60W Bass Amplifier

Low-cut and Bass controls

Output power: 40W into 8 Ohm and 60W into 4 Ohm loads

Amplifier circuit diagram:

Amplifier parts:

R1__________________6K8 1W Resistor
R2,R4_____________470R 1/4W Resistors
R3__________________2K 1/2W Trimmer Cermet
R5,R6_______________4K7 1/2W Resistors
R7________________220R 1/2W Resistor
R8__________________2K2 1/2W Resistor
R9_________________50K 1/2W Trimmer Cermet
R10________________68K 1/4W Resistor
R11,R12______________R47 4W Wirewound Resistors

C1,C2,C4,C5________47F 63V Electrolytic Capacitors
C3________________100F 25V Electrolytic Capacitor
C6_________________33pF 63V Ceramic Capacitor
C7_______________1000F 50V Electrolytic Capacitor
C8_______________2200F 63V Electrolytic Capacitor (See Notes)

D1_________________LED Any type and color
D2________Diode bridge 200V 6A

Q1,Q2____________BD139 80V 1.5A NPN Transistors
Q3_____________MJ11016 120V 30A NPN Darlington Transistor (See Notes)
Q4_____________MJ11015 120V 30A PNP Darlington Transistor (See Notes)

SW1_______________SPST Mains switch

F1__________________4A Fuse with socket

T1________________220V Primary, 48-50V Secondary 75 to 150VA
Mains transformer (See Notes)

PL1_______________Male Mains plug

SPKR______________One or more speakers wired in series or in parallel
Total resulting impedance: 8 or 4 Ohm
Minimum power handling: 75W

Preamplifier circuit diagram:

Preamplifier parts:

P1_________________10K Linear Potentiometer
P2_________________10K Log. Potentiometer

R1,R2______________68K 1/4W Resistors
R3________________680K 1/4W Resistor
R4________________220K 1/4W Resistor
R5_________________33K 1/4W Resistor
R6__________________2K2 1/4W Resistor
R7__________________5K6 1/4W Resistor
R8,R18____________330R 1/4W Resistors
R9_________________47K 1/4W Resistor
R10________________18K 1/4W Resistor
R11_________________4K7 1/4W Resistor
R12_________________1K 1/4W Resistor
R13_________________1K5 1/4W Resistor
R14,R15,R16_______100K 1/4W Resistors
R17________________10K 1/4W Resistor

C1,C4,C8,C9,C10____10F 63V Electrolytic Capacitors
C2_________________47F 63V Electrolytic Capacitor
C3_________________47pF 63V Ceramic Capacitor
C5________________220nF 63V Polyester Capacitor
C6________________470nF 63V Polyester Capacitor
C7________________100nF 63V Polyester Capacitor
C11_______________220F 63V Electrolytic Capacitor

Q1,Q3____________BC546 65V 100mA NPN Transistors
Q2_______________BC556 65V 100mA PNP Transistor

J1,J2___________6.3mm. Mono Jack sockets

SW1_______________SPST Switch

Circuit description:

This design adopts a well established circuit topology for the power amplifier, using a single-rail supply of about 60V and capacitor-coupling for the speaker(s). The advantages for a guitar amplifier are the very simple circuitry, even for comparatively high power outputs, and a certain built-in degree of loudspeaker protection, due to capacitor C8, preventing the voltage supply to be conveyed into loudspeakers in case of output transistors' failure.
The preamp is powered by the same 60V rails as the power amplifier, allowing to implement a two-transistors gain-block capable of delivering about 20V RMS output. This provides a very high input overload capability.

Technical data:

Sensitivity:

70mV input for 40W 8 Ohm output

63mV input for 60W 4 Ohm output

Frequency response:

50Hz to 20KHz -0.5dB; -1.5dB @ 40Hz; -3.5dB @ 30Hz

Total harmonic distortion @ 1KHz and 8 Ohm load:

Below 0.1% up to 10W; 0.2% @ 30W

Total harmonic distortion @ 10KHz and 8 Ohm load:

Below 0.15% up to 10W; 0.3% @ 30W

Total harmonic distortion @ 1KHz and 4 Ohm load:

Below 0.18% up to 10W; 0.4% @ 60W

Total harmonic distortion @ 10KHz and 4 Ohm load:

Below 0.3% up to 10W; 0.6% @ 60W

Bass control:

Fully clockwise = +13.7dB @ 100Hz; -23dB @ 10KHz

Center position = -4.5dB @ 100Hz

Fully counterclockwise = -12.5dB @ 100Hz; +0.7dB @ 1KHz and 10KHz

Low-cut switch:

-1.5dB @ 300Hz; -2.5dB @ 200Hz; -4.4dB @ 100Hz; -10dB @ 50Hz