Oscillation with Transformer-Balanced Inputs


Depending on several conditions, chiefly the microphone, the transformer-balanced inputs, such as those on the 302, 442, MixPre, MM-1, MP-1, and MP2, can get into an oscillating condition with a microphone that renders the audio unusable. This oscillation is often described as a "motorboating" condition since this low frequency oscillation sounds like a motor boat.


Microphones with low rejection to common-mode phantom voltages fluctuate their output level based on the phantom voltage supplied. This variation "wiggles" the bipolar +/-15 volt audio rails -- which subsequently fluctuates the phantom voltage rails, which fluctuates the output level and sets up the oscillation. Sound Devices field mixers and preamps are very portable products and have compact, efficient power supplies. The AA batteries in the product supply several voltages including, ±15 volts for audio, 48 or 15 V for phantom power, and 5 V for LED's. The power supply is suitable for most microphones, but is susceptible to oscillation with microphones prone to drawing current based on output level.


With microphones prone to this condition, several things can be done to minimize and/or prevent the condition.
  • Engage the high-pass filter.
  • Use 48 V phantom even if not required.
  • Power the microphone(s) from an external phantom supply.
  • Power the mixer from an external, DC power supply.
Additional capacitance in the mixer would also help the condition, however, these units have the most capacitance that will fit into their chassis. The same microphone that exhibits this condition with the MixPre or MP-2 may not exhibit the problem with another preamp with a larger phantom supply with more capacitance.

Microphones Know to Exhibit this Problem

  • AKG Blue Line Series
  • AT 4073
  • AT 4053b
  • Sanken CUB-01, CS-1
If you have experienced the above condition with a microphone not listed, contact Sound Devices so we may make this list inclusive.