Suppressing audio turn-on POP

[Oznog]

I have an LM386 circuit which will have headphones hooked up at the time it turns on.

The headphones are capacitively coupled which means there will be a pop as the LM386 output stage biases. Also the rest of the amp has biased circuits which could make a pop even if the output amp didn't.

It is difficult to get a low impedance series element to disconnect the headphones during this time. A dual relay is too large, pricey, and takes too much power. Since this is not a lot of current, I was wondering about just shorting the output during this period. The digital pots can disconnect the audio input during this time no problem so there's only pop current. At first I thought about shorting past the headphones with an N-channel FET to ground, but then I remembered about the intrinsic diode in MOSFETs. Putting an NMOS/PMOS back-to-back would be an undesirable level of complexity (board space, component count, cost). I thought about using a transmission gate like a 4016 either in series or to short the pop to ground but in either case the resistance is much too high.

Any other ideas? I could always do a bipolar transistor. It would mean the speakers would still see a 0.3v pulse due to the limited saturation voltage, and the other prob is the signal needs to be switched on by a microcontroller which means there's going to be a brief between power application and microcontroller execution. With the FET I could have left a 10k pullup resistor to Vdd on all the time and it wouldn't drain the battery when off, and the microcontroller only pulls the gate low after the turn-on delay expires.

Any ideas what I should do here?

 

[mramos1]

Microcontroller to drive two emmiters to gound after a short delay maybe?

I wonder if an RC will work to drive the transistor to ground and leave out the microcontroller.

Not big on the analog, I normally drag out a simulator.

 

[Roff]

Keep in mind that anything you do that causes a big, fast risetime transient will cause a pop.
Well, this is pretty weird, but it is just an idea. I don't think it is practical, because of the reason I put on the schematic, but you did ask for ideas.
The idea here is to make both terminals on the headphone jump up to the bias voltage simultaneously.

 

[Oznog]

Keep in mind that anything you do that causes a big, fast risetime transient will cause a pop.
Well, this is pretty weird, but it is just an idea. I don't think it is practical, because of the reason I put on the schematic, but you did ask for ideas.
The idea here is to make both terminals on the headphone jump up to the bias voltage simultaneously.

Stereo headset's common must be grounded. Already been through this. So cap coupling the ground side like this is not going to work.

 

[Roff]

Keep in mind that anything you do that causes a big, fast risetime transient will cause a pop.
Well, this is pretty weird, but it is just an idea. I don't think it is practical, because of the reason I put on the schematic, but you did ask for ideas.
The idea here is to make both terminals on the headphone jump up to the bias voltage simultaneously.

Stereo headset's common must be grounded. Already been through this. So cap coupling the ground side like this is not going to work.

Now you tell me. :lol:
Is this because the hardware already exists, and the jack is connected to GND?

 

[mramos1]

Ron H always kills my chance to get out the programmer, compiler and program a chip.

Of course the last time he did, he save my butt.

 

[Roff]

Ron H always kills my chance to get out the programmer, compiler and program a chip.

Of course the last time he did, he save my butt.

He did?

 

[mramos1]

Yes you did and thanks. uC and bridge idea.