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Thread: USB Audio DAC mod + LaserBoy Correction Amp

  1. #351
    Join Date
    Jan 2009
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    Quote Originally Posted by weartronics View Post
    With pleasure, I have drawn the schematic for you this morning:


    All channels (X, Y, I, R, G, B) share one offset buffer, and all channels use exactly the same differential amp circuit. The X, Y channels are bipolar differential, and the R, G, B, I channels are unipolar differential, but this is accomplished in software.

    I suggest that you use a +/- 5V power supply with rail-to-rail op-amps. It means the output voltages are limited to safe levels without needing any additional circuitry. I used the IA0509S DC-DC module (from the original correction amp kit) to provide the +/- 5V power supply from USB, and TLC074 op-amps.

    The offset adjustment potentiometer can be any value between e.g. 10k and 100k. The 47k resistors can also be any value between e.g. 10k and 100k, as long as they are all exactly the same. The 100R resistors are not critical either - their purpose is only to limit transmission line effects and short-circuit current.

    I'm aware of many ways to improve this design, but I choose not to implement them because additional complexity is unnecessary. The schematic above works well.



    As you can see from my screen captures in other threads, my work is in Linux. I tested the DAC using C-Media cards based on CM106L and CM6206. It works well using the ALSA-provided USB audio driver as well as my own kernel-mode driver on Linux. I also used a Windows computer to run some of the free Zoof-ware through the drlava EasyLase wrapper without any problems. What "setting may be out of whack" specifically?
    Thank you! I'll be trying that out shortly..

  2. #352
    Join Date
    Aug 2009
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    Bristol, England
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    The CM6206 is specified as 3.1V peak to peak, so a little gain is indicated (No big deal).

    Regards, Dan.

  3. #353
    Join Date
    Mar 2008
    Location
    Sydney, Australia
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    315

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    Quote Originally Posted by Stoney3K View Post
    Throwing in the same resistors on the input and feedback of an op-amp, will give you a gain of, er... exactly, one. That would work great as a buffer/inverter stage but you're not doing much amplification...
    The resistors are the same value, but look more closely at the configuration. The differential amp in that schematic has a gain of, er... exactly, two.

    Quote Originally Posted by Stoney3K View Post
    what's the max output swing on the C-media boards?
    The C-Media DAC outputs are 3.1Vp-p, so the gain of 2.0 can reach 6.2Vp-p. This corresponds to more than 8 degrees optical angle of my scanners (MediaLas CatWeazle LC II) with factory tuning. Of course, you can change one resistor to increase the gain if you want, but that schematic is exactly how I built my amplifier.

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