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I am building a 9 V battery-powered microphone pre-amplifier and I'm encountering a design problem which I don't know how to solve.

I have an op-amp that serves as main amplifier which is powered by a battery using a ground and 9 V rail. In the context of this question I think this circuit is sufficient to explain my situation (values are not accurate and circuit is not complete):

enter image description here

My main problem is that whenever I close SW1 to turn on the device, I get a loud pop sound at the output because C4 is not yet charged (at least that's what my simulation is telling me).

I'd prefer to use the smallest circuit possible and a circuit with very low quiescent currents. There are two ways I've considered:

  • Somehow make the main amplifier supply rise very slowly when SW1 is closed
  • Somehow short R5 when the device is turned on. I've considered using an n-channel MOSFET, but I'm not sure if I can reliably make sure it's always shorting R5 fast enough.

Would one of these be the way to solve this and if so, what would a circuit look like?

I've also looked at this question. Since I'm not hooking up a speaker: Would disconnecting my output ground using a MOSFET work like in answer 1? Unfortunately I don't understand answer 2 at all.

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    \$\begingroup\$ I've seen many commercial (HiFi) appliances do this with a simple relay + timer. \$\endgroup\$ Apr 27, 2021 at 17:08
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    \$\begingroup\$ How is the amplifier volume controlled? Does it have a mute input or a way you can start it without the volume at max? Not sure that's a cure, necessarily. But if available, it should be tried. A common way is a delayed relay. But that may be more than you want to bite off. \$\endgroup\$
    – jonk
    Apr 27, 2021 at 17:08
  • \$\begingroup\$ I've considered a relay, though, do relays typically operate at less than 1ma? \$\endgroup\$
    – theIpatix
    Apr 27, 2021 at 17:25
  • \$\begingroup\$ You could try a capacitor-multiplier PNP to ramp up the supply slowly, but stages downstream will still have a click when you reach +8.8V and level out. \$\endgroup\$
    – glen_geek
    Apr 27, 2021 at 17:45
  • \$\begingroup\$ @thelpatix, look into latching relays. Only take a pulse to engage and disengage them. But then you'll need a delay, and way to "reset it" on power-off. \$\endgroup\$
    – rdtsc
    Apr 27, 2021 at 17:55

1 Answer 1

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I think that C3 is causing the POP. It is eliminated by slowing down the biasing like this: POP

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  • \$\begingroup\$ At least in simulation this indeed seems to remove the giant output step I get. I guess I'll have to wire up an actual example and test it. As for the component values: They we're not accurate in the schematic I provided. Maybe a poor attempt by me to break down the circuit to the minimum for the question. \$\endgroup\$
    – theIpatix
    Apr 27, 2021 at 19:38

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