LME49720NABD: Amplifier Noise

Hi, Roger,

Could you provide more details about this issue? Do you have a block diagram or an schematic that could simplify your problem description?

Thank you.

Best regards,
Luis Fernando Rodríguez S.

Hi, Roger,

Excuse me, do you have any update on this? Could you provide the information that I requested in my last post? This is just to have a better approach to this issue.

Thank you.

Best regards,
Luis Fernando Rodríguez S.

I need to make some corrections on the first message I sent you. The phono preamp originally had a NE5532 bipolar input opamp. I upgraded it to the TLO72 then later to the TLE2072. Both chips worked perfectly except that the TLE2072 had superior sound compared to the previous two opamps. After reading some data about the LME49720na I was made aware that the maximum capacitive load is 100pf. The output of the moving magnet preamp circuit is connected to the line amp stage with a 2.0mfd ppmt capacitor followed by a grounded 10k resistor. The spec. sheet says to put a series resistor before the load if the value of load capacitance exceeds 100pf.  I did note that as I increased the preamp output level the noise level would decrease. Do you think that the excessive capacitive load is causing the noise? There is a 10k resistor in series with the volume pot. but of course it is after the phono stage output capacitor connected between the function selector switch and the volume control. If this could be causing the noise what value of resistor do you recommend I put in series with the phono stage opamp output and the 2.0mfd coupling capacitor. I should be able to send you the schematic by saturday if you need it.                                     

                                                                                                                                                                                                                                           Thank You

                                                                                                                                                                                                                                               R. Bell

Hi Roger,

yes a schematic would be good...

I think, what you describe is more a high pass filter to stop DC offset voltages, not what is usually called a capacitive load.

Kai

Hi, Roger,

Thank you for your clarifications above.

I have few suggestions to you regarding the noise issue:

- First of all, you need to verify that all the power supplies are stable. Ensure that all the noise is eliminated from these sources. Any parasitic signal at the supplies may result in a noise issue at the output.
- Ensure that all the details of the layout example of datasheet are respected in your design.
- Please connect at least a 470ohms resistor between the amplifier output and the capacitive load. If the noise persists, could you disconnect the capacitive load and verify if the noise is still at the output?

Best regards,
Luis Fernando Rodríguez S.

Luis:

         Should I remove the 2.0 mfd coupling capacitor and then direct couple the phono preamp with a 470 ohm resistor to the line amp? If I connect the resistor in series between the capacitor and the opamp output, it will create a series rc filter causing  a change in the low frequency cutoff of the phono amp output. If I am in error about this series rc coupling effect please let me know. I have already installed .1mfd bypass caps and 10mfd voltage buffers on the opamp power terminals.I am partial to direct coupling between amplifier opamp stages as long as there very little or no dc offset on the outputs of the opamps. The way I look at it, the 2.0mfd coupling cap would be recognized by the phono opamp as a large capacitive load and may be causing parasitic oscillations and low frequency noise from the lme49720na opamp. The existing circuit worked flawlessly with the NE5532, the TLO72 and the TLE2072.

Hi Roger,

give us a schematic please. This tells more than 1000 words...

Kai