LM4871: higher gain

Michael Gilge

Part Number: LM4871

The data sheet states that the LM4871 should be used in low gain configuration. Our input signal to the LM4871 has an amplitude below 0.5 Vrms.

What would be the maximum gain recommend for the amplifier?

We are currently using a 5V supply, 4 Ohm load and Ri=21k, Rf= 47k and the volume is still not sufficient.

Thanks,

Michael

over 8 years ago

0 José Luis over 8 years ago

TI__Mastermind 20085 points

Hi Michael,

It is always recommended to set the gain to 2V/V. You can increase the gain, but the quality audio could get worse.

Best Regards
José Luis Figueroa
Audio Applications Engineer

0 Michael Gilge over 8 years ago in reply to José Luis

Expert 1720 points

Hi José,

what do you mean by 2V/V? What could be the maximum value for Rf (given that Ri=21kohms)?
It is a speech application, no HiFi music.

Best regards,
Michael

0 José Luis over 8 years ago in reply to Michael Gilge

TI__Mastermind 20085 points

Hi Michael,

This means gain = 2. You can select any value for Rf, but you should take into account so that optimal performance, the gain should be set to 2 or lower.

Best Regards
José Luis Figueroa
Audio Applications Engineer

0 Michael Gilge over 8 years ago in reply to José Luis

Expert 1720 points

Hi José.

thanks. Which preamplifier do you recommend to get to 1-2 Vrms? We only need mono.

Best regards,

Michael

0 José Luis over 8 years ago in reply to Michael Gilge

TI__Mastermind 20085 points

Michael,

You could have a look to the OPAs, these kinds of devices are used like preamplifiers.

Best Regards
José Luis Figueroa
Audio Applications Engineer

0 Michael Gilge over 8 years ago in reply to José Luis

Expert 1720 points

Hi,

I am driving the LM4871 from a headphone output. I would assume the impedance of the headphone output is low, e.g. 50-120 ohms.

Should I match the low impedance with the LM4871 design by selecting Ri = Rf = 100 ohms and adapt Ci accordingly?

Do you have a reference for the LM4871 driven by a headphone amp?

Best regards, Michael

0 José Luis over 8 years ago in reply to Michael Gilge

TI__Mastermind 20085 points

Hello Michael,

if the headphone impedance is very low, It could be necessary to match the devices impedances.
I don't have any reference for the LM4871 with this kind of application since this device is designed like output device.

Best Regards
José Luis Figueroa
Audio Applications Engineer

0 Michael Gilge over 8 years ago in reply to José Luis

Expert 1720 points

Hi,

is it possible to go as low as 100 ohms for the input impedance? Ri = RF = 100ohms?

Or what are the limits?

Best regards,

Michael

0 José Luis over 8 years ago in reply to Michael Gilge

TI__Mastermind 20085 points

Michael,

I think that your application could work, but it is always recommended to use resistors higher than 1kΩ.

Best Regards
José Luis Figueroa
Audio Applications Engineer

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LM4871: higher gain