In our EE lab, we had to build the following amplifier. It is from this lab manual, page 40.
A week ago, I built it successfully, with 2n3904/3906 and d44vh10/ d45vh10 for the power amplifiers (Q3,Q4). It was driving an 8ohm speaker and all worked fine. I put my circuit in a box and came back this week to demonstrate its operation to the lab instructor but suddenly my circuit just oscillated at a very high frequency and nothing I did could correct it. I have rebuilt it twice now and somehow it is stable until I apply a signal and increase the volume - then it just goes crazy. At one point it worked perfectly fine when \$R_L\$ was a 10K resistor. Once I connected the speaker, it just went crazy and the transistors started heating up. What could be causing this instability and how can I correct it?I have noticed the issue is at the output stage - when I remove the op amps and connect the midpoint between the diode pairs to ground, it still goes crazy so I think the problem is within the transistor part.
- Vsupply = +/- 15V
- Op Amps: ua741 8-dip