I am currently trying to design a portable bluetooth audio DAC, and have made a schematic that I think might work.
I know that the bluetooth wiring works (have tested direct audio from the chip, but not through the amplifying circuit).
I have been getting quite confused on how many capacitors/resistors to use, and what ohms. I want to maintain audio quality/reduce noise, as much as possible.
Am I on the right track, and will the op-amp circuit work? (I haven't tested it on a breadboard, as some of the parts haven't arrived yet).
The raw audio output volume from the bluetooth chip is about half what I would like, so the amplifier circuit should be wired to produce 2x gain.
Everything is powered by a single battery.
Thanks!
I know that the bluetooth wiring works (have tested direct audio from the chip, but not through the amplifying circuit).
I have been getting quite confused on how many capacitors/resistors to use, and what ohms. I want to maintain audio quality/reduce noise, as much as possible.
Am I on the right track, and will the op-amp circuit work? (I haven't tested it on a breadboard, as some of the parts haven't arrived yet).
The raw audio output volume from the bluetooth chip is about half what I would like, so the amplifier circuit should be wired to produce 2x gain.
Everything is powered by a single battery.
Thanks!
You have DC bias across the potentiometers, which is bad news. What value are they BTW?
Best not to put bipolar opamp bias current through the wiper of a pot, it will be very noisy.
Are you trying to use a virtual ground?
The opamp circuit is unusual, a unity-gain buffer with a noise gain of 2. This will clip prematurely. Also not sure how it will handle cable capacitance.
Best not to put bipolar opamp bias current through the wiper of a pot, it will be very noisy.
Are you trying to use a virtual ground?
The opamp circuit is unusual, a unity-gain buffer with a noise gain of 2. This will clip prematurely. Also not sure how it will handle cable capacitance.
The parts have arrived now, and I tried out the circuit on a breadboard.
It all seems to work, but power supply noise is audible, and the audio has a slightly underwater sound.
The potentiometer is 10K ohms.
Yes I was trying to use a virtual ground, as not having that led to distorted output (in testing before I had the capacitors).
Could I avoid the clipping issue by moving the 10k resistor from the direct opamp output onto the loopback wire?
Thank you for the feedback!
I have edited the circuit to hopefully make the part values clearer and fix the weird opamp circuitry (not sure where to put the opamp bias current where it will still work, do I need to move it to the inverting input?):
It all seems to work, but power supply noise is audible, and the audio has a slightly underwater sound.
The potentiometer is 10K ohms.
Yes I was trying to use a virtual ground, as not having that led to distorted output (in testing before I had the capacitors).
Could I avoid the clipping issue by moving the 10k resistor from the direct opamp output onto the loopback wire?
Thank you for the feedback!
I have edited the circuit to hopefully make the part values clearer and fix the weird opamp circuitry (not sure where to put the opamp bias current where it will still work, do I need to move it to the inverting input?):
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I ended up fixing audio quality issues with the circuit attached, not sure if it's better to have the potentiometer before or after the opamp?
Thanks for the help 🙂
Thanks for the help 🙂
That's more like a x2 gain amp now 🙂
If the non-inverting inputs are coming straight from the caps on the DAC outputs, you have no DC bias. I would suggest setting up a buffered virtual ground at mid-rail voltage to reference the signals against in a single-supply setup like this. You AC couple in, and AC couple out, but the opamp works around the mid-rail voltage. Again to repeat, no DC on the pots please, DC across the track will lead to large DC offsets as the knob is turned. So AC only on pots, and capacitor on the wiper (which you have now).
Ensure opamp inputs have a DC bias current path or they won't work.
If the non-inverting inputs are coming straight from the caps on the DAC outputs, you have no DC bias. I would suggest setting up a buffered virtual ground at mid-rail voltage to reference the signals against in a single-supply setup like this. You AC couple in, and AC couple out, but the opamp works around the mid-rail voltage. Again to repeat, no DC on the pots please, DC across the track will lead to large DC offsets as the knob is turned. So AC only on pots, and capacitor on the wiper (which you have now).
Ensure opamp inputs have a DC bias current path or they won't work.