MIXER ANATOMY: CHAPTER 2

Even though microphones produce alter-

nating current (AC), they have a definite polarity orientation. Be sure that you don’t

reverse the wires between pins 2 and 3.

Did you pick up a cool retro mic at a yard sale that has a 1/4" phone plug instead of an XLR? Chances are it’s a high impedance mic and it will require a matching transformer in order to sound its best with your mixer. Plug-in transformers available from several sources will not only match the impedance and signal level, but will adapt the plug too.

Phantom Power

Most condenser mics require phantom power, a system whereby the mixer supplies a DC voltage to the mic’s electronics through the same wires that carry audio.

The phantom power on all Mackie mixers (except the Onyx series) is globally controlled by the PHANTOM switch on the rear panel (one switch for each group of 8 mics on the 8-Bus). A PHAMTOM LED on the front panel will remind you that phantom power is turned on. The Onyx mixers have a 48V switch and LED indicator on each channel for selective control of phantom power.

“Phantom” owes its name to the ability to disappear when it’s not needed, like when connecting dynamic mics (Shure® SM57/SM58, for instance) that don’t require external power.

The most common phantom power supply is 48 volts. All Mackie mixers have a 48 volt phantom supply with the exception of the PPM series, which provide 15 volts. Some microphones will operate just fine on lower voltages while others require the full 48 volts. Check your mics’ specs.

Here’s what the phantom powering circuit looks like:

2

1

0 V

3

48 V 48 V

There’s 48 volts between pins 1 and 2 and between pins 1 and 3. Since pins 2 and 3 are both at the same DC potential, the voltage difference between them is zero. That’s why an unpowered mic doesn’t “see” the voltage.

 

 

 

 

18

Compact Mixer Reference Guide

 

 

 

Barenekked Preamps!

People have thought highly of Mackie mic pre- amps for a long time, so when it was time to put the engineers to work on some new designs, they brought us the Onyx series, which includes the 800R. This answers a popular customer request for a rack-mounted unit with eight mic preamps, but no other mixer features. The 800R has a few neat tricks up its sleeve which we’ll discuss later, but we wanted to mention it in this discussion about microphone preamps.

Dynamic and ribbon mics don’t like to

see DC voltage applied to their sensitive insides. Normally it won’t be, but a shorted

or miswired mic cable can cause the full voltage to appear across the sensitive microphone element. Be sure that your cables are in good condition before applying phantom power.

Also, unless you know for certain that it is safe to do so, never plug single-ended (unbalanced) micro- phones, instruments, or electronic devices into the MIC input jacks if the phantom power is on.

Practice safe phantom powering! Plug in

your mics with phantom power switched off! The XDR preamps used in many Mackie

mixers have extra protection against damage from “hot plugging,” but your mics don’t. Get into the habit of plugging mic cables in with the phantom power off, then turning it on.

Line Inputs

Line inputs on Mackie mixers are on 1/4" TRS (tip- ring-sleeve - the parts of the mating plug) jacks. All Mackie line inputs are balanced and present a moderately high impedance load, around 10 kΩ, depending on the model and the particular input. As we’ve seen from our block diagram tour, LINE IN jacks on channels with both line and mic inputs share circuitry (but not phantom power) with the mic preamps.

You can use these inputs for virtually any line-level signal. With proper adjustment of the TRIM control, they’ll accept signals over a range of approximately –45 to +16 dBu.

Some Mackie mixers are equipped with stereo line inputs. These have two line-level input jacks which share a common set of controls. To learn how signals are routed from these inputs, consult the block diagram or quick start guide for your mixer, or check the Controls section of this manual.