by LARRY KLEIN
----Technical Director Klein at the employees' entrance of the AKG plant
in Vienna.
Power vs. Volume
Q. I recently bought a new turntable and a very expensive phono cartridge. Every thing works fine except that the volume control on my preamplifier must be turned up much higher for the same level of sound. Did I lose amplifier power as a result of mismatching, and, if so, how can I adjust for it?
GEORGE GARLIND Brooklyn, N.Y.
A. This question, in various permutations, . is a common one-and, unfortunately, one that is difficult to answer simply. First of all, it is necessary to understand that the volume-control setting on an amplifier, pre amplifier, or receiver does not necessarily have a one-to-one relationship with either the amount of sound coming out of the speakers or the amount of power that the amplifier is supplying to them. Think of the volume control simply as a valve controlling the amount of signal passing through its particular section(s) of the amplifier. In other words, a volume control can only reduce the level of the signal going through it; a volume control that is turned full up is, in effect, removed from the signal path. Since your new phono cartridge supplies less signal to your preamp than was provided by your previous cartridge, the volume control must be set higher to achieve the same sound level. This is completely nor mal, and the same thing could happen if you were to switch to, say, a new tuner or tape deck and their output-signal levels were lower than those of your previous units. And if you were to switch to lower-efficiency speakers or a power amplifier with lower input sensitivity, your preamplifier's volume control would in those cases also have to be set higher to get the same audible level.
As long as the signal level supplied by a phono cartridge or other program source is never so high that the amplifier being used is overdriven (overloaded), then the volume-control setting has nothing to do with the power rating. For example, suppose you have two power amplifiers, one of which is rated at 40 watts per channel and the other at 100 watts per channel. And suppose further that the 40-watt amplifier can be driven to its full output with a 0.25-volt input signal whereas the 100-watt unit needs about 0.5 volt from the preamp to reach the same 40-watt level.
(A still greater input signal is, of course, need ed for it to reach the 100-watt level.) What will happen is that the 40-watt amplifier will play louder than the 100-watt one given the same normal preamp volume-control setting.
I suspect that a good part of the confusion between power and gain in hi-fi results from an improper analogy with sports-car performance. A car's ability to achieve a high speed in a short time is a pretty good indication that there's lots of power under its hood. Unfortunately, neophyte audiophiles often conclude (erroneously) that components whose volume controls are designed to "come on fast" are somehow providing more power than those units with a slower, normal control taper. As long as the volume control does not have to be set so high that amplifier noise becomes audible or so low that it becomes "touchy," you can safely assume that everything is operating at the right signal level and that full output from your amplifier-whatever its wattage may be-will be available when it is required by the music.
Carbon-fiber Components
Q. As a tennis buff, I've gotten used to seeing the buzz words "carbon fiber" and "graphite" in advertisements for high-price rackets. Now carbon fiber seems to be crop ping up in audio products such as tone arms and speaker cones. What exactly is this stuff, and what does it do for audio performance?
IAN HOLCOMB; Georgetown, Md.
A. I learned there are those who also consider audio a "high-price racket," so the association of these two areas is not that remote. In any event, my experts tell me that there are several different types of car bon fiber, but the one used in both tennis rackets and tone arms actually consists of separate, very fine strands of pure crystalline carbon in an epoxy-resin binder. This fiber-and-resin "composite," as it is called, has several properties that make it very attractive to designers of both tennis rackets and tone arms. It is, among other things, very rigid, low in mass relative to its volume, inherently well damped, and nonresonant. When carbon fiber is used in tone arms, it is either applied to the metal shank of the arm as a sort of coating to strengthen it and improve damping or else it is an ingredient in the molded-epoxy tubular shank itself. In making speaker cones, the carbon fiber is added to the "slurry" (a liquid mixture of wood pulp, water, and heaven only knows what else) from which the cones are molded. As part of the cone material, the carbon fibers help stiffen the cone and damp out internal resonances and vibration transmission. Besides tone arms and speaker cones, carbon fiber is also used in the audio field in making phono-cartridge head shells. In each case, the advantages sought from its use derive from the material's low relative mass, high strength, and high internal damping.
Line Inputs
Q. When writing about equipment, Julian Hirsch and Craig Stark frequently refer to "line" inputs or outputs. What kind of line are they talking about?
CHARLES DEVIS; St. Paul, Minn.
A. The term "line" used in connection with hi-fi equipment (as opposed to that used to facilitate social encounters) was adopted from broadcast and recording-studio usage. In such commercial applications there are actually several commonly used "standard line levels" which indicate whether the in put and output circuits of various pieces of equipment can be interconnected without running into problems of noise, distortion, or inadequate signal strength. (The term "level" refers to both the signal level and the effective impedance.) The specific levels and impedances involved in professional use need not concern us here, since they are of no practical significance in home hi-fi equipment.
As it appears in current hi-fi literature including the present IHF amplifier standard-"line" is synonymous with "high-level," which in turn refers to the characteristics of any jack designed to provide (or accept) audio-signal voltages from about 0.1 volt up to several volts. The IHF amplifier standard defines "line input" as: "Any set of input terminals off an amplifier whose primary function is to accept line-level input signals, normally construed to mean input levels in excess of 25 millivolts." I think I was out of town during the Standards Committee meeting at which the 25-mV figure (which seems too low to me) was adopted, but the intention of the figure is clear: to distinguish high-level inputs from low-level phono or microphone inputs, both of which are designed to accept voltages with an average signal level of, say, 10 mV or so. In home hi-fi usage, there's no agreed-upon impedance for the line input, although one can assume that a jack labeled "line" (or, more frequently, "aux," which should have the same characteristics) will usually have an input impedance of 50,000 ohms or higher.
Note that when the input impedance of a power amplifier is 25,000 ohms or less it may cause some low-bass loss in preamps not de signed to feed that low an impedance.
Because the number of questions we receive each month is greater than we can reply to individually, only those letters selected for use in this column can be answered. Sorry!
NEW PRODUCTS: Roundup of the latest audio equipment and accessories
Source: Stereo Review (USA magazine)