Hum but from what?

[ecc83]

Couple of days ago son* sent me from France two recordings of guitar. One performance, two microphones.

The one attached (original was 16bit .wav) was made with a BM-800 LDC (sic) the REALLY cheap jobby. But useful since it can be jacked straight into a laptop or, run on an XLR cable as is the case here into a Berry UMC204HD interface (I had it for 2 weeks, very quiet)

I am sure you folks can hear the hum click in at the start and then can be heard at the last gasp the clicks off? I have isolated the hum and attached a spectrum. some 50Hz and a bit more at 150Hz. That indicates inductive pickup of a transformer, if it were 'system' noise it would be at 100Hz.

Now, I have an identical microphone so I hooked it up to my KA6 and set gain such that "rains on plains" was hitting neg ten from 200mm. I then waved the mic over the mains traff side of our HT-20, all round a Tannoy monitor and close to a wall rat powering a USB HDD.

Not a peep of mains intruded! So, either son is living inside a MuHASSIVE hum field or his mic is in some way faulty, would not be at all surprised, QC on the BM/Sky mics is abysmal. I cannot see the former situation being the case otherwise electric guitar would be impossible! Naturally dad has sent him all this data but no reply as yet...KIDS eh?

*He apologizes for the poor playing. The exercise was for mic testing and he has only just begun to learn that Goldberg variation.

Dave.

 

[Steenamaroo]

Hi Dave,

it can be jacked straight into a laptop or, run on an XLR cable

Which is your son doing? If he's going straight into the laptop it's possible that either the microphone isn't putting out a balanced feed that way or the input he's using is unbalanced.
That'd be my first guess.
If he's going XLR I'd get the multimeter out and check continuity against all three conductors in his XLR cable.
If that checks out then I'd probably be checking if he's running on a laptop and, if so, what the laptop chassis is reading against mains earth.
See if there's any difference running it with the charger disconnected from the laptop.
If there's no issue there then I'd physically inspect the microphone, particularly the headbasket, to make sure it's well fitted/connected to the body,
although I'd expect different, and much worse, noise if that was the case.

Hopefully something in there is helpful.

 

[ecc83]

Hi Dave,



Which is your son doing? If he's going straight into the laptop it's possible that either the microphone isn't putting out a balanced feed that way or the input he's using is unbalanced.
That'd be my first guess.
If he's going XLR I'd get the multimeter out and check continuity against all three conductors in his XLR cable.
If that checks out then I'd probably be checking if he's running on a laptop and, if so, what the laptop chassis is reading against mains earth.
See if there's any difference running it with the charger disconnected from the laptop.
If there's no issue there then I'd physically inspect the microphone, particularly the headbasket, to make sure it's well fitted/connected to the body,
although I'd expect different, and much worse, noise if that was the case.

Hopefully something in there is helpful.

Err? Did say"or, run on an XLR cable as is the case here into a Berry UMC204HD interface (I had it for 2 weeks, very quiet)" and of course the mic then needs phantom power so all three conductors must be intact.
It is weird though...

Dave.

 

[rob aylestone]

Not sure about how balanced these are because while they work on phantom, they also work on 5V on one of the 3 cores, so it isn't conventionally balanced - could this be the problem?

 

[Steenamaroo]

Err? Did say"or, run on an XLR cable as is the case here into a Berry UMC204HD interface (I had it for 2 weeks, very quiet)" and of course the mic then needs phantom power so all three conductors must be intact.
It is weird though...

Dave.

Ah, I took that to mean that's how you have yours set up. Sorry - I see now.
Yeah, sounds like Rob's on to something.
You'd know more about it than me, Dave, but if it's one of these that can with 5V supplied on one core then maybe it's not running on phantom power the 'normal' way and maybe even not balanced at all.
I would have expected yours to act the same way, though, if that was the case so I'm betting on laptop/floating ground/usb issues.

 

[ecc83]

Fair suggestions chaps so I am now going to take mine apart. I suspect the output is 'impedance balanced' a technique quite common on many capacitor mics.

"Oil be bek"

Well, it looks quite 'symmetrical' in the output circuit.SMT so hard to follow but there is a 47R in hot and cold outs and a 1uF cap in each and they go to a transistor per side, There is a third transistor inbetween and if I had to guess I would say "Long Tailed Pair", i.e a differential amplifier (they only need one side for the mini jack)

Dave.

 

[ecc83]

Found a schematic (always try Google first!) Not an LTP but a 'concertina' phase splitter as per old valve designs. But yes, a symmetrical output.

Rather more sophisticated than I gave then credit for!

Dave.

 

[Steenamaroo]

Ah, good work. Nice to know.