Now whatever the story is, I do not see a problem measuring spikes on DC voltages but I think when higher speed and big currents are at stake when an amp is outputting music, things get extremely complex. Could some (type of) ringing trigger oscillations, too? I presume it can.
The HF ringing from diodes and transformers forms an RF mixer—carrier wave modulated by mains harmonics—which then broadcasts noise throughout the amplifier. This noise is received by leads and wires, amplified, and destabilizes the circuit and robs power. It may stimulate any resonant circuit looking for energy to itself ring. Stopper resistors and Zobels work by ruining the Q of oscillators, parasitic and otherwise, by increasing losses to the point the oscillation damps out. Hobbyists tend to not look for noise in amplifiers above 100 kHz, and certainly not at a few hundred kHz. Electrical engineers have the knowledge, equipment, and desire to find it.
To begin, for those trying to understand the background issues raised herein, Hagerman offers a description of snubbers and some plots. This is older work from over twenty years ago, and newer, lower-noise diodes are available. See:
http://www.hagtech.com/pdf/snubber.pdf
Calculating Optimum Snubbers by Jim Hagerman
July 24, 1995
I was reading my first issue of Audio Amateur when my eye caught the letter from Scott Morovich (TAA 3194 p.46) regarding snubber design in power supply diode circuits. I've been working recently on snubber design and everything he said was right on the money. Rick Miller's reply suggested using capacitors only as snubbers, but these may not have the effect he expected and the impedance of the circuit may no1 be as low as he thinks. A similar letter (p.49) gave me the impression that there may be a lot of misunderstanding out there about how snubbers work. I thought perhaps I could help by sharing my analysis of snubbers in power supply circuits_ The results show that snubbing the parasitic RF oscillations is relatively straightforward without having to resort to exotic diodes.
The basic snubber type in this analysis is a series resistor and capacitor usually placed across the power supply rectification diodes. Many readers have tried this, but coming up with values for the components can be a crapshoot. This analysis (once you get through the math) results in a few simple equations (boxed) allowing direct calculation of optimum values. At a minimum they provide a starting point for the all important listening tests
Cornell-Dubilier explains placement of the snubber across the diode, and the calculations for the snubber capacitor and resistor:
Much of CDE's emphasis is SMPS, of course, but it applies here as well. Diode Qrr is the same.
Here are some comments on diode noise by the late John Camille which appeared in Sound Practices in 1994.
It is not technical, and it is two decades old, but it specifically addresses diode Qrr noise from an audiophile and hobbyist perspective:
Development of a 211 Amplifier, Part 3: Reducing Diode Noise
by John Camille (Chimera Labs)
Sound Practices (1994, Fall)
Done properly, silicon diode supplies can be built that are quieter than untreated vacuum diode designs. One must remember that vacuum diodes also generate a significant amount of white noise that should be corralled in better designs.
Noise reduction for either vacuum or silicon diode rectifiers is a worthwhile undertaking. When silicon diode noise is controlled, the reliability factor and the virtually limitless lifespan of solid state diodes in properly designed supplies points toward the choice of silicon rectifier devices over vacuum tubes.
The primary culprit responsible for the noise generated by good quality silicon junction diodes is the turn-off characteristic. A reverse pulse is generated by the minority carriers crossing the junction after the majority carriers have galloped through. Tremendous strides have been made recently in reducing this effect in diodes designed for use in switched mode power supplies (SMPS). The processes used to create these fast turn-off devices avoid many of the noise and oscillation problems of older diodes.
However, reverse recovery pulses still exist. The energy distribution as a function of time varies with each device but the general trend is down at a rapid rate, as semiconductor designers seek to meet requirements for more efficient SMPS designs.
I have been doing empirical work with the simple-minded idea that the very fast fall time pulse excites the LC resonant circuit presented by the secondary winding of the transformer. The excitation of the LC circuit produces a damped wave burst of RF energy centered on the resonant frequency of the transformer. I have measured the burst frequency fundamental on different transformers at frequencies between 6 kHz and 165 kHz. Of course, the oscillation frequency (f0) is transformer and installation specific.
What all this means to the experimenter is that there are one or more transmitters buried in your amplifier. These transmitters produce 120 harmonically-rich pulses each second with a fundamental frequency (Q for each diode rectified supply. These pulses are radiated and conducted to other parts of the amplifier where they are detected and amplified along with the desired signal. Those beautiful wiring harnesses of old are real sonic killers for this reason. What you get is "diode grunge" that rides on the audio signal.
A more insidious problem is that these diode created bursts are also coupled back into the AC mains where they can affect unprotected low level stages elsewhere in the system.
...
Afterwards, I rationalized the cure, thinking that Schottky diodes have relatively few minority carriers, thus they provide little kick to the LC resonant circuit formed by the secondary winding. Since then, I have routinely replaced all of the pn diodes with Schottky diodes when I rebuild and recalibrate instruments for my shop.
I discovered the same pn burst problem during early work on the 211 amplifier. The attitude at the time was, "If I could see an artifact on the scope, it would be audible". In went the Schottkys on all low voltage and bias supplies. The high voltage supply for the 211 was another problem, however. A suitable bridge for the 1400 V power supply would require around a hundred 90V devices in the stackl Enter brute force techniques ...
...
The transformer-rectifier-filter interface must be short and sweet! I am wary of leads (antennas) over one inch long. My "new construction" supplies are fully shielded per a future article in SP. VHF RF construction techniques will make even the quietest amplifier quieter and sweeter! If you must bundle wires, use triaxial coax with proper grounding techniques. Think RF!
Johnson's paper, "
Simple, No-Math Transformer Snubber using “Quasimodo” Test Jig" sets forth a jig to find the ringing frequency instead of using calculations. His abstract contrasts his experimental approach to the traditional one (emphasis added):
Designing a snubber for a power supply transformer is a lengthy process, requiring both measurements and calculations. Usually a sinewave generator is connected to the transformer secondary, with primary shorted. The sinewave frequency is swept, in search of an impedance peak. Next, a known-value capacitor is connected across the secondary, and a second frequency sweep obtains a second impedance peak at a second, lower, frequency. The two measured peak-impedance frequencies provide two equations in two unknowns (inductance and capacitance), which are algebraically solved. Finally the extracted inductance and capacitance are inserted into the damping equation for this second-order RLC circuit, and a snubber resistance RS is calculated which provides the desired damping factor. It is assumed that transformer inductance is approximately constant across frequency, which is convenient for the math but unfortunately not true in real transformers.
This note presents another way to design a snubber, which requires no calculations, no assumptions, and only a single measurement. Rather than measuring the transformer parameters and then calculating the snubber, this procedure places an actual snubber across the transformer, observes transformer ringing directly, and adjusts the snubber until ringing is damped into non-existence.
The only thing I am not sure about is, is this nitpicking, which to me is normal trying to approach an ideal situation, or is it a real problem?
Sigh. Instead of pondering and water-cooler discussions, I would instead refer you to the published work I've cited which sets forth the physics, electrical engineering, and solutions. Draw your own conclusion based upon engineering analysis.
