Author Topic: Headphone and line out various questions  (Read 633 times)

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Offline tapirathTopic starter

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Headphone and line out various questions
« on: June 09, 2023, 09:47:40 am »
I have a mono CODEC IC with an integrated headphone amplifier and I came up with this output scheme.


Couple questions:

  • The ferrite - pi EMI filter is right off the datasheet but why would I care about really high frequency EMI in an audio output device?
  • Considering that the headphones will be 16-32 ohm, I had to come up with really huge 470u coupling capacitors, the R17 and R19 are chosen to bleed these huge caps as fast as possible when the output is unplugged but they will also let through couple mA during normal operation, would this affect the sound quality?
  • Considering that my CODEC is mono and that I want to hear the same audio on both ears, I simply doubled up the same signal, does this method have any downsides? Can I merge the L/R channels for the headphone with one single 1000u cap (did that with lineout but there I only need a 4.7u cap not a huge 1000u one)?
  • Any other comments?

much appreciated


« Last Edit: June 09, 2023, 01:09:51 pm by tapirath »
 

Online radiolistener

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Re: Headphone and line out various questions
« Reply #1 on: June 09, 2023, 11:26:07 am »
  • The ferrite - pi filter EMI filter is right off the datasheet but why would I care about really high frequency EMI in an audio output device?

There are at least two reasons:
1) it reduces audio noises.
2) it minimize RF effect on output amplifier and as result minimize audio distortions and unwanted audio interferences due to RF signal mixing on semiconductor elements such as transistors and diodes which is used in audio circuit

Probably you have hear annoying buzzing noise when a cell phone is working near chinese audio amplifier. This is because chinese audio amplifier lacks of such RF filters and shielding... As you can see, the result is very well audible at audio frequencies, despite the fact that the operating frequency of a cell phone is 900 MHz. This is due to a mixing effect of RF interferences on a semiconductor elements, such as diodes and transistors which are used in audio circuit.
« Last Edit: June 09, 2023, 11:34:09 am by radiolistener »
 

Offline tapirathTopic starter

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Re: Headphone and line out various questions
« Reply #2 on: June 10, 2023, 08:55:06 pm »
  • The ferrite - pi filter EMI filter is right off the datasheet but why would I care about really high frequency EMI in an audio output device?

There are at least two reasons:
1) it reduces audio noises.
2) it minimize RF effect on output amplifier and as result minimize audio distortions and unwanted audio interferences due to RF signal mixing on semiconductor elements such as transistors and diodes which is used in audio circuit

Probably you have hear annoying buzzing noise when a cell phone is working near chinese audio amplifier. This is because chinese audio amplifier lacks of such RF filters and shielding... As you can see, the result is very well audible at audio frequencies, despite the fact that the operating frequency of a cell phone is 900 MHz. This is due to a mixing effect of RF interferences on a semiconductor elements, such as diodes and transistors which are used in audio circuit.

Makes sense! Any ideas about the other points? I was especially wondering about the bleeding resistors. 470u is a big cap but I don't want the audio quality to suffer.

 

Online Benta

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Re: Headphone and line out various questions
« Reply #3 on: June 10, 2023, 09:20:29 pm »
I don't understand why you're doubling the headphone circuit.
Just connect L+R in the connector and you'll have sound in both ears.

 

Online Benta

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Re: Headphone and line out various questions
« Reply #4 on: June 10, 2023, 11:26:24 pm »
Let me rephrase: why duplicate the whole circuit? Your headphones don't give a hoot if they're supplied from one capacitor plus diverse or from two.
Just connect L+R together in the connector and only build half of your current thing.

 

Offline tapirathTopic starter

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Re: Headphone and line out various questions
« Reply #5 on: June 10, 2023, 11:28:17 pm »
Let me rephrase: why duplicate the whole circuit? Your headphones don't give a hoot if they're supplied from one capacitor plus diverse or from two.
Just connect L+R together in the connector and only build half of your current thing.

only because the 1000u caps looked physically huge and 2x470u needed considerably less real estate but in all honesty I didn't look hard enough. maybe there are smallish 1000u caps too.
 

Online Benta

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Re: Headphone and line out various questions
« Reply #6 on: June 10, 2023, 11:35:21 pm »
You still don't understand.
Delete:
C20, R19, C24, C25, FB2, D5.
Connect T and R at the connector.
Now, whether you want C17 to be two 470 uF in parallel or just one 1000 uF is another story.
 
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