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.
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:
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 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 across 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 wave form 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.
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