JBL GTQ240 Internal Adjustments, Mute Connection, 4.5Speaker-LevelInput Harness Color Codes

4.4 Mute Connection

The GTQ amplifier includes a mute input terminal. This amplifier will mute when a 5V to 12V DC signal is applied to the “mute” input and will be in normal oper- ation mode with either a ground or noth- ing connected to the “mute” input. This “mute” input is compatible with any aftermarket noise gate that puts out a control voltage of +5V to +12V DC.

4.5Speaker-Level Input Harness

Color Codes

Group 1 Left (+):		White
Group 1 Left (–):		White with
		Black Stripe
Group 1 Right (+):		Gray
Group 1 Right (–):		Gray with
		Black Stripe
Group 2	Left (+):	Green
Group 2	Left (–):	Green with
		Black Stripe
Group 2	Right (+):	Purple
Group 2	Right (–):	Purple with
		Black Stripe

5. Internal Adjustments

5.1Speaker-Level Input Impedance Adjustments

The speaker level inputs of the GTQ amplifiers come factory-set with

100k ohm input impedance. This will pro- vide the lowest distortion operation from the speaker outputs of most modern head units by reducing the power the amplifier in the head unit must deliver to practi- cally, nothing. On some older or lower- priced head units this load will not facili- tate proper fader operation. To allow for this, we have provided the ability to change the input impedance of the speak- er-level inputs to 15 ohms. This is accom- plished by moving the jumpers shown on the diagram below. This input is also capable of directly accepting signals, when in the 100k ohm setting, from many Balanced Line Drivers such as those sometimes used in competition vehicles. If the head unit has 4 channels of built-in amplification and/or an electronic fader control, you should leave the jumpers in the factory-set position.

If the head unit has 2 channels of amplifi- cation, with a speaker-level fader, the jumpers should be set to the 15-ohm position. This will always be a rotary-type control, not one controlled by electronic pushbuttons.

	JUMPER SELECTOR
	SELECT 100K OR 15 OHMS
100K	INPUT
Ω
15	LINE
30
30

5.2Crossover Frequency Adjustments

The GTQ amplifiers include built-in frequency selectable crossovers. One crossover is connected in series with the amplifier circuitry and the other cross- over is connected to the preamp level out- put jacks. These crossovers can be set in either the “F” (full bandwidth operation), “L” (subwoofer operation), or “H” (satel- lite operation).

The crossover frequencies are set by chips inside the amplifier. These chips are simply a set of resistors, connected across the pins and molded into a single package. The crossover frequencies may be changed to any value desired by changing the resistor network. JBL has chips available in the popular values list- ed in the table which follows. If none of these suit your system, you may purchase compatible resistor networks from a local electronics store, or you may build your own custom values from discrete resis- tors mounted on a 14-pin DIP Header using the instructions which follow.

Frequency	Resistor		JBL Part
	Value		Number
50Hz	47k	Ω	1-23-750
80Hz	33k	Ω	1-23-817
120Hz	22k	Ω	1-23-820
200Hz	12k	Ω	1-23-821
250Hz	10k	Ω	1-23-810
375Hz	6.8k	Ω	1-23-822
500Hz	4.7k	Ω	1-23-815
650Hz	3.9k	Ω	1-23-823
2.5kHz	1k	Ω	1-23-824
5kHz	470	Ω	1-23-816

5.3 Custom Chip Construction

Regardless of whether you build or buy it, the necessary resistor network has the following configuration:

•Each resistor in the package has the same value.

•If you know the crossover frequency you want, you can calculate the resistor value necessary by solving the following equa- tion:

Resistor	2,500,000
Value =	Frequency in Hz
in ohms

•Use the following equation if you have a resistor pack of a known value, and want to find its crossover frequency:

Frequency	2,500,000
in Hz	= Resistor Value in ohms

•To build chips from discrete resistors, solder the resistors to a standard 14-pin “DIP Header” according to the diagram. If a DIP Header is not available, you may bend the leads of 1⁄4-watt resistors 90 degrees, trim them to 3mm-length, and insert them directly into the chip sockets.

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