Ultrasonic Oscillation

Also many SS amp manufacturers use something called a Zobel Network. It amounts to a series inductor (coil) and a capacitor or just the inductor. This is used as a filter and is located just after the output devices prior to the signal hitting your speaker terminals. It could be as simple is one of your pieces of audio gear is unstable for some reason. I once found a fet constant current source was acting like an R.F. generator in an old CAT preamp. Once a device becomes non-linear it's possible it can oscillate.

Thank you both for your comments and explanation. If I understand you right, this is something that occurs internal to the device rather than user or environmental. When this occurs, say within an amp, does the damaging effects move in direction of the speaker outputs or also in the direction of the preamp/source? Or conversely, if the pre is in oscillation and damages the speakers do I assume that the amp is damaged also. Two pair of damaged speakers and a couple of pieces of suspect equipment, I am a bit frustrated but I feel much better being provided explanations of a possible problem. I appreciate your thoughts very much. Again, thanks. Dave

This can occur due to loading effects on the amplifier, and not just due to parasitic feedback as described above.

Or it could be due to an amplifier that was not designed with enough "stability" in the first place.

For any amplifier using feedback (and most do), if there is not enough "margin" built into the design, they can oscillate at extremely high frequencies, often a few megahertz. This high frequency oscillation can create beat components down into the infrasonic (50-100KHz) range that could fry speakers depending on the magnitude and power levels of the oscillation, and depending on the speaker itself.

In some power amplifiers, the use of high capacitance interconnects can trigger unstable oscillation. Likewise, if a speaker system has too much capacitance, it can also be triggered. Note this is capacitance in parallel with the load (or speaker), not coupling capacitors in cross over networks.

The capacitance loading forms another response pole with the output impedance of the amplifier. If this pole (or corner frequency) is within the closed loop bandwidth of the power amp, instability will result and the amp will oscillate.

Note that defective or failed devices in the output stages of the power amp (such as a weak power transistor) can increase the output impedance of the power amp and trigger osillation that was not there before.

Oscillation in the pre-amp is also possible, but it should not be getting through the power amp as most have RF filtering on the input stages that should prevent this from getting to the power stages. Of course, if the power amp is broadband and amplifies the RF, then it will be transfered to the speakers.

Best way to detect this oscillation is with the old fashioned oscilliscope. Find a tech who has one and look at the power amp outputs with the pre-amp disconnected. Then hook up the pre-amp and check again. Then add the speaker cables and keep checking.

If RF oscillation is there at high enough levels to burn tweeters, it should be easy to see on any decent scope.

Do you think that the addition of a sub (REL Strata III) correctly connected to the amps speaker posts could create this instability in the amp? Again, thanks.

Italian, one other possibility to consider is that if you have been clipping (overloading your amp with too much volume) then that will also damage tweeters.

Usually tweeters only have to handle a few watts but if you overload the amplifiers, the distortion that the amp makes can destroy the tweeters in a matter of seconds.