Slow ramp up/fast ramp down 48V phantom power (circuit design) - diyAudio
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Old 17th June 2011, 04:13 PM   #1
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Default Slow ramp up/fast ramp down 48V phantom power (circuit design)

So. i want to eliminate the pop when turning on phantom power in any preamp, but i also don't want to wait for the power to ramp down.
using ktechlab for simulating, i found that several designs had a quite high voltage drop, in addition, all the designs that i found also ramped the voltage slowly down.

so i made up a "new" design, using a Sziklai pair and a FET, when the circuit is off, everything is earthed,( effectively shorting the cap?)
the question really, is would this work?
it simulates nicely in ktechlab(BUT the simulator is odd), giving almost 10mA into any microphone load with a voltage of 48.3V

Any comments on the schematic is welcomed.


the schematic
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the scope view from ktechlab:

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Old 18th June 2011, 03:39 PM   #2
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Would'nt this be easier? I did not test it for component values. E
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Old 18th June 2011, 04:17 PM   #3
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you have to understand it is the active components turning off and on inside the mic as well as the differential discharge from the phantom blocking caps inside pre.



like the 1970's equipment that had a " plop " in the speaker when turning it on.

That is why it should have been told to you, that you pull the slider down.
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Old 19th June 2011, 04:47 PM   #4
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Quote:
Originally Posted by DavesNotHere View Post
you have to understand it is the active components turning off and on inside the mic as well as the differential discharge from the phantom blocking caps inside pre.

That is why it should have been told to you, that you pull the slider down.
I am rather new to electronics, I joined this forum to learn.
the reason I wanted slow rise phantom power was because I have read that a slow rise implementation reduces most of the pop when turning on phantom power


"pull the slider down" is a very useful, and very correct approach, but my little m-audio interface can turn on phantom power totally silent with an akg condenser plugged into it. with absolutely no pop delivered in or out So to pull the slider down and leave the issue at that would be to accept a compromise, and I do not like to accept compromises

so if the mic is silent, and the preamp makes the noise, the issue should be dealt with at the preamp.

E:
I am sceptical about the thermistor approach, because of phantom powered microphones are specified to always draw less than 10mA of current, and some drawing 1-2mA meaning a theoretical variation of 0,48W to 0,096W(2mA) on the thermistor. depending on the power dissipation of the NTC thermistor, it seems logical that the voltage would vary, giving higher voltages at higher temperatures, and very low voltages if the power applied to the resistor would be lower than the temperature coefficient(BAD BAD sentence, but I hope you get what I mean). also, the voltage applied immediately would be at a possibly quite large step up, depending on the ratio of the NTC and the resistor. But it was a very simple and elegant solution, It could work perfectly if it was always the same load, i think

But I am no expert, I'm actually a rookie, I simply try to make intelligent observations.

the reason I posted was to get a yes/no if the design is actually usable, and maybe some feedback to improve my design.
-M
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Old 19th June 2011, 08:14 PM   #5
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You did get some pretty good feedback from Mickeymoose you chose to ignore. Simplest is usually the best solution and the Mickeymoose solution allows you to set the charge / discharge rates separately. It uses fewer parts.

G
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Old 19th June 2011, 08:42 PM   #6
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I suspect it wont work.
The capacitor charging up at the base of a transistor is too fast.
The threshold is 0v7 which will pass pretty quickly.

I would just charge up a capacitor from the 50v slowly and feed the phantom power from another resistor after that. Keep it simple.
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