Guitar Amplifier Circuit

This is a medium power guitar amplifier is recommended especially for portable amplifier. The amplifier is a combination between a simple integrated audio driver, LM391-80 and a stage of output power designed contratemps with bipolar transistors.

Thermistor NTC, which is in temperature contact with output power transistors, allows to separate AO LM391 power stage when it heats too much.

The start of the thermal protection is at a NTC flow current NTC of approximately 200uA. Electrolytic condenser asure a “smooth start” to remove clicks that may occur when connecting the amplifier. It can happen that the protection can be too sensitive, in that case certain adjustments are admitted in the value of R4 or thermistor.

The standby current can be adjusted with P1. This potentiometer is initially 0 and then is adjusted to achieve a current of 50mA. Current can be increased up to 400mA if you want low distortions.

The power transistors are placed on the same side of printed circuit board, so that it can be mounted on a heatsink jointly with the thermistor. The heatsink must be large enough and have the thermal resistance of maximum 10K / W.

L1 has 20 turns with 0.8mm emailed copper, coiled over R21. C9 is a ceramic capacitor.

Guitar Amplifier Circuit

Guitar Amplifier Circuit

Guitar Amps

General 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.

Features:

Sensitivity:

• 35mV input for 40W 8 Ohm output
• 42mV 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

Treble control:

• +9/-16dB @ 1KHz; +12/-24dB @ 10KHz

Brightness control:

• +6.5dB @ 500Hz; +7dB @ 1KHz; +8.5dB @ 10KHz

Bass control:

• -17.5dB @ 100Hz; -26dB @ 50Hz; -28dB @ 40Hz

Circuit Diagram:

Guitar Amps

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