High-Frequency Oscillation

Why toroidal output transformer can make High-Frequency Oscillation?

A well-designed toroidal output transformer can have excellent high-frequency response because of its core material, construction and winding arrangement. That is very good for audio, but it means the transformer does not naturally attenuate high-frequency signals as much as the EI transformer.
How to Reduce High-Frequency Oscillation
1. Add a small compensation capacitor Add a small capacitor to the feedback or gain stage to reduce high-frequency gain. A typical starting value is 47–220 pF. (parallel to R7 & R17)
2. Add a Zobel network Add a series resistor and capacitor across the speaker output. A common starting point is 10Ω + 100 nF. This can help stabilize the amplifier at high frequencies.
3. Add grid-stopper resistors Install a 1kΩ–10kΩ resistor directly at the power tube grid pin. This helps prevent high-frequency oscillation in the power tube. The PCB has R6 & R16 resistors. Try to increase to 10KΩ.
4. Improve grounding and feedback wiring Keep the feedback connection and ground return as short as possible. Make sure the feedback wire is separated from the output signal and high-current wiring.
How to test
Before changing multiple components, use an oscilloscope to check the amplifier for high-frequency oscillation.
Connect a suitable dummy load to the amplifier and monitor the output with the oscilloscope.
Then gradually increase the signal-generator frequency:
10 kHz → 20 kHz → 30 kHz → 50 kHz → 100 kHz → 200 kHz → 500 kHz
Watch for:
1 Increasing ringing or overshoot.
2 A high-frequency waveform riding on the audio signal
3. Sudden oscillation
4. A significant increase in amplitude at a particular frequency
You may see something like:
20 kHz — clean
50 kHz — slight ringing
100 kHz — significant ringing
150 kHz — oscillation
This will help identify the frequency where the amplifier becomes unstable.
If you do not have an oscilloscope available, you can still listen to the amplifier with high-power music. You may not be able to hear the high-frequency oscillation directly, but the music may sound abnormal, harsh, or distorted, which can be an indication of instability. However, an oscilloscope is strongly recommended for confirming the problem.