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Headphone Amp front end design.
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Jbliss:
Hi All

Thanks for all the help so far, here is my first go at the Schematic of my headphone Amplifier Design.
Would really appreciate some feedback on where I have possibly gone wrong. (Still learning)

Thanks
JB
SiliconWizard:
First off - your schematic is rather neatly laid out. Maybe I would have flipped (mirrored) the XLR symbols so as to avoid crossing the + and - lines, but that's a detail.

I was first a bit wary of the 39.2 ohm series resistors at the heaphone outputs, but apparently this is what they recommend on the example applications in the TPA6120A2's datasheet. Still think that may be a bit on the high side, dissipating unnecessary power and giving too much attenuation for lowish impedance headphones (such as 16 ohm which they claim this chip supports). This will certainly work, but you may want to test this and maybe lower their value a little depending on the headphones you're going to use.

Another point is about the separate grounds. I don't know how you're going to do the PCB layout, but this split ground thing is a recurrent topic of discussion here. I personally would just use two big ground planes (on each side, going for a 2-layer board) and avoid splitting the grounds and joining them in one point, which is usually a bad idea in spite of still being regularly suggested. Obviously others may have a different take on this.

Lastly, I would have used the opamps (op1611) as non-inverting summing amplifiers instead of inverting. Not that it will matter a whole lot, but you would avoid an unnecessary phase reversal and possibly a marginally higher noise (although it would probably again not matter here). It would load the volume pots less as well. Just a thought.

Other than that, I haven't seen anything problematic so far.
Jbliss:

--- Quote from: SiliconWizard on October 31, 2018, 05:25:25 pm ---First off - your schematic is rather neatly laid out. Maybe I would have flipped (mirrored) the XLR symbols so as to avoid crossing the + and - lines, but that's a detail.

I was first a bit wary of the 39.2 ohm series resistors at the heaphone outputs, but apparently this is what they recommend on the example applications in the TPA6120A2's datasheet. Still think that may be a bit on the high side, dissipating unnecessary power and giving too much attenuation for lowish impedance headphones (such as 16 ohm which they claim this chip supports). This will certainly work, but you may want to test this and maybe lower their value a little depending on the headphones you're going to use.

Another point is about the separate grounds. I don't know how you're going to do the PCB layout, but this split ground thing is a recurrent topic of discussion here. I personally would just use two big ground planes (on each side, going for a 2-layer board) and avoid splitting the grounds and joining them in one point, which is usually a bad idea in spite of still being regularly suggested. Obviously others may have a different take on this.

Lastly, I would have used the opamps (op1611) as non-inverting summing amplifiers instead of inverting. Not that it will matter a whole lot, but you would avoid an unnecessary phase reversal and possibly a marginally higher noise (although it would probably again not matter here). It would load the volume pots less as well. Just a thought.

Other than that, I haven't seen anything problematic so far.


--- End quote ---

Thank you! Great idea regarding the OP1611 as non inverting!
On the topic of grounding I was basing the grounding system off this https://www.rane.com/note151.html I suppose my idea was to run a trace from C3, C23, C33, C43 back to there respective Pin 1's. The pin 1's would be tied to chassis Ground at a star point where both grounds would meet. This could be a terrible idea? any opinions on the best way to do this ?
Thanks Again
JB
NiHaoMike:

--- Quote from: SiliconWizard on October 31, 2018, 05:25:25 pm ---I was first a bit wary of the 39.2 ohm series resistors at the heaphone outputs, but apparently this is what they recommend on the example applications in the TPA6120A2's datasheet. Still think that may be a bit on the high side, dissipating unnecessary power and giving too much attenuation for lowish impedance headphones (such as 16 ohm which they claim this chip supports). This will certainly work, but you may want to test this and maybe lower their value a little depending on the headphones you're going to use.

--- End quote ---
The power into a pair of headphones at usual listening volume is well below -20dBm. In fact, it can often be helpful to increase the resistance to get the usable SNR up, especially when using mass market parts.
Richard Crowley:

--- Quote from: SiliconWizard on October 31, 2018, 05:25:25 pm ---First off - your schematic is rather neatly laid out. Maybe I would have flipped (mirrored) the XLR symbols so as to avoid crossing the + and - lines, but that's a detail.
...
Lastly, I would have used the opamps (op1611) as non-inverting summing amplifiers instead of inverting.

--- End quote ---
Or, he could simply un-cross the XLR + and - lines    >:D
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