Hardwiring, Layout Problems | Solutions To These Problems |
Power Supplies and Capacitors | Interstage Signal Feedback Through the Power Supply | Types of Distortions | Resistor comparisons
Home Page | Reviews |
Tube Preamplifiers | Tube Amplifiers |
DIY Parts/Price list | Warranty |
Specials | Contact, Directions | Audio Links | NEW: Forum
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 covered some 50 years ago in the RCA Radiotron Designers Handbook. So the problems discussed here are not 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. Why? Because the power supply, especially the "filter" capacitors (decoupling capacitors) corresponding to each signal stage, is in the "signal path". One of its purpose is to provide a perfect ground, signal wise. This can be easily demonstrated by the figures and explanations to follow.
Let's take a look at a typical perfect audio circuit (fig.1) and its' A.C. (Audio Component) "equivalent circuit" (fig.2).
.......
Let's set the stage.
Now notice that in a PERFECT circuit (Fig. 2), the capacitors, C1 and C2 (perfect capacitors) , are missing; with a line drawn to ground for C1. The impedance of both Perfect capacitors is zero ohms. This has several interesting implications. It implies that the capacitors are of infinitely large value (infinite ufd.), has zero inductance and zero internal resistance. It also implies that the *DF and *DA (dissapation factor and dielectric absorption) are zero. In other words, the top of R1 is perfectly grounded, signal wise. The Thevenin model of fig. 1,2 appears below.
Notice C1 has a line through it. Being perfect, C1 (and the rest of the power supply in parallel to C1) is not there signal wise. The only purpose of showing it is to demonstrate where it would be in the thevenin circuit.
Now let's get back to the real world. In the real world, C1 is not infinitely large. 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' reactance (XR1' in table 1) at different frequencies.
.......
Notice the impedance 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 (XR1) (complex mathematics involved in order to add them, not simply R1 + XR1) at any given frequency, with phase shifts. (Remember, to simplify our discussion the arrow is infinite impedance.) This means that the stage gain and phase varies with frequency.
This would imply that we should make the value of C1 as large as possible, in order to minimize gain and phase changes. But this poses a problem in the real world. NO capacitor is perfect, as *DF and *DA, internal resistance, inductance are always present to some extent. Different brands and types, such as electrolytic and tantalum capacitors are by far the worst in terms of DA and DF, and can approach 8% and become resonant, and turn into inductors as low as a few khzs.
Polypropylene capacitors have a DA of only approximately .02%. But even polypropylenes have internal inductance and, depending whether foil or metalized, fairly high internal resistance. Thus construction, terminal techniques, and even wire size can make a sonic difference. Polypropylenes also have the problem of large physical size.
So what does this mean? Incorporating bad capacitors, with their high DA and DF percentages, means lack of true inner detail, the grunge and noise floor elevate, and smearing of the sound. The larger the value, the more problems with the capacitor itself; thus inferior musical reproduction. Superior parts are definitely needed in the power supply.
The power supply should address at least three points.
This is a tough challenge and nearly impossible to accomplish. The more the number of stages the tougher the challenge.
I hope you see how much of a problem designing just the power supply can be, let alone the rest of the circuit design. In fact a common power supply for all stages is nearly impossible to design correctly and work properly. Of course the problem of frequency dependent feedback through the power supply is solved if one uses separate power supplies for each stage.
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 the ability of the insulating material to give up electrons or for the molecular structure to relax.
See "Picking Capacitors" by Walter Jung, Audio Magazine, Feburary, March 1980, for much more details and explanation.