"Phantom" powering 9v IEM amps

[Matt Riley]

Hi all,

 

So, I'm starting to become slightly annoyed by the maze of wall warts cluttering up our stage, having invested in some of the above Millenium headphone amps (to supplement our Sennheiser wireless IEM packs) which were, of course, supplied sans UK power supply! This got me thinking... is it possible to inject very low current 9v power (no more than a 9v battery) over the balanced aux returns on stage like phantom power to save from having to put wall warts everywhere. I was contemplating building a power injector using one of these, injecting 9v over the aux returns with a custom XLR wired up at the other end with both an XLR - M and a power connector (using spare cables from the power supply) to supply this power to the beltpack.

 

• Has anyone thought about doing this before?
  
• Is it safe (I reckon so as the current will be the same as touching the exposed terminals of a 9v battery)?
  
• Is it likely to damage the units (or any active monitors plugged into the same sockets?)
  
• Can you think of any other pitfalls?
  
• Can you think of any modifications to make this work better?
  

[sam.spoons]

I'd say if you supplied 9V phantom power (as in +9V on pins 2 & 3 and 0V on PIN 1/ground) you'd be fine. You would need to decode this supply at t'other end though. Simpler to use an XLR cable to feed unbalanced audio on pin 2 and 9V on pin 3 (both with respect to pin 1/ground) but you could have problems (possibly not damaging but probably noisy ones, others more knowledgable on the electronics side would be better qualified to comment than me though) if you plugged it into a normal balanced input.

 

BTW I have that very device on my guitar pedalboard and, while my needs are simple, it seems to be a well designed and built piece of kit.

[dbuckley]

As mentioned, going unbalaned plus power on one XLR cable is easiest, and that is the way that most party line intercom systems work, eg Clearcom, or the Blue Room's own Bluecom.

 

Your needs for power and IEM is very common in an allied field, namely TV production. Of course, there is different terminology, they use a thing called IFB, which at the receive end (ie the bit the talent wears) is more or less identical to (mono) IEM. Google for wired IFB.

 

That said, power over balanced audio is indeed possible, and possible a number of ways. Easiest to implement is using transformers, but that is quite an expensive solution.

 

I'd be tempted to make up custom cables and connectors using starquad cable, and have balanced audio on one pair of the quad, and the +ve power on both of the remaining pair, using the screen for common return. So some soldering and heatshrinking at each end to break out a power jack. These cables will end up a bit fragile at the connectors, so I'd carry spares. If money is no object, 5 pin XLRs.

[timsabre]

The trouble is that headphone amps need quite a lot of current and the current draw is very variable. This makes balanced line power (proper phantom power) quite difficult to achieve. The comms style unbalanced line with power on one of the pins is much easier and more likely to be successful.

[Simon Lewis]

Although not too cheap, "scroller cable" gives you the twisted pair and a heavier gauge pair of conductors for dc.

[Matt Riley]

Hi all,

 

I was hoping to use existing conventional balanced multicore infrastructure, so I might start by experimenting with unbalanced. The devices in question run off a c.100 ma supply so not crazy current and are designed to run off a 9v battery. How does conventional phantom power (mic side) deal with the issues suggested above? Transformers in the mic and mixing desk?

 

I'll get experimenting!

 

Matt

[Brian]

The mixer end is normally a pair of 6k8 resistors from the +48v rail to pins 2 and 3. The problem is that the resistors need to be a high value so as not to load the audio down but if they are too high then the phantom current flow will cause too high a volts drop.

 

In you case, drawing, say, 50mA will drop the phantom down to 0V so that won't work.

 

Transformers are also used, however, a DC current of 50mA through your average audio transformer will seriously run the risk of saturating the transformer core (= distortion and clipping) unless you use large (ie expensive) transformers.

 

The proper way to do this, and still maintain balanced audio would be to inject power onto the audio line using constant current sources as they have an inherently high resistance and won't load the audio side of things. At the receiver end you'd need to extract the DC using constant current loads and then transform that into a sensible 9v for the amps.

[Doug Siddons]

Just a thought but something like this might be an alternative for you