How to Make 60W Bass Amplifier Circuit With Bass Preamplifier

 This design adopts a well-established circuit topology for the power amplifier, using a single-rail supply of approximately 60 V and capacitor coupling for the speaker(s). The advantage of a guitar amplifier is that the circuit is very simple, even for relatively high power output, and some degree of built-in speaker protection, which, thanks to the capacitor C8, prevents the supply voltage from reaching speaker in case of output transistor. Fail. The preamplifier is powered by the same 60V rail as the power amplifier, allowing the implementation of a two-transistor amplifier block capable of providing approximately 20V RMS output. This provides a very high potential for input overload.

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 (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:

Bass Preamp

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

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 shown for C8 is the minimum recommended value. One 3300µF capacitor or two 2200µF capacitors in parallel would be a better choice.  The listed Darlington transistor types may be oversized for such a design. You can replace them with MJ11014 (Q3) and MJ11013 (Q4) or TIP142 (Q3) and TIP147 (Q4).  Transformer T1 can also be 24+24V or 25+25V (ie 48V or 50V central voltage point). Obviously, the center faucet must not be connected.  Switch SW1 inserts a low cut function when open.  In all cases where a Darlington transistor is used as an output device, it is essential that the sense transistor (Q2) is in thermal contact as close as possible to the output transistor. Therefore, a TO126 package style transistor was chosen for easy screwing into the heatsink, very close to the output pair.  R9 should be adjusted to measure about half of the source voltage between the anode of C7 and ground. Better tuning can be done using an oscilloscope, to achieve a symmetrical cropping of the output waveform at maximum output power.  To regulate quiescent current, temporarily remove fuse F1 and plug the Avo-meter's probe into the two wires of the fuse holder.  Set volume control to minimum and Trimmer R3 to minimum resistance.  Turn on the circuit and adjust R3 to read about 30-35mA current.  Wait about 15 minutes, see if the current changes, and readjust if necessary.