Ringing-Transformer - A. Kimmel
On the Ringing-Transformer Controversy
Alan Kimmel
This controversy reminds me of the Shakespeare play Much Ado About Nothing. In that play a couple plans to marry, but before the wedding the groom is led to believe his bride is being unfaithful, when in fact she has remained faithful.
A good audio transformer is designed to pass music and voice faithfully. When you send a square wave into a xfmr you are exciting the xfmr in a non-musical and non-linear fashion, and you are doing so at frequencies up to 10X the square wave's fundamental frequency. If the fundamental frequency is high, ringing is often the result. Artificial waveforms like square waves are rare in music, and probably non-existent in voice. But people have learned that if they feed high frequency square waves into an audio xfmr, they can make the xfmr ring, and some of those people get disturbed about it only because they see the ringing on an oscilloscope, despite being unable to hear it. Some of them even decide that the transformer cannot faithfully pass music before they even hear anything through the transformer.
Unless severe, ringing is all but ignored by the human ear. Two of the reasons for this are that the ringing frequency is much higher than the fundamental frequency of the square wave and the ringing amplitude is also much smaller than the square wave's amplitude.
In a good xfmr with good bandwidth, if you drive it with a circuit you know to be a high fidelity circuit, the music coming out of the xfmr will also be high fidelity. If you have pure music going into the xfmr, you will have the same coming out of the xfmr. A well designed audio xfmr leaves music intact, but may or may not leave a high frequency square wave intact. A good audio xfmr is low in the distortions that the human ear is sensitive to, but may or may not be low in other (inaudible) distortions. This is key. It's interesting that some people make much ado about ringing, which is more difficult to hear than square wave rounding. The human ear is more sensitive to rounded corners of a square wave than to ringing or overshoot. Show me an amplifier that rounds the corners of a square wave and I'll show you an amplifier that rolls off the high frequencies. Your ears will detect HF rolloff much sooner than they will detect ringing.
If a high frequency square wave causes an amplifier to ring, what effect does such an amplifier have on music played through it??? THIS is the ringing issue. Hint: Some of the best reviews by both customers and reviewers go to amplifiers that ring on square waves, and sometimes not. That is, there is no clear correlation between whether or not an amplifier rings on square waves, and whether or not the amplifier faithfully reproduces music.
Mr. Cy Brenneman reminds us that if someone wants to ensure a transformer has minimal ringing, he must terminate the transformer with its characteristic load impedance.
Many Japanese audiophiles believe their ears over conventional test equipment. These audiophiles have heard every kind of amplifier there is and many of them are fond of amplifiers which are filled with audio transformers: Input transformers, Interstage transformers, and Output transformers. (Some of these amplifiers even have several interstage xfmrs.) No doubt many of these amplifiers ring like crazy when fed a high frequency square wave. Yet some of these amplifiers are highly respected for their faithful reproduction of music.
As a designer of audio equipment, over the years I have worked to develop audio circuits that not only reproduce music well, but also perform well. My guiding philosophy included maintaining low distortion of every kind of waveform including square waves. I discovered that good audio xfmrs are available which can help with things like isolation, impedance matching, etc. I did notice that when one of my circuits would send a perfect high frequency square wave into a xfmr, the result was often a ringing square wave. But when I played music through the circuit and xfmr combination, the music was just as pure and lifelike as before. Other listeners around the world confirm that the purity of the music is unaffected. I then decided that the ringing we see on a scope when a high frequency square wave is fed into a xfmr is little or nothing to be concerned about. It is inaudible; it does not affect music.
All the ears that have auditioned every kind of amplifier can't be wrong. Today there are audio products designed to be low in the distortions that the human ear is sensitive to. Some of these products may or may not give you the best looking square waves ever seen across the audio band; and some of them may or may not give the lowest THD ever measured, but some of these products can give you pure music because, as mentioned, some of these amplifiers are low in the distortions that the human ear is sensitive to.
(Note: I am not suggesting an amplifier must measure bad to sound good. There is little correlation between conventional parameters and musical faithfulness. Some amplifiers that measure great sound great; others that measure great sound bad. Likewise, some amplifiers that measure poorly sound bad, while others that measure poorly sound quite good.)
Mr. Jack Elliano of Electra-Print Transformers explains that when you hit an inductor with a square wave, you are essentially making that inductor into an Ignition Coil, and so it will ring, similar to an ignition coil.
Mr. Brenneman asks: "What musical instrument makes square waves?" Bottom Line: An amplifier that rings when fed high frequency square waves may ring slightly or not at all when playing music. (And if the amplifier does ring slightly during music, you just won't hear it.)
Last updated: Thursday October 11th, 2007 at 03:02:44 PM