Distortion

The monolithic view of distortion, namely clipping, is false. There are many more forms of distortion than merely clipping, although clipping is quite important, if not the most important. Typically, amplifier designers, faced with severe cost restraints only include one or two of the following list of distortions. However, Pritchard Amps uses all of them.

CLIPPING

Clipping in preamplifiers occurs when the signal approaches the power supply voltages which may be a voltage and ground. The nature of the clipping is determined by the non-linear character of the components: triodes, transistors, op amps, feedback, etc. Some distortion "emulators" use various types of diodes to limit the voltages. The master volume control view of amplifiers makes the triode distortion paramount. Triodes have a unique output (plate) resistance that becomes larger and larger as the plate voltage rises and the plate current drops. Triodes also have a unique link between grid conduction and plate voltage. The plate voltage becomes a reflection of the grid voltage, thus high impedance sources produce well-rounded saturation waveforms. These phenomenons create the vaunted "soft-clip" of vacuum triodes such as the 12AX7.

Pritchard has captured these phenomenons in his circuitry. This circuitry type is disclosed in Pritchard's U.S. Patent 5,434,536 and related foreign patents.

XGPA Triode	12AX7	JFET	Pentode	Bipolar Transister

BIAS SHIFT WITH CLIPPING

Clipping with bias shifting is a well-known concept. In mild forms it provides a pleasant changing array of harmonics as the bias shifts during clipping distortion. In extreme forms it can cut amplifiers off for unpleasantly noticeable periods. In less extreme forms it can create substantial even harmonics that can produce output transformer saturation and reduce output power in spite of being generally thought to be beneficial.

One form of distortion that Pritchard did not emulate is the bias shift induced crossover distortion found in vacuum tubes. The reason for this omission is simple; it produces unpleasant high-order harmonics.

HARMONIC GENERATION NON-LINEAR CHARACTERISTICS

Pritchard is the first inventor in this patent niche to recognize the importance of harmonic generation without clipping. It makes clean tones warm and full-bodied. It also seqways gracefully into clipping making the amp distort slower. Non-linear characteristics in vacuum triodes and pentodes are particularly important to the operation of instrument amplifiers, particularly guitar amplifiers. The non-linear characteristics produce both harmonics and intermodulation distortion. Harmonics are produced when the non-linearity operates on a single frequency. Intermodulation distortions are produced when the non-linearity operates on two or more frequencies. They differ in their relationships to the original signals. The harmonics are two, three, four,... times the fundamental frequency. The intermodulation distortion components are frequencies at the sum or difference frequencies of their original signals and their harmonics. Thus, the harmonics are related to the original signal, the intermodulation distortions are not. The low order harmonics up through the sixth are musical. About 80% of the harmonics above the sixth are discordant, not musical, and offensive. Intermodulation distortions may or may not be pleasant depending upon the nature of the input signal. In instrument amplifiers that deal with a single source, the concern is not so great as it would be for amplifying multiple signals.

The vacuum tube output stage has a unique form of intermodulation distortion. It is formed by the combination of the power supply ripple with the input signal and that is why an amplifier sounds differently on 60 Hertz power than on 50 Hertz power such as found in the United States and Europe respectively. Intermodulation distortion has been theorized as a source of amplifier "fat" by Pritchard. Techniques for implementing fat creation have been disclosed in some of Pritchard's patents: 5,761,316; 5,761,317; and 5,848,165 and their foreign counterparts in particular.

SCALE DISTORTION i.e. COMPRESSION

Scale distortion is created by gain changes within the amplifier. There are two sources, both in the output stage. The gain of vacuum pentodes drops with screen voltage to form one type of the well-known tube amplifier compression. So when the power supply voltage sags, so does the amplifier gain. This is particularly noticeable in amplifiers that do not use output stage feedback. Please note that feedback Presence controls reduce the output stage feedback at mid and high frequencies.

The other type of scale distortion that is not so well-known is derived from the non-linear gain characteristic of vacuum pentodes, particularly noticeable in Class AB amplifiers. Even though balanced push-pull amplifiers suppress even harmonics, Class AB amplifiers translate the even harmonic production into significant odd harmonic production. Since the gain of vacuum pentodes goes up with increased plate current, the result is a previously unrecognized character, the expansive third harmonic. It has the opposite effect of the clip-induced compressive third harmonic and adds life to the amplifier. Disclosures to this character can be found in many of Pritchard's patents.

FREQUENCY DISTORTIONS

Frequency distortions are quite familiar; tone controls are their primary source. However, amplifiers can suffer a reduction of bandwidth as they distort. Basically, in distortion the negative feedback becomes ineffective and so does its bandwidth widening characteristic.

The history of the Pritchard Amps is marked by exaggerating the translation of vacuum tube characteristics to Pritchard's patented circuits. Initially these exaggerations point out the nature of the effect. Later, the level of these effects were altered to be above and beyond - to create the greatest amplifiers ever.