Break down 741 schematic for the newbie?

Discussion in 'General Electronics Chat' started by daroc26, Feb 6, 2011.

  1. daroc26

    Thread Starter Active Member

    Apr 4, 2009
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    Hi All,

    This schematic is really troubling me. I'm having difficulty understanding the connections. Especially, the pots R2 and R3. I'm also not sure about the 741's pin 4, is this connected to ground rail, and pin 7 is this connected to +v?

    On the 386 does pin 3 go to ground rail?

    I had this setup earlier with an electret microphone. When I connected a 9v battery, I heard a click in the speaker, but no vocal.

    Any help would be much appreciated!!!

    Thank you!
     
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  2. Audioguru

    New Member

    Dec 20, 2007
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    Pin 7 of the 741 opamp connects to +9V together with pin 6 of the LM386.
    The negative terminal of the +9V battery connects to ground.
    Pin 4 of the 741 connects to the negative terminal of a second 9V battery that has its positive terminal connected to ground.

    Each battery should have a bypass capacitor in parallel with it on the circuit board. 10uf is fine. The polarity must be correct.

    The schematic shows pin 3 of the LM386 connected to ground so why ask about it?
    Pin 4 of the LM386 is also connected to ground and also are a few other parts.

    The LM386 is missing a capacitor in series with a resistor at its output that are shown on its datasheet.

    An electret mic will not work if it is not powered. The very low input impedance (R1) of the inverting opamp will load down an electret mic so much that it will barely work.
    That circuit is designed for a low impedance dynamic mic (has a coil and a magnet).
     
  3. beenthere

    Retired Moderator

    Apr 20, 2004
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    The control functions are pretty well identified.

    1. R2 - see the Ebook http://www.allaboutcircuits.com/vol_3/chpt_8/5.html

    2. R3 - The 386 has a fixed gain. To vary volume, you control the input signal level.

    3. If you look at the notation, you will see the op amp's pin 4 is connected to -9 volts.

    4. The notation clearly shows pin 7 tied to +9 volts (you will have to use two 9 volt batteries)

    5. There is a connection dot that means the 386's pin 3 is connected to ground.

    6. Electret mic's require a connection to a voltage source - http://www.epanorama.net/circuits/microphone_powering.html
     
  4. daroc26

    Thread Starter Active Member

    Apr 4, 2009
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    Thanks for the replies! I did not see that a second battery was needed for the 741, thanks for the insight!

    Beenthere, I went to the epanorama site. If I use the basic circuit, would I connect the output to a 386 amplifier?

    Thanks again.
     
  5. beenthere

    Retired Moderator

    Apr 20, 2004
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    On the mic schematic, there is a point called "OUT". I would eliminate the C1 capacitor in the 741 schematic, and attach the "OUT" to R1. The 10uF capacitor should have its positive terminal attached to the 2k2 resistor, so the marked negative terminal goes to the 741's R1.
     
  6. Audioguru

    New Member

    Dec 20, 2007
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    The 741 opamp is 43 years old and has poor performance for audio because it has a lot of hiss and it needs two batteries.
    Add a 10uF capacitor between pin 1 and pin 8 of the LM386 to boost its gain 10 times and add all the required parts that are shown on its datasheet like this:

    EDIT: I changed the schematic.
     
  7. daroc26

    Thread Starter Active Member

    Apr 4, 2009
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    0
    So with this schematic, the 741 is eliminated, and I could use one battery source, 9v.

    Okay, this looks like something I could follow.

    Great!
     
  8. marshallf3

    Well-Known Member

    Jul 26, 2010
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    Guru is correct.

    I still don't know why the 741 even exists anymore. It was a great little device in its day but using half of a TL082 cuts the power requirements by about an order of 10 and you get frequency response that goes all through the audio range.

    They have improved them:
    http://www.national.com/ds/LM/LM741.pdf

    Better yet:
    http://focus.ti.com/lit/ds/symlink/tpa741.pdf

    but these aren't the same thing as a common 741 referenced in most circuits.

    I prefer http://focus.ti.com/lit/ds/symlink/tl082.pdf but if only using half of it tie the inputs to ground.
     
  9. Audioguru

    New Member

    Dec 20, 2007
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    No.
    A typical LM741 or ua741 opamp has a supply current of 1.7mA when it has a 30V supply. A typical TL081 has a supply current of 1.4mA and a TL082 has a supply current of 2.8mA which is more than a 741.

    No.
    An LM741 is an ordinary lousy old 741 opamp. No improvement.

    No.
    It is not an opamp, it is a low voltage bridged power amplifier complete with biasing resistors.

    Why?
    A TL082 is a dual opamp. If you need a single opamp then use the TL081 single opamp. A TL084 is a quad opamp (it has 4 opamps inside).

    A TL07x is the same as a TL08x but is selected for low noise for audio circuits. They cost the same.

    The fairly old TL08x and TL07x opamps have a fairly serious problem called "Opamp Phase Inversion". The output suddenly goes high if an input voltage goes within a few volts of the negative supply voltage.
    But I have used TL07x opamps in thousands of circuits and have never had the problem occur.
     
  10. marshallf3

    Well-Known Member

    Jul 26, 2010
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  11. Audioguru

    New Member

    Dec 20, 2007
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    The TDA2822M bridged little power amp is almost the same but comes in a normal though-holes DIP case.
    They use two amplifiers to drive the speaker in a bridge so the output voltage is nearly doubled then the output power is nearly 4 times as much as a single amplifier. Also they do not need a huge output coupling capacitor.
    There are (or were) about 120 similar bridged car radio amplifiers available.

    You can't bridge two little LM386 amplifiers because their output current is too low and they are not matched like the two amps in a monolithic IC. The heating is nearly 4 times as much but the output power is almost the same as one amp. There are some bridged LM386 guitar amps designed by morons who don't know this because they didn't read the datasheet.
     
  12. marshallf3

    Well-Known Member

    Jul 26, 2010
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    Yea, but we've seen a lot better. What was that one we talked about several weeks ago?
     
  13. Audioguru

    New Member

    Dec 20, 2007
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    Most semiconductor manufacturers sell flea power amplifier ICs. But most are in a tiny surface-mount case that is difficult to cool.
     
  14. marshallf3

    Well-Known Member

    Jul 26, 2010
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    Yea, but we came up with a good one not long ago. Class T if I recall but I think you did me better with a far better IC.

    Simple search shows the LePath of which I personally bought one of and it would operate on 9V but it will take 24V. Pots are cheap but easily lubed or exchanged with simple ones from most any supplier.

    They're almost giving these away on Parts Express or Amazon
     
  15. Audioguru

    New Member

    Dec 20, 2007
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    Tripath invented class-T amplifier ICs. Their performance wasn't as good as larger manufacturers like National Semi and Texas Instruments.
    Tripath went bankrupt a few years ago but many Chinese amplifiers are sold today with their ICs or poor copies of them.

    National's LM3875 and LM3886 are excellent linear (class-AB) amplifier ICs.
     
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