LM4674: Schematic review

Hi Shu-Cheng,

datasheet says "Superior click and pop suppression eliminates audible transients on power-up/down and during shutdown."

May I ask, why you have put switches to the outputs?

Kai

Hi Shu-Cheng,

I will review your schematic and will come back to you tomorrow with further information.

Best Regards
José Luis Figueroa
Audio Applications Engineer

Hi Kai,

Because we have a lots pop noise issue before and couldn't resolved, so we reserved this way.

For now , this switch only reserved, not use in future.

Thanks!!

Hi Shu-Cheng,

only a minor issue: C8091, C8092 are ceramics with very low ESR. Sometimes resonances can be seen in combination with B548, because of the little ESR in this loop. It can be helpful to use an aluminium electrolytic cap here and parallel it by a X7R ceramic cap (or caps) with small package. The aluminium electrolytic cap only to dampen the resonance and the ceramic cap to filter out the high frequency noise of LM4674.

Kai

Hi Shu-Cheng,

The pop noise is caused by an unbalanced input. INR- and INL- pins should have the same network than INR+ and INL+, so you should add a 75Ω resistor at INR- and INL- to balance the inputs and reduce the pop noise.
You should take into account that the input capacitors should be made of materials with temperature coefficients of X5R, X7R or better. Besides, you should select capacitors with a tolerance of 10% or lower and resistors with a tolerance of 1%.

Best Regards
José Luis Figueroa
Audio Applications Engineer

Hi Sirs,

Thanks for your quick reply.

Please refer our update as below

The pop noise is caused by an unbalanced input. INR- and INL- pins should have the same network than INR+ and INL+, so you should add a 75Ω resistor at INR- and INL- to balance the inputs and reduce the pop noise.

we will add 75 ohm between INR- and INL-.

 

You should take into account that the input capacitors should be made of materials with temperature coefficients of X5R, X7R or better. Besides, you should select capacitors with a tolerance of 10% or lower and resistors with a tolerance of 1%.

Our input capacitors with X7R a tolerance of 10% and 75Ω resistors with a tolerance of 1%.

 

Only a minor issue: C8091, C8092 are ceramics with very low ESR. Sometimes resonances can be seen in combination with B548, because of the little ESR in this loop. It can be helpful to use an aluminium electrolytic cap here and parallel it by a X7R ceramic cap (or caps) with small package. The aluminium electrolytic cap only to dampen the resonance and the ceramic cap to filter out the high frequency noise of LM4674.

After our discussion, we keep our design. Because we don’t need to rigorous of sound quality.

Because we found our ground is not good enough, so we update our schematic as below

Kindly let us know if still have any suggestion. Thanks!!

SCHEMATIC1 _ PG31_ AMP_NS_LM467_20180321-new.pdf

Hi Shu-Cheng,

I think José Luis wanted to say that the both inputs of each differential input of LM4674 should look identically. This means a circuit like this:

This scheme assumes, that the sources driving these inputs do not have relevant source impedances. If they have a relevant source impedance, then you must also add it to the 75R resistors. Example: If the drivers have a source resistance of 100R each, then you would have to add 100R resistances to each 75R resistor in the INR- and INL- lines.

Kai

Hi Kai,

Nice to meet you.
It seems that I misunderstood what you meant.
When we have pop niose, we just can fine tune R556, R9994, R9995 and R567 to balance right?
I modify schematic as attachment.
Can you double check for us?
Thanks.

AMP_NS_LM4674SQ.pdf

Hi guys,

The Kai's assumption is right about the input resistors.
Another thing that could help to reduce the pop noise is to remove R9919 and R9920 if this is possible in the design.

Best Regards
José Luis Figueroa
Audio Applications Engineer

Hi Jose,

I know Kai's assumption is right about the input resistors, so I put on the resistors of 75 ohm and reserved layout place to fine tune.
I will remove R9910 and R9920.
Do you have any suggest for our design?
Thanks.

Marco.

Hi Marco,

I don't have any extra suggestion about your design. Everything else seems to be right.

Best Regards
José Luis Figueroa
Audio Applications Engineer

Hi Jose and Kai,

Thanks for your help.

Marco.

Hi Jose and Kai,

This information is good for us and I will product the PCB and verify it.

Thanks a lot.

Marco