Ultrasonic oscillation is basically a parasitic oscillation that would render the circuit (whether preamp or power amp) unstable &/or non-linear. 'Ulrasonic' would imply that the oscillation frequency is very high (compared to the audio signal of 20Hz-20KHz). An ultrasonic oscillation could occur when part of the output signal is fedback to the input (of the pre or power) such that it enhances bonafide input (of the pre or power). A bonafide input of the pre is the output of the CD player, for example & a bonafide input of the power amp is the output of the preamp. When this happens it is called positive feedback & it is generally not good at all & can have devastating effects such as destroying the electronic devices inside the pre or power, damaging connected speakers. The reason for this is that when a circuit (parasitically or ultrasonically) oscillates it generates high frequency power & gets hot. The device(s) at this point are often operating outside their SOA (safe operation area) & will eventually break-down. Also, an oscillating circuit is not linear so signals amplified thru an oscillating circuit have (a lot of) distortion. Thus, speakers connected to a distorted, amplified signal will get damaged over time (quickly).
The output can get coupled back to the input in various ways:
* thru internal capacitance that is used as part of the electronic circuit,
* thru ground (which is common to both input & output) current flowing thru the ground impedance,
* thru the power supply. When 1 power supply is used for multiple circuits, one circuit might modulate (wiggle) that power supply due to the input signal & that wiggle can get transmitted to another circuit running off that same power supply thereby creating a coupling.
The designer of electronic audio gear can avoid ultrasonic oscillations by
* making sure that input & output wiring is not adjacent,
* putting a metal can/shield over sensitive part(s) of the circuit,
* use of bypass capacitors in the power supply (these are thin/skinny capacitors often 10,000uF or 6800uF - something in the region of 10X smaller than the main power supply cap) to provide a low impedance AC path to ground,
* paying careful attention to how the ground routing (on PCBs) & wiring (to the chassis) is done such that the power ground current (of the output transistors) is not shared with the signal ground current (of the amplifier driver stage) & that these 2 currents are starred/Kelvin-connected to the chassis ground thereby providing a low impedance path to ground for both.
Hope that this info helps.
The output can get coupled back to the input in various ways:
* thru internal capacitance that is used as part of the electronic circuit,
* thru ground (which is common to both input & output) current flowing thru the ground impedance,
* thru the power supply. When 1 power supply is used for multiple circuits, one circuit might modulate (wiggle) that power supply due to the input signal & that wiggle can get transmitted to another circuit running off that same power supply thereby creating a coupling.
The designer of electronic audio gear can avoid ultrasonic oscillations by
* making sure that input & output wiring is not adjacent,
* putting a metal can/shield over sensitive part(s) of the circuit,
* use of bypass capacitors in the power supply (these are thin/skinny capacitors often 10,000uF or 6800uF - something in the region of 10X smaller than the main power supply cap) to provide a low impedance AC path to ground,
* paying careful attention to how the ground routing (on PCBs) & wiring (to the chassis) is done such that the power ground current (of the output transistors) is not shared with the signal ground current (of the amplifier driver stage) & that these 2 currents are starred/Kelvin-connected to the chassis ground thereby providing a low impedance path to ground for both.
Hope that this info helps.