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Circuit review?
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StillTrying:

--- Quote from: paulca on April 05, 2018, 07:18:31 pm ---A virtual ground is used when you only have a single supply.  So you need to create an artificial ground of (usually) 1/2 Vcc so that the amplifer can amplify both + and - signals.  There for you need an AC coupling cap and a ground reference resistor on the inputs... and potentially the outputs, but I'm relying the headphone being a resistive load to ground to pull the output down.
--- End quote ---

I'm afraid that you've convinced me so much that at a virtual ground is needed, that I'm sticking by my previous post. :)
paulca:

--- Quote from: StillTrying on April 05, 2018, 08:50:33 pm ---
--- Quote from: paulca on April 05, 2018, 07:18:31 pm ---A virtual ground is used when you only have a single supply.  So you need to create an artificial ground of (usually) 1/2 Vcc so that the amplifer can amplify both + and - signals.  There for you need an AC coupling cap and a ground reference resistor on the inputs... and potentially the outputs, but I'm relying the headphone being a resistive load to ground to pull the output down.
--- End quote ---

I'm afraid that you've convinced me so much that at a virtual ground is needed, that I'm sticking by my previous post. :)

--- End quote ---

The interesting difference in that circuit is that the feedback loop is biased to actual ground via a cap to block DC.  I've seen both variations.

The other thing I have seen is biasing the feedback to "VGND", but also putting a decouling cap on VGND to GND near the amp to absorb and noise.
StillTrying:

--- Quote from: Twoflower on April 05, 2018, 07:52:22 pm ---Not sure about the virtual GND. My first thought: No way. But the more I think about it I think it might work. As it's only used as reference for the OPAs. But for my taste the 2x 4.7kOhm is a bit too high to help stabilize it at the center. At power on it rely on identical cap-values as voltage divider. In real world they differ +/-20% (or +/-10%) so does the GND until the resistive voltage divider equalize it. The result will be a offset at the output during power on.
--- End quote ---

I agree the 4k7s seem a bit high, but I can't see an uneven current anywhere, and with the VGND's decoupling caps being 10X the value of the output coupling caps. I think the VG will only ever move about 5-10%, - in theory.  :)

Edity random thought: I don't think the input 22u caps are needed.
BrianHG:
This will be the only 1 comment I will make on this thread.  Paulca, you do realize that having headphone jacks right next to the left and right of the volume knob not only means it will be difficult to turn, even considering a bit of height difference, you wont be able to place a knob on the volume knob, or even use professional high quality headphone jacks which may have a larger diameter that typical earphone bud jacks.
paulca:

--- Quote from: BrianHG on April 05, 2018, 09:41:47 pm ---or even use professional high quality headphone jacks which may have a larger diameter that typical earphone bud jacks.

--- End quote ---

I haven't seen a pair of headphones in a long time that had only a 1/4" jack plug.  Most have been 3.5mm with a 1/4" screw over.

This is desktop audio, not a mixing desk.  Size matters.

I have to get all of things I am prototyping over the past month onto one single board and case that I hope will be under 6" across!  So I need to practice with tight layouts.

Going panel mount on the pots and jacks is starting to seem like the right approach for the final all-in-on though.  Even for part selection, it gets a lot easier when you aren't mounting it onto the PCB to pick those things.

I noted the pot is very close to the jacks.  It makes a lot less sense when you add plugs with mouldings, making a knob over the shaft of the pot not likely to work.

Also as I found on my DAC, getting 2 PCB mounted jacks and a PCB mounted USB to work when it comes to a case requires... inovative and fairly messy solutions.  Like jack holes big enough for the plastic moulded barrel of the plugs!

For litlte small prototypes I'd prefer they were PCB mount, all in one units though.
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