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The internet, for the first time, allows a free flow of information between individuals and companies from all parts of the globe. This not only allows for increased learning, but also for alot of misleading and false information to be spread as gospel truth. This white paper will show some of the audio hype that is currently being spread as truth.

On some forums, I have seen alot of misinformation concerning how low a load a tube preamp can drive. The "load" I refer to is Rg, or the grid resistor for the following stage.

I have been hearing 50 to 100 times the output impedance of a tube preamp is necessary. Of course they also recommend a buffer output stage if the input Z of an amplifier is 20k-50k or so. But adding another stage compromises the fidelity. Is a buffer stage necessary? What are the criteria?

1. Any descent preamp design will drive loads lower than 50 or 100 times the output impedance (Z) of a preamp. Look at almost any 12AX7A circuit. It's output impedance is seldom lower than 30k and almost always higher. According to somes viewpoint (with 30k output Z) the load needed would require a high value of 1.5 megohms to 3 megohms. Obviously, this never occurs in high end audio.
   
   RCA Radiotron Designers Handbook, 1960, written by 26 engineers, page 482 states: "The output grid resistor (grid resistor of following stage Rg, sometimes called the load resistance of the previous stage).... and there is no appreciable advantage in either gain or distortion through the use of a resistance greater than 4 RL (RL = plate resistor value)". I recommend a minimum of 15 times the output Z, just to be on the safe side.
   
   Since the first stage of an amp driving the driver tube, and/or the 2nd stage driving the output tube almost never sees a ratio of 50 to 100; why should the output of the preamp be any different?



2. The factors affecting the low frequency Response on the output of a preamp are the output coupling capacitor and load resistance. Too low a value of load resistance, or too small of output coupling capacitor will lessen the bass. This has virtually nothing to do with the actual output Z of the tube.



3. The factors affecting the high frequency Response on the output of a preamp are the interconnect cable capacitance and input capacitance of the amplifier. So again, the value of Rg is virtually out of the equation.



4. Distortion can be affected by the ratio of load resistance (Rg) divided by the output impedance of the preamp. But as cited earlier from the RCA Designers Handbook, virtually no distortion changes occur until the ratio of Rg to RL drops to approx 4. Another way to put it is Rg is 4 times RL. I recommend 15 times to be safe. But this is a far cry from 50 to 100 times others propagate at truth.
   
   Another question is how does lowing Rg affect the harmonic distortion? Lowering Rg increases the lower, even order distortion products, mainly 2nd and some 4th orders. Odd orders like 3rd and 5th are actually decreased. The result is that the preamp would actually sound slightly fuller and smoother, not harsher.

I hope this helps in ones understanding and ones filtering out misinformation concerning this subject.

No Gainstage Preamplifier Needed?

One of the relatively new forms of marketing strategy is that "X" brand amplifier doesn't need an active gainstage preamp, thus 'one can get rid of an unnecessary gainstage' or 'one can use a higher sonic quality passive preamplifier instead of a gainstage preamplifier' (the inference is that one has again been saved a gainstage). In order to fully understand what is happening, one needs to understand the history of audio.

Back in the 60's and even later, receivers were popular, meaning a tuner, preamp, (basic) amplifier, and whatever gadgets, such as a cassette deck or reel to reel tape machine, were enclosed in a single housing. However, separates were available and increasing in popularity. These separates, enclosed in their own separate housing, included tuners, gainstage preamplifiers, and (basic) amplifiers. Integrated amplifiers were also available, which meant the gainstage preamplifier and (basic) amplifier were integrated into one housing. In fact, if one checks in the Library of Congress, or even Stereophile magazine (I have about 8 issues from 2001), the definition of an "integrated amp" is explained as a (basic) amplifier and preamplifier combined.

In any audio system, a certain amount of voltage gain is necessary to increase the signal source voltage in order to drive the output stage of an amplifier to full power. Thus, if a CD player's output signal is 2 volts RMS, and we need 30 volts RMS to drive the output stage of our amplifier, the necessary total gain is approx 23.5db. There is no way around the fact that the signal from the source must be increased.

Now back to our discussion. So how can an amplifier have 23.5db of gain when it doesn't need a "gainstage preamplifier" stage? The answer is the preamplifier gainstage has been incorporated in the "amplifier" itself. But hold on. I thought that was what an integrated amplifier was, a preamplifier and amplifier put together? And you would be right. So what happened? Actually a couple of definition changes took place.

1. First, the "integrated amplifier" is now called simply an "amplifier", smearing the difference between a (basic) amplifier and integrated amplifier.
2. Secondly, the preamplifier gainstage incorporated into the "amplifier" is simply called a simple gainstage, or even amplifier gainstage. "Preamplifier" is left off the name.

So now the public has seen an example of marketing strategy; that one can save a gainstage, when actually the preamplifier gainstage has simply been added to the (basic) amplifier.

Of course this looks good to the consumer, saving a gainstage, and is no doubt intended to increase sales for the company. Some companies have accepted this strategy and are now spreading it as audio advancement. But it is simply a marketing ploy.

In conclusion, one has to be very careful what one reads, as there is alot of hype and false information out there

These articles are written as a public service.

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