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I'm trying to replace the obsolete AN6878 led driver (logarithmic response, used for audio level indicator) with a modern and available solution. I'd like to keep the same functionality of the original.

datasheet

this is the bit of the circuit that I'm trying to replace

enter image description here

looking at the datasheet, I think I can replace it using an opamp and some comparator IC, but I only get a basic idea and don't know how to procede. Can anyone help me understand?

Edit: I’m open to software solution, if I only could understand the exact voltage values where the various leds go on

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No, a comparator won't do (also, how's that more "modern"?). You can try to approximate the logarithmic response using discrete semiconductors and a lot of hand-tuning, then use a parallel output DAC plus LED-driving transistors.

There's a reason these ICs were popular even back when ICs were expensive! The alternative was pretty complex to implement.

For someone with a bit of background in math and/or software: What about simply using your opamp and a diode to precision-rectify your audio signal, then sample it with the ADC of a microcontroller? From there, it's a bit of averaging and whatever response you want. Heck, if you're willing to write more software (and maybe omit the rectification and instead bias to mid-ADC-range-voltage), you could even achieve weighting according to psychoacoustic models, if a "perceived loudness" display is actually what you're after.

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  • \$\begingroup\$ Thank you, yes I could do that in software, but I need to understand the various voltage levels where the leds go on and replicate that \$\endgroup\$ – Dimitri Petrucci Oct 8 at 22:50
  • \$\begingroup\$ so, measure. You'd need to do that anyway, or you could actually build a scale that is meaningful, not identical to some 1970's IC... \$\endgroup\$ – Marcus Müller Oct 9 at 7:35
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If you wanted to replicate this with monolitic IC's then the datasheet would be a good place to start. Instead of a regular op amp the first amplifier is probably a log amp, and then followed by comparators. The resistors set a range of voltages for the comparators from lowest to highest, the external resistor between 6 & 7 sets the range for all. The comparators need enough current to drive whatever LED's are selected.

enter image description here

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If you are performing this repair as an exercise in engineering, then you could definitely build a replacement circuit using op-amps. After all, that is what the original IC used in it's design. It would be a somewhat complex circuit, however, when compared to the compactness from integration of the original chip.

If you simply want to "future proof" the circuit using parts that are currently available, you could just use one of the LM391x series chips, changing whichever resistor values that need changing in the "glue" circuitry to match the performance of the original circuit. You also might possibly need to add a logarithmic op-amp to the input stage, depending on the LM391x series chip you use.

If you just want to repair the item, it appears as if the original part is still available to some extent, and NTE even makes a "drop in" replacement chip, the NTE1500.

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