By Larry Klein, Technical Director
above: Klein using his calibrated finger to check the stylus compliance of an oversize plastic mock-up of the Shure V15 Type IV.
DC Amplifiers
Q. I'm a little puzzled over the use of the initials DC in connection with amplifiers. I have two questions: (1) Does it mean "direct coupled" or "direct current"? (2) What is its significance?
H. LANDSMAN; Forest Hills, N.Y.
A. answer to question (1), both; and to, (2), I'm not sure. Many years ago, when I worked as a test-instrument repair man, one of my specialties was DC oscilloscopes. The various amplifying stages of a DC scope are directly coupled to each other without the use of interstage d.c. (direct current) blocking capacitors. This type of oscilloscope has the advantage that very low frequencies (down to 0 Hz) can be handled, and here is no capacitor-caused phase shift fo distort the waveform.
The direct-coupled amplifier in the scope was also a direct-current amplifier; for example, connecting a flashlight cell to the scope's input terminals would produce a trace deflector on the oscilloscope screen that was directly proportional to the d.c. voltage applied.
With one of today's DC power amplifiers you could also connect a flashlight battery to its input jack and get a large d.c. voltage at the peaker terminals-if there were no d.c. locking capacitor at the amplifier input and/ or no designed-in low-frequency rolloff, and if you didn't blow a fuse or trigger the amplifier’s protective circuits (which are designed to deep d.c. from reaching the loudspeaker).
In the early days of transistor design most power-amplifier circuits included a very large capacitor between the output transistors and he speaker. Its purpose was to prevent the normal d.c. operating voltages present at the power-output stage from reaching the speaker. But although capacitors will block d.c. and ass a.c., they do not do the job perfectly. As he frequency of the, a.c. audio signal becomes lower (approaches d.c.), the capacitor begins to impede its passage (capacitors also cause phase shift, but we need not go into that ere). Therefore, in order to get very low audio frequencies through to the speaker with out loss, the output capacitor had to be very large electrically and physically, meaning expensive and space-consuming. These are both important practical considerations in amplifier design. So it is evident that there are good technical and economic reasons for eliminating the coupling capacitors at the output of the amplifier. As far as the practical-as contrasted to the theoretical-advantages of d.c. (or DC) design in the rest of the amplifier, I'm not sure what my position is. I do know that we don't want frequencies below 10 Hz or so (from record warp or amplifier noise) to reach a loudspeaker since (1) they could cause the speaker voice coil to be driven beyond its linear limits, (2) they use up power that would best be applied to the audio range, (3) they could produce severe IM distortion, and (4) they wouldn't be audible as "sound" anyway.
The promotional literature for many of the recent new amplifiers stresses the use of DC design as though it were a recent development. But, in point of fact, I know of at least one DC power amplifier that has been on the market for at least six years. There is no question that DC circuits have a theoretical advantage in respect to infrasonic response and absence of phase shift. But whether these theoretical advantages translate in any specific product into audible advantages must ultimately be left up to the ears of the listener.
If astute readers come away with the impression that I'm fence-straddling on the issue of DC design, they are absolutely correct!
Slow Woofer
Q. I am hoping that you will be able to settle a long and drawn-out argument. I was told (and believe) that when you can actually see the excursion of a woofer it is due to some flaw in the record or the reproduction system.
Woofer excursion that obvious must have some cause. My friend states, "It's supposed to do that!" Who is correct?
TIMOTHY POULIN; Orono, Me.
A. I think you are right on several points. Although you can see a woofer cone move when it is delivering a loud, very-low-frequency signal, it should not be fluctuating wildly on normal program material. In general, a woofer cone's response has to do with the speaker system's internal damping;
but we certainly can't ignore the electrical contributions of the amplifier and the record player. A turntable with severe infrasonic rumble-say, below 20 Hz or so-can produce visible-if not necessarily audible speaker-cone excursions. And warped records can cause even more severe effects at even lower frequencies.
To a great degree, what reaches the speaker cone at infrasonic frequencies depends on the frequency response of the preamp and power amp at those frequencies. If either unit rolls off the very low frequencies, then obviously the woofer cone won't be faced with the difficult task of coping with their presence. It is for this reason that many engineers feel that amplifier response down to d.c. (0 Hz) buys nothing but trouble. But in any specific case that's something to be resolved by (1) the audiophile's decision as to the way he wants his amplifier to perform at low frequencies, (2) his ears, and (3) his woofer cone.
Amplifier/Speaker Output
Q. I often see ratings of power amplifiers that read: "x watts into 8-ohm loads." Why is it that the power rating increases when 4-ohm loads are used?
ALAN DUVERNAY; Marion, Ohio
Q. The rear panel of the 80-watt-per-channel amplifier I'm about to buy has terminals (and front-panel switching) for two pairs of speakers. Can you tell me whether the 80 watts will be divided between the two speakers in each channel, or what?
EDWARD C. WINGATE; Cornelia, Ga.
A. It may come as shocking news to most of my readers (I know it did to me long ago) that amplifiers don't really put out watts, they put out volts. Once that is understood, then Ohm's Law can be applied to answer both of the above questions. A transistor amplifier is essentially a constant-voltage de vice, meaning that a certain signal voltage applied to its input will provide a certain signal voltage at its output, to be applied across a "load" such as a precision test resistor or loudspeaker. The relevant part of Ohm's Law is W = E^2/R, where W is watts, E is output-signal voltage, and R is the impedance of the load resistor or speaker. Let's take as an ex ample an amplifier that, with a given input voltage, puts out 20 volts. Across an 8-ohm load, this is the equivalent of 50 watts. How ever, if we substituted a 4-ohm load, the amplifier would then be putting out 100 watts-that is, if it had the ability to do so.
(Power-supply and output-transistor limitations usually reduce the 4-ohm output to something like 150 percent of the full rated 8-ohm output.) That takes care of Mr. DuVernay's question. However, Mr. Wingate's query brings in the matter of speaker impedance. If the two sets of speakers to be connected have identical impedance characteristics, then whatever voltage (and hence power) the amplifier is putting out would be divided between them.
However, no two 8-ohm speakers of different make or model are likely to have impedance curves that match at all frequencies. There fore the signal-voltage division on music be tween the speakers (if they are connected in series) is likely to vary from moment to moment depending on what frequencies are involved. If the two speakers in each channel are connected in parallel (the usual switching arrangement), then the voltage across the pair will remain constant but the current through each will vary with frequency. And, as a result, so will the wattage.
C Cells by the Sea Shore
Q. I am quite befuddled over the do's and don't's of leaving the C-cell batteries in my cassette recorder at my summer house.
The instructions say remove them if the recorder is not to be used for a while. I wonder if you could shed some light on this subject.
NANCY HENDERSON; New York, N.Y.
A. Most batteries (cells, that is) used in flashlights, recorders, and radios are rated as to their "shelf-life." This means that they can be stored under normal conditions for a specified amount of time without losing a significant amount of strength. The shelf life is determined basically by the amount of internal chemical change taking place within the cell under no-load (non-operating) conditions.
After a while, depending on the type of C cell involved (alkaline, carbon-zinc, etc.) the battery will discharge itself and possibly corrode its external shell. It is this corrosion, which can eat away at the parts within a recorder, that the instructions are obliquely warning against. There also appears to be an electro chemical process that takes place slowly at the points where the cell makes electrical con tact with its holder. If any corrosion is ob served these should be cleaned with fine sand paper. In sum, it's good practice to remove the batteries and store them in the refrigerator; this slows down their internal chemical processes and thereby substantially extends their shelf life.
Infra vs. Sub
I notice that lately your magazine is using the term "infrasonic" instead of "subsonic" to refer to the very low frequencies. Are you being fancy-or what?
FRANCIS LEFFERTS; San Francisco, Calif.
A. Not fancy, just correct and in accord with modern usage. You might also have noticed that we use "ultrasonic" instead of "supersonic" to describe events at the other end of the audible spectrum. One takes a supersonic flight but gets teeth or jewelry cleaned by an ultrasonic device, and we have long had both infrared and ultraviolet light sources. And, yes, we know that there are many exceptions and inconsistencies in these usages, but it is unlikely that the philological chaos will ever be completely resolved.
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!
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Source: Stereo Review (USA magazine)