Royer Labs SF-2 - Manual (Page 8)

Hum, Noise & Mic Orientation
All ribbon microphones are susceptible to picking up stray alternating magnetic fields, which
show up in the audio signal as hum. Power transformers (such as those found in guitar
amplifiers) and alternating current motors are the most likely sources of hum. In-wall wiring,
solid-state light dimmers, and electrical utility transformers are other likely sources.
A well designed microphone provides shielding to minimize the effects of such stray magnetic
radiation. Complete isolation is not possible however and unpowered ribbon microphones can
manifest this condition to a greater degree because of their higher gain requirements. With
vintage ribbon microphones that often employ large, bulky magnet structures and are often
poorly shielded, the problem can be even worse.
The SF-2 reduces induced hum in three ways. Because the essential gain is provided at the
microphone, there is no need to push the preamplifier for excessive (hum producing) gain. Royer
SF microphones (SF-2, SF-12, SF-24 and SF-24V) are also designed to minimize the effects of
stray radiation by integrating the transducer barrel as part of the magnetic return circuit, further
reducing hum potential. Lastly, all SF-series ribbon microphones incorporate toroidal impedance
matching transformers, which repel magnetic radiation.
Hum can only come from external sources and simply repositioning a ribbon microphone is the
best way to control it. If hum is detected, the microphone is in the proximity of an alternating
magnetic field.
While listening to the mic’s output (preferably with headphones), move the mic around. It will
“find” the noise source quite easily. For example, if you are miking a guitar amplifier and suspect
the amplifier’s power transformer to be the source of hum, move the mic around the amplifier.
You will probably find that the hum is louder as you approach the amplifier’s power supply and
quieter when you move it away. Eliminating the hum can be fairly simple and requires using the
“null” point of the microphone. The null point is the position that places the microphone’s
magnetic components away from the lines of the noise's radiation. Slightly rotating the
microphone is usually all that is required to cancel out the hum. This is similar to the procedure
guitarists use to eliminate hum from single-coil guitar pickups, turning around slowly while
listening for the hum to cancel. There will be a position where the hum disappears from the mic’s
output. Hum and other noise can even be picked up by microphone cables. Since the output of
Royer Active Series mics is 15 dB to 25 dB higher than with traditional ribbon microphones, any
noise picked up in the mic cables will be at a corresponding lower level and thus less likely to be
objectionable.
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