LM4861: Crossover Distortion

Edward Zimmer

Part Number: LM4861

I am having problems with crossover distortion with this amplifier. We have it set up like it is in the typical application figure 2. However, for Rf we are using 100k and for Ri we are using 40k. We are getting this weird blip at the crossover. I hooked up a potentiometer to the line to see how the resistive load affects its. It looks like the glitch starts happening at about 107 ohms. Anything over 107 ohms I get a clear waveform. Whenever the potentiometer was below 107 ohms the cross over distortion would start and continue to get worse as resistance decreased.

Does anyone have any experience dealing with crossover distortion in these amplifiers?

CH1 is differential between pins 5 and 8

CH2 and CH3 are the signals at pin 5 and 8 respectively.

Thanks,

over 3 years ago

0 Ivan Salazar over 3 years ago

TI__Guru** 114555 points

Hi Edward,

Thanks for the thorough explanation of your test. I assume '107' is kilo-Ohms on Rf, is that correct?
Are you following any of the data sheet diagrams or could you share your schematic to double check?

Thanks and Best regards,
-Ivan Salazar
Applications Engineer

0 Edward Zimmer over 3 years ago in reply to Ivan Salazar

Prodigy 10 points

Hey Ivan,

Rf is set to 100kilo-Ohms. 107 is referring to 107 Ohms put across the outputs (pin 5 and pin 8)

I am following figure 2 from the datasheet. (Typical Audio Amplifier Circuit)

Thanks,

0 Ivan Salazar over 3 years ago in reply to Edward Zimmer

TI__Guru** 114555 points

Hi Edward,

I misread the "load" in your description. Let me analyze this and share further comments later today or tomorrow.

Best regards,
-Ivan Salazar
Applications Engineer

0 Edward Zimmer over 3 years ago in reply to Ivan Salazar

Prodigy 10 points

Hey Ivan,

Wanted to see if you have any update on this?

Thanks,

0 Ivan Salazar over 3 years ago in reply to Edward Zimmer

TI__Guru** 114555 points

Hi Edward,

Apologies for the delay. I'm going through the data of another crossover distortion case from some time ago with a similar amplifier, I'll get further comments shortly.

Best regards,
-Ivan Salazar
Applications Engineer

0 Ivan Salazar over 3 years ago in reply to Ivan Salazar

TI__Guru** 114555 points

Hi Edward,

Can you clarify what is the frequency of the signal you're trying to play?
If you test with lower frequency signals, do you still see the same distortion?

Best regards,
-Ivan Salazar
Applications Engineer

0 Edward Zimmer over 3 years ago in reply to Ivan Salazar

Prodigy 10 points

Hey Ivan,

I tested the amplifier from 15kHz to 60kHz. The distortion is present in all frequencies across that range.

Thanks,

0 Ivan Salazar over 3 years ago in reply to Edward Zimmer

TI__Guru** 114555 points

Hi Edward,

Please notice LM4861 is an audio amplifier, thus the intended bandwidth of this amplifier is <20kHz. Perhaps there's some high frequency instability going on in the system. You may try limiting the bandwidth, could you share a schematic so we can review and add comments on it?

Best regards,
-Ivan Salazar
Applications Engineer

0 Edward Zimmer over 3 years ago in reply to Ivan Salazar

Prodigy 10 points

Ivan,

I am using the schematic in the datasheet with slight variations. Attached is the schematic we were seeing the crossover distortion with.

0 Ivan Salazar over 3 years ago in reply to Edward Zimmer

TI__Guru** 114555 points

Hi Edward,

Can you try adding back the feedback capacitor to form the low-pass filter? The cutoff frequency would be higher than 60kHz anyways, and it may limit any high frequency oscillations.

Best regards,
-Ivan Salazar
Applications Engineer

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LM4861: Crossover Distortion