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Phantom power hum

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Phantom power hum

PostPosted: Sun Nov 29, 2020 2:15 pm
by musicegbdf
Hi , have posted under newbie as returning to home recording.
In that post I have asked questions about setting up a new studio. I have a mix of old and buying new gear.
I have a another question.
I have purchased a AT 2020 mic. I am waiting on new PC to set things up , but decided to just check the mic in my old pre amp ( focusrite trakmaster platinum) . The preamp was only device connected and played into earphones.
With the new mic I am getting a ground hum which goes went phantom is turned off , and my Shure sm48 is fine , so seem to be linked to the phantom power.
My xlr cable is fairly old and only have one so maybe that could be it ?
Also is there a search function on this forum so I dont ask questions that have already been asked.
I cant see one.
Thanks

Re: Phantom power hum

PostPosted: Sun Nov 29, 2020 2:54 pm
by James Perrett
What happens if you have phantom switched on with the SM48 plugged in?

Do you have a meter that you can use to check the cable?

Re: Phantom power hum

PostPosted: Sun Nov 29, 2020 2:58 pm
by The Elf
I'd be suspicious of that cable. Get a proper mic cable, which it sounds like you need anyway, and hopefully it will fix the problem. And if it does, bin that old cable.

Re: Phantom power hum

PostPosted: Sun Nov 29, 2020 3:18 pm
by ef37a
The Elf wrote:I'd be suspicious of that cable. Get a proper mic cable, which it sounds like you need anyway, and hopefully it will fix the problem. And if it does, bin that old cable.

Nah, buy two! Really, the stuff you get on the net is easily good enough for home studio use, probably wouldn't last long ratting around a stage but fine at chez nous.

If the cable doesn't fix it you likely have a faulty pre amp but if you have test meter there are checks you can do...Funnily enough I am just about to post some noise figures for two spook boxes, look for "Ghosts in the Machine".

Dave.

Re: Phantom power hum

PostPosted: Sun Nov 29, 2020 3:39 pm
by musicegbdf
Actually made me think.
With nothing plugged in no noise , but turn on phantom I get the buzz.
Is the pre amp scrap for new mic (though ok for sm58.)
I tried plugging into a socket in a different room and buzz still there.
Will check out the Ghost post as well.

Re: Phantom power hum

PostPosted: Sun Nov 29, 2020 3:54 pm
by ef37a
musicegbdf wrote:Actually made me think.
With nothing plugged in no noise , but turn on phantom I get the buzz.
Is the pre amp scrap for new mic (though ok for sm58.)
I tried plugging into a socket in a different room and buzz still there.
Will check out the Ghost post as well.

You could have a "punched thru" regulator there chap and a lot more volts on the XLR than you should! Don't plug any more mics into the pre amp until you have had a meter on it.

Dave.

Re: Phantom power hum

PostPosted: Sun Nov 29, 2020 4:09 pm
by musicegbdf
Ok Dave , will do . Will the Sm58 be ok with phantom turn off or is it scrap now ?
Thanks
Ray

Re: Phantom power hum

PostPosted: Sun Nov 29, 2020 4:15 pm
by Hugh Robjohns
If the hum only occurs when phantom is turned on, the chances are very strong that the preamp is faulty.

There are various different ways of generating the +48V phantom supply, but most involve several electrolytic capacitors and it may be that one or more have perished.

It's not likely to be the cable since if one leg was broken the dynamic mic wouldn't work, and if the screen was broken the capacitor mic wouldn't work at all.

Re: Phantom power hum

PostPosted: Sun Nov 29, 2020 4:22 pm
by musicegbdf
Ok sounds like it is toast then ... expect the value of it is not worth fixing. I have a friend who is a retired electronics engineer might give it to him to play with .
Will have to see if there is a wiring diagram to give him.
Will keep the 2020 to use only on the audiobox for now.
Wont be able to test that till I set up my new laptop.
I do appreciate the help. What a good forum this is.
Ray

Re: Phantom power hum

PostPosted: Sun Nov 29, 2020 5:49 pm
by musicegbdf
Dave you are right.
I read up how to check the voltage .
It is 105 volts with the phantom power on.! !
Finger crossed it hasn't screwed the mic ...
Hopefully as it was only on for a max 30 seconds :cry:
With PW off there is just over 30 volts.

Re: Phantom power hum

PostPosted: Sun Nov 29, 2020 6:52 pm
by ef37a
musicegbdf wrote:Dave you are right.
I read up how to check the voltage .
It is 105 volts with the phantom power on.! !
Finger crossed it hasn't screwed the mic ...
Hopefully as it was only on for a max 30 seconds :cry:
With PW off there is just over 30 volts.

OOoof! I suspect Hugh is right that they use a voltage multiplier circuit and then a "floating" 78 style regulator. You r friend could probably fix it even without a map but even if not, if the pre amp is usefully low noise with the dynamic he could strip out the phantom supply for safety and you could get an external supply for capacitors.
I suggest you pay a bit more than I did for my two! Canford Audio or Studiospares probably have decent ones.

Just FYI. Have we ever seen such boxes reviewed in SOS? Shame if not.

Dave.

Re: Phantom power hum

PostPosted: Sun Nov 29, 2020 7:02 pm
by musicegbdf
Not sure if it was reviewed. The dynamic mic has no noise . Was curious that it was putting out 30v with phantom off.
I see second hand ones fetch around £100. I might start off just using the presonus box preamps to start with.
When I first got it , i seem to recall it did the same and it went back to focusrite under warranty.
I didnt know that was the cause .
That was more than 10 years ago though.
I can give it to my friend as a challenge for him.
Either that or it can be a large paperweight.

Re: Phantom power hum

PostPosted: Sun Nov 29, 2020 8:08 pm
by Trevor Johnson
Really, the stuff you get on the net is easily good enough for home studio use

True Dave, except that the cable is usually fine but the tolerances of the XLR connectors is often variable - especially the male connectors. Nothing worse than struggling to remove a 'tight' connector from what is probably a PCB located socket on an expensive piece of kit. I know, having recently been there!

Re: Phantom power hum

PostPosted: Sun Nov 29, 2020 8:20 pm
by Hugh Robjohns
ef37a wrote: Have we ever seen such boxes reviewed in SOS? Shame if not.

You mean like this one? https://www.soundonsound.com/reviews/canford-audio

Or this one? https://www.soundonsound.com/reviews/triton-true-phantom

Re: Phantom power hum

PostPosted: Sun Nov 29, 2020 8:30 pm
by Hugh Robjohns
musicegbdf wrote:I read up how to check the voltage .
It is 105 volts with the phantom power on.! !

Eek! That sounds very scary! And slightly unlikely...

Using a multimeter on DC Voltage setting, with black probe on pin 1 of the XLR and the red proble in either pin 2 or pin 3. Should be 44 to 52 Volts.

With PW off there is just over 30 volts.

It normally takes from a few seconds to a few minutes for the phantom voltage to decay back to zero if there is no load. So hopefully that was a decaying voltage rather than a static one. If it was permanently 30V DC there is a second major fault somewhere!