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Bill Waslo is Right - Your Amp is Oscillating

Posted by Charles Hansen (M) on September 7, 2017 at 13:45:16

In Reply to: What I didn't know posted by Macncheez on September 6, 2017 at 19:42:59:

If a solid-state amp is running hot for no apparent reason, it is almost *always* because it is oscillating - usually in the MHz range - so you will never hear it.

The fact that it does so with certain configurations of speaker cables is simply due to the capacitance of those cables. The vast majority of solid-state power amplifiers have an emitter-follower circuit in its output stage. Nominally this has a maximum voltage gain of <1, so there is no way for it to oscillate.

But bipolar transistors turn out to have a mechanism whereby a capacitive load at the output causes an emitter-follower to have a negative input impedance at the base, and the darned thing will oscillate. (This is the basis of the well-known Colpitts oscillator circuit.)

That is clearly what is happening with your amplifier. Normally amplifier manufacturers do all sorts of things to prevent this from happening, including using an inductor at the output (to isolate the amplifier from the capacitive load) and adding Zobel networks (to "swamp out" potential capacitances that can cause problems). Many of these band-aids degrade the sound quality, but it is better to have a mediocre amp that works than a great sounding amplifier that doesn't work... :-)

I am extremely surprised that your McIntosh would do this. It's not like it was designed by some beginner in his garage. But clearly they have dropped the ball on this particular design. I would contact them to see if they have an update.

All of this first became a well-known problem with the introduction of the very first "high-end" speaker cable - imported from Japan by Polk Audio (!), it was called "Cobra Cable" and it looked a lot like Kimber Kable, with many fine strands woven together - much in the same way a rope is woven. All of those strands create a very high capacitance, and Cobra Cable blew up many, *many* high-end amplifiers.

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