Need feedback on a subsonic (high-pass) filter design - Page 1
Forum section: electronics / beginners

author: MWisBest
author_rank: Contributor
timestamp: February 20, 2018, 05:47:18 pm
content: 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, Icoulddo 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.

author: danadak
author_rank: Super Contributor
timestamp: February 21, 2018, 12:05:33 am
content: If you operate the OpAmp off split supplies, then signal ground is chassisground is power rail common ground(s) is RCA shield ground.If you do a single supply then grounds the same, and either you AC couplethe 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+filterprohttp://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.

author: floobydust
author_rank: Super Contributor
timestamp: February 21, 2018, 12:41:32 am
content: 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 as1W 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.

author: MWisBest
author_rank: Contributor
timestamp: February 21, 2018, 03:45:05 pm
content: Quote from: danadak on February 21, 2018, 12:05:33 amThanks for the resources! I appreciate it. I'm nottooconcerned 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.Quote from: floobydust on February 21, 2018, 12:41:32 am12dB/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 notidealhaving 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.