Phantom power and interface Pres going bad

I think the only way to figure it out would be for a service tech to discover the actual problem. It is likely that one of the parts just wore out, but there is a possibility that there might be a clue about how some pattern of usage caused the problem.
 
When a manufacturer is looking at components to put in a device they have lots of choices. For example, some capacitors are rated at 500 hours of use before failure (Mean Time Before Failure) and some are rated at 10,000 hours before failure. Guess which cost more. :-)
 
best regards,
mike
 
 
 
 

Can't hurt, but also shouldn't be necessary. A properly-designed phantom power supply should tolerate you placing a dead short across the jack's contacts and leaving it that way for a week. It is possible that heat buildup may have caused premature failure in the 48V regulator or one of its filter capacitors, as Mike suggested. No way to know without having somebody open it up and diagnosing the real problem.

gswitzI have an old M-Audio interface that the pres went bad on after a while.

Which one? I was a designer there way back when, it might be one I worked on, and if it is I might be able to provide some pointers...
 
A couple questions:

1. Do any of the front panel LEDs still illuminate?
2. Does the preamplifier still work with a dynamic microphone?
3. Is the output noisy, with or without a microphone connected?

gswitzA friend of mine has a Zoom device that has gone bad.

Can't really help with that one, sorry.

gswitzI'm starting to wonder whether plugging in mic cables while phantom is on might have caused this.

 
Phantom Power is, at best, a compromise. It probably causes as many problems as it solves! Well, almost.
 
For lower priced preamplifiers the 48VDC is usually generated using what's called a voltage doubler, which can in fact double the voltage, but it does limit the current - the lack of a 'real' 48V power supply is challenge #1 for several reasons, but I'll spare you. Current is not a big challenge - the maximum current allowed for phantom power is on the order of 10mA.
 
The 48VDC is applied to the "plus" and "minus" pins of the microphone input through a pair of 6800 ohm resistors, and matching is important, but not critical for the phantom power. It is critical for Common Mode Rejection Ratio, since an imbalance between the resistors will create an impedance imbalance at the input. That's challenge #2.
 
Phantom power applies the same voltage to both pins 2 and 3 so that the microphone element does not "see" a potential difference. Again matching the impedance from each signal pin to ground becomes important, since any imbalance will place a voltage across the element.
 
AND... this is where several myths about phantom power arise. Hot-plugging phantom powered microphones is considered a bad idea, but in fact there is little difference between hot-plugging an XLR connector and flipping a switch. (For the record, there are ways to ramp up the power to avoid thumps and other problems, but you don't find these refinements in lower cost devices.)
 
HOWEVER, hot plugging a 1/4" TRS connector with phantom power applied is a really bad idea. Each side of the power supply gets shorted, briefly, and that not only causes very loud thumps (think a 36 dBu transient), but it can also damage the power supply, and the microphone element.
 
Another popular myth is that dynamic microphone elements can be damaged by phantom power. This is a case of "it depends", but almost all modern designs place dynamic elements (even ribbons) across the pair, so that there is no potential voltage applied across the element. I still try to avoid knowingly connecting dynamic microphones to phantom power supplies, but I don't lose sleep over it.
 
If you are still curious about phantom power I recommend the following papers:

• The minds at THAT Corp presented a paper at the 2001 AES convention that describes the challenges facing circuit designers. They followed this up with a second paper at the 2009 convention. These are must reads for anyone designing or repairing microphone preamplifiers!
• NTI (Neutrik Instruments) published an application note that goes into excruciating detail about testing phantom power. I really wish this had been around when I was designing!
• Eddie Ciletti wrote a great article for EQ magazine way back in 1999 that remains one of the best I've come across. If you read only one read this one (but really, read them all!)

Repairing your M-Audio preamplifier may be as simple as replacing a couple of capacitors (or quite a few, depending on the extent of the damage.) Well worth the effort I think.
 
If the preamplifier is still working for dynamic microphones you could also consider an external phantom supply. I have a couple, and they provide better, more stable, and quieter phantom power than many of my other preamplifiers.
 

wst3,
 
My M-Audio Interface is this one...
http://www.soundonsound.com/sos/may08/articles/fasttrackultra.htm
Fast Track Ultra
 
The Pres make noise all the time... when phantom is on or off. When nothing is plugged in. when the inputs are set to the front.
 
When inputs are set to the back, it works fine.
 
Lights appear to be working.
 
I've taken it apart and didn't see anything obviously wrong.
 
I don't really use it anymore. I have a new RME UCX which works great.

I hot plugged my air compressor into an extension cord last Friday.
 
White blue sparks spit out of the connection as the compressors plug was disintegrated by the arcing current.
 
I had to go buy two new plugs to replace the old ones. From now on I'm going to check the switch on the compressor before I plug that bad boy in.
 
:-)

Thanks, Johnny (Cactus Music). This makes sense to me and seems probable that this has happened. For the M-Audio device, Phantom is either on or off for the front panel jacks (set using a switch on the back). The power is not specific to the different jacks. As I often practice in hotels or on the road, it seems entirely reasonable that I plugged an electric guitar into one of the front panel jacks directly without turning of the Phantom Power using the switch on the back.
 
Mike,
 
The manual for my compressor actually gives the order of operations for plugging in the DBX and says it needs to be off while plugging or unplugging anything.
 

DBX 162SL Manual
When connecting the 162SL, refer to the following steps:
• Turn OFF all equipment BEFORE making any connections.
• Mounting the 162SL The 162SL requires a two rack-space height (3.5 inches) and a standard 19 inch rack-space width. It can be mounted above or below (leaving 1U of space above and below the unit)  anything that doesn’t create excessive heat.  This is to ensure proper ventilation. Ambient tempera- tures should not exceed 113°F (45°C) when equipment is powered.
Caution: Never remove the cover. There are no user serviceable parts inside.
• Make connections via XLR or 1/4” connectors.
• Plug in AC power cable and power ON the unit.

 
I couldn't find anything like this for the RME UCX. It doesn't say 'turn off the device before connecting anything'.

Bummer, Mike! I guess I just had musical compressors on the mind. I've been pouring over the manuals and looking for the info. Obviously I'm very interested in not having my current interface go bad on me.
 
Totally off the subject...
Mike, I also learned that you can use a number of RME interfaces together. I thought this sounded very interesting when it comes to expanding capacity.

gswitz
My M-Audio Interface is this one... Fast Track Ultra

 
Sorry, that one was after my tenure there. I can make a couple guesses, but that's about it.
 

gswitzThe Pres make noise all the time... when phantom is on or off. When nothing is plugged in. when the inputs are set to the front.
When inputs are set to the back, it works fine.
Lights appear to be working.

Making sure I understand...
when you plug something into a rear jack you get audio, pretty much as you'd expect
At all other times you get noise.
 
If that's the case then the problem is isolated to something that is not shared between the front and rear input jacks. Not sure what that would be, but the actual gain stages and such are all OK or the rear jacks would not work.

gswitz
I've taken it apart and didn't see anything obviously wrong.

Well that stinks! Or at least makes it a bit more challenging...

gswitz 
I don't really use it anymore. I have a new RME UCX which works great.

Well not to sound disloyal, but f you have an RME why are you still using the M-Audio? I have no issue with the gear that M-Audio makes, it is (my opinion, trying to be unbiased) as good as it gets at the price point. But the RME is in a whole different league!

understood!
 
Well, perhaps you can gift it to someone that knows how to fix it? Just a thought...
 

Some people still believe that leaving devices on all the time is good.
The opposite is true: heat kills components. (I am not talking about not using gear for years.)
 
I don't think you did anything wrong. You can try to send the interface in for repair. If it costs too much to repair, let them throw it away, and enjoy you RME interface.
 
Wilko
 

... then the current literally melted the male pin off the compressor's power plug. It looked like it was a welding rig.

 
Similar thing happened to me a couple weeks ago.  I was using my multi-meter to check my electrical panel and accidentally touched a 30 amp circuit.  My probe was instantly vaporized and multi-meter blown.  
 
It is very unlikely phantom power damaged your preamps.  But it is good practice to turn off phantom power and wait a good 15 seconds before unplugging condenser microphones.  Connection problems are usually the No. 1 cause of signal failure.  Capacitors can also blow if there is a power surge and improper current protection.  Keep in mind cheap gear will not last, Zooms break easily because they are built like toys, M-Audio is a little better, but not much.  Probably isn't worth trying to fix it as that would cost more than a brand new one.