Simple Hybrid Audio Amplifier

The debate still goes on as to which are better, valves or transistors. We don`t intend to get involved in that argument here. But if you can`t make your mind up, you should try out this simple amplifier. This amplifier uses a valve as a pre-amplifier and a MOSFET in the output stage. The strong negative feedback makes the frequency response as fl
Simple Hybrid Audio Amplifier - schematic

at as a pancake. In the prototype of the amplifier we`ve also tried a few alternative components. For example, the BUZ11 can be replaced by an IRFZ34N and an ECC83 can be used instead of the ECC88. In that case the anode voltage should be reduced slightly to 155 V. The ECC83 (or its US equivalent the 12AX7) requires 2 x 6. 3 V for the filament supply and there is no screen between the two triodes, normally connected to pin 9. This pin is now connected to the common of the two filaments. The filaments are connected to ground via R5. If you`re keeping an eye on the quality, you should at least use MKT types for coupling capacitors C1, C4 and C7. Better still are MKP capacitors. For C8 you should have a look at Panasonic`s range of audio grade electrolytics. P1 is used to set the amount of negative feedback. The larger the negative feedback is, the flatter the frequency response will be, but the smaller the overall gain becomes. With P2 you can set the quiescent current through T2. We have chosen a fairly high current of 1. 3 A, making the output stage work in Class A mode. This does generate a relatively large amount of heat, so you should use a large heatsink for T2 with a thermal coefficient of 1 K/W or better. For L1 we connected two secondary windings in series from a 2x18V/225 VA toroidal transformer. The resulting inductance of 150 mH was quite a bit more than the recommended 50 mH. However, with an output power of 1 W...

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