Hardwiring, Layout Problems | Solutions To These Problems | Interstage Signal Feedback Through the Power Supply | Resistor comparisons
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Thank you for checking out this paper on power supplies and capacitors. At the outset, I MUST mention that this article is a compromise between being technical and being easily understood. This involves clarity, simplicity, with the mathematics left out. (Engineers will know the mathematics and accurate formula and be able to more precisely fill in the blanks, so to speak.) Since the audience varies greatly and most have not majored in Electronics Engineering, I determined not to make this article overly demanding. I have therefore taken the necessary liberties to write as such. I simply want the general audience to get the overall concept that in the power supply, types and quality do make a difference. By the way, this subject was partially covered some 55 years ago in the RCA Radiotron Designers Handbook. So the problems discussed here are not necessarily new by any means.
Surfing the internet, I have seen many articles concerning audio designs, tube and transistor. However, the power supply is one of the few regions not intensively investigated, understood, or at least written about. It is also a weak link in any audio component. There are several areas to consider when designing a power supply.
There are plenty of articles addressing the first two points. However, I have yet to see an article that addresses the third point in any substantial way. In fact, many assume that a simple "de-coupling" capacitor addresses the problem. It does not, as we shall see.
We understand that a vacuum, or any active device, has resistance, capacitance, requires a DC voltage etc. The power supply "filter" decoupling capacitors, corresponding to each signal stage, is in the "signal path". One of its purpose is to provide a ground, signal wise. This can be easily demonstrated by the figures and explanations to follow.
Let's take a look at a typical audio circuit (fig.1) and its' A.C. "equivalent circuit" (fig.2) if the capacitors were perfect.
Now notice that in (Fig. 2), capacitors C1 and C2 (perfect capacitors) are missing; with a line drawn to ground for C1, and straight wire for C2. This has several interesting implications. It implies that the capacitors are of infinitely large value (infinite ufd.), has zero capacitive reactance, zero internal resistance, and zero inductive reactance. In other words, the top of R1 is perfectly grounded at all AC musical signal frequencies. The Thevenin model of fig. 1,2 appears below.
Notice C1 has a line through it. C2 is also a line. The only purpose of showing C1 is to demonstrate where it would be in the thevenin circuit since no capacitor is perfect. Not by a long shot.
Now let's get back to the real world, in which C1 is not zero impedance. How is the circuit influenced by C1 at different frequencies, in otherwards music? Let's suppose we use a value of 100uf for the value of C1. Fig. 4, the equivalent circuit shows this value of 100uf and its impedance/capacitive reactance at different frequencies.
Notice the impedance/capacitive reactance of C1 changes with frequency (this is simplified, as reactance is not resistance). This means the real value of the Plate Load is not simply R1, 22k ohms, but is 22k ohms "Plus" the impedance of C1 (complex mathematics involved in order to add them, not simply R1 + C1 impedance) at any given frequency, and we need to include phase shifts. This means that the stage gain and phase varies with frequency.
This would require that in order to obtain the widest frequency response and in order to minimize gain and phase changes, we should make the value of C1 as large as possible. But this poses a problem in the real world. NO capacitor is perfect, as *ESR, *DA, and inductance/self resonance are always present to some extent. Capacitor types, such as electrolytic and tantalums are by far the worst in terms of ESR and DA, which can approach 8%, with self resonance and inductance raising its head as low as 2 khzs. (See "Picking Capacitors" by Walter Jung and Richard Marsh.) This may be modified by current high frequency capacitors.
Polypropylene capacitors have a DA of only approximately .02%. But even different polypropylene capacitors have different internal inductance and varying ESR, depending upon dimensions, and whether foil or metalized. Thus physical size, conductor thickness, terminal techniques, can and will make a sonic difference. For instance we were able to listen test several brand polypropylene capacitors.
The power supply should address at least these points.
This is a tough challenge; the greater the number of stages the tougher the challenge.
So what does all this mean? I hope you see how much of a problem designing just the power supply can be, let alone the rest of the circuit design. It takes superior quality parts, superior circuit design, even the layout when dealing with power supplies. Even then a common power supply for all stages is nearly impossible to design correctly and work properly. Using CCS (tube or SS) introduces its own problems. Of course the problem of frequency dependent feedback through the power supply is solved if one uses separate power supplies for each stage. However, virtually no manufacturer does.
As mentioned in my opening comments, this article was meant for the general audience, in simple language.
* DF stands for dissapation factor and DA stands for dielectric absorption. Extremely simplified, DF is the reactance of the foils, terminations, and leads. DA is dependent on two molecular level properties: the permanent "dipole moment" and the "polarizability" or the induced change in dipole moment due to the presence of an electric field.
Also see "Picking Capacitors" by Walter Jung, Audio Magazine, Feburary, March 1980, for more details and explanation.