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I am working on a project "cat projector" which shall "shout" at the cat when she jumps on the table in the living room.

For the logic I use an Arduino board which plays/streams the sound at 22.1kHz from a SD-Card and using the MCP4821 DAC to generate the signal. This works perfectly fine (as you can see on the measurement). The output on the DAC is between 0-4V.

The measured output signal of the DAC is shown here:

The signal at the output of the DAC.

I plan to use a regular 8Ω speaker with 0.2W or 1W. I also have a 5V source and a 9V source which I can use for the amplifier (both sharing the same ground). The DAC is powered from the 5V source.

Here a circuit which already contains the exact output signal from the measurement.

schematic

simulate this circuit – Schematic created using CircuitLab

What is a good amplifier circuit, to convert the DAC output (0V - 4V) into a suitable signal for the 0.2W or 1W speaker?

My best (failed) solution so far was this one:

schematic

simulate this circuit

Final Solution

Just for documentation, this is the working circuit I ended up with. It is using the LM386N-1 with a 8Ω, 0.2W speaker. The sound quality is usable, with small distortions on high frequency tones.

The final solution

Using C1/C5, there is no hearable noise if the DAC is turned off (0V level). The actual measurement looks like this:

The measurement of the solution

The yellow signal is measured in front of the speaker. The blue signal is measured at pin 3 (input+) of the amp. Note the different scales: Yellow 1V, blue 50mV.

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  • \$\begingroup\$ With no attempt at a design this question should be closed. \$\endgroup\$
    – Leon Heller
    Commented Oct 25, 2014 at 13:07
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    \$\begingroup\$ I tried several designs without success. Using LM386N-1 as op-amp in different configurations, also pure transistor based solutions. All without success, with extreme distortion. I already asked the question with an complete design, but merely got hints. So I reformulated my question, in the hope there is a way to transform the signal into something "amplificable". \$\endgroup\$
    – Flovdis
    Commented Oct 25, 2014 at 13:25
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    \$\begingroup\$ @LeonHeller I added one of the failed designs for you. Actually I can not simulate it, because I couldn't find a LM386N-1 for the simulation. \$\endgroup\$
    – Flovdis
    Commented Oct 25, 2014 at 13:36
  • \$\begingroup\$ Not much of an audio person but I think for that circuit the voltage gain will be 20. Try changing R4 to something like 4K7 or maybe a little less and see how it goes. \$\endgroup\$
    – PeterJ
    Commented Oct 25, 2014 at 13:53
  • \$\begingroup\$ The LM386 is NOT an op amp! \$\endgroup\$
    – Matt Young
    Commented Oct 25, 2014 at 14:22

1 Answer 1

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Google "LM386" and take one of the examples in the datasheet:

enter image description here

With the default 20x gain you will probably need ~ 1:40 attenuation between the DAC output and the LM386 input. Use a log potentiometer for the 10k to set the attenuation, once you know the correct setting you can swap it for two resistors.

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  • \$\begingroup\$ Actually the attenuation was the problem all the time. Now, at least I get a more or less usable result. Thank you very much. This circuit was the first I tried, but I always had a signal in the range ±1V which way too much. \$\endgroup\$
    – Flovdis
    Commented Oct 25, 2014 at 15:52

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