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I am using an LM4875 as a pre-amp for an audio circuit. Schematic here. The intended function is to "condition" audio inputs from different sources so they have the same peak-to-peak voltage when passed to the power amps.

My understanding of the circuit is that it applies a variable inverting gain at AMP1 which is set by the DC volume control. Then it inverts it again with a gain of 1 at AMP2 and powers RL.

It has an input called HP Sense which can be used to turn AMP2 off if HP Sense is driven high. This allows the circuit to ignore RL and instead power headphones instead of a generic speaker.

If I do not plan to use this HP Sense function can I simply use the output of AMP2 and break the AMP1 output connection right after it's junction with AMP2 negative input? Basically, can I use Vo2 and leave Vo1 as a NC?

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If you're only going to use one output, you should use Vo1 since that's what the chip is designed for. But I'm not sure why you'd want to do that. The chip is designed to drive a speaker with differential ("bridged") outputs. Among other things, this lets you connect the speaker directly to the IC with no decoupling capacitor. It also looks like the amplifier can provide more output power in bridged mode.

Also, if you want to do signal conditioning, a power amplifier IC is probably not the best choice. I would suggest either a proper automatic gain control circuit, or (if you need digital control) something like a PGA2310.

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  • \$\begingroup\$ I will try to change it to use Vo1, but the bridged mode does not work with my application. Too little power. I am working with a circuit made by someone else and I am just trying to understand it/make it work. I'll see if I can make a change to something more suitable. \$\endgroup\$
    – Nate San
    Feb 28 '17 at 0:02
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If you never want the diminished gain that the HP Sense operates, why not put a pulldown on the HP Sense line and connect your output across Vo1/Vo2? Re-read the part of the datasheet on "BRIDGE CONFIGURATION EXPLANATION" as to the benefits of using that.

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  • \$\begingroup\$ Well this amp delivers significantly less power than I need, even with their differential advantage. The output is passed into a power amplifier after this, and their current design is not set up to accept this kind of input. \$\endgroup\$
    – Nate San
    Feb 27 '17 at 23:45
  • \$\begingroup\$ That's a good reason ... in THAT case, I'd stick with the configuration explicitly mentioned in the datasheet, use Vo1 to supply the next stage, and hold HP Sense high. The reason for this is a bit of "engineering cargo cultism" Sometimes, but not always, amplifiers will react badly when unloaded ("Amplifiers Oscillate, Oscillators Won't" is an aphorism from my youth). \$\endgroup\$ Feb 27 '17 at 23:47
  • \$\begingroup\$ Yeah, this IC and the circuit it sits in was not designed by me. So I am just trying to make it work. I agree with your assessment and I'll try to swap it so V01 is the primary. \$\endgroup\$
    – Nate San
    Feb 28 '17 at 0:00
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Seems reasonable - ground HP sense (pin 3) to disable the headphone operation and leave Amp1 output (pin 5) open. Take your signal from Amp 2 (pin 8).

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  • \$\begingroup\$ Fantastic. That's what I hoped to do. \$\endgroup\$
    – Nate San
    Feb 27 '17 at 23:45

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