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Microphone Month 4

Speaker Thumps, Drive Rack PA and Murphy’s Law

Q: “How do I avoid the funny ‘Kapow!’ sound that my speakers make when I turn off my Drive Rack PA?”

This Tech Tip actually applies not just to Drive Rack PA users but also to all of you who operate sound reinforcement systems. There is a specific order of powering on and off your system components that you must learn and follow to avoid damage to your loudspeakers and other gear.

Always make sure that your power amps are the last item turned on and the first turned off – every time you use your system!

Many pieces of audio gear emit a power spike, or a transient “thump,” when powered on and off. This uncontrolled signal can damage speakers that aren’t protected against such abuse. Learn to follow this sequence when you power up:

  1. Make all mic, mixer, outboard effects, and output cable connections with all devices OFF.
  2. Turn on your outboard effects units (if any) first.
  3. Turn on your mixer next.
  4. Then turn on the Drive Rack PA.
  5. Finally, turn on your amplifiers or powered speakers.

When powering down, reverse the sequence:

  1. Turn off power amplifiers and/or powered speakers.
  2. Turn off the Drive Rack PA.
  3. Turn off your mixer
  4. Turn off all outboard gear.

It’s then safe to disconnect your cables.

Years ago an InSync reader reminded us to take extra precautions with some power amps, pointing out, “Some of the better amps out there have huge filter capacitors that can take… on the order of a minute to discharge after I turn off the power. During that minute, the amp is effectively ‘on,’ and an upstream transient will still pound the speakers.” So wait a minute after powering down your amps, then continue to turn off gear in sequence.

Drive Rack PA users in particular notice this phenomenon because the unit emits a unique “sssss – POP” sound when it is turned off prior to the amps. dbx posts this warning in the user manual:

• IMPORTANT- It is imperative that the power amps are turned off prior to cycling power to the Drive Rack PA.

To guard against Murphy’s Law – the accidental shutdown of the Drive Rack, your mixer, and outboard gear during a gig, your best bet is to use an uninterruptible power supply (UPS).

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