2. 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
4. 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 outputFrequency response:
50Hz to 20KHz -0.5dB; -1.5dB @ 40Hz; -3.5dB @ 30HzTotal harmonic distortion @ 1KHz and 8 Ohm load:
Below 0.1% up to 10W; 0.2% @ 30WTotal harmonic distortion @ 10KHz and 8 Ohm load:
Below 0.15% up to 10W; 0.3% @ 30WTotal harmonic distortion @ 1KHz and 4 Ohm load:
Below 0.18% up to 10W; 0.4% @ 60WTotal harmonic distortion @ 10KHz and 4 Ohm load:
Below 0.3% up to 10W; 0.6% @ 60WBass control:
Fully clockwise = +13.7dB @ 100Hz; -23dB @ 10KHz
Center position = -4.5dB @ 100Hz
Fully counterclockwise = -12.5dB @ 100Hz; +0.7dB @ 1KHz and 10KHzLow-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 3300uF capacitor or two 2200uF 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).
5. 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.
author: EEE Projects website: eeeproject.com