Ultrasonic Oscillation

Hi. I have had recent issues with my Snell Ds, tweeters not working. Anyway this led me to test of all of my equipment including my GMA Europas. Their tweets were dead also. I dropped an email to Roy at GMA and he suggested that I may consider the possibility of ultrasonic oscillation in either my amp or pre-amp. They are the Odyssey Candela pre amp, and Stratos amp. Could you please explain what it is, and more importantly what are the causes. Soon I'll get a diagnostic locally and sending the pieces to Klaus for repairs if needed. I look forward to your insights. Thanks much, Dave
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.
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.
Atmasphere, clipping. I associate this with volume, loudness. I have old ears, on a clock with six o'clock being zero volume I drive the volume no more than 1/4 (9:00). Can this low level create a clipping situation? Please know, to all, I've learned alot through this thread, you all have provided a wealth of knowledge. Thanks much, Dave
Italian, the volume control has nothing to do with the actual volume. With today's super high-output digital sources (a pet peave- there is no reason to have so much output) a 9:00 setting on the volume control might easily clip a lot of amps.

IOW it depends on how much output the source has too.

Another possibility is ultrasonic noise from the digital source.

Anything you connect to the speaker output could potentially create an unstable situation, if the capacitance of the wiring is too high.

But this is very unlikely with a sub unless you are using exotic cables.

Did you upgrade interconnects prior to burning out your HF drivers?

SEVERE clipping can do this, but it would be so loud and unpleasant I doubt if you would not notice it.

Unplugging a connection to an amp or preamp with the volume up (that results in a burst of noise) can do this. But since you have destroyed TWO sets of drivers, this is unlikely.
Yes, i did switch to a set of DH Labs special edition ICs from pre to amp. Thin wires. Have a set of MAC palladium ICs from CDP to Pre but had been using them a while. Dave
Hi, thought I'd follow up on this thread. I took suggestions to have my equipment bench tested, both sets of speakers as well as pre and power amp checked out OK. Set everything up, and now five months later HF still dead. Again and again yet I swapped all equip and wires. Problem still exists. I have been told it's not likely but can it be in my house wiring? I have also ran power to other outlet elsewhere in the house via heavy duty extension cord and same problem. Any suggestions, what can I possibly be missing? Yeah, I'm totally frustrated and at a loss. Thanks much, Dave
Italian, It sounds to me like your amplifier has too little power and you are pushing it too hard. That is why tweeters fail! Its not because of a malfunction.

So you either need a more powerful amplifier or more efficient speakers.
Hi Atmasphere, thank you for your response. I appreciate it very much. And a consideration that I had not considered. I have used my GMA Europas(4 ohm/88db/7-120W @ 8 ohm amp)for years with both an Odyssey Stratos amp at 150W @ 8 ohm and a Denon POA 1500 at 150W @ 4 ohm. I don't play my sytem at loud volumes. The GMA's are known as an easy load and are known to be driven with low power amps from what I have read here over the years. That really is what is vexing, until this year system played fine. Even the Snell's that are a more difficult load are pushed by tube systems of a lesser power then the amps above from what I have read also. This weekend I'll be taking the system to my daughter's house to see if the problem persists there no matter how dumb that may sound. No stone unturned. Again, thanks much. Dave
One other thing to look at is corrosion on the ground contacts of your RCA connectors. That can toast tweeters too.
Not sure why I read this old thread but I did. A very interesting read, and now I'm curious as to how the situation resolved. ???
Yeah, we ought to have an unsolved mysteries category, it would be interested to see what the outcome turned out to be. I liked the one with the system hum that baffled the OP for quite a while and was finally traced/linked to the dc cabinet lights in the kitchen being on.
Hello all, I was going through some threads discussing palladium and came across an old thread of mine. So Timru here's the rest of the story and no it wasn't the kitchen lights although I only wish. As part of my desperation and frustration I dragged all of my equipment to Music Technology and Bill Thalman's gave it all a clean bill of health. I called him back to discuss and he came to my house to listen and said the high freqs were performing as they should. So now what to do, listen to my wife who said the problem had to be me. Many times. I had my hearing checked and now with hearing aids music is beautiful. I now consider the ear horns as one more critical piece of equipment in my system. Am I hearing what others may hear, most likely not but in my limited ranges a great joy.
And that's the rest of the story. Regards, Dave