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Computer Amplifier

  1. Jul 27, 2006 #1

    I have a few questions about creating a multiple output stereo computer speaker amplifier. I am using a standard audio output from the computer. Basically what I want is to have:

    1. Both quarter inch and eighth inch stereo jack input, but only one at a time will be used. I figure if I put them in parallel that would solve that problem.

    2. I want 4 stereo 20 - 60 Watt outputs. I would like to use any combination of the speakers at a time (say all four pairs, just one, etc at the same time). I am thinking about using opamp with a feedback loop. This is the one I am currently considering:

    http://www.national.com/ds/LM/LM675.pdf [Broken]

    I would use the "Non-Inverting Amplifier" design from the datasheet (The first schematic). The plan would be to have a total of 8 amplifiers (left and right for 4 speaker outputs). My understanding is that the input current to each amplifier is low enough such that it is ok to have all 4 pairs to run at the same time, can I make this assumption?

    3. I also want to have 2 eighth inch jack stereo headphone outputs. I figure I can just hook those up directly to the inputs. Will it be a problem to use two in parallel at the same time (ie: can one use two headphones in parallel off of one standard audio card)? How might I solve the problem of the low resistence of the two parallel headphones? Also, my understanding is that the input resistance to each opamp is high enough to where it will not be a problem to have say a headphone in parallel with a speaker amplifier, am I correct to assume that?

    I guess my question is what do you think about this design idea. Is there a significantly better way to solve my design needs? Are there any other problems that you forsee in this design?

    Thanks in advanced,
    D Dean
    Last edited by a moderator: May 2, 2017
  2. jcsd
  3. Jul 28, 2006 #2
    I can't remember off the top of my head, but I think you'll want a push-pull circuit. I loaned out my electronics book so I can't look anything up.
  4. Jul 28, 2006 #3
    Most of the designs that I've looked at are built with a bipolar network: most being a common-emitter (or differential pair) feeding a push-pull output stage. I figured that since I plan to have 8 separate amplifications it would be a lot easier if I could just use a chip like the one in my first message. Looking at the schematic for the opamp it is basically that (with more voltage amplification stages and filtration).
  5. Jul 28, 2006 #4
    Does anyone know the standard for the output voltage and maximum current for a PCI sound card. I realize the max current may fluctuate with different devices. I have the Sound Blaster 5.1 PCI Card from soundblaster.com. They aren't too detailed about the device specifications.

  6. Jul 29, 2006 #5


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    Science Advisor
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    The designations Line In/Line Out have a general standard.
    You can look this up. Your card should be close to that.

    For a power output, if they don't tell you, then you can determine it with test equipment.
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