Balanced Input Connectors, Trs & Xlr

FRONT AND REAR CONTROLS

11. BALANCED INPUT CONNECTORS

(1/4” TRS & XLR)

These 1/4” (6.3mm) TRS (Tip/Ring/Sleeve) phone jacks and XLR jacks are compatible with balanced inputs and are wired as Tip/Pin 3 = (+), Ring/Pin 2 = (-), and Sleeve/Pin 1 = Ground. Normal maximum input level at clipping is 1.15V RMS (+ 4dB). Input impedance is 10K Ohms. Since the TRS phone jacks and XLR jacks are wired internally in parallel, you can parallel this unit with another amplifier by using either the LINE 1/4” jack or the XLR jack (depending on which you’re using to input your signal) to output the signal to the input jack of the other amplifier.

The 1/4” TRS phone jacks can also be used for unbalanced inputs. For TRS phone plugs, simply connect the unused side of the balanced input to ground. For 1/4” TS phone plugs, no change is necessary for compatibility with this input. Balanced input signals are recommended as they are less prone to AC hum. For long cable runs a source of less than 600 ohms output impedance is needed to avoid signal loss. For short cable runs an unbalanced signal input should be suitable.

For stereo (two-channel) operation, use the inputs for both CH-A and CH-B; for parallel or bridged mono operation, use only CH-A input. (See 13-MODE SELECTOR SWITCHES below for more explanation.)

12. CLIP LIMITER SWITCH

This switches the internal Clip Limiter ON or OFF for both channels. The limiter prevents excessive signals from overloading the amplifier (and thus flattening the peaks of the waveform and introducing distortion) by automatically lowering the amplifier gain to minimize overdrive. This limiting is primarily used to protect full-range speakers from high frequency distortion or damage to the horns caused by bass overdrive. However, when driving a subwoofer, defeating the limiter can produce a desired extra “punch” to bass instruments, such as kick drums.

13. MODE SELECTOR SWITCH

The SPA Series amplifiers offer 3 modes of operation: PARALLEL, STEREO & BRIDGED. Slide the switch to one of the three positions for you application.

•PARALLEL (MONO) INPUT — This mode allows both channels to operate in parallel with the same signal and without requiring a Y-cord. In this mode the inputs for both channels are internally connected, so that you only need to feed a signal into one of the channels. This still allows independent control of each channel. It also enables easy “daisy-chaining” with other amps by using the other set of input connectors.

(Note: Do not select this “Parallel” mode when feeding the amplifier 2 separate signals.)

(Note: Do not use both unbalanced and balanced cables in the same set-up as that can unbalance all the connections when daisy-chaining, resulting in hum.)

•STEREO INPUT — This is the most common mode generally used, and allows independent control of 2 separate signals such as stereo playback, main and monitor live mixes, and bi-amp operation (highs in one channel and lows in the other).

•BRIDGED MONO — This mode combines the power of both channels to drive a single speaker. In this mode the amp produces 4 times the peak power and 3 times the sustained power into a 4 or 8 ohm speaker than each channel can deliver separately in stereo or parallel mode.

(CAUTION: In this mode the amplifier can deliver high power into a speaker. Make sure that the speaker, connectors and wiring can handle this output. Note that for prolonged overdriven outputs into a 4 ohm speaker the breaker may trip, so care must be taken not to overload the amplifier in such operation.)

Connect the input signal to CH-A input for bridged mono operation.

14. LF FILTER (30HZ) SWITCH

The low frequency LF filter rolls off frequencies below 30Hz when this switch is turned to the ON position. This filter improves the audio by limiting unwanted inaudible low frequency speaker cone movement. Unless a preceding device, such as a mixer, already has such a 30Hz roll-off, this filter should generally be employed for optimum audio and to protect your speaker from excessive cone excursions beyond its rated limit at sub-audio frequencies. Generally this switch should only be left “OFF” if you are monitoring the whole signal for frequency content, such as for studio playback.

15. FAN

The fan speed is varied continuously automatically to maintain the proper internal operating temperature. Rear- to-front airflow keeps amps and racks cool.