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DAC questions
lerouxb Posted: 17 Mar 2020, 04:03 PM
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For your flash synth: Did you use an audio amplifier IC with the DAC or did you find that the buffered output on its own was strong enough? I'm trying to figure out if I need to buffer it any further in order to drive some small in-ear earphones.

I'm looking at the datasheet and the output impedance with the buffer on seems to be in the range 2 to 3.5 kohm. Although that's sample and hold which probably doesn't apply. It also talks about a resistive load of 5 or 25 kohm depending on whether the load is connected to VSS or VDD. But then I guess speakers aren't resistive loads. So I have no idea what to make of it. Given that earphones are probably in the range of 8, 16 or 32 ohms. Although that's impedance and I can never wrap my head around how that would map to current requirements.

I included a little audio amplifier chip and a digital potentiometer on my prototype board, but it is way too loud even at 0.5x gain. In fact even at the lowest volume (as set by the digital rheostat) and then I'm getting a very fuzzy/noisy signal. I've been trying to find info on what kind of voltage you should be outputting into earphones and no one seems to be answering the question online. But I've come to realise that the DAC's voltage range is way too big for headphones so I don't want to amplify the voltage - if anything I need to drop it and just buffer it so I can get lower output impedance. But now I'm wondering if the DAC's built-in buffer would be good enough..

The remaining problem then is that the DAC's voltage at the top end would be too high. So I can output use lower values, but then I lose lots of bits of precision..

I really wonder what smartphones that have headphone jacks do.

All of this must be common problems and I've come to realise I lack the technical vocabulary to know what to search for ;)

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mit Posted: 17 Mar 2020, 08:03 PM
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yeah whatever

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On the flash synth I used the buffered DAC output with a passive filter circuit to block DC and control the output impedance.

Dropping the volume if it's too loud is easy - just use two resistors as a potential divider. There is no loss in quality in doing that. Best to have the DAC as loud as it will go and attenuate it with resistors, then you don't lose any bit-depth.

The resistors only act as a perfect divider if there's no load on it, as soon as you plug headphones in it'll change the characteristics. If you're not short of power it's best to buffer the output after the divider. I don't do that on the flash synth because it's all a bit of a hack, just some careful component choices make it viable with headphones.

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