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Audio Amp 1V gain?

Discussion in 'Audio' started by Casablanca, Jan 1, 2010.

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  1. Casablanca

    Casablanca

    5
    0
    Jan 1, 2010
    Hello,

    This is my first post on this forum and I look forward to participating in the future.

    I am trying to teach myself the basics of electronics. I have bought the textbook and lab manual that is used at my local community college for the first course in EE Technology.

    I have run into a road block in the lab manual. It tells me to buy on-line a 1 watt power amp kit that has a voltage gain of 1. (Later in the instructions, I am supposed to connect a speaker to the output of the amp and supply the amp with a 1KHz sinewave. )

    I have found on the Internet 2 audio amp kits that have a power gain of 1 watt, but both of the amps have a voltage gain of at about 30db.

    First, does anyone know where I can find a kit that is described in my manual? If not, can you direct me to a schematic for such a circuit?

    Second, I am confused. If the amp only has a voltage gain of 1, is the current being amplified, at the max, to 1 amp. so that the formula P = VI results in a power gain of 1 watt?

    When are voltage gain audio amps used and when are current gain audio amps use? Does it make any difference?

    Thanks for your help.
     
  2. Resqueline

    Resqueline

    2,848
    2
    Jul 31, 2009
    Welcome to the forum. We'll be happy to discuss with you & will try to assist you to the best of our abilities.

    An amplifier with a voltage gain of 1 is also called a unity gain amplifier, or a buffer amplifier. If you take an over-unity gain amplifier and provides it with a "volume" control it can be adjusted to have unity gain (which is the way most audio amps works).
    You are correct in that it only amplifies the current as needed to sustain the same voltage on the output as on the input.
    Power gain is however not expressed in Watts, but in dB. Voltage gain can also be termed in dB. One has to differ between these.
    When stating how many watts an amplifier can put out, one also has to state into what impedance this is. It has a voltage limit, a current limit, and a power limit.
    Usually an amplifier amplifies both voltage & current, but in the old days you had piezoelectric phonograph pickups that could put out 1V. Since the impedance of it was very high, and speakers has a very low impedance, the amplifier would have to be able to reduce the voltage (to 1/10th) and increase the current (10000 times). So then it had under-unity voltage gain, and over-unity current & power gain.
    A transformer can be said to have voltage or current gain (not both at the same time) but it always has unity (0dB) power gain (or slightly less).
     
  3. Casablanca

    Casablanca

    5
    0
    Jan 1, 2010
    Hi Resqueline,

    Thank you for the quick and helpful reply.

    My most pressing problem is to find a kit for a unity gain amp. If I buy one of the 1 watt audio amp kits marketed on the Internet and adjust the voltage of the output to match the voltage of the input, do I now have a 1 watt amp with a voltage gain of 1 (as the lab manual requires)?

    Thanks again.
     
  4. Resqueline

    Resqueline

    2,848
    2
    Jul 31, 2009
    Yes, it should do the trick afaik, but you match the input attenuation to -30dB by means of a volume control (at the input, but maybe that's what you said) that you also must buy. It's just an audio taper (logarithmic) potentiometer somewhere around 10k-100k ohm.
    Or if the kit already has a volume control you're set to go. (The stated gain would be with the volume maxed.)
     
    Last edited: Jan 2, 2010
  5. Casablanca

    Casablanca

    5
    0
    Jan 1, 2010
    Hi Resqueline,

    With your help, I have been doing my homework on unity gain amps (also called buffer amps).

    The “1 Watt amp” (that’s the phrase the book uses) my lab book directs me to make is to allow me to connect a 6 ohm speaker to the output of my sine wave generator. I found in the spec’s for my sine wave generator and the output impedance is 600 Ohms. Obviously, if I simply connected the 6 ohm speaker directly to the generator, there would be a huge impedance mismatch.

    So, I now understand that the unity gain amp we have been talking about is a circuit to match the high output impedance of the generator (the generator’s spec says the load impedance is 600 ohms) to the low impedance of the speaker.

    After I construct the amp, and before I connect the speaker, I will add a log pot to the input. Using my 2-channel scope, I will adjust the pot of the amp until the peak-to-peak value of the input sine wave from my signal generator equals the peak-to-peak sine wave coming out of the amp. Do I now have a unity voltage gain amp?

    Once I have adjusted the amp I will then connect the speaker to the output of the amp. Even though the current coming out of the signal generator is relatively small, the buffer amp will provide the current necessary to drive the speaker.

    Do I now have the procedure correct?

    Thanks again for all of your help.
     
  6. Resqueline

    Resqueline

    2,848
    2
    Jul 31, 2009
    Yes, if you were to get 1V with the speaker connected directly to the generator it would have to be set at an output level of 100V.

    For good use of audio equipment (& other power sources) the load impedance must be at least 10 times higher than the source impedance.
    The only exception is tuned RF circuits where source & load impedances are to be equal for maximum power transfer.

    The output impedance of that amplifier is probably around 0.5 Ohms so the amplitude will drop slightly when you connect the speaker. Therefore you have to adjust the pot with the load connected to get it exact.

    The generator will see no difference when you connect the load, all needed current will come from the amp & its power supply.
     
  7. Casablanca

    Casablanca

    5
    0
    Jan 1, 2010
    Thanks again for all of your help. Actually, I believe I have learned much more by talking to you than I will in performing the experiment.

    I really appreciate you taking your time to pass on so much knowledge to me.
     
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