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I'm working on an FPGA implementation of a retro '80s computer (see http://fpgabee.toptensoftware.com) and am trying to setup some 8-bit audio support. I'm using a sigma-delta DAC and a R/C low-pass filter.

What I currently have is a 3.3 V pin from the FPGA driving a circuit.

If I connect a pair of iPhone headphones I can hear the audio, it's sounds quite good although the volume is a little low.

What I'd like to do however is connect this to my PC for recording or to pass-through to my speakers. I tried connecting the above circuit directly to both line-in and microphone sockets on my sound card but the results were terrible. Nothing audible on the line-in, and very low volume and lots of noise on the microphone socket.

So I'm looking for is a simple circuit (I presume a amp of sorts) that will let me get a clean signal into the PC. I'm not talking about hi-fi quality stuff here - this is 8-bit audio driving a very simple dac.

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    \$\begingroup\$ Have you tried a very simple op-amp? \$\endgroup\$ – Ignacio Vazquez-Abrams Aug 4 '13 at 0:39
  • \$\begingroup\$ For the microphone input I think most only like about 100mV in, you might be able to try a divider to attenuate it instead of an amp as well. \$\endgroup\$ – PeterJ Aug 4 '13 at 1:01
  • \$\begingroup\$ @IgnacioVazquez-Abrams I happen to have an opa2314 setting here from another project, but don't know how to hook it up. \$\endgroup\$ – Brad Robinson Aug 4 '13 at 1:25
  • \$\begingroup\$ @PeterJ wouldn't that make it even quieter? Already on the mic input I have to ramp up the gain in the PC to even hear it. \$\endgroup\$ – Brad Robinson Aug 4 '13 at 1:26
  • \$\begingroup\$ That's what TI application note 31 is for. \$\endgroup\$ – Ignacio Vazquez-Abrams Aug 4 '13 at 1:27
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If you are looking for a simple audio interface with volume control, you can do a quick search on internet for such, but if you want recommendation based on personal experience, I would suggest LM4811, which is a simple, low cost and easy to use headphone amplifier from TI. Typical connection diagram for LM4811

http://www.ti.com/product/lm4811

If you work for a company, you can register at TI and ask for a couple of samples. They are not easy to solder, but you may ask someone for help to get it on a test board.

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  • \$\begingroup\$ It's not exactly clear that this will solve the actual problem the poster faces. \$\endgroup\$ – Chris Stratton Nov 13 '13 at 22:57
  • \$\begingroup\$ @ChrisStratton this will bring the unamplified audio to an amplified line-level. That is exactly what OP wanted (with an extra digital audio control). What is unclear about that? \$\endgroup\$ – Passerby Nov 14 '13 at 1:22
  • \$\begingroup\$ It's quite unlikely that is the actual problem. And no, what you have depicted is not really a line level output, though it might be adapted to one with component changes. \$\endgroup\$ – Chris Stratton Nov 14 '13 at 2:29

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