Op Amp Preamp

Discussion in 'The Projects Forum' started by Wendy, Oct 3, 2010.

  1. Wendy

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    OK, I'm helping my boy with his first project, basically a really simple amplifier to go between his computer speakers and the computer. Basic enough, so I drew up a schematic and am posting it for review.

    [​IMG]

    The op amp isn't really set, I'm socketing it to allow for easy change out, in case my first choice isn't good enough.
     
    Last edited: Oct 3, 2010
  2. R!f@@

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    Which device do you have in mind Bill.
    Something like TDA2030
     
  3. Wendy

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  4. sage.radachowsky

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    Bill, I don't really understand that power supply. I'm not familiar with that setup. What do you call the two ground symbols? One looks more like "Earth" (with the lines) and the other reminds me of a signal ground and you've labeled it "common". I don't understand why the "common" is on top of a Zener like this.


    EDIT:

    Aha, okay, now I started to understand. The "common" is a reference point about halfway between the ground and the +12V. The output of this circuit will be positive and negative around the common based on the AC component of the input signal above a certain frequency, which is perhaps about 10 Hz for the first 3 dB falloff.

    Is this the typical way of doing audio signals?
     
  5. Wendy

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    Don't know. I typically use ground for negative, and I needed another symbol for midpoint, which is what the triangle is.

    There isn't really a set way to draw something like this, drawing schematics is an art. My way is definitely not the only way, and I may get some argument that it is incorrect from some.

    It is legitimate however, and easy to understand, which is what is important with any schematic.
     
  6. R!f@@

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    Sorry Bill..those IC's cannot handle speakers. They are low impedance OPAMPS used as a preamp.

    If you are going to make a computer desktop speaker amp then u should go for what I suggested
     
  7. Wendy

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    You didn't read deep enough. Computer speakers have amplifiers in them. Nowdays you can't find any without.

    While I don't understand why he wants it really, I am told some games get really quiet but still have sound. <shrug>
     
  8. R!f@@

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    I read it....pre amps cannot drive speakers. but power opamps can.

    I am rather confused on what u want Bill

    wait...

    is it that u want to amplify the signal level of the computer output?
     
  9. R!f@@

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    Yup
    that is what you are after, speaker out from the PC to a preamp and from it to the powered speakers. Ur circuit is good But I doubt that ur supply diagram would work
     
  10. Wendy

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    What's the problem with the power supply?

    One of the nice things about zener regulators is (unlike a solid state power supply regulator) they can sink and source current and still maintain a fairly good regulated voltage, in this case ±30ma or better. Try it by itself, and see what happens if you inject or draw current. A lot of folks hate them, but I've always had pretty good luck using them in my designs. The exact voltage (and regulation) isn't that critical in this design, just noise.

    The actual power supply is a switching 12V regulated unit, it may offer some problems with switching noise. I have a lot of coils if that is the case for a pi filter. Or other wall warts.
     
  11. R!f@@

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    Hehehe.
    Nothings is actually wrong.

    My point is that opamp u want works with dual supplies. I don't think it can work with single ended supplies..
    If you have just +12VDC, then you shud choose an OPAMP that works with a single ended supply like 741... I think. Not sure whether 741 works with a single ended supply. But my point is that if you plan to use that OPAMP then u shud have ± Voltages with reference to ground :)
     
  12. Wendy

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    Uhhh, no...

    Creating a Virtual Power Supply Ground

    I just created a primitive one. It is a myth op amps must have dual supplies, you can always get a single power supply to work if you have the extra circuitry (which I do) and the voltage is great enough (it is).
     
  13. R!f@@

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    Hmm..that works for low power and battery operated devices
    But not practical in High power amplifiers. I have never seen virtual ground in amplifiers
     
  14. R!f@@

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    But remember to have output coupled through capacitors, same as the input.
    And one more thing, do not use the existing PC speaker supply for that preamp
     
  15. Wendy

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    Most amps don't use simple op amp circuits, and have a single source output to a speaker (why would you need a virtual ground there?). I've seen a lot of things in the field in over 30 years, some of it stuck.

    If you're going to make a high power amp (which this isn't) it is probably easier to create the voltages you need with two power supplies. There is absolutely no reason you couldn't use a virtual ground for any power level. You could even do it with switching mode power supplies, but the key concept is the virtual ground must be able to sink as well as source current, most regulators can't do this.

    Just because you haven't seen an idea implemented doesn't mean it isn't practical under the right circumstances.
     
  16. Wendy

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    Now you're reading things I didn't say I was doing. :D There is an old expression in my part of the country, don't teach grandma how to suck eggs (implication, she was doing it before you were born).

    The design absolutely depends on power supply isolation, which is why it doesn't need capacitors on the output. The computer is expensive, but the computer speakers are cheap, and the capacitors would have to be quite large.

    The only question I have is whether the speakers have a 600Ω input impedance, which is the norm as I understand it.
     
  17. R!f@@

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    Heheh..ok ...I get it :D

    In order to find the input impedance of ur speaker, u have to see what type of amp chip is used in it.
    But it will work since normally, those type of speakers will have a high impedence input and since ur opamp can cope 400Ω, u have nothing to worry about :p
     
  18. tom66

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    Bill, I like that supply with the virtual ground. But I am confused about the use of R1 and R2 in parallel. Why not use a single ~120 ohm resistor?

    I don't think you'll need the 100nF cap, they are usually used for circuits which have lots of logic switching noise, and yours won't.
     
  19. sage.radachowsky

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    I understand what you're saying about the power supply and how you've drawn it, Bill. I tend to use "V+" and "V-" for the power rails for dual-supply op amp circuits, and the ground symbol as the ground. I like to have a good solid supply. But I see how this one works.

    This is shooting from the hip, but my intuition says that C3 should be a lot larger than 0.1uF because the feedback loops can draw somewhat heavily on that common at audio frequencies... A Zener's voltage does wiggle a bit depending on the current drawn, so that common voltage will vary a bit in relation to the supplies, by some amount. If you use a bigger cap it will be more steady.

    On the topic of "single-ended supply" op amps, I take it to mean mainly that the op amp performs well all the way down to the negative supply, so that if that is the signal ground, then it will work as expected to the lower end of the input range. But all op amps really have a single supply, consisting of a positive and a negative supply connection. The distinction is really about the input and output ranges and performance at the lower extreme.
     
  20. Wendy

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    The total ohmage I'm using will dissipate over ¼W, and I'm using ¼W resistors, so I'm splitting the load and doubling the wattage.

    As for the 220µF and 0.1µF, call me a traditionalist. It can't hurt, and might help. Remember, electrolytics work best at low frequencies, not so well at high ones, but the 0.1µF takes over from there.

    Zener circuits are regulators, it will absorb any frequencies by simply maintaining a constant voltage. At least this is the theory. I have found them to be quite quiet in other designs. The 0.1µF capacitor there is to absorb the slight amount of noise generated by the zener, which will be a hiss.

    With good virtual ground, there is no difference between a single power supply voltage or a dual. It depends on the virtual ground acting like a ground of course.
     
    Last edited: Oct 4, 2010
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