Advanced Training 2009Advanced Training 2009

Topics to be coveredNoise

Grounding SafetySystem Gain ConsiderationsAuxiliary Fed Subwoofers

What Causes Noise?

Induced Electricity from External Electrical or Magnetic Fields

-Lights-HVAC-Motors-Fans

Ground Currents-Improper Grounding Methods-Faulty Equipment-Faulty Connections!!

How to Fix NoiseInduced Noise

-BALANCED CABLES!

Unbalanced Cable Tips

Expensive and exotic cables, even if double or triple shielded, made of 100% pure unobtainium, and hand-made by a team of virgins, will have NO significant effect on hum and buzz problems!

•Keep cables as short as possible.•Use cables with heavy gauge shields.•Bundle signal cables.•Maintain good connections.•DO NOT ADD unnecessary grounds.•Use ground isolators at problem interfaces.

How to Fix NoiseGround Currents

-Signal Ground Lifts-Transformers

Ground Lift

For shielded balanced audio cables, the shield shouldALWAYS be grounded at the driver — whether or not the receiving end is grounded.

Transformer Isolation

Properly Operating Equipment

Defective Equipment

• NEVER, EVER use devices such as 3 to 2-prong ac plug adapters, a.k.a. "ground lifters," to solve a noise problem!

• Interrupting any portion of Equipment Ground (for example, by breaking off the ground pin of an AC plug or using a "ground lift" plug), or adding a series impedance (like an inductor or "choke") that reduces fault current is unsafe, and could cause electrocution.

Defective Equipment with Lifted Ground

Consequences

• Between 1980 and 1989: 153 people died by contacting short-circuited, damaged, or improperly installed wire or equipment.

• For the year 1990, the National Safety Council reported that electrocutions were the fourth leading cause of work-related traumatic death.

• Improperly connected grounds in audio equipment may lead shock back to Monitor Engineer, FOH Engineer, etc….YOU!!

Power considerations

• Ensure that all circuits used share a common ground i.e. come from the same breaker panel

• Mics must be pin 1 grounded to the nearest console

• NEVER lift equipment ground for ANY reason. It’s against the law and unsafe!

System Gain Structure

• Two widely used methods: Unity Gain and Optimal Gain

• Unity Gain: Setting all ratios to Unity or 1/1. Level is unchanged

• Optimal Gain: Set clipping point of all pieces in the signal chain equal.

Pros and Cons

• Unity gain:– Maximum headroom– Slightly higher noise

floor

• Optimal Gain:– Reduced Headroom– Lower noise floor

Headroom?

• Difference between peak and RMS values

• The closer a circuit is to clipping, the greater the distortion

• Leave at least 6dB of headroom

• Avoid earlash• Amps have less

available headroom above +4 dBu, but are designed to run at higher levels

Back to system gain structure

• Set amp inputs wide open

• All gear in the signal chain set at Unity

• Control system volume at the console

ConventionalSubwoofers

• Currently Our System is Set Up in the Following Fashion

Conventional Sub Downfalls

• Lower harmonics tend to muddy up the low end.

• High Pass Filters on Channel strip generally have gentle slopes of 12 dB

• Low Frequency Stage Volume can enter through VOX

System Auxiliary Fed Auxiliary Fed SubwoofersSubwoofersPresets are in the Ashly for Aux Fed Sub Setup

Aux Fed Sub Advantages

• Only Signals with “meaningful” low frequency content will be sent to subwoofer

• Other content will be rolled off at a steeper rate

• Gain Structure Remains Intact

Aux Fed Sub Disadvantages

• Need to fade master fader, and aux fader• Additional Crossover is sometimes needed

Aux Fed Sub Instruments

-Bass Guitar-Kick Drum-Low (Floor) Tom-Low mic on Piano-Low mic on Leslie (Organ)-Electronic Keys-CD/TAPE Playback