API 312 - Phantom Issue

Hi there.

I've got an issue going on with my phantom power/or maybe it's normal.

I've got an 8 channel rack of 312 style cards that I built some years back.  I just replaced an op-amp and did some voltage testing before I re-racked.  I noticed that when I turned on the 48volt supply, I'm only getting about 46 at the xlr inputs.  I'm pretty sure it measured 48 prior.  The voltage at the input to the mic preamp connector is 48volts.  I also noticed that when the 48volt was turned off, there was 1.39vdc still left at the xlr input(measuring from pin1 to pin2 and pin1 to pin3).

Is this normal operation?

Thanks,
Nate

Some Phantom power supplies have a resistor and a LED indicator. When switched OFF the DC drops from 48vdc down to about 2.2 to1.4vdc. You should have a REAL BLEADER across the phantom so the voltage drops to zero vdc.
The low voltage can cause failure to some microphones.
Duke

Yes.  I've got an resistor/led combo for each channel.  I'll look into the REAL BLEADER. 

Just posted this for reference...I'm noticing that the C7 on all channels is connected to each input xlr pin 1 then to the xlr grounding lug.  This is grounding to the actual mic preamp enclosure then.  The main power common coming from my external power supply isn't connected to the enclosure.  That is probably my problem.  Maybe each of the C7's should be disconnected from the ground lug on the xlr jack and wired to the input common from the power supply. 

I have also heard a 'thump' when I turn the 48v on an off.  So for all the 8 channels, should I connect C7(-) back to the 0v/common coming from my power supply?  I've got 2 power one supplies in a separate chassis and they are sharing a 0v/common.  One supplies the 48v and the other the 16 +/-. 

dawsonaudio said:

Hi there.

I've got an issue going on with my phantom power/or maybe it's normal.

I've got an 8 channel rack of 312 style cards that I built some years back.  I just replaced an op-amp and did some voltage testing before I re-racked.  I noticed that when I turned on the 48volt supply, I'm only getting about 46 at the xlr inputs.  I'm pretty sure it measured 48 prior.  The voltage at the input to the mic preamp connector is 48volts.  I also noticed that when the 48volt was turned off, there was 1.39vdc still left at the xlr input(measuring from pin1 to pin2 and pin1 to pin3).

Is this normal operation?

Thanks,
Nate

Click to expand...

Phantom power is not a precision voltage... 46V is likely more than enough.

Since phantom power is supplied via buildout resistors (6.8k) , the impedance of your meter could load down the voltage.

JR

When I test my BAE phantom, it's reading 48.6vdc.  When switched off, the voltage drops quickly to zero-ish.  My API phantom starts at 42.3vdc and creeps up really slowly. 

Since the enclosure itself never connects to the power supply 0v/common, I should probably connect the C7's to the incoming 0v/common.  Does that seem correct?

Here is a link to the guts of my api mic preamp rack.  The power supply is a separate box. 

https://drive.google.com/file/d/1KBAghGNe-vRFvRMyC53qcNzsvGk4DzPN/view?usp=sharing

So I rewired one of the channels phantom power.  I connected the c7 capacitor that previously connected to the xlr input jack pin 1 and ground lug (essentially grounding it to the enclosure) and reconnected it to the input 0v/common lug on the power connector jack.  48 volts right away and drops off to zero pretty quickly when turning off. 

So should I connect all of these capacitors to the individual cards where the 0v is wired or wire them directly to the 0v/common lug?  Not sure if there would be any kind of 'looping' happening either way or which is best...

Also, I'm not sure if I should connect the entire enclosure to the 0v/common or keep 'floating' it like I am.  Pin 1 on all of the input xlr's hasn't been connected to any kind of ground all of this time.  Hmm....

Thanks.

Note R13 on that CAPI drawing, if it exists, which you may be shorting out. 

dawsonaudio said:

Since the enclosure itself never connects to the power supply 0v/common, I should probably connect the C7's to the incoming 0v/common.  Does that seem correct?

Click to expand...

No. Don't do that!

Pin 1 of the XLR in should be connected to the chassis over the shortest possible distance (this distance determines the wavelength of RF that can be radiated within the enclosure). Pin 1 of the XLR is the shield and the chassis is the shield for the device. So phantom "barrows" the shield of the cable / XLR pin 1 / chassis as the ground return. The 0V (called "audio common" in your schematic\) MUST be connected to the chassis ground somewhere. If it did not, phantom would not work at all. If you have external supply, the proper thing to do would be to have two separate ground wires within the supply cable, one for the chassis ground and one for 0V. Then of course you have +48, +16 and -16 lines so five in total. The chassis ground conductor should be bound to the chassis near the supply cable entrance. Same at the other end in the supply except there, you might have a bolt that also connects to earth ground of the mains plug and the power supply common at the output filter caps right next to where the 0V ground conductor is connected. So that output filter cap ground is where the two grounds are in fact connected together. If the two supplies are different modules, then have the chassis ground, 0V and phantom ground connected to only the +-16V supply ground (again, right next to the output filter cap grounds if you can) and then run a preferably short and thick ground wire from that ground point over to the phantom ground but make the +48 conductor physically close to that wire before running out the supply cable with everything else (just twist them together just enough so that there aren't any gaps bigger then a cm or so).

If you are not measuring continuity between pin 1's and 0V, it might be because you already have it wired correctly and you have the supply cable disconnected. Or maybe something has some corrosion built up on the screws that connect the various chassis grounds together. How are the XLR pin 1's connected to the chassis? Some XLRs have a metal spike that stabs the metal panel. That's the best option.

R13 doesn't exist on my build.  Should I add it? 

Pin 1 of the xlr input/s have always connected to the xlr jacks additional grounding lug which is bolted to the enclosure physically.  The phantom power has always worked this way.  I've powered mics many times with my current setup...

My power cable is supplying +16/-16/48/0v(common).  I have the mains ground tied to the two power one supplies in my power supply chassis.  So the power supplies are sharing the common wire.  From there, it goes to the preamp enclosure but never connects to the enclosure physically.

If it’s not there, no. 

dawsonaudio said:

I have the mains ground tied to the two power one supplies in my power supply chassis.  So the power supplies are sharing the common wire.  From there, it goes to the preamp enclosure but never connects to the enclosure physically.

Click to expand...

If you're saying that 0V and chassis are actually not connected, that statement must be false or phantom would never have worked at all.

What I described above is the ideal scenario. If your power cable only has 4 conductors, then you will have to just connect chassis to 0V at the supply entrance. Otherwise everything else I've said should be followed wherever possible for optimum performance.

My guess at this point would be that your phantom issues are ground related. All I can recommend is that you go through each step of what I said above (make pin 1 to chassis connection short, check continuity between panels and re-tighten screws if necessary, make chassis to 0V using chassis bolt next to supply entrance, ...) and try to make it as close to that as possible.

I agree that there is something wrong with my wiring.  It's also always hard to describe over typing vs. seeing in person so thanks for your help here.

I've never had the chassis (the enclosure where the preamp cards are located) connected to the incoming 0v/common (from the power supply unit) with a bolt.  I'll do that chassis bolt with a star washer this evening and connect it to the incoming 0v. 

I hope I'm understanding this correctly.  If not, I'll draw it out in a bit and send a photo attachment.

Thanks again,
Nate

Ideally, the negative of C7 should be connected to pin 1of the XLR and to its shell.
And the common 0V should also be connected to chassis at a point that may be optimized by experimentation.
In most cases, the location of that point should not be critical. However, in severely polluted environments (strong RF or interference), it may matter.

The 48 volt works fine now.  Thanks for the insight here.  I placed a ground lug inside near the supply feed and attached the 0v to it.  I'm wondering now how the 48 volt ever worked before.  Thanks again.

Audio1Man said:

Some Phantom power supplies have a resistor and a LED indicator. When switched OFF the DC drops from 48vdc down to about 2.2 to1.4vdc. You should have a REAL BLEADER across the phantom so the voltage drops to zero vdc.
The low voltage can cause failure to some microphones.
Duke

Click to expand...

Hey, Duke.  As a matter of fact, this API-lack-of-bleeder-resistor issue has been destroying the FET buffers of my active AEA microphones.  I've measured around a volt and a half sitting on the 212L inputs 15 minutes after turning off phantom.  See here:

https://groupdiy.com/index.php?topic=77263.0

Anyway, this is an issue across our entire Vision console, and I'd like to spend the summer modifying all of the mic pres with a proper bleeder resistor.  Any idea where the best place to put one might be?  I was thinking of putting a large value resistor across the 10µ reservoir cap...

sircletus said:

Anyway, this is an issue across our entire Vision console, and I'd like to spend the summer modifying all of the mic pres with a proper bleeder resistor.  Any idea where the best place to put one might be?  I was thinking of putting a large value resistor across the 10µ reservoir cap...

Click to expand...

If the circuit is anything like the above CAPI schematic, put a 10K from the unused lug of the 48V switch (SW1) to ground. That way, it only drains when switched out so you can drain quick with 10K and it saves every minutia of juice when 48V is on. If it's not the same circuit, just put a 100K across the LED. An extra 500uA isn't going to make a lick of difference.