Specifications; Theory Of Operation - Fender FRONTMAN 15B Service Manual

FRONTMAN 15B
Product Release No.
:
PART NUMBER:
POWER AMPLIFIER SECTION:
Power output:
Rated load impedance:
Distortion at rated power:
Power bandwidth:
Compression range:
PREAMP SECTION:
Input impedance:
Sensitivity (@ 1kHz):
Equalization range:
PHYSICAL SPECIFICATIONS:
Width:
Height:
Depth:
Weight:

SPECIFICATIONS

PR 359 (This is not a model number)
22-1503 120 V
22-1533 240 V Aus
22-1543 230 V UK
22-1563 230 V
22-1573 100 V
22-1583 230 V Nord
15 Watts
8 Ω
5%
40 Hz to 20 kHz
19dB
202k Ohms
4mV at onset of compression
HIGH +/- 15 dB at 8 kHz
MID +/- 15 dB at 800 Hz
LOW +/- 15 dB at 40 Hz
13-1/4 inches (34.3 cm)
15 inches (38.1 cm)
9-3/4 inches (24.8 cm)
21 lbs. (9.5 kg)
The input circuitry is a low pass filter with the -3 dB down point at 22 kHz. This sets up the initial
response of the amplifier and provides RF suppression. U1B is an "Auto Pad". It provides excellent
headroom and low noise. At upper volume control settings, it also rolls off low frequencies at 219
Hz. U1A makes up the tone controls. They provide 15 dB of boost or cut at 40 Hz (Low), 800 Hz
(Mid), and 8 kHz (High). U2B is a special tone shaping circuit used for sonic enhancement.
U2A is a unity gain amplifier which drives the power amp stage. In conjunction with U4, it acts as
the gain reduction circuit for the Deltacomp. The output of the power amplifier is sensed through
R23 by the Comparator U5A. As the amplifier output approaches clipping, the output of U5A goes
negative. This charges Capacitors C20-C23 through the time constant made up with R41 (3.9K x
8.8µF = 34.3mSec). This single-pole charge is the attack time for the Deltacomp. As the
capacitors charge, the voltage at the base of transistor Q1 goes negative, turning on this
Darlington device. Current from Q1 controls the output voltage of U4 (Operational
Transconductance Amplifier). The output of U2A also drives the inverting input of U4. The inverted
output from U4 mixes with the non-inverted input to U2A causing a cancellation effect. This
attenuates the signal to the power amp, thus eliminating clipping. When the power amp output is
reduced, the output U5A goes positive. Blocking Diodes CR8-CR11 force the Capacitors to
discharge through R40. This 4-pole discharge is the Release for the Deltacomp.
The power amplifier is contained within U3. CR1 and CR2 keep the output centered during
clipping. CR3 and CR4 provide flyback protection for the output of U3.
2
FRONTMAN 15B

THEORY OF OPERATION