Pre-Amplifier

Discussion in 'Homework Help' started by amjg92, Mar 27, 2011.

  1. amjg92

    Thread Starter New Member

    Dec 14, 2010
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    Hi,
    I am stuck on a hw problem and need some help. the problem is I dont know where to begin from.
    I need to design an amplifier with 50 signal voltage gain. I am stuck at the very beginning - the question asks for a pre-amp then a power amp.
    i know what is a pre-amp but I have no idea from where to begin to design one? Can anyone halp pls
    Thanks,
    A
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
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    Preamps typically have some specific input. That is, one for a phono cartridge will be different from one for a microphone, and both are different from one that works with a standard line level input.

    Google has lots of useful links, including DIY sites.
     
  3. amjg92

    Thread Starter New Member

    Dec 14, 2010
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    I want to design for an audio amplifier, i think that the input will be an ipod mp3 cd player etc. but not a mic what are DIY sites pls?
     
  4. beenthere

    Retired Moderator

    Apr 20, 2004
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  5. hobbyist

    Distinguished Member

    Aug 10, 2008
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    To design a unit, using an input signal from a device as chosen, you need to do some preliminary work.

    First do a investigation to find out the output signal voltages and output impedance of the device being used, as the input signal.

    If no data can be found, suitable for investigation, then a emperical testing must be undertaken, to gather such information.

    This is done by, using a multimeter, and connecting it to the output of the input signal device, with the setting of the multimeter on DC range.

    Turn on the input device, and take it to max volume, and check the DC voltage reading on the meter for the highest volt reading, these readings will be varying up and down, because they are DC voltages modulated by audio frequency signals. (voltage spikes).
    So get a close approximation, and write down that value as........ "Vinmax"

    This will determine the maximum input signal you can amplify without much distortion on the output of your amp.

    Now to get a close aproximation of the device output impedances.

    If you have access to a osciloscope, then hook up the input device to a oscilloscope, and again max. volume, and set the frequency range on the O.Scope, to where the audio signal coming from the input device is easy to see the modulation as it reaches peak voltages, then looking at the peak amplitudes determine roughly what the voltage peaks are at.

    Then take a capacitor of around 1000 uf and connect it to the output of the device. ,and if you don't have a rersistor sub box, then hook up a 2K ohm rersistor to the other end of the capacitor and connect this resistors, other end to ground, as you touch it to ground, look at the waveform on the scope, and see if there is any significant change in its amplitude.

    If no change occurs, continue to lower the resistor value, and keep on checking until the waveform drops significantly, when it does, then begin to experiment further by raising the resistancew value, a little at a time until you rerach a point where the waveform just begins to show some degregation in its output. Then write down this resistance value as ........"Zinmin"

    Now you have both the input signal voltage and the input impedance to work with in the beginning of this design.

    Now that's the input side. You still need to determine what the output side is going to be, in order to match the input to the output.

    So now you need to secure the data of your output device.

    Is it a audio device, or some visual lighting effects, a relay driver, what is the purpose of the output to be amplified.

    When this is chosen, then you need to know what is the impedance that it presents, to your circuit.
    From there you are now ready to do your design.
    You now write down all the parameters,
    Voltage supply, Voltage gain, imput and output impedances, and any other data gathered from previous steps.

    Now to begin your preamp stage, you design its imput impedance to be around 2- to 10 times greater than "Zinmin". This will ensure the least input signal attenuation.

    Now start working on the stage gain, to get the required gain, when you calculated all the values, then build it on a breadboard, and using a audio signal generator, and a oscilloscope, hook up the circuit to the generator, and set it for the ,,,"Vinmax" at a audio frequency of choice, and hook the output to the scope, and check for signal amplification, distortion, change the frequencies and voltage amplitudes, and run it through a lot of tests to see if it performs as required.

    Change circuit component values if needed to get the requirements wanted.

    Once your satisfied, then begin to work with the power stage, to match the output impedance of the preamp to the input of the power stage as well as matching the voltage amplitude of the preamp to the voltage amp of the power stage, with the required output current needed to drive the output load.

    And build and test and work back and forth between the stages until you can get the best results your looking for.
     
  6. Adjuster

    Well-Known Member

    Dec 26, 2010
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    You cannot use a DC meter to measure the signal source amplitude. There is no good reason to suppose that the signal source will have any DC content at all. Even if the signal started that way, it would very likely have passed via a DC blocking capacitor if it was accessed at an output normally presented to the outside world. AC signals should be measured with an AC meter, or preferably an oscilloscope.

    As for estimating output impedance by loading down the output, having first turned up the volume full blast: this may be OK for really low level signals, but don't ever do this to even a small power amplifier. If you do that, you are likely to blow it up.
     
  7. Audioguru

    New Member

    Dec 20, 2007
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    A multimeter set to DC or even set to AC will not accurately measure the voltage of an audio signal.

    Why bother trying to measure a signal source anyway? The assignment asks to design an amplifier with a gain of 50. The amplifier must have a preamp and a power amp.

    The preamp can have a gain of 10 and the power amp can have a gain of 5.
     
  8. hobbyist

    Distinguished Member

    Aug 10, 2008
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    NO, your misunderstanding the above procedure, this is NOT for looking for any DC level of voltage, but rather to see where the PEAK voltages are running at from the input device, that way there there can be a rough estimate of what voltages will be presented to the designed amp.

    So as to know what kind of design for the preamp would be needed to handle the input signal. This is for a ballpark estimate, has NOTHING to do with any DC level, but rather peak signal voltages, so as to have a starting point of what voltages shall be presented, so as to know how to
    go about the initial design, if the input peaks are too large, than you wuold know that the design of the preamp would need to be designed to handle such large signals, that way there you have a basic starting point too know where to begin the design, for the input signal.

    Otherwise you don't know what signal voltages are going to be presented, so when its built and tested, and the volume is at max. then there is the very real possibility of overdriving the amp, however by having a idea of what the max. peak voltages are, then the design can be implemented around those input signals too be able to handle the input signals at max, as well as normal audio output.

    And of course as far as testing for output distortion by loading the signal device, is to get a BALLPARK ESTIMATE, of what input impedance too make the preamp, that would give the least attenuation to the input signal coming in, because without having a knowledge of this will result in a very real posibility of making the input impedance to low, which would then cause the signal input to become attenuated to the point where distortion could present itself, however by having SOME idea of where the impedance should be, will help in designing the input stage, so as to have a high enough impedance so as to not cause the signal input to be attenuated, but rather passed through into the next stages.

    So I hope this better clears up the vague misunderstanding that seemed to have been presented, by the way I wrote the above procedures.

    These procedures were to address the OP original question of where to start, these procedures were to show this person, what needs to be done, to get started in suach a design assignment.
     
  9. Audioguru

    New Member

    Dec 20, 2007
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    A DC multimeter cannot show the peak of an AC signal because it is too slow.
    It is made so that it reads a steady DC voltage even if it has some AC on it.
    So 0.0VDC with AC reads 0.0V.

    A moment ago I set my signal generator to a 2V RMS 1kHz sine-wave and measured the DC on its output. It measured 0.0VDC.
     
  10. hobbyist

    Distinguished Member

    Aug 10, 2008
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    yeh, I agree a hundred percent, that a steady state sinewave, will not show up on a DC setting, unless the frequency was very low for the meter to pick it up.

    I guess I was thinking in terms of audio frequencies can become low enough to be recognized by a meter, to at least give some indication of what voltages can be present with the input device.

    Sort of like ripple voltage readings when checking a power supply across the filter cap.

    The only reason I am saying this, is becaus having some idea of what the signal voltage value is, helps determine what the supply voltage needs to be, among other factors.

    But again,
    please remember I am only trying to help these students at a BASIC level, where I reside as a hobbyist in this field, then from there they can advance to the professional level, where you guys reside.
     
    Last edited: Mar 28, 2011
  11. Audioguru

    New Member

    Dec 20, 2007
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    If the audio frequency is extremely low so that a DC voltmeter responds, then the meter or display will wobble positive and negative and not be readable because it is trying to follow the AC signal.

    The school project is missing many details like what is the input signal from, is there a volume control and what is the required amount of output voltage swing. without knowing these details then it will be lucky for the amplifier to work.
     
  12. hobbyist

    Distinguished Member

    Aug 10, 2008
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    Hi,

    That's the point I'm trying to make, lets say that as the meter is wobbling back and forth and the display shows 180mV then fluctuates up and down, well at least now you have a general idea, of how large the signal could go, so as to make the supply voltage large enough to handle signals double that if needed.

    Am I that far off base with this.

    The reason I say, this is because I did this when I wanted to design and build a power amp for a discarded CD ROM player, from my computer, and I remember looking for peak voltage readings, to have some idea where to start the ddsign for the supply, as well as thew amount of gain I needed, ect...

    Like I said before I'm only a hobbyist in this field.
     
  13. Audioguru

    New Member

    Dec 20, 2007
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    The meter reading fluctuates up and down so quickly that you see just a blur. It is not made to accurately wobble with the signal like a speaker will try to do. It averages 0VDC.
     
  14. hobbyist

    Distinguished Member

    Aug 10, 2008
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    OK,

    Thanks for your time, and patience.
     
  15. amjg92

    Thread Starter New Member

    Dec 14, 2010
    12
    0
    Thanks for the answers. I used an oscillascope and found out a couple of results. You have directed me on the right track if i need anything else i will ask again
    Thanks
     
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