Question on LCR circuit

[matevana]

I've often used an inductor, capacitor and resistor wired in series and shunted across a driver's terminals to suppress a peak below the crossover point.

Sometimes I get better results when the capacitor is omitted from this circuit. What affect does an inductor and resistor have by themselves, when they are wired in the same manner? 😕

 

[PeteMcK]

high pass filter

 

[matevana]

high pass filter

It doesn't behave like a high pass filter. I am familiar with inductive high pass circuits, but they typically have the resistor in series and the inductor in parallel with the load. In this case, picture an LCR notch filter with the capacitor removed.

 

[skorpion]

Matevana, to answer your question, the inductor and resistor will basically have no effect by themselves in such a filter.

Let my small Spice example illustrate this. The circuit in the following picture is constructed to have a notch effect at 1 kHz with the chosen 4 ohm impedance speaker:

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The corresponding frequency response will be (magnitude measured in dB):

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Removing the capacitor from the filter will result in this response:

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/Erling

 

[matevana]

Matevana, to answer your question, the inductor and resistor will basically have no effect by themselves in such a filter.

Let my small Spice example illustrate this. The circuit in the following picture is constructed to have a notch effect at 1 kHz with the chosen 4 ohm impedance speaker:

Hi Erling,

Interestingly enough, experimenting with different value coils and resistors in series, and then shunted across the woofer's terminals did seem to act as a shelving filter of sorts.

I was able to borrow an SPL meter and found that the combination of a 1 mH coil and various resistors wired in series, then shunted across the woofer's terminals basically acts like a notch, in this case only affecting frequencies between 200Hz and 630Hz. Frequencies above and below were unchanged and the lower the resistors value, the greater the attenuation.

The columns below represent the test tone level in decibels with resistor values of 5, 7, 12 and 15 ohms (rated 10 watts) respectively at various frequencies between 200Hz and 630Hz.

200 Hz, 76, 76, 77, 77

250 Hz, 76, 77, 78, 79

315 Hz, 73, 75, 76, 76

400 Hz, 71, 72, 74, 74

500 Hz, 75, 76, 77, 77

630 Hz, 68, 69, 71, 71

Interestingly enough, when the capacitor was restored making a true LCR notch, the notch's center frequency just shifted to the right, but the Q remained basically the same. I'm guessing the filter works without the cap, since its seeing some capacitance from a standard LR2 circuit which sits between the input and the notch. Does that make any sense? Also, the amount of attenuation is inversely proportional to resistance, which seems counter intuitive. 😱

 

[skorpion]

The sim of course illustrates what happens in an ideal circuit with for example a loudspeaker with constant impedance and linear FR.

In reality you don't have a perfect loudspeaker and other components in the circuit interacts with the LCR depending on configuration.

/Erling