, where the resonant frequency is w=(LC)^-0.5

Thread Starter


Joined Feb 29, 2004
mmm, audio signal range is 20 - 20KHz, so that mean that the resonant filter should allow all that bandwidth? how does it work in the signal path? (I've heard about choke filter only on power supplies)
does any one know the values of a standard choke filter used in the input signal path?
(I have two squematics: neither of them show the value of the inductor, just X1 or L1)
Anathema :)


Joined Jan 22, 2004
:) hi anathema,

don't worry about the value of the inductor, just buy that from your neighborhood electronic store and tell them you need that filter to be used for mic/line driver. that's a ready made item. you don't have to make it. they can tell you the value for that. :)


Joined Jan 22, 2004
:) hi,

allow me to enlighten you about inductors ( commonly called coils ). when an alternating current (sinusoidal waveform) starts flowing thru an inductor the magnetic flux that happens in the inductor (L) changes with the current (I). now when a 2nd L is place close to the 1st L (just like in a power transformer) alternating voltage will start to flow in the 2nd L and this effect is what we call "mutual inductance" whenever there is changes in the magnetic flux in a L this also affects itself and this is what we call "self inductance" w/c measures the L character to set an induced voltage as a result of change in its I.

unit of inductance is measured in HENRY meaning when 1A of I flows thru a given L in 1 second resulting to 1 volt is induced to flow in the 2nd L. the mutual inductance between the coils is said to be 1 Henry. by self inductance 1 volt is induced in the 1st L only.

as to how it functions "Lenz's Law" is used (google search on "Lenz's Law"). when I starts to flow in the L, the L resist the flow. however when I decreases, the L allows the I to shortly continue at the rate the I has started.

according to Lenz's Law - the direction of induced current in a coil is such that it opposes the change in magnetic field that produced it.

now back to your problem, when a coil and a capacitor whether they are in series or parallel w/c is your case, the resulting circuit produces a special characteristic, such that impedance of the circuit changes with the frequency of the voltage. current will flow easily as if unimpeded at a given or desired frequency and on the other hand current will have difficulty flowing at another frequency.

you said the value of the inductor is not specified but you have the capacitor value.
below are some equations you can use in solving your problem; :D

fo = 1 / 2 * pi * (sq.rt. of L * C)

L = 1
------------------------- squared
2 * pi * sq.rt. of C / fo

C = 1
--------------------------- squared
2 * pi * sq.rt. of L / fo

use the capacitor value of 220pf and the frequency of 15khz to 20khz and you will have your inductor value. (hint your answer would be no less that 275 and not more than 300) :rolleyes:

i hope this will enlighten and solve your problem :)

as a side light but still on the same topic ( boy am i really in the mood for this? :D :D :D :D )

this kind of circuit is already quite uncommon in audio & low frequency circuits. op-amps have been used and they function the same. this circuit is more commonly used in RF, generation of high voltages such as car ignition coils. theoretically at resonance the impedance is infinite using ideal components. :eek: practically a series or parallel circuit cannot be identified from its other in some circuits. ;)