Common Issues with Guitar Amplifiers
Guitar amplifiers employing output devices like TDA7293 (100 W) or LM3886 (68 W) often suffer damage due to excessive supply voltage in the quiescent state. Transformers are frequently utilized so close to their specifications that this problem can stem from a high mains input voltage. In numerous countries, domestic AC outlet voltage is permitted to surge up to 10% above the nominal value. As replacing the transformer isn’t a practical solution, the author devised a relatively straightforward electronic remedy: a voltage limiter for the symmetric supply to the amplifier. The circuit follows the classical voltage regulator setup of a Zener diode linked to the base of a pass transistor. However, in this version, the conventional bipolar transistor is replaced with a power MOSFET. The circuit is symmetrical concerning both negative and positive supplies; hence, we will discuss the positive half only.
Voltage Limiting Circuit Operation
The input voltage, which does not exceed 50 V, feeds the series of Zener diodes, D1, D2, and D3, through resistor R3. The resistor serves to cap the current through the Zener diodes at approximately 5 mA. This arrangement of Zener diodes offers the advantage of distributing their dissipation among them, as well as enabling flexibility in setting the total voltage drop via careful component selection. The cumulative voltage across the diodes, totaling 39 V with the given values, must exceed the desired limiting output voltage by the gate-source voltage of the MOSFET. C1 contributes to voltage smoothing across the Zener diode chain.
Consequently, this circuit not only limits voltage but also diminishes supply ripple, including the hum component. R1 and C4, in conjunction, stabilize the HEXFET by preventing oscillation. In the absence of a load, the output voltage might be higher than anticipated. With a small load, such as the quiescent state of the output stage, it drops to the desired level. Thus, the circuit offers stabilization rather than regulation of the output voltage. The operation of the negative half mimics the positive half, except for the polarity of voltages, necessitating the use of a P-channel MOSFET.
Handling HEXFET Variability and Cooling
It’s noteworthy that the gate-source voltage of the HEXFETs can vary significantly, up to a few volts. This variance can be compensated by selectively choosing the Zener diodes in the chain and regulating the current through them. However, for most applications, the precise voltage at which limiting initiates is not critically significant.
Sufficient cooling is imperative for the HEXFETs. Ideally, they can be attached to the amplifier’s existing heatsink. Alternatively, a separate heatsink with a thermal rating of 2.5 K/W should be employed when necessary.