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10 | JL Audio - MHD900/5 Owner’s Manual

channel filter is a low-pass filter (attenuates

high frequencies). For each filter, the user can

select from a shallow (12dB/octave) filter slope

or a more aggressive (24dB/octave) slope.

1) “High-Pass Filter” Controls (Front and Rear

channels) or “Low-Pass Filter” Control (Sub

Channel): These switches allow you to select filter

slopes or defeat the filter(s) completely.

“Off”: Defeats the filter completely. Useful when

using an external crossover, such as one built into

a head unit or signal processor.

“ 12dB”: Configures the filter to attenuate

frequencies above or below the selected

filter frequency at a rate of 12 dB per octave

(Butterworth alignment).

“ 24dB”: Configures the filter to attenuate

frequencies above or below the selected filter

frequency at a rate of 24 dB per octave (Linkwitz-

Riley alignment).

Depending on the speaker system and the

vessel, different filter slopes may be required to

produce a smooth transition between the sound

of different speakers in the system.

In high-pass mode, the sharper “24dB” setting

will do a better job of protecting small speakers

with limited power handling. In low-pass

mode, the “24dB” setting can also help by more

aggressively removing lower midrange output

from a subwoofer system.

The shallower “12dB” octave setting

allows for more output overlap between

the speakers being crossed over. In many

cases, this will result in a smoother “blend”

or transition between speaker systems.

Experiment to find the slope or combination

of slopes which best matches the acoustic

requirements of your system.

2) “Filter Freq. (Hz)” Control:

This control selects the frequency at which the

filter begins to attenuate the signal and refers

specifically to the frequency at which output

has been attenuated by -3dB.

The filter frequency markings surrounding this

rotary control are for reference purposes and are

generally accurate to within 1/3 octave or better. If

you would like to select the filter cutoff frequency

with a higher level of precision, consult the charts

in Appendix B (page 18) of this manual.

“80 Hz” is a good starting point for most systems

where a subwoofer is connected to the Subwoofer

Channel and component systems or coaxials to

the four main channels.

INFRASONIC FILTER Subwoofer Ch. Only)

The “Infrasonic Filter” is a 24 dB/octave high-

pass filter, with a fixed cutoff frequency of 30 Hz.

This filter is designed to conserve amplifier power

and protect subwoofer systems, without audibly

degrading the sub-bass output.

With ported enclosures, the use of the

“Infrasonic Filter” is highly recommended to

protect the speaker(s) from excessive excursion

below box tuning.

With sealed enclosures, the use of the filter

is less necessary, but can still help protect the

speaker system from excessive excursion.

The “Infrasonic Filter” can be completely

defeated by selecting the “Off” position on its

switch. This bypasses all signal from flowing

through the circuit.

The output of the amplifier decreases for a

given input voltage when the “Input Range”

switch is placed in the “High” position. The

output increases with the switch in the “Low”

position. While this may sound counter-

intuitive, it is correct as described.

CHANNEL CONTROL SECTIONS

To the left of the “General Setup” section on

the Control Panel of the amplifer are three sets of

controls: one for the “Rear Channel Controls”

one for the “Front Channel Controls” and

one for the “Subwoofer Channel Controls”.

In each of these sections you will find a

rotary control labeled “Input Sens.” (Input

Sensitivity). These controls are designed to match

the input sensitivity of each channel section of

the MHD900/5 to the specific signal source that

is feeding the amplifier. These controls must be

adjusted, with care, following the procedures

outlined in this manual. Failure to make correct

adjustments can result in weak output, excessive

distortion and/or undesirable noise in the audio

output of the amplifier!

Note: The Channel Control sections also house

“High-Filter” or “Low-Pass Filter” and “Filter

Freq.” controls. These are crossover filter controls

and will be discussed in the next section of this

manual. This section will explain the Input

Controls only.

“Input Sens.” Controls

Once the appropriate “Input Voltage” range

has been selected in the “General Setup” section

of the amplifier, the “Input Sens.” controls are

used to match the source unit’s output voltage to

the input stage of each pair of amplifier channels

for maximum clean output. Rotating the control

clockwise will result in higher sensitivity (louder

for a given input voltage). Rotating the control

counter-clockwise will result in lower sensitivity

(quieter for a given input voltage). To properly

set each pair of amplifier channels for maximum

clean output, please refer to Appendix C (pages

18, 19) in this manual.

After using this procedure, you can then adjust

the relative level of each channel pair by adjusting

the input sensitivity downward on any of the

channel sections, if they require attenuation to

achieve the desired system balance.

Do not increase the “Input Sens.” setting for

any amplifier or channel section in the system

beyond the maximum level established during

the procedure outlined in Appendix C (pages 18,

19). Doing so will result in audible distortion and

possible speaker damage.

Be aware that all three “Input Sens.”

adjustments will need to be made, regardless

of how many input cables are feeding the

amplifier. These controls will allow you to set

the appropriate relative levels for the “Front”

channels relative to the “Rear” channels and the

“Sub” channel (and any other amplifier channels

in the system).

CROSSOVER CONTROLS

Crossovers are groups of electronic filters

designed to control the frequency ranges

that each speaker in a system will be tasked

with reproducing. Proper crossover setup is

critical to sound quality and reliability.

The MHD900/5 has three individually

adjustable filter controls, one for the “Front”

channels, one for the “Rear” channels and

another for the “Sub” channel. The “Front”

and “Rear” filters are high-pass filters (they

attenuate low frequencies), while the “Sub”

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