of gain (see Figure 4B). For all versions of the amplifiers, the gain remains the same regardless of
whether the input is balanced or unbalanced.
Note: The polarity of the balanced inputs can be reversed by changing four jumpers on the Main
Board near the barrier strip inputs. Cross wire JP103 and JP104 to change the input polarity of CH1
and cross wire JP101 and JP102 to change CH2.
CAUTION: To avoid the risk of electric shock, refer all internal adjustments to qualified service
p e r s o n n e l .

Input Sensitivity

The input sensitivity of the amplifiers are set at the factory to 1.5V

R M S

for rated output. The input sensitivity

can be changed to 0.775V

R M S

by adding two jumpers on the Main Amplifier Board. See page 14 of this

o w n e r’s manual for more information.

Output Wiring

For Normal Stereo or Dual Mono operation, use the red and black binding posts associated with each channel.
These are multi-way binding posts that can be used with standard single or double banana connectors, bare
wire (up to 8 gauge) or spade lugs.
Use heavy-gauge wire for speaker connections. The greater the distance between the amplifier and the
speakers, the larger the diameter the wire should be to minimize power losses across the wire and improve the
damping of the speaker. Wire thickness specifications, or gauges, get larger as the wire gets thinner; thus 14-
gauge wire is thicker than 18-gauge wire. Use the following chart as a guide:

Wire Length

Gauge of Wire

Up to 8 ft.

18 gauge

Up to 12 ft.

16 gauge

Up to 20 ft.

14 gauge

Up to 30 ft.

12 gauge

Up to 50 ft.

10 gauge

This will insure that the resistance of the speaker wire is less than 2.5% of 4 ohms, resulting in a transmission
loss of less than 0.1dB. Class II (NEC) wiring can be used.

Polarity

Loudspeakers must be connected with consistent polarity for correct phasing between them. Incorrect phasing
will do no physical harm, but frequency response will be affected. The key is to make sure that both speakers
connected to the speaker terminals are hooked up the same way. Connect (–) at the speaker outputs to (–) on
the back of the speaker, and (+) at the speaker outputs to (+) on the back of the speaker.

Dual Mono

For dual-mono operation, turn the amplifier off and move the Stereo/Mono switch to the DUAL M O N O
position. Connect the input signal to CH2 input (CH1 input becomes disabled). Connect the speakers to the
Speaker Outputs on the amplifier in the same way that you would for normal stereo operation. Both speaker
outputs will carry the signal that is connected to the CH2 input.

Bridged Mono

For bridged-mono operation, turn the amplifier off and move the Stereo/Mono switch to the BRIDGED MONO
position. Connect the input signal to CH2 input (CH1 input becomes disabled). Connect the speaker(s) to the
two (+) speaker terminals. The CH2 red post is the hot (non-inverting) side, and the CH1 red post is the low
(inverting) side (see Figure 6).
When connected in this way, each channel “sees” one-half the impedance of the speaker that is connected between
them. If an 8 ohm speaker is used, each channel will see a 4 ohm load. Therefore it is not recommended that anything
lower than 8 ohms be connected in this mode of operation. Use Parallel Mono operation for lower impedances.
Note: Be sure to set both of the input level controls to the exact same setting for equal power distribution per channel.
Note: In bridged mono operation, the output connections are actually a balanced output configuration. This means
that neither output terminal may be grounded (both have voltage present).

Caution: Extreme care must be taken if using 1/4˝ phone plugs as speaker connectors with the amp in bridged-
mono mode. The reason for this is when connected to a bridged-mono output, the shell of the plug is “hot” and
could cause a nasty surprise if it comes in contact with something or someone that is grounded. If you must
use phone plugs at the speaker connection, at least insulate the shell of the plug with shrink sleeving.

Parallel Mono

Parallel-mono operation is useful when you are running sustained high levels into a single load, or when
driving a low impedance load. To configure the amplifier for parallel-mono operation, contact Carver
Professional Technical Service Department for Field Application Bulletin pm125-1. It contains detailed
instructions describing the simple procedure for modifying both the pm125 and pm420 power amplifiers for
parallel-mono operation.

Clipping Eliminator

This circuit prevents the input signal from exceeding the point where it would drive the amplifier into hard
clipping. It has no effect on the signal until it reaches the point where clipping would occur. The larger the input
signal the more the signal is reduced to keep the output just below clipping. The Clipping Eliminator circuitry
is inactive when the unit is shipped from the factory, but can be activated by installing jumpers in sockets JP9
and JP6 (see instructions on page 13).
Note: If the Input Level control(s) are turned down far enough, a sufficiently large input signal can drive the
input differential amplifier into clipping. The Clipping Eliminator circuit cannot remedy this kind of clipping.
Likewise, it has no effect on clipping that occurs prior to the amplifier inputs (at the mixer or equalizer stage,
for example).

This manual is related to the following products:

pm125