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Audio tone generator

Discussion in 'Audio' started by Doug3004, Oct 18, 2017.

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  1. Doug3004


    Sep 5, 2014
    I have a transducer I want to use for something.
    The part is a "bass shaker" type thing, basically the voice coil from a speaker. You attach this to a chair and connect it to a bass-channel output of an audio amp so that you can feel the bass better, but I'm just using it as a agitator to try to remove bubbles from a liquid mix. The bass shaker is a common inexpensive one stated as 50mm across and 30mm high, 4 ohm coil and 20 to 25 watts. The typical ranges stated for it are 20 to 80 Hz, but people say it will go as high as 500 Hz. To get the best effect out of the device it would need to be fed an AC signal, not just pulsing-DC.

    My first thought is to just get a little "Arduino-style" DC motor driver, since apparently most of those let you reverse the motor direction when the voltage is still above zero. They use PWM to control the motor voltage (up to 12v usually) so both the frequency and amplitude could be controlled this way.

    I'm thinking there might be an easier way however, but without building the entire thing from scratch. It would just be an analog circuit with a couple knobs to control the frequency and amplitude, from 20 to 500 Hz and up to maybe 6 or 12 volts?... It doesn't need to be precise in frequency or power, nor does it need to display the frequency or power.

    I don't know what this might be called however..... When I search Google for "tone generator" all I get is results for telephone line testers. Is there a name for this thing, or a common (cheap-China) product sold that can do this?
  2. Harald Kapp

    Harald Kapp Moderator Moderator

    Nov 17, 2011
    A signal generator for this application can easily be built from a 555 timer IC used as an astable multivibrator.
    This IC will not have enough power. A simple class AB driver will increase the output power, e.g. as shown here (bottom of the page, with diode biasing).
    The output capacitor will remove the DC portion of the signal and let only AC pass to your speaker. Or use an IC design. Your favorite internet platform probably has off the shelf modules for that purpose. You may have to add the output capacitor for removing the DC portion.
  3. Doug3004


    Sep 5, 2014
    I have a project going on that involves driving a solenoid coil at 20 to maybe 200 Hz. I have ordered a variac to try but am wondering about using an audio amp instead. Most other common power converters don't output the AC wave, they end up giving a pulsing-DC and so aren't as useful.

    Looking on the China sites, they sell "audio power supplies" that are a rectifier and a few smoothing caps, and these have three output terminals that are ground, + and - voltage (of whatever the wall socket voltage is). There is no transformer on board at all. There is at least 10-15 different kinds of these little boards.
    And they sell (transistor-based) "sub amps" that have three power inputs that specify { +50 }, { -50v } and [common] power connections. There's probably at least ten different kinds of sub amps ranging from 100 to 300 watts, and ALL of them have the three +50, -50 and common power inputs.

    What I can't find is the transformer needed to connect the first board to the second? A search there for "audio transformer" just gets little line-level ones, and when I look for "audio power transformer" I just get isolation transformers (1:1) or transformers set up to feed 6v to tube filaments.
    What might this wall-voltage-to-50-volts transformer be called? Is there some special name for it?
    Or is something else entirely used here?
  4. Externet


    Aug 24, 2009
    To raise bubbles, I would use a table shaker, which is a table with springs as legs, with a motor attached underneath with an eccentric weight on its shaft, speed controlled (dimmer?) Simple to build.

    Like ---->

    To do the audio approach, to generate audio, this works


    Fed to a household stereo or car subwoofer amplifier to drive your transducer.
  5. Doug3004


    Sep 5, 2014
    I can't use a shaker table as the liquid is sprayed on a roller that is part of a big & heavy piece of equipment. Shaking the whole entire thing isn't practical.

    I like using the little China modules to build stuff because that way the whole thing is already separated into different stages, and if it stops working you can easily figure out what stage failed and replace it. That's not nearly as easy to do if it's all combined onto one cramped PCB.

    I guess it would probably be simpler to just use one of the function generators to drive a car sub amp running off a big-amp 12v dc power supply. The $80-$100 price for that isn't an issue here, I just wondered if there was not a cheaper-but-uglier way to do it because it doesn't need to look pretty.

    I ordered a variac to try using; it will be fixed at 60 Hz but might work well enough as it is. I've never had one before and I couldn't find any good guide on what kinds of current you get when drawing lower-than-input voltages. ......From testing with a Dremel tool as a vibrator I am estimating this thing needs to run maybe 75 watts constant output at the most, and maybe 2-4 amps. That works out to 25-30 volts, but I dunno if the variac can put out that much current at such a low voltage setting. Seems to me that the current would have to be basically proportional to the voltage/center tap setting, since the winding wire is all the same thickness?

    If it can't put out enough current then the car-sub-amp option is probably easier.
    And I needed a variac anyway. ;>)
  6. duke37


    Jan 9, 2011
    A Variac is rated by the output current and is limited by the brush connection. Too much current can heat the winding and give a rough surface for the brush to slide over. Do you have a Variac or variac?
    An 8A Variac is a big,big lump.
  7. Audioguru


    Sep 24, 2016
    Why were you looking at 300W Chinese amplifiers when your transducer has a maximum allowed power of only 25W?
    Why do you need a variac? It will melt or it will blow up the transducer because 25W into 4 ohms has a voltage that is only 10V RMS.

    Most bridged car amps produce 25 Whats, not 25 Watts into 4 ohms. 25 fake Whats is actually about 14 real Watts. A bridged amplifier that produces 25 Watts into 4 ohms needs a power supply that is about 17VDC or 18VDC.
  8. Doug3004


    Sep 5, 2014
    I didn't have one, but I ordered one. A China-made 20 amp, 2000VA one that tops out at 130 volts, price is ~$120. I don't know who really makes it, lots of places sell these models (there is a 5-amp and a 10-amp one also).

    If necessary I can make a step-down transformer and run that off the variac. My assumption is that the coil I'm using will need not-many-volts but possibly several amps, and I don't know anything off the shelf that can do that--AND that allows adjusting the power level up and down easily.

    I'm not using the original transducers, since they were way too weak. The way they are built just isn't ideal for my use here.

    What I'm using now is a coil of 22-awg magnet wire wound around an air core that is 1 inch wide and roughly 1 inch deep. This is two 1/4-lb spools of this wire.

    Also the car amps I was looking at are single-channel sub amps, and those cost $50 for an amp rated at 500 watts or so. That is the lower-end amps and so even if they only really put out 50% of their rated power, that amount is still way more than I should need.
  9. Alec_t


    Jul 7, 2015
    You're aware that Variacs are auto-transformers, hence don't provide mains isolation?
  10. Doug3004


    Sep 5, 2014
    Yea I know that part.
    The variac has two outlets as its 'output', and so the transducer is going to have a fuse + a normal electrical cord+plug. Because of having a normal power cord, the transducer would need a fuse to blow if it ever gets plugged into a normal outlet.

    Part of the problem I have right now is that (with a home-made coil) I don't know how much power it really needs to work well... And so I have to start with something to figure out how much volts / amps is really necessary here.

    Overall I think I like the function generator+car amp idea the best, but the variac will provide some better insight I think.
  11. duke37


    Jan 9, 2011
    Winding your own coil without proper design of the magnetic circuit will lead to very low efficiency. I would not be capable of deriving the equations.
    You should consider making the system resonant so that the transducer only has to provide the energy lost per cycle rather than the total energy per cycle
  12. Doug3004


    Sep 5, 2014
    About that variac... what would be a test to determine if it was really isolated or not?
    Some of the reviews claim that it is, but others not.

    I tried checking the ohms between the hot side of the variac's plug, and both sides of one of the output sockets. Both prongs of the output sockets showed very little resistance with the variac's own plug, <1 ohm. This is what I would expect from a regular variac, and I assumed this one was the same. There is a voltmeter between the output points, would that be interfering with the measurement?

    Some reviews on amazon are claiming it is isolated, and others say it's not. Because of the way that some sellers list their items when multiple sizes are available, sometimes the reviews of one size end up listing for other sizes.

    Anyway--it won't work as it is for me. The transducer coil runs way too much current even at only ~3 volts, it hits 3 amps pretty fast, which is just barely moving the variac knob off of zero. I will have to look at the audio amp method I suppose.
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