3.2 Negative Output Resistance (Rs) in Ace-Bass Amplifier
Whenever a driver element or any of resistors R413 - R418 in
the current sensing circuit has been replaced, the negative
output resistance must be reset to equal the positive resistance
of the parallel voice coils in the drivers.
3.2-1 Set jumper below trimpots in position "TRIM", and dis-
connect one speaker cable from the circuit board.
3.2-2 Connect a signal generator to the input (J303, pin 1 or
pin 4), and an AC voltmeter across the amplifier output.
3.2-3 Set signal generator to 20.3+-.5Hz, and adjust level until
voltmeter at amplifier output reads -16dBU (123mV).
3.2-4 Reconnect the speaker cable to the circuit board (so drivers
are connected to amplifier output), and adjust trimpot
Rrs till voltmeter reads OdBU (775mV).
NOTE 1:Voicecoils of drivers must be at room tempera-
ture when
R
rs
is set.
NOTE 2:Voltages in 3.2-3 and 3.2-4 are correct.
Negative output impedance of Ace-Bass amplifier
makes output voltage larger
when amplifier load
increases.
3.2-5 Reset jumper in "RUN" position.
NOTE: Do not touch trimpot Ra (R410)
This adjustment
requires calibrated microphone equipment and can
only be made at the factory. Ra only effects the
sound pressure level at 20Hz. Change of one driver
will cause maximum .5dB change in this level
(typically less than .20dB), so no correction is
required in the field.
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