Note, the author has posted an update to this story. Find it here.
When it comes to hardware that is subject to abuses of “guests” and
others, none is under duress more than microphones and headsets. Here’s an area
crying out for a cost-effective solution for reliability, guest comfort, ease
of use, security/theft control and hygiene.
headset adapter is shown as used by Victor Osorio of HRRZ(AM/FM) in Honduras.
With the advent of computers, and more specifically the microphone audio
interface, an interesting solution has appeared: the computer headset
microphone. These units, predominantly imported, are reminiscent of event
location headset mics used in sports reporting, although lighter in gauge and
less sophisticated. Light on the head and with a reasonably good sounding mic,
they remind us of the benefits of mass production. Costing under $10, sometimes
as cheap as $7 in bulk, they are available from sources as convenient as RadioShack
(at a price premium) and from many national computer discounters. You can even
order in case lots.
The only bug-a-boo? Normally you need an interface to provide the
operating voltage for the unit’s electret mic, and electronically make the conversion
from essentially high-impedance unbalanced to a nominal 150 ohm balanced
output, to mimic a regular microphone for station use.
Let’s design and build that interface. We’ll include desirable extra features
such as an individually operated headset level control and simple processing to
enhance voice band audio response for increased intelligibility.
I’ve tried over the years to offer ideas for simple yet valuable gear
for your station that can be expensed from petty cash and built on a
time-available basis. Call this another such “therapy” project.
Radio World has posted a
larger version of the accompanying schematic on our website at www.radioworld.com/fitchHeadset.That linkincludes
a fairly complete parts list, with as many RadioShack stock numbers as we could
match. (As usual, our version is not the only way to build one, nor the ultimate
in design optimization; it’s just a departure point.)
Mics in these units are DC powered and the computer
typically supplies 5 volts (unloaded). The tiny load that is the mic’s FET
amplifier drops the voltage to about 2.5 volts, and the voltage variation
around this axis point is the actual audio from the mic. Most computers use an
internal series resistor of about 10 k-ohms; that’s what we’ve used as well.
Buc’s interface is at lower right.
The design of our “box” is centered on an input power supply of a single
DC voltage, nominally between 7 and 15 volts. A 12-volt DC 100 mA wallwart, bought
or liberated from your junk box, will work. In a rare situation where the mic
is away from all hard power, a good 9 V DC battery will supply power to the box
for between 5 and 10 hours of continuous use with a 45 mA or so drain; think
Duracell or Energizer 9 volt.
This voltage enters the box through a side connector and immediately is
dropped to a regulated 5 volts via a 7805 three-pin regulator. At this point 5
volts is supplied to the mic through that 10 k, 1/2-watt resistor.
The ripple on this 5 volts is low, 3 to 5 mV, which is necessary for a
mic of this type.
That regulated 5 volts also is fed to a DC-to-DC converter to develop
the 12 volts + and – for the op amps.
One 1458 (essentially a pair of compensated 741s in a single eight-pin
DIP) is used as a pair of cascaded filters to shape the audio into the voice
band. The other is used as a unity gain cascaded pair to provide a pair of
separate outputs, one plus and one minus polarity. These voltages, from
essentially near zero impedance generators, pass through a 68 ohm resistor in
each leg, creating a balanced source impedance of 136 ohms, which is close
enough to 150 ohms for our matching purposes.
A DPDT switch selects mic audio with or without processing.
Audio for the headset could come from a multiplicity of sources but we
expected the program audio to come at speaker level. The audio enters the box
in a 1/4-inch mono chassis mount jack. To quiet the audio in the headsets fully,
we’ve chosen a single circuit jack with a normally closed contact on it. When
nothing is plugged in, the headset line shorts to ground.
The headset audio passes to the on/off switch on a 1 k audio tapered
pot. When you turn “on” the pot, the audio is connected to the top of the pot.
The bottom (ground end) of that pot is connected to a 100-ohm resistor that
goes to ground. This prevents the audio from being turned off completely unless
you turn off the audio with the switch.
People are people and do strange things. I had a guest turn down her
audio and then suddenly go ballistic when she couldn’t hear anything. This
little bit of audio at the end is helpful in always keeping some audio on.
This interface unit
for a single mic was hand-assembled on DIP proto-boards. You can use just about
any scheme you want for wiring and assembly. If your talent work in pairs, it
might be prudent to build two mic channels into the same box. The DC-DC converter
specified should just be able to handle four op-amps.
Early field use has complained that some RF from RPUs or nearby
transmitters is getting into the mic appearing as a low sizzle sound. A 0.01 uf
ceramic disc capacitor across the mic “high” input to ground eliminates most of
this. Several of the various mic/headsets that we’ve been using have little
shielding in the mic cord; this appears to be the most likely entry point. You
may find that some brands of mic headsets have less propensity for this (probably
more shielding); make these units your first choice.
Now when you have guests in studio or on location, you can make them
feel special by taking a new, spiffy clean mic headset for them out of the box.
You can even let them take it home as a useful souvenir of their appearing on
the radio with you when done. At the least, you’ve taken a “wear item” and made
its maintenance a disposable issue.
Charles S. Fitch, W2IPI, is a registered professional consultant
engineer, member of the AFCCE, senior member of the SBE, lifetime CPBE with
AMD, licensed electrical contractor, former station owner and former director
of engineering of WTIC(TV) in Hartford, Conn., and WHSH(TV) in Boston.