weird(?) DC voltages at output of mic preamp

You are seeing the amplified blocking capacitor charging and discharging currents. Any time the DC voltage across a coupling cap changes, that is a form of AC, so it passes through the capacitor with an RC time constant of the capacitance times the resistance that the current has to pass through. If this pop takes several seconds, a coupling cap is way larger than needed to pass frequencies no lower than about 20 Hz.

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John Popelish

Thanks John.

I would like to clarify a couple things: If this is an AC voltage, why does it show up as DC on my meter? Also, the pop isn't audible for several seconds, but my meter does show the voltage for several seconds (not sure if that was clear in my original post). Are your reponses still valid in this case?

Thanks

preamp

answered

big

(i.e.

output

DC

quickly.

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The only DC that capacitors block completely is steady, unvarying DC. AC comes in two forms, sinusoidal and exponential. They can combine as a decaying sinusoidal, also. AC frequency is naturally measured in radians per second (2*pi of those per cycle), and the natural measure of exponential frequency is its time constant.

Capacitors pass current in proportion to the rate of change of voltage across them (I=C*(dV/dT)), regardless of whether it alternates direction or only varies on one direction. The amplifier is probably saturating at the beginning of the decaying pulse, so the output is steady till the decay goes low enough that the amplifier comes out of saturation. If the decaying pulse lasts long enough, your DC meter can measure it as it decays (especially the starting saturated part). Your solution is either to shunt the audio signal to ground while this decay occurs so it doesn't show up at the output, cancel it with an equal and opposite decaying voltage or shorten the duration by making the coupling cap much smaller (making a high pass filter) so that the pop is not long lasting enough to be a problem.

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John Popelish

Thanks again John.

I have 2 100v 100uF caps at the input (to block the 48v from the IC) - the schematic on the data sheet (it's an SSM2017) shows 2 47uF caps. Could the extra size be a factor here (I don't know how much smaller I could get away with without rolling off the bass too much)? Also, I have a 470uF 250v cap as a smoothing cap in the DC circuit before the regulator for the phantom power. Is the added voltage handling of the cap a factor also? I also have a

1 uF cap as a blocking cap at the output (at 100v I think - I used a poly cap here). Also, if I reduce the cap values, will this reduce the amount of voltage hitting the IC as well as the duration?

Thanks

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this

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Do you have a link for this schematic?

The closest thing I could find was this replacement chip:

It certainly makes the problem worse. The application note on my data sheet (page 2) shows the phantom power connected to the mic side of the 47 uf caps through 6k8 resistors, and another 4R7 after them before the clamping diodes. So the caps charge with very nearly a

6K8*47u=.3 second time constant, while the diodes conduct. But after the cap current falls to what will not drop 15 volts across the 10k to ground resistors (about 1 time constant), the time constant becomes (10k+6k8)*47u= .8 seconds.

So the inputs settle toward ground at that rate. Since the amplifier has a differential input, it starts to work as soon as the common voltage on the two inputs reaches the input common mode limit, which is about 2 volts inside the supply rails. But the output will still stay saturated a while after that if the two input capacitors are not well matched, so that one input settles toward zero faster than the other. You can test for this problem by looking at the DC polarity out of the amplifier during settling, and then switch the two input capacitors and see if the DC switches directions.

Regardless, the input caps have a long time constant with the input loading (grounding resistors) compared to 20 Hz. 1 uf should be enough. 2 uf plenty. And then you can use film caps with much better leakage and tolerances than electrolytics.

I doubt that it is involved in the problem.

If the amplifier is saturated, the output cap is not the problem. The huge over voltage takes place at the input, before all that gain.

It won't affect the peak much, only the duration.

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John Popelish

Thanks John.

I d> >

the

away

phantom

poly

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Either or both is fine.

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John Popelish

What is the reason for turning the phantom power off and on? I've used dynamic mics on phantom powered preamps with no ill effect. Obviously the condenser mic requires the power. I've also had no problems plugging the condenser mic into the preamp 'hot'. Just curious. GG

Well, the unit has to be switched on and off in the first place, so I'll get the DC problems whether I have a separate switch for phantom or not.

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True, but a simple power on mute system would take care of things without having to have a 'smart' mute that would mute, toggle phantom power, un-mute. The mute system would be more complicated than the actual preamps.

GG