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Engineering and Design, page 2

With the introduction of the internet, forums were established that allowed just about anyone to pass along information from anywhere. Most of the time we know little if anything about the poster or reviewer. How a poster or reviewer's information was gathered, processed, and the resulting conclusion is sometimes rather interesting. Usually it consists simply of, "we stuck in a part and the sound was better," in a component, so no accurate or stringent listening tests were performed.

For instance, when comparing vacuum tube "T" to vacuum tube "U", is each tube operated under its optimum conditions? What design is being used? Is the design actually accurate in order to give an accurate accounting of "T" and "U"? If the design is bright, then a "dark" sounding tube will be ranked most accurate. Rankings of tubes and other parts on the internet are often suspect at best. What about capacitors?

Consider 0.47uf capacitor "A" being touted as sounding better than 0.47uf capacitor "B". One needs to ask:

Is the value 0.47uf the correct size to give an accurate representation of the music? Actually no. Almost all designs use insufficient size coupling capacitors for an accurate assessment. As a result, the darker, more inaccurate capacitor "A" is chosen, when in actuality "B" was the more accurate capacitor. Why is this important? Because the bass may have been increased, but the midrange and highs are negatively affected. If the proper value was used, "B" would have been chosen over "A". Not only would the bass have been sufficient, but the midrange and highs would have been more accurate as well. However, since "A" was chosen by improper testing, "A" is on the market while "B" became extinct, and sound quality suffers.

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The Proper Size Power Transformer for Preamplifier Class A Operation.

What size power transformer is optimum for pure class A operation, say in a preamplifier? First, Class A means the power supply voltage remains constant, average current constant as well. The only requirement of a power transformer is staying cool. There is an optimum size, so too small or too large is actually detrimental due to heat of cost (which you pay). A large transformer may make for "good" marketing and profit, but I am about the sound.

If, for arguements sake, a larger transformer (heavy) is better for "regulation"; then using a tube rectifier with its high resistance actually defeats the very purpose of using a large transformer in the first place.

According to the RCA Tube Manual, a 5AR4 rectifier tube has 160/ 200 ohms impedance per plate at 450 volts/225ma and 500 volts/160ma, respectively. At currents in the low milliamps, such as preamplifiers, the tube's plate resistance is much higher. Even in power amplifiers, the high resistance defeats the very purpose of the large power transformer.

Amplifiers operated in class AB or B operation is another matter entirely. These classes of operation causes the average current to vary wildly, thus large power supply voltage changes occur unless a large, low resistance power transformer is incorporated for better voltage regulation. Unfortunately as mentioned above, rectifier tubes have high resistance which negates the large power transformers regulation ability. Swing chokes are often added to help regulate the voltage, but they add reactance and have their own problems. (See more concerning chokes below.)

As one can see, there are many considerations when choosing a power transformer for a preamplifier. A large power transformer means nothing, or is simply an expensive patch for other problems in the particular design the manufacturer used.

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Rectifier Tube vs Solid State Rectifier

This is an interesting subject I had to deal with in my own designs. Do solid state (SS) rectifiers cause sonic problems? Proponents claim that solid state diodes produce noise while tube rectifiers soften the sound. Well, I can measure down to microvolts and have yet to measure any noise voltage from solid state diodes. However, tube rectifiers do cause the plate voltage to vary, especially when bass is produced.

Secondly, the proof is in the pudding. Virtually no one has yet been able to tell if my 11A is in the system or out, so the preamplifier is virtually perfect. No review, by magazine or customer has ever mentioned any problems caused by the rectifiers I use. So the real question is why are tube rectifier proponents having sonic problems? An improperly designed power supply is one answer. Doing a few equations or using a computer program does not tell the whole story as the models are not complete.

Now I do avoid solid state transistors/FETs in the direct signal path because they create their own problems and of course are not isolated from the signal circuitry. If regulators, constant current sources, or Mu circuits are used, tubes are a must since they are in the direct signal path. Using a tube rectifier with a Solid State regulator, current source, or top device in a Mu circuit is irrational. Using a regulator in place of the decoupling capacitor places the regulator in the direct signal path. (Do a thevenin equivalent circuit for proof.)

I want what sounds the best, not what is popular, or expensive, or is "good" marketing strategy.

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Chokes

Chokes create sonic problems. Some claim that a choke is superior to a resistor. Some simply claim that chokes are solid engineering and have always been used over the years.

Unfortunately, all is not that simple.

  1. Good resistors can have virtually no "sound" of their own.

  2. Chokes cause eliptical loadlines, wherever they are placed in the signal circuitry.

  3. A choke adds its own distortions and reactance, thus artificially flavoring the sound.

  4. Chokes used between the "decoupling" capacitor and previous capacitor creates problems as the reactance of the choke varies with frequency. So at 20khz, a 5hy choke has 600,000 ohms impedance separating the two capacitors. At 20hz however, only 600 ohms separates the two capacitors. Though simplistic, it is as if the "decoupling" capacitor is actually changing value with frequency.

How can a component sound accurate with such compromises?

It is true chokes have been used for decades, but for reasons quite different than most believe and post.

  1. Not in any particular order, because they limited surge current through rectifier tubes when the circuit was powered on. There are whole sections in the RCA Radiotron Designers Handbook and other engineering books covering this very subject.

  2. The power transformer could be physically smaller using chokes, less peak current, thus saving money.

  3. Filter capacitors were of limited quality, so a choke was a way of reducing the number and physical size of filter capacitors, thus improve reliability.

  4. Since filter capacitors were also of limited physical size and uf, reducing hum was difficult. Chokes were a reliable way of helping to reduced hum at the speaker.

So while chokes performed some basic functions, improving sonic quality is another matter. Here are some other basic principles.

  1. A choke's magnetic field does not fluctuate instantaneously, but takes time which is dictated mainly by the core material. It is a form of electrical inertia, thus adding distortion.
    Resistors, on the other hand, have virtually no such field to create distortion, and the current fluctuates instantaneously.

  2. A choke does not change total reactance in a linear manner vs frequency because its DC resistance affects the total reactance at low frequencies more than at high frequencies.
    For example a 10 henry choke's reactance at 20hz is approximately 1250 ohms (plus the DC wire resistance) and rises to 125,000 ohms at 2khz, and 1.25 million ohms reactance at 20khz. A capacitor's reactance, on the other hand, is near zero ohms at high frequencies and rises to only a few dozen ohms at 20hz. And a decoupling capacitor does not perfectly isolate the choke from the signal circuit, especially at low frequencies, where the choke is most nonlinear. So the power supply has a changing reactive component vs frequency.

  3. A choke has lower and high frequency limits. A non-inductive resistor is virtually, totally linear from DC well into the hundreds of kilohertz (khz).

  4. A choke has self resonant and ringing issues. At what frequencies depends on the core material, size, and winding technique, associated capacitance etc.

  5. Chokes "pick up" stray magnetic and electrical fields, so proper placement and shielding is required to minimize the problem.

  6. When used in low current or small signal situations, like grid chokes of small tubes, the hysteresis curve is the most non-linear, creating the most distortion. The choke is directly in the signal path.


  7. When large signals are present, such as driving the grid of output tubes or the plate load of a tube, core satuation, increased harmonic distortion, eliptical loadline problems becomes an issue. Distortion rises as the frequency is lowered. High distortion, low value coupling capacitors may lead to "False bass".

From the Radiotron Designers Handbook.

"(A) When two or more input frequencies are applied to a non-linear amplifier the output will include the sum and difference frequencies located about each of the higher input frequencies. For example with input frequencies of 50 and 150 c/s, the output will include frequencies of 50, 100, 150, and 200 c/s. Even if the lowest frequency is very much attenuated by the amplifier, the sum and difference frequencies tend to create the acoustical impression of bass. With more than two input frequencies the effect is even greater, so that fairly high distortion has the effect of apparently accentuating the bass."

"(B) Owing to the peculiar properties of the ear, a single tone with harmonics may be amplified, the fundamental frequency may be completely suppressed, and yet the listener hears the missing fundamental."

"These two effects assist in producing "synthetic bass" when the natural bass is weak or entirely lacking. It should be emphasized that this is not the same as true bass, and does not consitute fidelity."

This is one reason that even 2nd harmonic distortion is objectionable. (And intermodulation distortion is approximately 3 times that of harmonic distortion.) This is exaggerated as the music becomes more complex.

So we see that chokes are anything but benign while adding to the cost. One actually pays more for inferior sound, but more "iron" makes for "good" marketing.


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Tubes

Well, back to vacuum tubes. A design consists of many many more parts than tubes and as such influences the sound more. As such it makes sense to use the most accurate parts, and then choose the best type tube. If we wish to use one stage with a wide bandwidth, we need a tube with high Gm and low output impedance (Z).

It is interesting that in all areas, such as medical, mechanical engineering etc, parts, materials have flaws. Nothing is perfect by any means. Yet some preach that electronic parts are magically perfect, or at least so accurate our ears cannot detect the flaws. Yes, maybe from a harmonic or IM distortion point of view, but there are other flaws and distortion producers, such as ESR, DA. Again we find insufficient computer models when it comes to parts. It also demonstrates the lack of basic research performed by some. It might be worth mentioning that college courses/text books only give the tools to research deeper. Text books and/or classes were never an all inclusive, totally in depth knowledge base. Consider a Master's or Doctor's degree. Consider large corporations that have in depth research departments.

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Distortion and Frequency Response

I like distortion to be as low as possible and the bandwidth as wide as possible. For instance, if a preamplifier is -1db at 20hz, there will be a problem. At 40hz, -0,4db, at 80hz, -0,15db. Too much bass creates masking problems, masking distortion since it changes one's perception.

If the highs are off -0,5db at 20khz, -0,2db at 10khz, -.07db at 5khz. The specs can be good while the perceived perception is either good or questionable. However, if the specs are poor, the music will never be optimum. Oh it can sound good, but not optimum. The music should sound natural.

Well that is it for now. Just because a part or component is expensive, large, and/or weighty does not mean it is better. Just because one uses a computer program or mathematical equations (does look intelligent though) does not mean it is accurate, or distortion free.

In conclusion, I think one gets a glimpse how complex it is to produce a component.

These articles are written as a public service.

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SAS Audio Labs

Steve Sammet
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Morton, Illinois USA 61550
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