Electrical – Why does it take over 4 seconds to eliminate audio popping in this circuit

amplifieraudio

I made a simple audio mixer circuit with its output going to headphones. Right now I'm only testing with a single audio signal at the input, which is music from a smartphone. I hear a pop sound after I apply power (2 x AA) to the circuit, unless power has already been applied for at least approximately five seconds. Even if I leave power applied for a minute, and then I disconnect and reconnect power, there is a pop sound. What about this circuit would take five seconds to stabilize? VCC stabilized nearly instantly when I measured on the scope.

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I'm using a dual op amp IC, the LM4808 (http://www.ti.com/lit/ds/symlink/lm4808.pdf).

There are four 470 uF decoupling caps in parallel at the output (for a desired cutoff), and a 1k resistor to Ground that I included as a discharge resistor, but it doesn't seem to have any effect.

Best Answer

Here's what the data sheet says about Cb (the 10 uF capacitor in your circuit): -

Bypass Capacitor Value

Besides minimizing the input capacitor size, careful consideration should be paid to the value of the bypass capacitor, CB. Since CB determines how fast the LM4808 settles to quiescent operation, its value is critical when minimizing turn-on pops. The slower the LM4808's outputs ramp to their quiescent DC voltage (nominally 1/2 VDD), the smaller the turn-on pop. Choosing CB equal to 1.0μF or larger, will minimize turn-on pops. As discussed above, choosing Ci no larger than necessary for the desired bandwidth helps minimize clicks and pops.

So I would definitely experiment with this value and initially try 1 uF. Also, it makes little sense to use both amplifiers (not op-amps BTW) of this package - attach your headphone decoupling capacitors to the first stage. Cascading two stages might make the popping problem worse.

You should also note that having nearly 2000 uF coupling your headphones (32 ohms or thereabouts) has a high pas cut-off of only 2.5 Hz and this is too low for audio. Make it more like 220 uF and you might find the pop is reduced.

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