USB -> I2S and Analogue

[marce]

[Redshift187]

Well I have one assembled, and it works pretty well for analogue. The background is very close to silent, close enough that I can't tell if it's working even with my very efficient IEMs, which was one of my main goals. I had some troubles with the proto service, the slot holes for the 3.5mm jack and the buttons were ignored and done as just plated round holes, so those items didn't fit.

I had R16, the ground loop breaking resistor, as 10r. That was much too high and contributed to instability. I had extra, so I paralleled 4 to give 2r5, and that seems to work just great, although I haven't since tested the ground loop problem I get with my main amp.

My HD415 headphones connected directly made the output extremely unstable (they have a meter long cable), with all sorts of oscillations. Adding a separate jack with 100r in series with the signal cured that problem and also lowered the volume enough that the volume control is actually usable again (and in addition limits the output power to a bit lower than the max specified by the PCM2706)

Photos to follow this weekend.

[SoNic_real_one]

Originally Posted by Redshift187

I had R16, the ground loop breaking resistor, as 10r. That was much too high and contributed to instability. I had extra, so I paralleled 4 to give 2r5, and that seems to work just great, although I haven't since tested the ground loop problem I get with my main amp.

That's the worse thing that is vehiculated around here - the "ground loop breaking". There is absolutelly no need for that.

[Redshift187]

Originally Posted by SoNic_real_one

That's the worse thing that is vehiculated around here - the "ground loop breaking". There is absolutelly no need for that.

And how do you propose I get rid of the bad ground loop between my media PC and my safety earthed amp?

[SoNic_real_one]

Ground between cases is not in the signal path. Is like conecting the fridge and washer toghether on the same ground plane with your equipment. There is no circulation on that ground that will go into your signal input, because is just ONE wire.
Using a resistor in the signal path ground is just forcing the signal to return via external ground, because now that's the path with lowest resistance. Signal will travel now thru the power sockets and electric cords instead of shielded wire.
Instead of traveling via a shielded cable, now you just created a huge ground loop in the SIGNAL path. And now you are picking up all the RF noises with that loop.

[Redshift187]

Originally Posted by SoNic_real_one

Ground between cases is not in the signal path. Is like conecting the fridge and washer toghether on the same ground plane with your equipment. There is no circulation on that ground that will go into your signal input, because is just ONE wire.
Using a resistor in the signal path ground is just forcing the signal to return via external ground, because now that's the path with lowest resistance. Signal will travel now thru the power sockets and electric cords instead of shielded wire.
Instead of traveling via a shielded cable, now you just created a huge ground loop in the SIGNAL path. And now you are picking up all the RF noises with that loop.

The resistor is between digital and analogue ground and not in the signal path ground.

[SoNic_real_one]

Still SIGNAL path. Digital signal needs to go into the DAC. Analog signal exits the DAC. Do you think there is optocoupler in the DAC? The switching transistors in the conversion stage use the common ground.
Look on ANY DAC datasheet and see if you have separated grounds with resistors.

[Redshift187]

The 100R resistors I added to the output are on the bottom, covering the cuts I made in the output traces. I jumpered the ground loop breaking R16/C16 to join the analogue and digital grounds.

This is my second one built and working, the first has volume/mute buttons that work just great. (the first one isn't as pretty due to all the desoldering/resoldering I did while I figured out the instability problems)

Photos: