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Capacitor coupled amplifiers: Evil or Not

Posted by BigguyinATL (M) on December 9, 2014 at 09:12:23

Every now and then I visit the Elliott Sound Labs site for a refresher on almost anything audio - especially amplifier circuit design issues. From a psycho acoustic perspective, I always have considered that one of the more important aspects of stereo reproduction is the preservation of dynamics. Live signals show more dynamic range than most recordings. Of course this has lead to me experimenting with Dynamic Range "anti-compressors" in the 70's and 80's with some success. it is a genuine shame that with the digital era producers, in most cases, do not take advantage of the dynamic range afforded them.

Of course, with a high dynamic range comes tremendous responsibility - to preserve it in playback. This leads to one of my main thread topics over the years, clipping issues. Many think of clipping as a phenominon that creates a square wave - lopping off the signal - and creating all kinds of odd order harmonics - blowing out tweeters and other harsh & obviously audible phenomenon. It is my experience, that the clipping is one of the key culprits in a lot of audiophile observations. When we say the sound "loses detail", the "image focus drifts" and a host of other concerns that are far more subtle than what we believe are issues come into play...

OK finally to the topic at hand. In Audiophiledom, it is common to say let's avoid as many components as possible in the circuit path - with those nasty capacitors holding us up as prime targets. In a power amplifier, however, DC output at the terminals is certainly not desired. We do not want those main drivers (the low pass section) operating with an offset, even for a short while.

Elliott shows that even an AC coupled input amplifier when clipped for just 20 milliseconds with an asymmetrical waveform gives 2.7 volts DC output (almost two batteries in series) which would significantly disturb the loudspeakers' sound in the low frequencies.

His conclusion here makes a lot of sense:
"To what extent have the effects described here influenced reviewers (who typically never use any instrumentation, and usually never know if the amp under test is clipping or not)? I have no idea, but it is not unreasonable to assume that some degree of clipping must be experienced from time to time, and that will affect the outcome of a subjective test ... but with absolutely no technical detail to indicate the actual cause of the problem should it exist. Reviewer-speak will obfuscate the real issue(s), and the lack of instrumentation leaves us in the dark.

Finally, it must be pointed out that this shows that clipping with real-world (speech or music) signals creates not only the harmonics that have been described in innumerable web pages, but also generates a subsonic signal that is potentially very damaging to drivers, but is never mentioned. This signal has the capability to cause driver damage at worst, or unwanted cone modulation and additional loudspeaker distortion at best - neither can be considered a desirable outcome.

The moral of the story is to avoid clipping at all times - even momentary (supposedly inaudible) clipping will generate an unwanted low frequency or subsonic signal whose frequency will be completely unrelated to anything in the programme material. All it will achieve is cone displacement and increased intermodulation distortion"


"The hardest thing of all is to find a black cat in a dark room, especially if there is no cat" - Confucius

• clipping behavior   (Open in New Window)

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