Mackie M1200/M1400 owner manual Thru, LOW CUT Filter, Constant Directivity

THRU

Someday you’ll do a show at Carnegie Hall and realize that one M•1200 or M•1400 ampli- fier is just not going to do the job — you’ll need a six-foot-high rack full of ’em. That’s what the THRU jack is for. Simply plug the signal source outputs into the first amp’s INPUTs , patch from that amp’s THRU jacks to the next amp’s INPUTs, and so on, daisy-chaining as many amps as you can afford (assuming your console has low-impedance outputs).

A general rule of thumb is to

maintain a load impedance

10 times or more than the

source impedance to prevent

excessive loading. If your console has an output impedance of 100 ohms, then you can daisy-chain up to twenty M•1200/ M•1400 amplifiers, which presents a load of 1000 ohms to the console (input impedance of 20 kohms divided by 20 amplifiers = 1000 ohms).

The THRU jacks can also be used to relay the signal on to other devices such as a DAT or cassette recorder, enabling you to record ex- actly what the audience is hearing. The THRU jacks are wired straight from the XLR and TRS INPUTs — there is no electronic circuitry between — so the signal going into the amp is exactly the same as the signal coming out of the THRU jacks.

You can use the THRU jack

as an input, if necessary,

since it’s wired in parallel

with the other input connec- tors. You can also use the

1/4" TRS INPUT jacks as THRU jacks. Simply connect the 1/4" TRS INPUT jacks on the first amplifier to the TRS 1/4" INPUT jacks on the second amplifier using 3-conductor shielded cables with TRS plugs on both ends.

Warning: If you use a regular guitar cord with 2-conductor TS plugs, you’ll unbalance the sig- nal at the XLR input by grounding the low side

(–)of the signal (pin 3).

LOW CUT FILTER

Every woofer has frequency response speci- fications. It’s usually expressed in Hertz (or cycles per second), like “40Hz–300Hz.” The “40Hz” refers to the low-frequency point (usu- ally, but not always) where the speaker’s output drops by 3 dB, and will "roll off" com- pletely as the frequency goes down. There is no point in sending a woofer any frequencies it

can’t reproduce — you can’t hear it, and worse yet, it’s a waste of amplifier power that can be better used reproducing frequencies you can hear.

5dB

0dB

–5dB

–10dB

–15dB

1Hz 10Hz 100Hz 1kHz 10kHz 20kHz

Low Cut Filter Frequency Response

In order to match the output bandwidth with your particular speaker system, the M•1200/M•1400 amplifier has a tunable LOW CUT FILTER. The frequencies are clearly marked along the knob’s travel :

•Fully counterclockwise, the frequency is below 10Hz, effectively bypassing the filter.

•Center detent is 35Hz and labeled TYPICAL, since precious few woofers actually go below that.

•3/4 of the way up is labeled STAGE MONI- TOR, 100Hz, perfect for, well, stage monitors (they seldom reproduce below 100Hz; besides, it prevents low-frequency “leak- age” into the house).

•Fully clockwise is labeled 170Hz.

So, grab your woofer’s spec sheet and find the low roll-off spec. Then set the M•1200/ M•1400’s LOW CUT FILTER at the same frequency.

If you do this correctly (and make sure the meters are happy), you’ll never again see your woofer moving sporadically without au- dible signal. Your system will play louder and cleaner, and you may never blow another woofer again!

CONSTANT DIRECTIVITY

Compression drivers mounted on constant- directivity horns require compensation, in the form of a high-frequency boost, with its “knee” set somewhere between 2.5kHz and 6kHz.

Until now, you’d have to resort to external crossovers or worse yet, graphic EQ modules. Both of these are fraught with limitations, not to mention adding cost, rack space require- ments and complexity to your system.

The M•1200/M•1400 Power Amplifiers eliminate the need for any of these external devices — they have the compensation cir- cuitry already built in. And using it is a breeze.

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