Article 12

Bobby Nathan's

Keyboard Magazine "In The Studio"

Article # 12

CHOOSING A MIXER

Part 1

Choosing the right mixer is no easy task. Your mixer can become obsolete quicker than your synthesizers. To try and help avoid this, I've put together some insights that could help your next decision.

When considering any console it's a good idea to start at the top. In any console the input stage is the starting point. The input stage or "front end" contains a consoles mic and line level amplifiers. The mic pre-amplifier in any console is the hardest working part. It deals with very low signal levels and amplifies those low levels by great amounts. For the mic pre-amp to do this successfully, the trick is not to add noise (hiss) in the process. Be sure to audition the mic pre-amps in any console you're considering!

Look for built in phanthom powering. This will save you money later. A phanthom power supply alone can cost a few hundred dollars. Some consoles do not even have a way to interface external phanthom power supplies. Phanthom power is needed to drive active direct boxes. As I had mentioned in my article entitled "Interfacing Keyboards" in the December 1985 issue of Keyboard Magazine, active direct boxes are better to use than passive DI boxes because (1) active DI boxes can handle transients better, (2) they are quieter, and (3) they preserve the rich harmonics preset in all complex analog and digital synthesizers. Be sure than the voltage of the Phanthom supply is at least +12 to +48 volts. Although +12 volts will work, I'm definitely in favor of the higher voltage swing of a +48 volt phanthom power supply, especially if you're planning to drive a number of active DI boxes.

Whether you'll be using a consoles mic or line inputs keep in mind that they way the front end handles transient response is another most important consideration. Transients are severe peaks in audio level. They happen too fast to even be seen on most VU meters. They are most commonly caused by very percussive sounds. Drums and percussive instruments and also acoustic piano are all very transient sounds. When a transient sound hits the front end of your console, if the level exceeds the the headroom of the mic or line amplifiers the result will be distortion. The greater the amount of headroom a console offers, the wider the range of transient response. A good feature to look for is an overload indicator L.E.D. . When lit, the overload indicator tells you that there are transient peaks overloading or clipping your consoles front end.

Look for a pad. A pad is switchable device, placed before the mic pre-amplifier or line stages, to cut input level. A good pad consists of a switch selectable at -10 or -20 db of cut and a trim potentiometer to fine tune levels in between those amounts. By using a console's pad correctly, input levels can be trimmed to match the consoles range of headroom. On line level inputs very rarely is the pad necessary. Line level is more consistent where as mic level covers a much wider range of input levels requiring the help of a pad.

Be aware of the type of input connectors that are supplied. The standard configuration calls for balanced female XLR cannon jacks on the mic level inputs and unbalanced 1/4" phone jacks on the line level inputs. Looking for this configuration will save you money later. The need for costly interface boxes can be eliminated. Don't choose a console with 1/4" unbalanced phone jacks on its mic inputs. An unbalanced mic input is totally unacceptable. Unbalanced cable runs are susceptible to RF and other sources of noise. Be aware that the mic pre-amp has to boost the signal great amounts to start off with. Most keyboards with 1/4" phone jack outputs can usually output a hot enough signal for line inputs on most consoles. Keyboards whose output is line level and terminate in 1/4" unbalanced jacks and are over twenty feet away from the console will have to make use of Direct boxes. The direct box interface's with the balanced XLR jacks of the consoles mic level inputs. Remember if you're using DI boxes keep their unbalanced input cable runs short as possible. Keyboards with balanced XLR connectors are usually at line level and output too hot of a signal for XLR balanced microphone inputs unless the console has a pad on the microphone input of each channel. Line level input's on most mixers are usually 1/4" unbalanced phone jacks. This can be accepted because line level is less susceptible to noise and RF. Ideally though, the line level inputs should be balanced. On higher quality mixers, both the mic and line inputs show up on custom connectors. These multi-pin connectors allow the user to configure his own connector scheme.

The equalizer is the next component of a console to scrutinize. It is usually next in the signal path meaning after a signal passes thru the mic or line input stages it next passes thru the EQ section. It is important to have an EQ (equalizer) in/out switch. Another good feature to look for is EQ cut/boost knobs with center detentes. At this center point the knob is neither cutting or boosting frequencies. This is most useful when you need only to EQ a certain frequency and the EQ section has three or four bands.

Now at this point I'd like to clarify a big misunderstanding concerning parametric EQ. A true parametric equalizer has three separate controls. The first is the frequency sweep knob. This control selects the desired frequency. The next part is the control that selects the amount of cut or boost. The third part selects the width of the "Q". The Q is responsible for how wide or narrow the cut/boost control affects the neighboring frequencies. The Q of a parametric EQ has exactly the same function as the Q, emphasis, or resonance control found on all analog synthesizers. A example of narrow and wide Q is as follows l l l l l l l l l l l l l l l l l l l l l l l l l l

2k 4k 6k 500hz 4k 7.5k

An equalizer is not truly parametric unless it has the three basic parts represented on three separate knobs. There are other types of EQ's called Semi or Quasi parametrics. These others have either a fixed or switchable Q. Another type called Sweep EQ has a fixed Q, a sweepable frequency select and a separate cut/boost control. There is yet another type called selectable EQ. Selectable EQ has a cut/boost control and either a click stop control or a three way switch to select preset frequencies.

Which type is better ?

That is truly up to your ear. Just because a EQ is parametric does not mean it sounds musical. Most parametric EQ's are great for non musical sounding EQ'ing. For example to take the ring out of a house Pa system without affecting the overall sound. Because a parametric EQ can get very narrow and selective over what frequencies it affects, it is called a notch filter. Very much like the notch filter setting on the Oberheim modular 4 and 8 voice synths. What is more important is how bands an equalizer has. One of the most musical EQ's is the graphic type, but unfortunately it is not found on many mixers on each individual channel. A three band EQ consisting of sweepable low and high bands and a parametric midrange is most powerful. A three band with selectable frequency low and high bands and sweep or parametric midrange is another good choice. Look to see that the frequency range of each band overlaps that of each other.

Low and high pass filters are another powerful feature. A low pass filter can trim of hiss for example and still leave the high band of the EQ section to equalize other frequency characteristics of whatever the source of that channel is. The high pass filter does the opposite. It filters low frequencies that cause rumble and leave the low band of the EQ section free for other work. The simplest type of low or high pass filters are preset. The amount of cut is preset and is usually between -12 and -18 db. THey were designed to only affect frequencies above ( low pass ) and below ( high pass ) the preset frequency. The frequencies can be variable and represented by either a sweepable control or a selective switch.

______________ ______________ l l l l l l High Pass curve Low Pass curve

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