I took the risk to experiment with this and I guaranty that I do not have any problem by connecting the VGND to GND to my project. The only result was to have clearer sound . The reason that I took the risk was because with that much noise my project was useless . Also as I use this project in a very loud situation (live gigs in a bar) I found that I could hear and other kind of noises like clock sync so I found that I had to short another two pins together (pin 10 + pin 11) to get read also from this kind of noise . In this image you are going to see how I short them together https://imgur.com/ksMECm5 . I am not an expert, however as I described before I had to find a solution immediately as I had to use it in a very loud place, so I do not know what I did exactly however it works perfect with no problems and damages. With this two hacks I manage to have a very silent project that is usable for me . Here is my project that still alive https://imgur.com/h96Blzy
Teensy 3.6 + Audio Shield huge noise
- Thread starter zioaxiom
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I took the risk to experiment with this and I guaranty that I do not have any problem by connecting the VGND to GND to my project. The only result was to have clearer sound . The reason that I took the risk was because with that much noise my project was useless . Also as I use this project in a very loud situation (live gigs in a bar) I found that I could hear and other kind of noises like clock sync so I found that I had to short another two pins together (pin 10 + pin 11) to get read also from this kind of noise . In this image you are going to see how I short them together https://imgur.com/ksMECm5 . I am not an expert, however as I described before I had to find a solution immediately as I had to use it in a very loud place, so I do not know what I did exactly however it works perfect with no problems and damages. With this two hacks I manage to have a very silent project that is usable for me . Here is my project that still alive https://imgur.com/h96Blzy
Here's the pdf: https://www.analog.com/media/en/technical-documentation/application-notes/AN-1056.pdf
Here's an attempt to re-attach the screenshot:
The headphone output is floating above ground at 1.56V or so deliberately so the chip doesn't need output capacitors
nor a negative supply (the outputs are centred on 1.56V - measured with my Audio shield rev D). If you have to connect
headphone ground to ground, use a capacitor so you don't short the virtual ground driver within the chip.(*) When
you shorted the two grounds you'd have been putting DC through your headphones as well as drawing a lot of current
internally to the chip.
My suspicion is that the headphone driver internally is three opamps, one each for shield, L and R to the 'phones,
and the shield gets a DC value to remove DC bias. Typical opamps survive indefinite short-circuit, but its not a
great idea to do this, nor to put DC through phones as the diaphragms will be end-stopping.
(*) There is a risk of oscillation doing this however.
What approximate capacitance should that be?
Hi Mark,
I'm sorry for the dumb question, I'm not an electrical engineer...
I assumed the impedance to be around 11k and cutoff frequency 100Hz to reduce the noise and it gave me 1.47x10^-7.
The scope picture looks slightly better, but the noise is louder.
I tried even 90nF, 47uF 1000 uF, looks like 47uF works slightly better than others, but the noise is still worst than without the capacitor...
I also tried to disconnect headphone ground from HP_VGND and connect it to the GND and add the 220 uF capacitors in series, but noise just got worst...
Line out doesn't seem to have any noise, I might try some luck with external amplifier.
Is there any chance you could give some hints (simple schematic and values based on some cheap earbuds)?
Instability is one possibility as I stated, the headphone ground output is not meant to be loaded w.r.t. real ground, it seems it might be
the output from an internal opamp on the chip AFAICT, which wouldn't tolerate capacitive loading.
Basically the headphone output is designed to float w.r.t. ground and that's it. The datasheet specifically warns you about this,
you can ignore the warning, but don't expect it to work. If you want a clean signal from the chip use the lineouts. If you want
to connect a headphone/earphones to the chip, use the headphone pins. Don't mix the two if you can help it.
The internal amps on the chip that drive the headphone outputs are likely to add measurable amounts of distortion and noise to
the raw DAC performance too - another reason to use them only for monitoring with headphones.
the output from an internal opamp on the chip AFAICT, which wouldn't tolerate capacitive loading.
Basically the headphone output is designed to float w.r.t. ground and that's it. The datasheet specifically warns you about this,
you can ignore the warning, but don't expect it to work. If you want a clean signal from the chip use the lineouts. If you want
to connect a headphone/earphones to the chip, use the headphone pins. Don't mix the two if you can help it.
The internal amps on the chip that drive the headphone outputs are likely to add measurable amounts of distortion and noise to
the raw DAC performance too - another reason to use them only for monitoring with headphones.
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