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I am using a VS1053 (on a SparkFun Music Maker shield) to provide audio for my project. I am taking a line out signal and inputting it into an op amp for signal processing, ultimately being input to an Arduino.

The op amp(s) are using Vcc and ground from the Arduino as their power supply. The module with the VS1053 also uses the same supply connections.

But line out is referenced to the audio ground created by the VS1053. In order to see the input signal, somehow the op amp and VS1053 must have a common reference.

An early circuit connected the audio signal ground to the circuit ground. But this is explicitly prohibited in the VS1053 data sheet. It seems to work but probably does do as a matter of luck. And will likely fail eventually.

How do I connect the line output of the VS1053 to an op amp?

Thank you.

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The three signals, L, R and GBUF are for driving headphones and will have a DC offset on them. GBUF carries common signal voltage, mostly for low-frequency signal. It’s a way of boosting the loudness of the headphones without using a higher supply voltage.

For line-out use, GBUF should not be grounded or connected at all, other than the filter that’s on the shield board. Instead, leave GBUF unconnected, and use blocking capacitors for L and R to to your line-out to the op-amp. Reference the line-in to the shield board ground.

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  • \$\begingroup\$ Thanks. I can only only use one channel in this application. So, feed R (for example) to the op amp through a cap. Leave GBUF unconnected. The R channel signal will be with respect to circuit ground, shared by the Arduino, the Music Maker shield and the op amps. Is this correct? Also, I assume that the input to the op amp in this configuration will be 0V to 2.4V. \$\endgroup\$ – djsfantasi Jul 13 at 18:50
  • \$\begingroup\$ Yes, that will work. \$\endgroup\$ – hacktastical Jul 13 at 19:09
  • \$\begingroup\$ You also have the option to feed GBUF to the (-) side of your op-amp. That is, treat R and GBUF as a diff pair. You will get more bass signal that way. \$\endgroup\$ – hacktastical Jul 13 at 19:11
  • \$\begingroup\$ Also note that I can only use single supply op amps. \$\endgroup\$ – djsfantasi Jul 13 at 19:15
  • \$\begingroup\$ That’s not a problem. As long as it’s a closed system and you don’t care about driving normal line-out, it may be a benefit: GBUF could be used as virtual ground for the (-) side of your op-amp. You could DC-couple everything in that case. \$\endgroup\$ – hacktastical Jul 13 at 19:37
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Feed line output L and R through capacitors to external circuits. The signals have DC component which will be filtered out with the capacitors. GBUF is the earphone ground with the same DC component to eliminate the need of capacitors with earphones. Note: GBUF needs a capacitor and a loading resistor for stability, no matter is GBUF used or not. It's shown in the application notes in the datasheet.

BTW. No capacitors to separate DC are needed if the external circuit is a properly designed differential amp and you connect its inverting input to GBUF. The DC is eliminated by subtraction.

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