Need feedback on a subsonic (high-pass) filter design

[MWisBest]

I'm working on designing a subsonic (~25Hz high pass) filter for car audio purposes. Basically I'm making a ported box for my subwoofer, and need to avoid overexcursion below the port tuning frequency. Since the subwoofer is very high power I really need to do this in the line-level RCA inputs to the amplifier.

My idea is to use some op amps for the job. First a buffer, then the second-order high pass filter, then perhaps another buffer just in case. Since I only have +12V for power, I'll need to generate a negative voltage rail. I figure an LT1054 should do the job? Then +9V and -9V linear regulators to get something stable?

My main concern is I have absolutely no idea how to handle differences in signal ground (the RCA shield, this is an unbalanced signal) and chassis/power ground. I am a complete newbie with op amps, so I'm a little clueless here. Do I just reference the op amp signals to the signal ground, tie the output RCAs to signal ground, and use chassis ground for the power supply portion? Do I need to tie the signal ground to my chassis ground? Could I somehow be creating a (unwanted) DC offset on the output? Or am I just overthinking everything here.

Thanks!

Edit: Alternatively, I could do a balanced output signal if that would simplify everything, but I'd prefer to avoid it if it's not necessary. I have no idea how good (or not good) the balanced receiver is on the amplifier input.

[danadak]

If you operate the OpAmp off split supplies, then signal ground is chassis
ground is power rail common ground(s) is RCA shield ground.

If you do a single supply then grounds the same, and either you AC couple
the input to filter or offset the signal into OpAmp front end stage.

For the filter it should be a linear phase design for audio quality.

Use TI Filter wizard or Analog Devices to do design.


http://www.ti.com/tool/filterpro&DCMP=hpa_amp_general&HQS=NotApplicable+OT+filterpro


http://www.analog.com/designtools/en/filterwizard/


If you want to fool with digital filter try PSOC 5LP familiy, tool is free, low cost board is $ 10.
However difficult to get linear phase.






Regards, Dana.

[floobydust]

Similar to turntable rumble filters, basically what you are building. I'm not sure what slope you need, -12dB/oct maybe too little.

For the filter circuit, it's easiest to modify the filters of the sub power amp, or build the add-on inside, if you have the service manual.

Making an external op-amp filter circuit needs a bipolar PSU from a small DC-DC converter, such as 1W PEM1-S12 +/-12V 42mA. They have silly input V spec 10.8-13.2V not sure about >+14V.

A single-supply circuit will have high level thumps during turn on/off, which will damage the loudspeaker if this is out-of-sync with the power amp's turn on/turn off and internal muting circuit.
Either way, you need voltage regulation or engine cranking will cause LF excursions and a surprise.

[MWisBest]

If you operate the OpAmp off split supplies, then signal ground is chassis
ground is power rail common ground(s) is RCA shield ground.

If you do a single supply then grounds the same, and either you AC couple
the input to filter or offset the signal into OpAmp front end stage.

For the filter it should be a linear phase design for audio quality.

Use TI Filter wizard or Analog Devices to do design.

http://www.ti.com/tool/filterpro&DCMP=hpa_amp_general&HQS=NotApplicable+OT+filterpro

http://www.analog.com/designtools/en/filterwizard/

If you want to fool with digital filter try PSOC 5LP familiy, tool is free, low cost board is $ 10.
However difficult to get linear phase.

Regards, Dana.

Thanks for the resources! I appreciate it. I'm not too concerned about linear phase... there's so many issues in a car to begin with, and with the amplitudes and frequencies at play here it's only so audible. But I'll be mindful of it.

Similar to turntable rumble filters, basically what you are building. I'm not sure what slope you need, -12dB/oct maybe too little.

For the filter circuit, it's easiest to modify the filters of the sub power amp, or build the add-on inside, if you have the service manual.

Making an external op-amp filter circuit needs a bipolar PSU from a small DC-DC converter, such as 1W PEM1-S12 +/-12V 42mA. They have silly input V spec 10.8-13.2V not sure about >+14V.

A single-supply circuit will have high level thumps during turn on/off, which will damage the loudspeaker if this is out-of-sync with the power amp's turn on/turn off and internal muting circuit.
Either way, you need voltage regulation or engine cranking will cause LF excursions and a surprise.

12dB/oct is fine with a reasonable Q for my purposes.

That's an interesting idea to build this add-on inside the sub amp, it surely has +/- rails for op amps already... I don't have the service manual for this one but it's not rocket science.

Why does the op amp need a DC-DC converter like that instead of just an LT1054? Does it need some sort of isolation? I understand your concern about turn on/off thump, I think I have that covered though: I can simply keep my device turned on. If I use constant +12V for power plus the ignition/accessory +12V as an instant-turn-on and delayed-turn-off signal I should be covered, no? Obviously that's not ideal having the circuit idling even if the radio is off, but it seems like a fine way to avoid turn on/off thumps/pops. As for engine cranking, that's a good point if I ever make this an actual product to sell, but for personal use I should be fine just not turning up the stereo before engine cranking like an idiot.