12 January 2017
In this column, Kristen answers questions
about all aspects of electronics, including
hardware, software, circuits, theory, radio
troubleshooting, and anything else of interest
to the hobbyist. Send all questions and
comments to: Q&A@nutsvolts.com.
Q & A n WITH KRISTEN A. McINTYRE
Phantom Power and Stopping a Cough
QI am interested in creating a mute (a.k.a., “cough”) button for the microphones used by the hosts in our podcasting studio. The microphones are Audio Technica AT831B
Lavalier phantom powered from our mixer. I would like to
build the circuit in a small desktop box with a button on
top and XLR IN and XLR OUT on the back to insert the
box between the mixing board and the microphone. I have
seen a few “mute” schematics that suggest shorting pins 2
and 3, but I think this might short out the phantom power.
Do you have a schematic that might work for me?
AI think that the schematics you’ve found are on the right track. I have a collection of Nagra udio tape recorders (beautiful machines with superb audio quality, by the way), and that’s
where my experience with this comes from. Let’s first take
a look at how phantom power works.
Since some types of microphones and accessories
require voltage and a little bit of power to operate,
engineers came up with the idea of impressing a little
power supply voltage on the signal wires. Since the audio
signal is changing with time, it can be separately extracted
or injected with either inductors or capacitors, depending
on the implementation. In the case of balanced audio such
as we see in three-wire systems using XLR connectors,
we can take advantage of the three conductors to further
separate the AC component from the DC component.
The convention for these balanced systems is to raise
the two differential audio signals roughly equally above the
shield/ground wire. There’s a whole article that could be
written on whether the shield is the same as the ground,
or even if they should be connected to each other. Ward
Silver’s article from the November 2016 Nuts & Volts tells
the story well. An example of a simple circuit to implement
phantom power on a balanced three-wire system is shown
in Figure 1.
As a side note, loop-start telephone systems like POTS
(Plain Old Telephone Service) lines are an example of
phantom power that isn’t balanced. The phone company
supplies roughly 48 volts open-circuit to the telephone
instrument. That becomes a power supply that’s used to
pass current through a carbon microphone where the
resistance varies with sound pressure. A certain impedance
across the incoming pair causes the phone switch to sense
that the phone is off-hook, and the audio is impressed
as small AC variations on the line. There are many more
things (ring voltage, hook flash, DTMF, etc.) that can be
signaled in a loop-start system. It’s a really clever design.
Of course, when we build equipment, we are never
sure what will be connected to any terminals that we
expose, so we have to plan for wires to be shorted and
other bad situations. Phantom power is no exception, so
that is why there are resistors shown in the figure to limit
the worst case current supplied by the phantom power
supply. Even if you were to short all of the wires from the
XLR connector together, a properly designed device should
be fine. We never know for sure if things are properly
designed, though. Let’s just assume that for the moment.
The audio signal coming from a microphone in a
balanced or differential audio system is derived from
subtracting the voltages from the two differential audio
pins. In the case of XLR connectors, that’s pins 2 and 3. If
there is a voltage between the shield/ground (pin 1) and
the other two, it is not considered to be part of the audio
signal. What this implies is that pins 2 and 3 will generally
be close to each other in voltage.
In order to build our cough button, we don’t need
to short the entire phantom power system. All we need
to do is bring the voltage difference between pins 2 and
3 to zero. That means we can indeed simply short them
together with a switch (Figure 2). To be a little paranoid,
you might use a very low impedance — but not zero — to
connect them together.
An important consideration, though, is making sure
that noise can’t leak into the system from the switch.
It is probably a good idea to use a metal box to shield
the switch from stray electromagnetic fields, and to
connect that box to the shield/ground on pin 1 of the XLR
• Phantom Power and Stopping a Cough
• Extending Wi-Fi Range on the Cheap
• PBX and Auto Dialer
n FIGURE 1. One way to supply Phantom Power