wjm@whuxk.UUCP (12/02/83)
John Wray's comments about tests showing that people prefering the sound of tube amplifiers in double-blind tests are interesting. I've heard about similar tests and suspect that the difference is in the nature of the harmonics created by the active devices. Two amplifiers can have the same TOTAL harmonic distortion (say .01%) but one may be preferred by most people due to the specific harmonics contributing to the distortion. Psychological research seems to indicate that people find even-order harmonics sound "mellow" and less offensive than the "grating" sound of odd-order ones. Bipolar transistors tend to have greater third-order distortion while tubes (and their solid-state kin, FET's) tend to have more second-order distortion. In addition, most tube and FET amplifier circuits tend to clip more gradually when the input signal sends the amplifier stage to the power supply reference while bipolar circuits tend to clip sharply, with a rapid rise in distortion. Since bipolar transistors are subject to thermal runaway (the device resistance DECREASES with increasing temperature, causing the heat producing current to rise with temperature, making the device hotter and hotter until it self- destructs), elaborate protection circuits are required and these circuits can cause distortion of the audio signal. On the other hand, tubes and FET's are self-limiting since their resistance INCREASES with temperature, so these protection circuits are not required. Also, bipolar transistors are usually subject to crossover distortion when the signal voltage goes from positive to negative or vice versa (which happens 40,000 times a second at 20KHz) since a bipolar transistor will not start to conduct until the base-emitter voltage reaches 0.7 volts (polarity depends upon whether the transistor is NPN or PNP - also this value is for silicon devices - it is 0.2 volts for germanium ones). Clever bias circuits have been developed to minimize this distortion but the only way to totally eliminate it is to: 1. Use true Class A bias - in which the transistor conducts 100% of the time. This causes the device to get quite WARM, so Class A amplifiers are quite large and quite expensive. Also, this bias current has a rather significant impact on your electric bill. 2. Don't use bipolar transistors as the active elements. Until 1980 or so, the only other option was to use tubes. Unfortunately, tubes have their own drawbacks - they require power for their heaters, they are physically large, they have to be replaced periodically, and tube amplifiers generally have to use output transformers - that cause tube amplifiers to be expensive and power hogs. Fortunately, it is now possible to obtain power FET's (field-effect transistors) which provide a reasonable solution to this problem, since they operated on the same principle as vacuum tubes (using an electric field on a control element to control the power-handling output circuit) they have the same general characteristics and advantages. However, they have many advantages over tubes, since they are solid-state devices. They are long-lived, do not require heater power, compact, and unlike tubes come in both polarities (p-channel and n-channel) so output-transformerless circuits can be used. Personally, I've been quite pleased with the sound of my FET power amp (a Hafler 220) and have heard generally favorable comments about many other FET designs. Bill Mitchell Bell Laboratories, Whippany NJ whuxk!wjm
emrath@uiuccsb.UUCP (12/07/83)
#R:whuxk:-34200:uiuccsb:5700016:000:502 uiuccsb!emrath Dec 6 17:54:00 1983 I don't buy that argument that you shouldn't use bipolar transistors because of crossover distortion (unless you run them class A). Tubes and FETs are usually used in a class A-B configuration for power amps, too, and they will suffer from the same kind of distortion as a result of the configuration (maybe not to the same degree). A good class A-B with 50-100mA of bias on the output stage suits my purposes, but I wouldn't use tubes because of the output xformer and unless it doubled as a toaster.