Help: Phantom soft start circuit into Pre Amp board

Hi everyone

Ive just built my first mic pre and Im happy with it. There is just one issue that I'm facing. When I turn on Phantom power, there is a huge spike before it levels out. This is seen in the attached image. I was wondering if there are any simple soft start circuits out there that I may adapt and add to the Phantom power input of the circuit that will ramp up the phantom power slowly (a second or so) so that I do not get this spike going through the output.

Thanks guys.


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Sorry, I forgot to mention that channel 1 (yellow) is the phantom power and channel 2 (blue) is the measurement on the output of the pre amp. The PSU is the one from "Self on audio" - First image.

Testing the Phantom power signal on the oscilloscope looks like there is very little ripple. - second image. Phantom on channel 1 (yellow)


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The microphone preamp might be unstable at intermediate voltages during the ramp up, best to ramp up quickly but smoothly, using an RC time constant of perhaps 1ms. Perhaps 100R, 10µF, after the phantom switch before the 6k8's

Perfect balance for phantom power switching is unobtainable, you have microvolts of signal against 50V of common-mode change, thats asking for CMRR of 150dB or so. There's always going to be a thump/click on applying power suddenly. If nothing else the two large electrolytic coupling caps that go between the amp and microphone will not be matched to ppm precision, 10% is more likely, so charging them both to 48V will lead to a few 100µC of charge difference between them, on a timescale of 1ms that's 100mV or so, on a timescale of 100ms that's 1mV or so (perhaps my filter should be 100R 1000µF?)

[ Actually thinking about it the 6k8 and 47µF values have a time constant of 0.3s anyway, the filter I propose removes the sharp leading edge of switching - 6k8's matched to 0.1% will lead to currents matched to 7µA, so a few mV equivalent. ]
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Yes exactly that - in fact the cap can be larger, the large time constant after the 6k8's will diminate.

Thinking about it a perfectly silent switch on or off is pointless, the microphone itself will have a thump from the circuits powering up, but you can try to keep the high frequency energy down, that hurts the ears most. A cleverly designed desk would interlock preset gain with phantom power switching to prevent loud thumps ?


Paid Member
2003-06-12 7:04 pm
Maine USA
> A cleverly designed desk would interlock preset gain with phantom power switching to prevent loud thumps ?

You would think. I've never seen it.

Experienced board operators "KNOW" to pot-down before touching the Phantom switch. While the thump may be mild, sometimes the monitors are turned up high. And as you suggest, some mikes have very rude start-up transients.