p. 19
features overview
3
rear panel
12
GCL (Gain Comparator Limiter) switch
This switch is used to enable or defeat the GCL limiter circuitry. It is also a conventional
push-push type, requiring a small "tool" to activate. The IN position is DEFEAT; the OUT
position is ENABLE. Normally, the GCL function should be enabled to minimize the pos-
sibility of either or both channels going into clipping or overload. With this feature defeat-
ed, a severe overload could cause the mains circuit breaker to trip. The Crest GCL limit-
ing system will be covered in greater detail later in this manual.
13
Fan grill
A 2-speed DC fan supplies cool air to the amplifier. THIS INTAKE SHOULD NEVER BE
BLOCKED! The fan switches to high speed automatically when the unit requires addi-
tional cooling. At idle and cool, the fan should be in low speed. The fan should never
stop unless the amplifier is switched OFF or the AC mains power source is interrupted.
14
Combo input connector
The combo connector offers both female XLR and ¼" phone jack balanced inputs for
each channel. The XLR is wired with pin 1 as ground, pin 2 positive input, and pin 3
negative input. The ¼" phone jack is a tip/ring/sleeve (3-conductor) type, with the tip
being positive input, the ring negative input, and the sleeve ground. It is important to
realize that the XLR, ¼" jack, and barrier strip inputs are all in parallel; therefore a bal-
anced input to the associated channel can be accomplished using a male XLR, a 3-con-
ductor phone jack, or bare wires connected to the barrier strip.
As an alternative, the ¼" input can also be used with a regular tip/sleeve (2-conductor)
type plug commonly found on single-conductor shielded patch cords. In this case, the
input becomes unbalanced, with the tip as positive input, and the sleeve ground (the
ring being grounded by the sleeve of the plug). An additional unique feature of this ¼"
input jack is something called a "quasi-balanced" input. The sleeve of this jack is con-
nected to chassis ground through a relatively low-value resistance that is part of a
ground loop elimination circuit. This circuitry will provide hum-free operation when rela-
tively short ¼" cable patches are made to this input from various outputs on this amplifi-
er, or from other equipment that shares the same rack with this amplifier. The quasi-bal-
anced circuitry is "automatic" and virtually "invisible" in normal usage. This feature can
be defeated with a jumper on the barrier strip from the "-" input terminal of that channel
to the ground terminal.
15
Input barrier strip
A barrier strip is provided for input connections using bare wire or spade lug connec-
tions. CPX amplifiers employ low-noise, electronically balanced input circuitry. This cir-
cuitry offers a very wide dynamic range capable of handling virtually any input signal
level, while providing excellent common mode rejection to minimize hum and reduce
interference. This strip accepts balanced and unbalanced audio signals. The "+" and "-"
terminals are the positive and negative inputs to the respective channels. The GND ter-
minal is the common ground to both channel inputs. For use with an unbalanced
source, connect the "-" input terminal of the channel to ground with a jumper. If the "-"
input is left floating, a 6 dB loss in channel gain will result and the floating input terminal
may pick up outside noise.
