A bit new to passive filters, so wondering what is the formula of frequency cutoff for the two pole Low Pass RL filter? And how Gain/Frequency graph would look like? Thank you!

 

The second RL loads down the first RL causing a VERY droopy response.

 

for the two pole Low Pass RL filter?

LC filters are seldom used for low frequency filters due to the large inductors required and the poor tolerance of inductors.
RC or active filters using op amps are more common.

 

The interaction between the two stages can be reduced if the impedance of the second stage is made ten times the impedance of the first stage.

 

LC filters are seldom used for low frequency filters due to the large inductors required and the poor tolerance of inductors.
RC or active filters using op amps are more common.

With speaker crossovers being the exception.

 

The second RL loads down the first RL causing a VERY droopy response.

An equal-value RCRC filter has a Q of 0.333. I suspect an RLRL would be the same.
The best Q you can achieve is 0.5 (critically damped), where the impedance if the second part is >10 times that of the first.

 

An equal-value RCRC filter has a Q of 0.333. I suspect an RLRL would be the same.
The best Q you can achieve is 0.5 (critically damped), where the impedance if the second part is >10 times that of the first.

Below is the LTspice sim of two cascaded RC filters, showing the difference for them being equal, and the second having 10 times the first impedance:
The equal filter is about 6dB down at 1kHz while the other is about 3db down, with their high frequency rolloffs being nearly the same.

 

Below is the LTspice sim of two cascaded RC filters, showing the difference for them being equal, and the second having 10 times the first impedance:
The equal filter is about 6dB down at 1kHz while the other is about 3db down, with their high frequency rolloffs being nearly the same.

View attachment 302163

RCRC filters are really useful - you can achieve 12dB/octave roll-off, there is no overshoot or ringing, no problems with pickup in inductors (or getting inductors of high enough inductance) and no problems with op-amp phase-shift at high frequency spoiling the attenuation. The low Q is the only drawback.

 

RCRC filters are really useful -

Yes, for things like signal ripple reduction where a sharp rolloff is not necessarily needed.

 

Everyones graphs are similar, I appreciate the help, but still a bit confused about Cut Off frequency... when calculating by the hand getting around 80Hz, when you guys and other forum people get about 47Hz. Formulas I'm using:

 

Your equation for the -3dB frequency must include the Q.