60W Bass Amplifier

Low-cut and Bass controls

Output power: 40W on 8 Ohm and 60W on 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________47µF   63V Electrolytic Capacitors
C3________________100µF   25V Electrolytic Capacitor
C6_________________33pF   63V Ceramic Capacitor
C7_______________1000µF   50V Electrolytic Capacitor
C8_______________2200µF   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

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____10µF   63V Electrolytic Capacitors
C2_________________47µF   63V Electrolytic Capacitor
C3_________________47pF   63V Ceramic Capacitor
C5________________220nF   63V Polyester Capacitor
C6________________470nF   63V Polyester Capacitor
C7________________100nF   63V Polyester Capacitor
C11_______________220µF   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

Notes:

• The value listed for C8 is the minimum suggested value. A 3300µF capacitor or two 2200µF capacitors wired in parallel would be a better choice.

• The Darlington transistor types listed could be too oversized for such a design. You can substitute them with MJ11014 (Q3) and MJ11013 (Q4) or TIP142 (Q3) and TIP147 (Q4).

• T1 transformer can be also a 24 + 24V or 25 + 25V type (i.e. 48V or 50V center tapped). Obviously, the center-tap must be left unconnected.

• SW1 switch inserts the Low-cut feature when open.

• In all cases where Darlington transistors are used as the output devices it is essential that the sensing transistor (Q2) should be in as close thermal contact with the output transistors as possible. Therefore a TO126-case transistor type was chosen for easy bolting on the heatsink, very close to the output pair.

• R9 must be trimmed in order to measure about half the voltage supply from the positive lead of C7 and ground. A better setting can be done using an oscilloscope, in order to obtain a symmetrical clipping of the output waveform at maximum output power.

• To set quiescent current, remove temporarily the Fuse F1 and insert the probes of an Avo-meter in the two leads of the fuse holder.

• Set the volume control to the minimum and Trimmer R3 to its minimum resistance.

• Power-on the circuit and adjust R3 to read a current drawing of about 30 to 35mA.

• Wait about 15 minutes, watch if the current is varying and readjust if necessary.