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 C16, 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.
- Sensitivity:70mV input for 80W 8 Ohm output | 63mV input for 100W 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% @ 70W
- Total harmonic distortion @ 10KHz and 8 Ohm load: Below 0.15% up to 10W; 0.3% @ 70W
- Total harmonic distortion @ 1KHz and 4 Ohm load: Below 0.18% up to 10W; 0.4% @ 100W
- Total harmonic distortion @ 10KHz and 4 Ohm load: Below 0.3% up to 10W; 0.6% @ 100W
- 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
- The value listed for C16 is the minimum suggested value. A 10000uF capacitor or two 6800uF 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 and MJ11013 or TIP142 and TIP147 .
- 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 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.
- Set the volume control to the minimum and Trimmer R20 to its minimum resistance.
- Power-on the circuit and adjust R20 to read a current drawing of about 30 to 35mA.
- Wait about 15 minutes, watch if the current is varying and readjust if necessary.