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Resistors and?

Discussion in 'General Electronics Chat' started by Electronman, Apr 3, 2009.

  1. audioguru

    audioguru Well-Known Member Most Helpful Member

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    Most piezo transducers have a max allowed sine-wave signal of 24V RMS which is 68V peak-to-peak.

    A bridged car amplifier has an output of 14 watts into 4 ohms which is 21V peak-to-peak. If the amplifier is not bridged then the signal is about 11.2V peak-to-peak into 8 ohms which is only 1.96 Watts RMS at clipping.


    A step-up transformer driven from two small transistors can be used. A power amplifier is not needed since the power to the piezo transmitter transducer is very low.

    The ratio of the feedback resistor to R10 determines the gain. C2 stops the opamp from amplifying the DC voltage.
     
  2. Electronman

    Electronman New Member

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    The power supply is a 12 to 14V DC so whats the peak to peak output for the audio which Is not a pure sine??? I am getting confused!

    .
    I have to know the impedance of the transducer. Does a ohm meter do it for me

    But it amplifies the dc Offset, or maybe I misunderstood you?
     
  3. audioguru

    audioguru Well-Known Member Most Helpful Member

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    Audio power is measured with a sine-wave input and the volume control is increased until the amplifier clips the sine-wave.

    The power of a square-wave is exactly double the power of a sine-wave that has the same peak-to-peak voltage because half the power is added in the harmonics. That is why car amplifiers are rated with severe distortion that produces square-waves.

    The datasheet for the transducer lists its impedance which is probably 10k ohms. You do not need to know the impedance. Simply feed it with enough voltage swing. An 8 ohms to 1k ohms transformer steps up the voltage 11.2 times.

    An ohm meter measures DC ohms, not AC impedance. The meter cannot measure how much AC current the capacitance of a piezo transducer passes.
     
  4. dave

    Dave New Member

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  5. Roff

    Roff Well-Known Member

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    C2 blocks DC, so the gain to the 9V modulator bias (R3 and R4) is 1. C2 is a very low impedance (<<10k) at audio frequencies, so the gain is 1+R11/R10.
     
  6. Electronman

    Electronman New Member

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    Sorry I do no undrestand? I Do know that C2 is thereto block DC component,
    But Can not understand you. Besides why You desided to block DC and why It does not block DC offset That creates modulation index? Whats the formula for that caps? X= 1/jwc ?


    I fried my R transducer when I connected it to a 1:10 transformer at 12V power supply, lol. it was powerful for the first time.
    how do they calculate the impedance at the company and which freq they use to do so?
     
    Last edited: Apr 28, 2009
  7. Electronman

    Electronman New Member

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    Ron,
    I simulated your last modification circuit by Pspice. Bellow is the result.

    I think there is a Small fault yet? Please take a look at the waveform at the time domain. Why the modulated signal is not so good at peaks or envelopes or whatever you call them, that seems to be clipped at peaks?!

    Besides when I change the value of the R9 and use a higher resistor I get a strange waveform?!
     

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  8. audioguru

    audioguru Well-Known Member Most Helpful Member

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    You have pin3 input of the opamp at 13.5VDC which is way too high. The opamp will not work if it is higher than about 11V (with your 14V supply). Ron has it at 10.5V.

    Your audio input level is way too high. Its peak forces pin3 to a voltage that is much higher than the supply voltage that will destroy the opamp. Your signal is 3V peak. Ron's signal is 0.25V peak.

    Ron's circuit has the R11 trimpot to adjust the audio gain so that the input level can remain low enough.
     
    Last edited: Apr 29, 2009
  9. Roff

    Roff Well-Known Member

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    Why do you ask for help, and then change the op amps and the modulator bias voltage?
    LT1366 is a rail-to-rail op amp. I chose it for this application. If you want to use TL072, you will have to live with less output swing, which is probably not a bad thing.
     
  10. Electronman

    Electronman New Member

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    please do not be angry Ron,
    First I tried Your bias So I changed the values of those resistors to inject more offset to see if the waveform wants to be ok.
    Besides In your before modification You used a pot For those 2 resistors so I thought they are a Pot yet.

    Are you talking about that behaviuor of several op amps that audioguru told about it before? If so I must say that a looked around for a high speed opamp to use instead of tl072, I just found LM318, When I used it I had a distored bad output.

    auduguru, What is your idea of using LM833 instead tl072?
     
  11. Roff

    Roff Well-Known Member

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    Did you try simulating exactly what I posted, before you made the modifications?

    You don't need high speed op amps for the two that are in the schematic.
     
  12. Electronman

    Electronman New Member

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    Audioguru,
    Please take a look at the red rectangle in the bellow picture and tell me if that strange wave shape at those points is due to that behavior of tl072 you told me before?
    If not, why I have those strange waveforms there?

    Ron,
    I simulated your last circuit again with the values you used, I took that strange waveform again till I noticed that I have forgotten to trim the audio input signal (Thanks audioguru). Sorry for my fault.
    Anyway I am glad that you and audioguru are helping me thanks for it.
     

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  13. Electronman

    Electronman New Member

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    But You tlod this "LT1366 is a rail-to-rail op amp. I chose it for this application. If you want to use TL072, you will have to live with less output swing, which is probably not a bad thing."
    So I tought I am wrong of using tl072??!

    I have a LM833 Opamp too, Maybe it does work better than tl072. I will try it too.
     
    Last edited: Apr 29, 2009
  14. Electronman

    Electronman New Member

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    Ron,
    A technical question, How did you notice that those resistor values must be used there (R3 & R4 at your modification)? Just using a voltage divider formula?
     
  15. Roff

    Roff Well-Known Member

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    For 100% modulation, the upper sine wave needs to swing from vcc/2 to vcc, and the lower sine wave needs to swing from vcc/2 to 0V. In order to make this happen, the DC bias on the upper sine wave needs to be 75% of Vcc if you are using rail-to-rail op amps. When you invert the upper sine wave around vcc/2, the lower sine wave will be biased at 25% of vcc, which is what you need.
    If you are not using rail-to-rail op amps, tell us what you want to use and we can tell you the bias voltage that will give you the highest possible amplitude 100% modulation without audio distortion or overmodulation.
    Below is a simulated output waveform of my last circuit, but with Vcc=+14V and Vin=0.7V p-p (0.35V peak). This is the highest-level 100% modulation waveform you can achieve with a 14V supply. If you make the bias voltage lower, you can still get 100% modulation, but at a lower amplitude. If you make the bias voltage higher, you will not be able to achieve 100% modulation without clipping the peaks of the audio.
    The ratio of R3/R4 is calculated from this relationship:
    R3/(R3+R4)=0.75
     

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  16. audioguru

    audioguru Well-Known Member Most Helpful Member

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    Their spec's are almost identical.
    The same power bandwidth and very low distortion.
    The TL072 has FET inputs so it has a very low bias current and has low noise at high source resistances.
    The LM833 has ordinary transistors at the input for normal bias current and has low noise at low source resistances.

    They sound the same.
     
  17. Electronman

    Electronman New Member

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    Thanks Ron,

    I agree with you about that Vc/2, but can not understand why you inserted the 75% and 25% when the modulation index is 100% yet?

    I want to use TL072 as single ended (using resistor dividers).
     
  18. Electronman

    Electronman New Member

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    Both TL072 and LM833 worked fine one the circuit. You mentioned something about a reverse problem in op-amps, so are you sure I can use those low speed op-amps yet?

    Besides your ECM Preamplifier worked just fine with a headphone at the output, I changed the values and the sensitivity was so amazing.
    Now it is able to detect the ants marching!

    But I have a problem using it with the last Ron's modification. It has a lot of hoom when I connect it to the input of the Ron's modification.

    Ron,

    Can I change the values of the op-amp's resistors in your last modification to detect the ants marching too?!
    Or can I use it with a dynamic mic somehow too?
     

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    Last edited: May 3, 2009
  19. Roff

    Roff Well-Known Member

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    You have seen from the waveform that I posted that it works. Do you think it should be different? You should be able to study the waveform and the explanation I posted to understand it.

    What supply voltage do you want to use?
     
  20. Electronman

    Electronman New Member

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  21. Roff

    Roff Well-Known Member

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    On the last circuit I posted, if you want to use TL072 at vcc=14V, change R3 to 220kΩ. This will give you about 9.63V at the junction of R3 and R4. This is about optimum for 100% modulation at max amplitude without audio distortion. If you want more audio gain, change R11 to a 500kΩ pot, R10 to 3.3kΩ, and C2 to 10uF. This will give you a maximum audio gain of about 150. This is the highest audio gain you can get and still maintain at least 20kHz audio bandwidth. If you can tolerate less bandwidth, you can make R10 smaller, but be aware that you will have to make c2 larger to maintain your low end rolloff frequency.
    If you want more gain at full audio bandwidth, you will need a preamp.
     
    Last edited: May 4, 2009

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