Op amp suggestions needed

Discussion in 'The Projects Forum' started by ArlingtonMark, Jun 18, 2013.

  1. ArlingtonMark

    Thread Starter New Member

    Jun 18, 2013
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    My first post on this site. I spent 25 years in the Navy fixing information systems and digital combat systems. So, my electronix expertise is pretty thin.

    I have an Infinity Basslink in my car's trunk. It provides the level of bass re-enforcement that I like. The only problem is that its cut-off frequency is 200hz. I'd like to change that to ~400hz.

    The block diagram for my Basslink starts on page 36 of the first attachment. In that schematic pot VR203 (0-50kohm) controls the low freq cut-off of op amp U204B, a New Japan Radio NJM4558L. The second attachment is the spec sheet for the op amp. When the pot is fully CW the cut-off freq is 200hz. Obviously, the pot is not the issue here, but the op amp.

    So, what I'm asking for is suggestions for a replacement op amp so that the highest cut-off freq is ~400hz. Yes, I've checked Mouser, Newark and Digikey - no help.

    Thanks,
    Mark
     
    Last edited: Jun 18, 2013
  2. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    Actually, the pot and near-by capacitors create a frequency filter. The POT is the issue, not the op amp.
     
  3. ArlingtonMark

    Thread Starter New Member

    Jun 18, 2013
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    Thanks for your reply. You are correct-the op amp and the associated pot are being used as a dual channel low pass filter. However, I think the pot isn't the issue; the pot us fully CW (0 ohms) the cut off is 200hz; that's the system highest cut off frequency. I'm not sure which caps are involved; I'll do some more research.
     
  4. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    The resistor network near the non-inverting input of U204B is pretty ugly to decider and change. There are 22k ohm resistors in parallel with each side of a dual pot that is cross wired (one pot increasing and one decreasing as you turn clockwise).

    The simplest first attempt would be to cut the feedback capacitor C215 by half and the input capacitor c216 by half.

    You could model it in pspice to confirm.

    You will draw a bigger load on the sub since less of the input power will be filtered out. You might want to cut some gain overall of it starts sounding muddy.

    Also, if you are using a Dolby receiver, make sure to set that to a 400 hz cutoff or both units will be covering the 200-400 hz range and it will sound odd. Remember, bass frequencies are difficult to determine the source with the human ear. Higher frequencies are more directional. At 400hz cross-over, you will notice where the sub is located in the room. It will start to sound odd if it is not aligned with the other speakers in the room.
     
  5. tubeguy

    Well-Known Member

    Nov 3, 2012
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    I agree with GopherT's suggestion to cut the values of C215, C216 in half, but as he mentioned raising the frequency will make the sub-woofer location more critical.
    Usually bass frequency's are considered non-directional below around 180hz.

    Edit: Re-Read 1st post, is this system in your car ?
     
    Last edited: Jun 18, 2013
  6. tindel

    Active Member

    Sep 16, 2012
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    What you have here is a "Unity Gain Sallen-Key Low-Pass Filter". Which is a fancy term for a "Unity Gain 2nd order low pass filter". Which is a term for a really good low-pass filter.

    Really good filter - very hard to manipulate easily within a system.

    http://en.wikipedia.org/wiki/Sallen–Key_topology

    I played with the circuit for a while but couldn't find a circuit I would suggest (~400Hz with little to no Q). A lot of Q has the potential to start oscillations in your system, so be sure that your design is robust to part variation before trying it! Not to mention that Q will also boost the frequencies near the cutoff frequency before rolling off - ah - physics!
     
    Last edited: Jun 19, 2013
  7. crutschow

    Expert

    Mar 14, 2008
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    Why do you want to raise the crossover to 400Hz? There's little bass energy above 200Hz, which is typically handled by your normal mid-range speakers. And boost in those frequencies may give a boomy sound to the music.
     
  8. ArlingtonMark

    Thread Starter New Member

    Jun 18, 2013
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    Yes, it is in the trunk of my car.
     
  9. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    Exactly. A sub woofer isn't designed to produce mid range sound well.
     
  10. ArlingtonMark

    Thread Starter New Member

    Jun 18, 2013
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    ....is too high a freq for a subwoofer? I assumed that a sub would be able to replicate sound up to ~500hz. Am I incorrect?
     
  11. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    Check the manufacturer's specs for the specific speaker you plan to use. Also, just because the speaker CAN reproduce sounds up to 500 Hz, doesn't mean it will sound nice with the rest of your components for all the reasons listed above.

    A SUB-woofer is intended to be for frequencies below a woofer - that is what sub means (below). It is supposed to provide the frequencies you feel rather than hear. Granted, it bleeds over into the audible range of low frequencies and gives body to the bass, but it is supposed to make low frequencies feel natural like they do in real life when a drum is hit, a book falls on the floor or a car crashes into your house.
     
  12. crutschow

    Expert

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    Perhaps. But why do you want to boost frequencies that are already well produced by your other speakers? It will likely just give a muddy sound. There's a good reason for subwoofer frequencies to be normally limited to below 200Hz. You haven't explained why you want to extend that.
     
  13. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    Depends on the design of the speaker/enclosure. Most subwoofers don't reproduce mid ranges very well.
     
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