LM391 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 amp specifications

  • frequency band (at 3dB/8Ω): 11Hz … 20kHz
  • THD at 1kHz: 0.006% at 1W/8&Omega/Ir = 400mA
  • THD at 1kHz: 0.02% at Ir = 400mA
  • THD at 1kHz: 0.02% at 65W/8&Omega/Uin = 873mV
  • THD at 1kHz: 0.006% at 80W/4&Omega/Uin = 700mV

Guitar amp circuit schematics

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