Noise leaking from PSU to mic amp

You need to revise this board radically, because you are trying to make it do two things which are fundamentally incompatible - deal with large, high voltage switching signals and tine signals from a microphone.

First - as you suggest - slice across the ground plane so that the amplifier and power supply are completely separated. Now link the ground across to a single point, which should be the ground point for the mic connector. You will still get some pickup until you have attended to the grounding around the input, and that is a bit too detailed to go into here, but get some references to star point grounding in order to do this properly.

d Pearce Consulting

Just a thought, but if you could increase the switcher frequency to move the noise above the audio range, you could use a simple low-pass filter on the mic amp. Probably best used as a last resort after all the "good design practice" stuff fails.

Best regards,

Bob Masta dqatechATdaqartaDOTcom D A Q A R T A Data AcQuisition And Real-Time Analysis

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That's the first thing I would do. It might help a lot, but not totally cure the problem.

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Well, if the circuit doesn't work, then it doesn't matter now little time you have. Use a saw and cut the circuits apart.

Try adding some toroid chokes to the incoming power leads, both + amd -.

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Welcome to the world of analog preamp meets switched power supply. ;-?

It will probably take a combination of isolation techniques to solve your problem.

Did you by chance use the autorouter to make this board? And aren't you a digital guy maybe? I have done a mike preamp and a complete DSP with fast parallel flash memory on a supertiny board for a cellphone with 2 switching supplies et al on 2 sq. inch, and no crosstalk at all. I suspect your problem can come from the voltage supply for the electret mike which must be heavily filtered and a wrong reference point for the mike preamp, which should be the positive supply. A ground plane is not always the reference, a complete understanding of the function is needed, determining which way currents are flowing into and out of the circuit. You will probably need a redesign, this cannot be just updated like a software, sorry for that.

No, simply limit the bandwidth of all audio amplifying elements to

Hi everyone,

Thanks loads for all your feedback. You're all right - I've designed the board badly.

I tried separating the ground planes, using a star configuration ground and putting the mic on its own supply but none of this worked. So I went ahead and cut the PCB in half! Which worked fine! It's a bit of a crappy solution but it's worked! Now I can propperly sheild the two PCBs from eachother.

Thanks, Jack

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You note in idea 1) grounding, and good grounding is essential, but I'm 99% sure that what you're seeing is magnetic coupling into a loop in the microphone preamp. Making the preamp circuitry be small would be a Good Thing. But with an inductor millimeters away I think this may not be enough.

Shielding the magnetic field from an inductor is rarely economically possible. Yes, you can buy mu-metal, but that's a very expensive and clunky afterthought. (I once worked with a mu-metal shielded phototube that we nicknamed "the planet crusher" because of the extreme weight of all the shielding!)

Toroidal inductors are preferred but not a cure-all (they still leak).

Geographic separation, if you can be flexible about PCB shape, may help you a lot since magnetic fields decrease like 1/(distance cubed). Simple orientation of the inductor may help by a factor of ten or so.

Those would maybe help with electrostatic radiation, but you're seeing magnetic radiation.

Tim.

Great, thanks a lot for your really detailed reply.

Jack

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Two comments: You may or may not be suffering from any magnetic pickup, but to the extent you are, sheilding won't help. Second, to the extent separation and shielding do help, the same result can be achieved on one PCB by adding two separated ground planes, and a small over-sheild with tabs soldered into the ground plane.

To the extent that Daniel / Jack is suffering from magnetic pickup, we can point out this pickup is proportional to the cross-section area (whose plane is at right angles to the magnetic vector), so reducing this area (e.g. keeping signal paths immediately adjacent to ground return paths, etc.) will help dramatically. Second, keep in mind the effectiveness of twisted-wire pairs in rejecting magnetic pickup; each half-turn's pickup is canceled the opposite polarity of the next half turn. It's at least theoretically possible to cancel all the pickup entirely in an analogous manner, with a loop of input wiring arranged to add in its pickup with the opposite polarity. :>)

Hi,

Thanks for the reply.

Yes, I tried separating the ground plane before I cut the board apart, but that didn't seem to work (although maybe I didn't separate the ground planes far enough - I only made a thin stanley knife cut).

Next time I design a PCB like this, I will do as you suggest.

Thanks, Jack