Audio BJT Amplifier Design

Design Philosophy

I developed this design after acquiring a new computer. Following the purchase of the processor, monitor, and printer, I decided against acquiring a set of speakers at that time. After transitioning to a “soundless” setup, I resolved to integrate speakers. This naturally provided a perfect opportunity to warm up the soldering iron and explore a new design. This original design is based on a well-established design, examples of which can be readily found in numerous texts. My variation included the addition of a second differential stage to replace the conventional common emitter-plus-constant current source. This modification introduces a second inverse feedback path, creating a signal path consisting of an inverting differential amplifier and a feedback mechanism utilizing the non-inverting inputs. To achieve this, the constant current source avoids bypass capacitors, resulting in a DC-only stage. The other feedback path employs the standard output-to-non-inverting input of the first differential stage. The intention is to incorporate substantial inverse feedback to linearize the output transistors, considering that the non-linearity inherent in these devices diminishes their performance as analog amplifiers.

Another design criterion was the utilization of components commonly available at a hobbist's store: “Radio Shack”. No “exotic” components requiring a special order are utilized here. Consequently, the output stage employs a semi-complementary design, although this is not optimal concerning fidelity. (Radio Shack doesn’t offer complimentary pairs of power BJTs.) Given that the amplifier is intended for use with “wide band” speakers—ranging from 60Hz to 18KHz—that this store sells, the gain is deliberately kept low. With the design presented here, the maximum output is 6.0W. Even considering that these speakers are rated at 3.0W, one could potentially damage the speakers if the volume is increased excessively. Output limiting is achieved through the initial voltage amplifier (Q1). This stage possesses a gain of 7.5, resulting in an output voltage slightly less than 7.5VRMS for an input of 1.0VRMS. The gain of the circuit from the output end of the volume control to the speaker is unity, due to the significant negative feedback. The output power could be elevated to 10W, as there is ample “headroom” in the power supply design. (This was done to optimize fidelity as much as possible with a BJT-based design.) If the output is pushed beyond its limits, heat sinking on the power transistors needs to be enhanced.

The selection of actual components is not critical. Any small signal “transistors anonymous” will function here. The only exceptions are Q8 and Q10. These two transistors must be 2N3904 and 2N3906, respectively, as these represent complimentary pairs. These transistors are fundamental to the characteristics of the entire final stage and must be balanced in their characteristics to prevent DC from reaching the speakers and to ensure symmetrical amplification. As for the differential pairs (Q2, Q3, Q5, Q6), the type isn’t so important, however, matching pairs are crucial. It’s advisable to obtain several dozen of each, PNP and NPN, and select pairs that closely match in VBE and hFE. Matching pairs in the differential stages help to minimize DC offset, as this design employs DC coupling to improve low-frequency response. Speakers are sensitive to any significant amount of DC, which can damage the voice coils and compromises fidelity by restricting the cone’s movement. The 1800pF capacitor connected to the drain of Q1 was included because one of these amplifiers showed a weak (10mVP-P) oscillation at 700KHz. If you include it, be certain to make the connection with the least possible lead length, as it is an RF component. It will have no effect on audio quality since its cutoff frequency is about 58KHz.

Construction: Amplifier

The entire unit is designed to fit within a plastic “project box” cabinet measuring 7 X 6 X 3 inches. The “wide band” speakers, with a diameter of 3¾ inches, fit within a cutout of 37/16 inches. The circuit board is 6¾ X 2¼ double-sided, copper clad. The circuit is built “dead bug” style, simply by making all ground connections through soldering to the unetched copper, with the remaining wiring constructed above the circuit board. If additional mechanical reinforcement is needed, this is provided by soldering 2.2MEG resistors to the copper ground plane for use as “tie points.” Connecting such large resistors between the circuit and ground does not affect operation since the impedance at that point is significantly smaller. To accommodate the circuit on the board, both sides are utilized. The preamp module is constructed on one side, while the final stage is built on the opposite side. The heat sink for the power transistors is fabricated from two pieces of steel or aluminum 1 X 1 (in.) angle stock. Measure off a 2¼ length and cut it in half. This provides the necessary gap to separate these heat sinks electrically. Sand off enough copper from one end of the circuit board to ensure that the heat sinks aren't being shorted out. These are attached to the circuit board with one screw and nut. (Also, be sure to clear the copper away from the screw heads on the preamp side of the circuit board, otherwise, you will short out the finals.) In the middle of one flange of the angle stock, bore a hole slightly smaller than the diameter of the 2N3053s. This hole should be enlarged by careful reaming, so that the transistors fit snugly. This is essential for good heat transfer out of the transistor and into the heat sink. Once this has been done, apply a thin coat of silicone heat sink grease and press into place. These can be set aside until you need them. Once the circuit is fully assembled, it functions well, and produces good sound, despite the inherent limitations of BJTs as analog amplifiers. There are a couple of potential improvements possible. The first would be the use of a full-complementary final stage. If this is implemented, then any complimentary BJTs with a PC of 5.0W or more will operate effectively. Again, the most important component of this will remain the input Darlington pairs. Even better would be the substitution of complementary (N-Channel/P-Channel) VMOSFET power transistors. These, of course, don’t require Darlington pairs in order to operate. If VMOS power transistors are used, it's important to include a 100 ohm resistor in series with the gate terminal, mounted close to the transistor body with the shortest lead length. This will help to prevent the possibility of RF oscillation. (Unlike the BJT, the VFET is a high-frequency device, and the inadvertent construction of VHF oscillators with these is quite easy.)

Parts List: Amplifier

Parts List: Power Supply