ADAU-1701 - direct headphones connection
Hi ADI!
We're working on a new prototype with our custom PCB design based on 1701; we got everything working until now with input from ext microphone (ampop amplified), 1701 with selfboot and external control with uC (volume + others...). Audio signal is directly coming out of 1701 to headphones using Passive DAC Output Filter (from datasheet - fig.8). we've tried to connect to 2 different speakers, 130 Ohm and 23 Ohm, and the first one is much louder. I need to know if there's a way to maximise output signal without extra amplifier, what would be the best headphone impedance (if any), or other way to maximise signal...
Thanks.
Audio Signal Processors
direct
SigmaDSP Audio Processors
speaker
adau1701
Hello aMartins,
Although the DAC outputs aren't specified for output power, you can modify your passive output filter to allow more volume. The ADAU1701MINIZ evaluation board's passive filter uses 1/10 the series resistor and 10x the shunt capacitor -- 56 ohms and 56 nF. It drives 32 ohm 'phones to a quite usable (but not ear-splitting) volume. Edit -- your 130 ohm speakers may still be louder than 32 ohm speakers since the ADAU1701's DAC has a nonzero output impedance:
There's a few downsides, including higher distortion (about 0.2 % -- see this post for more details on that). It may not be necessary to go to this extreme -- you could first try, for example, 150 ohms and 22 nF. Also you'll hear a "pop" at power-ON, which modern headphone amp ICs suppress.
Best regards,
Bob
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