Thursday, January 13, 2011

18W Guitar Amplifier Using Darlington Transistor

The aim of this design is to reproduce a Combo amplifier of the type very common in the 'sixties and the 'seventies of the past century. It is well suited as a guitar amplifier but it will do a good job with any kind of electronic musical instrument or microphone. 5W power output was a common feature of these widespread devices due to the general adoption of a class A single-tube output stage (see the Vox AC-4 model). Furthermore, nowadays we can do without the old-fashioned Vib-Trem feature frequently included in those designs. The present circuit can deliver 10W of output power when driving an 8 Ohm load, or about 18W @ 4 Ohm. It also features a two-FET preamplifier, two inputs with different sensitivity, a treble-cut control and an optional switch allowing overdrive or powerful treble-enhancement. 

18W Guitar Amplifier Using Darlington Transistor Circuit
Note:
  • SW1 and related capacitors C4 & C5 are optional.
  • When SW1 slider is connected to C5 the overdrive feature is enabled.
  • When SW1 slider is connected to C4 the treble-enhancer is enabled.
  • C4 value can be varied from 100nF to 470nF to suit your treble-enhancement needs.
  • 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 R9 to its minimum resistance.
  • Power-on the circuit and adjust R9 to read a current drawing of about 25 to 30mA.
  • Wait about 15 minutes, watch if the current is varying and readjust if necessary.

List Component:
P1           : 4K7  Linear Potentiometer
P2           : 10K  Log. Potentiometer
R1,R2        : 68K  1/4W Resistors
R3           : 220K 1/4W Resistor
R4,R6,R11    : 4K7  1/4W Resistors
R5           : 27K  1/4W Resistor
R7           : 1K   1/4W Resistor
R8           : 3K3  1/2W Resistor
R9           : 2K   1/2W Trimmer Cermet
R10          : 470R 1/4W Resistor
R12          : 1K5  1/4W Resistor
R13          : 470K 1/4W Resistor
R14          : 33K  1/4W Resistor
C1           : 100pF Ceramic Capacitor
C2           : 100nF Polyester Capacitor
C3           : 470µF Electrolytic Capacitor
C4           : 220nF Polyester Capacitor (Optional, see Notes)
C5           : 47µF/25V Electrolytic Capacitor (Optional, see Notes)
C6           : 1µF/63V  Polyester Capacitor
C7,C8,C9,C10 : 47µF/25V Electrolytic Capacitors
C11          : 47pF/63V Ceramic Capacitor
C12          : 1000µF/35V Electrolytic Capacitor
C13          : 2200µF/35V Electrolytic Capacitor
D1           : 5mm. Red LED
D2,D3        : 1N4004/400V 1A Diodes
Q1,Q2        : 2N3819 General-purpose N-Channel FETs
Q3           : BC182  NPN Transistor
Q4           : BD135  NPN Transistor (See Notes)
Q5           : BDX53A NPN Darlington Transistor
Q6           : BDX54A PNP Darlington Transistor
SW1          : pole 3 ways rotary switch 
SW2          : SPST Mains switch
F1           : 1.6A Fuse with socket
T1           : 220V Primary, 48V Center-tapped Secondary 20 to 30VA Mains transformer

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