Signal Gen + 48V = NOISE--Well sometimes...Why?

[Ethan]

Well, if you've been putting up with my many many questions lately you should know I've been experimenting with preamps... A strange thing I noticed today. SOME preamps will increase in noise considerably (10db or so) if a signal generator (that is OFF) is connected to the input while phantom power is on. SOME other preamps are fine and don't increase in noise when phantom power is on while the signal gen is attached to the input. I should state that the noise phenomenon does not happen in any of the preamps when the input is terminated with a resistor.

The noise is mostly 60Hz. My question: why does this happen with some preamps and not others? Color me befuddled

 

[CJ]

Ground loop.
Need a schematic of your house and everything in it.
.

 

[Ethan]

Are you serious C-Meister?
If it's a ground loop in my house wouldn't ALL preamps react this way?

 

[CJ]

Just jivin.
Do all the pre's have transformer inputs?

 

[gyraf]

Could be due to generator-output-termination-impedance-when-turned-off - or worse: generator's output capacitor frying because of applied phantom power.

Many, many outputs - not counting microphones - seriously dislikes seeing phantom power. TC electronics stuff and modern Lexicons very often blow their output stages in this case.

Jakob E.

 

[kvintus]

Many, many outputs - not counting microphones - seriously dislikes seeing phantom power

Yep. Tried that on an M-audio usb soundcard. Didn't like it. Tried it on a TC M-something. Didn't like it. Tried in on an RME soundcard. Guess what? didn't like it either.

So now I have to repair stuff instead of building new stuff... Annoying.

/Anders

 

[bcarso]

"Could be due to generator-output-termination-impedance-when-turned-off - or worse: generator's output capacitor frying because of applied phantom power. "

Agreed. And if it doesn't have an output cap, even a worse situation

The phantom power will be trying to partially power up the generator output stage, if it is of the typical function generator type. No telling what evil lurks there.

 

[SSLtech]

Yes indeed, relating to a recent post of mine about shoving phantom up the back end of SSM5142's. there are a number of different output types on gear- balanced and unbalanced- that will go nuclear if you show them phantom.

Some time ago,I had two ($50,000 each!!!) Digibetas that had been rented out to use in a live show that had the output drivers (An SMT proprietary device) which had BURNED through the PCB!!! -All outputs on both digibetas. -It appears that the people were using a console that had XLR mic ins, ¼" line inputs and they only had XLR cables, so they'd put the digibetas into the mic inputs and padded the inputs. -Then they engaged the (global) phantom power... An expensive mistake to be sure! -If course they denied everything for a while, but after I and a colleague insisted that we were prepared to declare that the "forensic" evidence proved that hypothesis beyond doubt (it didn't of course, -just a strong suggestion-) they 'confessed'.

Moral of the story for me: If some thing that wasn;t designed to run on phantom power objects when it sees phantom power, make sure it never sees phantom power again: -either protect it, block it, modify the output, make sure it never sees another mic input, or sell it! :green:

Keef

 

[bcarso]

OWWW.

The forensic argument is sometimes required.

Reminds me of my inventor friend who got a suit dropped by convincing the attorney for the plaintiff that, despite the friend's notes not being witnessed and done in pencil, the carbon in the pencil lead could be carbon-dated.

Which was true, although the accuracy of +/- 50k years was not brought up.

 

[Ethan]

Yeah I didn't figure it was "good" to do. I stumbled upon it by accident as I was using the signal gen output to terminate the mic pre input (200R) without phantom, and in one instance accidentally flicked the phantom on to see a lot of noise. So I experimented with other pres and found that some don't react this way. Wait...

That wouldn't make sense.
Some pres will exhibit this extra noise but some others won't. Albeit, it may be bad for the signal gen. what could be different in the mic preamps that some react with extra noise while others don't?

Wait again...
The few that I tried that DID freak out with noise were all transformer inputs. THe ones that didn't freak out were transformerless... Could that be it?
Thanks as always :thumb:

 

[bcarso]

Could be the transformerless ones shunted enough of the phantom power away from the signal generator to protect it.

 

[CJ]

What kind of generator?