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(source: Electronics World, Dec. 1963)
By RICHARD S. BURWEN / Consulting Electronics Engineer
Comments on "transistor sound," over-all performance, and reliability of transistors in hi-fi power amplifiers.
Editor's Note: Following are some additional comments on our article "Transistors for Hi-Fi: Panacea or Pandemonium?", which appeared in our September and October issues. Mr. Burwen specializes in audio and transistor circuitry and was responsible for the design of several high power transistorized hi-fi amplifiers and other audio equipment.
MY views on transistors for hi-fi are generally the same as those given by the authors of the original article. Here are some additional points not covered previously.
One factor that can make a transistor power amplifier sound cleaner than a tube amplifier is its superior clipping action under overload conditions. Some transistor amplifiers recover from an overload much faster than vacuum-tube amplifiers by virtue of d.c. coupling. This makes the distortion produced by overloading much less objectionable and may even be completely unnoticeable when the amplifier is occasionally overloaded as much as 5 db.
Another characteristic that can make a very noticeable difference in the listening quality of the amplifier is the damping factor. Fig. 6 in Part 1 of the original article showed that the acoustic response of the speaker system did not change much when the damping factor was changed from 4 to 1 to infinity. Had the impedance of the speaker system been much lower than rated value at certain frequencies, as is the case in some of the more efficient systems, there would have been a greater difference in the response curves. This difference can be as much as 3 or 4 db at certain frequencies and does produce a very noticeable difference in the sound quality.
With either vacuum tubes or transistors the amount of distortion produced depends primarily on the ingenuity of the circuit designer in using negative feedback. Both can produce very low distortion if enough feedback is used.
On a performance-per-dollar basis, transistors are still slightly behind tubes.
Silicon planar transistors, recently available at a low enough price, make practical preamplifiers which can provide noise figures of 1 to 2 db over the audio range without microphonics. They can even perform well with source impedances up to 500,000 ohms.
The power transistors that really solve the problems of hi-fi, silicon planar types, are too expensive, but prices are falling.
Transistors have been developed for military equipment, for example the "Minute Man" missile, which are so reliable that if 100,000 transistors were operated for 1000 hours, not more than one would fail. The same technology which produces such high reliability is being applied, although to a lesser degree, to the transistors now available for hi-fi equipment. Since they lack the basic wear-out mechanism of the tube, the heater cathode, it can be assumed that transistors will last many times longer in hi-fi equipment provided that they are properly applied.
It is true that transistors are very easily destroyed, especially when someone unfamiliar with transistor circuitry reaches into an amplifier with a test prod. Complete short-circuit protection for a d.c. coupled power stage is rather costly. One compromise solution which the author has used is to incorporate a fuse together with an output coupling capacitor which limits the low-frequency energy in the transistors at a slight sacrifice in low-frequency power output.
Whether it is tubes or transistors, it takes a lot of feedback and a lot of components to produce extremely high quality performance. In general, it takes three to five transistors to accomplish the job of a 12AX7, but there are some compensating savings on the power supply, size, and the amount of heat produced.
A power amplifier using silicon alloy power transistors has been built to deliver 25-watts power output. This amplifier uses d.c. coupling, has only millivolts of d.c. offset across the loudspeaker voice coil, recovers from overload instantly, incorporates complete overload protection for both short circuits and reactive loads, delivers full power with less than 0.05% total harmonic distortion up to 5 kc., and does all this over a wide temperature range with production transistors without matching and without adjusting their bias currents. With planar output transistors the same circuit will produce full power to 20 kc. with similar distortion characteristics. (Sorry, but we cannot supply the circuit and construction details.-Editor)
Thus, transistors right now can produce higher quality than has generally been available from tube amplifiers.
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