Looking for a low-frequency audio amp circuit (1 hz to 25 hz)

Discussion in 'General Electronics Chat' started by SumGuy98, Nov 1, 2009.

  1. SumGuy98

    Thread Starter New Member

    Oct 31, 2009
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    I'm going to glue an aluminum plate across the top of a small speaker to create a sealed chamber and attach a small port to the plate (1/8" diam) for a tube that will lead to a pressure transducer.

    I want to create pressure signals (square, sine, etc) in the range of 0 to 4 psi and this is one possible way to do it.

    I can use a function generator to create the signals I need, but I'm going to need an amplifier that has a low frequency response down to 1 hz or less (ideally - DC) into a 4 or 8 ohm load. Probably a few watts at most. Any circuit ideas?
     
  2. bertus

    Administrator

    Apr 5, 2008
    15,649
    2,348
    Hello,

    This amp is about 40 Watts.
    The circuit on this page will go down to 4 Hz (with a 1uF input cap, even lower with a larger one).

    From the page:


    http://sound.westhost.com/project12a.htm

    Greeting,
    Bertus
     
  3. SumGuy98

    Thread Starter New Member

    Oct 31, 2009
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    Is there a low-frequency audio amp that can drive a speaker directly, without a capacitor in series on the output stage?
     
  4. SgtWookie

    Expert

    Jul 17, 2007
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    If you run a lot of DC current through a small speaker, you'll burn it up. :eek:

    Since you're going for low frequencies, and fidelity isn't much of a concern, you might consider using an L272 or L2722 dual power opamp. They're cheap, and you could make a simple push-pull amp or voltage follower with it.

    There are plenty of other single-IC amps that are designed to be used in automotive audio systems that would do it, too.
     
  5. legac

    Well-Known Member

    May 4, 2005
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    I have learned that LF at certain frequency and certain amplitude would harm human body
     
  6. bountyhunter

    Well-Known Member

    Sep 7, 2009
    2,498
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    Yes, many top end audio amps are direct connected. IMHO, any really good audio amp should be direct connected to the speaker. It does mean that the amp must use dual power supplies (positive and negative) and have the output stage adjustable to "zero" the DC bias set point. It's definitely doable.

    You might want to refer to the data sheet for a TDA2030 or similar part to get some ideas.
     
  7. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    Maybe if it's really strong signal level. Subsonic signals applied to the body make people feel queasy but don't hurt them. I read that in the 50's, some movie theaters had them attached to the seats for horror movies to make the people feel more scared.
     
  8. SumGuy98

    Thread Starter New Member

    Oct 31, 2009
    4
    0
    The story so far:

    I bought a cheap 6x9 car speaker, rated for 30 watts, with a mylar cone. I connected it to an Akai audio amplifier and fed a sine-wave signal from a signal generator. At frequencies between 1 to 10 hz, I observed the cone of the speaker move, with an amplitude of about 1/4 inch.

    I then sealed the top of the speaker with 1/4" plexiglass and attached a small tube (1/8") running from the sealed chamber to a pressure transducer.

    With the top side of the speaker now sealed up, the speaker now seemed to barely move at all at those same frequency and amplifier settings. The pressure transducer did respond by generating a pressure signal, but the amplitude of the pressure signal is only about 1/10 to 1/20 of what I need (I need a signal that ranges from 0 to 2 or 0 to 4 psi).

    It now seems that a sealed, air-tight speaker (of the size and power that I am restricted to using) will not give me the kind of pressures I need.

    A 12-volt aftermarket windshield-washer pump will give me the peak pressures I need, but it seems hard to modulate it's rpm to give me a 1-hz to 10-hz pressure signal, and they don't seem to be designed for continuous operation (many minutes to several hours continuous is what I need).

    I might go back to the aquarium air bubbler and use a 3-way medical-grade stop-cock. A reduction-gear motor would drive the stop-cock at a variable rate (1 to 10 rpm) and the bleed-off from the open port should give me something approximating a square wave pressure signal.

    If anyone has any other ideas how to generate a low-pressure signal (0 to 4 psi) of variable frequency (1 to 10 hz) please let me know.
     
  9. SgtWookie

    Expert

    Jul 17, 2007
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    OK, so the entire sealed speaker idea wasn't so great.

    A 6x9 speaker has a surface area of pi*a*b (where a is 1/2 the width and b is 1/2 the length), so total surface area is 42.4 square inches. To get 4psi out, you'd need a force of 169.6lbs from the voice coil. Not too likely that's going to happen without a really big amplifier.

    Can you remove the plate from the speaker without destroying it?

    Perhaps if you could attach some kind of small diapragm or piston/cylinder to the voice coil of the speaker you'd be able to get the pressure you need. Maybe an old piston-type windshield washer pump?

    There's also the ratio of your plumbing's total interior volume to the swept/displacement volume of any kind of pump/diapragm you'll use. The greater your plumbing's interior volume, the greater sweep/displacement volume you'll need to reach the pressure you need.
     
  10. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
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    1/4" plex is not really compliant. A thin plastic diaphragm like mylar glued across the speaker opening should transmit pressure waves better. A 4" circular speaker might work better. Some for automotive sound systems have big power ratings.
     
  11. THE_RB

    AAC Fanatic!

    Feb 11, 2008
    5,435
    1,305
    I would think about some type of solenoid. It's very easy to get a solenoid that will push/pull 2 pounds of force and has a 1" area. hence 2 PSI.

    If you can get a small airtight piston+cylinder off something (maybe one of those little $20 car 12v air pumps?) and attach the piston directly to a large solenoid that should do it.

    If you had access to a lathe you could make a small iron piston (say 1/2" diam 1.5" length) with o-rings in a non-ferrous cylinder, then just wind 2 windings around it to make it a push-pull solenoid that will generate LOTS of PSI.
     
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