by LEN FELDMAN
Muting (T). Normally, when one tunes an FM receiver, an unpleasant form of wide band noise is heard between stations. Special circuits have been developed by component manufacturers which eliminate this form of disturbance as the user tunes from station to station. Usually, the circuits are a form of "gate" or switch which permits signals of predetermined strength to be heard and eliminates all other signals (including noise) from the audio chain. These circuits are called "muting circuits." In some receivers, the threshold is adjustable, so that extremely weak station signals are not "blocked out" along with the undesired noise. Alternatively, some receivers are equipped with "mute defeat" switches which negate the muting action altogether for those instances when it is desired to pick up very weak ( albeit noisy) signals.
NOISE AND HUM (T, P, A). While applicable to tuners as well as to preamplifiers and amplifiers, noise and hum in tuner sections are normally covered by the S/N ( signal-to-noise) specification ( which see below) . Noise and hum in preamplifiers and amplifiers is usually stated with respect to a voltage or power output reference level. Thus, in the case of a power amplifier, the noise and hum may be stated as "75 dB below full output," for example. To describe the noise and hum characteristic of a preamplifier or amplifier properly, a separate specification should be provided for all the available inputs. High-gain phono inputs are likely to have a poorer hum-and noise spec than high-level inputs such as Tape or Auxiliary inputs. Our qualitative charts relating to both low-level and high-level-input hum and noise (which is stated in dB) are shown in Figs. 12 and 13.
Peak Power (A). Although, strictly speaking, Peak Power Output might be regarded as a "specification," we have purposely not capitalized it here because the component industry rightly considers this "spec" to be irrelevant. Peak power implies the absolute maximum power that can be delivered by an amplifier regardless of distortion. Since distortion free musical reproduction is the objective of true high fidelity components, the use of such a specification is to be discouraged. It is mainly used by console and "packaged goods" manufacturers in an attempt to "look better" in print and has no bearing on the actual performance of a product. Mathematically, peak power works out to be twice continuous power. Now, since music or instantaneous power has been stated to be somewhat greater than continuous power ( see Music Power), some of these same manufacturers have come up with yet another power term-Instantaneous Peak Power, which is a further inflated and equally meaningless specification. Our discussion of it here is only to remind readers to judge amplifier power on an equal basis when comparing one amplifier with another and to disregard "ad copy" such as that just described.
Phasing (A). When a pair of speakers are connected to the output terminals of the stereo amplifier portion of a receiver, it is important that each speaker be in phase with its companion speaker. By "in-phase" we mean that the cones of both speakers should move in the same direction, instantaneously, when driven from a common, monophonic signal. Phasing is best checked by listening to a monophonic program, standing mid-way between the loudspeaker systems to be checked. The check is repeated with the wires to one of the speakers reversed at the speaker terminals. If the second check results in better, more-centralized bass response, the speakers are now in phase. If the second check results in a diminution of overall bass and a seeming "hole in the middle" or vagueness of spatial identification of sound, then the speakers were in phase as previously connected and the original connection to one speaker should be restored. In making these tests, wires to only one speaker are reversed.
POWER BANDWIDTH (A). Since power output specifications are generally stated for a mid frequency ( usually 1000 Hz ) only, it is important to present an idea of power capability at frequency extremes, as well. Power bandwidth is defined as the two extreme frequencies ( low and high) at which the amplifier can produce one half its rated power at its mid-band rated distortion. As an example, a 50-watt amplifier having 1% distortion at rated output would be said to have a power bandwidth extending from 15 Hz to 25,000 Hz if, at those two frequencies, it can deliver only 25 watts at 1% distortion.
Fig. 12--Qualitative graph of hum and noise for low levels (phono, tape,
etc.); dB figures are referenced to full power output.
Fig. 13--As above, but for high-level inputs (Aux, Tuner etc.)
Our qualitative graphic presentation of power bandwidth is shown in Fig. 14, and is to be interpreted separately with regard to low-end and high-end extremes.
Fig. 14--Ratings for Power Bandwidth
POWER OUTPUT (A). See MUSIC POWER and RMS POWER. Recorder Outputs (P). Featured on most receiver rear panels (and sometimes duplicated even as front panel jacks, for additional accessibility), the pair of jacks labeled "Recorder Out" are intended for connection directly to a tape recorder.
The recorder is then fed with whatever signal you are listening to. Because the feeding signal is purely electrical voltage, results obtained in making recordings in this way are greatly superior to "placing a microphone in front of a loudspeaker," since the inherent distortions of the microphone and speaker are eliminated from the process. Usually, the recorder outputs are so arranged as to be completely independent of tone-control or volume-control settings. Thus, the user can listen to the source material at any level or tonal setting without affecting the essentially "flat" characteristic of the signal being fed to his tape recorder. Correct recording level must be maintained by means of the tape recorder's controls rather than by any receiver controls.
Relays (A). Generally, two types of relays are used in the amplifier section of the modern stereo solid-state receiver. Thermal relays, as the name implies, are activated by excessive heat caused by excessive current flow in output circuits.
Such relays are set to open when the case temperature of output transistors reaches specified limits for the devices. Re-set is automatic, occurring when temperatures cool down to safe operating values.
Current-activated relays are more direct acting, responding to predetermined values of excessive current flowing to amplifier loads. One type is not necessarily superior to the other and the use of either will be dictated by the circuit needs and the design philosophy embodied in a particular amplifier circuit. The presence of one or both types of relays in a receiver is evidence of a level of sophistication beyond that of the simple fuse, whose replacement constitutes a minor annoyance to the user.
RIAA (P). See Equalization.
R.M.S. POWER (A). Also known as "continuous power," this specification represents the most conservative statement of power capability of an amplifier. It denotes the amount of power that an amplifier can deliver to a load when fed with a constant, sinusoidal tone. This power rating must be accompanied by a figure of rated distortion to be totally meaningful. Thus, an amplifier might be said to produce 50 watts of power, r.m.s., at a maximum distortion of 1%. R.m.s. power is usually stated on a "per channel" basis, whereas "music power" is usually given as the total for both channels. The amount of power required for a given music system installation depends upon many factors, including listening-room size, efficiency of speakers with which the amplifier is to be used, number of speakers to be driven simultaneously, and the listener's tastes in program material and level at which it is to be played. Perhaps the greatest variable of those mentioned is loudspeaker efficiency, which may vary from under 1% (for some air-suspension, bookshelf enclosure types) to over 15% for some of the larger corner enclosures.
The user should also remember that the use of two stereo speaker pairs simultaneously represents a splitting of the power to each system and allowance should be made for this contingency in selecting the proper power-amplifier rating in any proposed system.
SELECTIVITY (T). This specification describes the ability of a tuner to discriminate between the desired station and stations removed in frequency by one channel-width ( Adjacent-Channel Selectivity) or two channel widths (Alternate Channel Selectivity). Alternate-channel selectivity is deemed the more important of the two specifications, since the Federal Communications Commission is careful not to assign adjacent-channel frequency to two stations in one geographical area.
Still, with today's ultra-sensitive tuners, adjacent-channel selectivity may also be relevant in certain areas where, with a well-designed antenna it is no longer unusual to pick up two stations just one channel width apart. Qualitative graphs indicating both adjacent-channel and alternate-channel selectivity figures are shown in Figs. 15 and 16 and, as can be seen from the figures, the higher the number of dB's, the better the selectivity characteristics.
Fig. 15--Adjacent-channel selectivity; Fig. 16--IHF selectivity, alternate
channel.
SENSITIVITY, IHF (T). See IHF Sensitivity.
S/N (SIGNAL-TO-NOISE RATIO) (T). Although the "Least Usable Sensitivity" defined by the Institute of High Fidelity ( See IHF Sensitivity) requires a signal-to-noise ( and distortion) ratio of only 30 dB, tuner sections of receivers are able to effect quieting far in excess of this modest figure when fed with greater (and more typical) input signals at their antenna terminals. Accordingly, the Signal-to-Noise specification, usually abbreviated S/N, tells how much lower the residual noise is, compared to the desired program, when the signal strength is of the order of 1000 microvolts. In FM tuners, application of a 1000-microvolt signal results in the maximum signal-to-noise ratio, in that any further increase in signal strength will cause no further reduction in noise content. The small amount of residual noise remaining at this signal level may be random wide-band noise, hum, or a combination of both. The range of signal-to-noise ratios and their quality ratings is depicted in Fig. 17.
Fig. 17--Signal-to-noise ratios
Separation, Stereo FM(T). See FM Stereo Separation.
Speaker Selector (A). Many receivers are equipped with a switch enabling the user to select stereo pairs of speakers in more than one location. Usually, main and remote speakers are provided for, and the switch often enables selection of main, remote, or both sets of speakers to be played simultaneously. This is a useful feature, since more often than not the amplifier section of the receiver has ample power to drive two pairs of speakers to adequate sound levels at low distortion. In utilizing this feature, however, the user should be careful of one important point. Most solid-state amplifiers will work well with load impedances down to four ohms. In fact, most amplifiers deliver their greatest amount of power when coupled to a four-ohm load. However, applying a net load of less than four ohms to most solid-state amplifiers can damage the output transistors very quickly or, at very least, cause fuses and thermal relays to "pop" interminably. Two eight-ohm speakers connected to a single channel (as in the case of main-remote combinations) adds up to a net impedance of 4 ohms, which is fine. Two four-ohm speakers similarly connected, however, would result in net impedance of only 2 ohms-well below the safe limit. With three sets of loudspeakers connected ( one main location and two remotes ), even if each speaker had a voice-coil impedance of 8 ohms, the resulting net impedance would be only 2% ohms (%) and would also be unsafe for most amplifiers. These precautions apply only if more than one system is to be played at once. If your receiver selects only one pair of systems at a time, you need not concern yourself with these impedance considerations.
SPURIOUS RESPONSE REJECTION (T). There may be other forms of signal interference when tuning across the FM dial besides i.f. and image interference. False appearance of signals at improper frequencies brought about by complex mathematical relationships between the intermediate frequency or its harmonics and sub-harmonics are often the cause of such additional spurious responses. In a well-designed tuner, such false responses will be very minimal or hardly detectable. Nevertheless, many manufacturers publish this specification in dB. The higher the number quoted, the better the tuner in this regard. Typical spurious-response rejection figures for modern tuner sections are shown in Fig. 16.
STEREO INDICATOR (T). Almost before the first generation of stereo FM tuners and receivers was off the drawing boards in 1961, manufacturers realized that the new stereo broadcasting system provided a simple way in which to denote a stereo broadcast in the tuner or receiver circuitry. Today, nearly every stereo tuner or receiver is equipped with a stereo indicator light to tell you when you have tuned to a stereo broadcast. Despite this convenience, there is still some confusion in interpreting the light. Some circuits allow the light to come on even when the mode selector switch of the receiver is set for mono listening. As a result, many listeners THINK they are getting stereo reception when in fact they are really hearing the same material coming from each speaker because a switch was set wrongly. Conversely, some circuits are arranged so that so long as the receiver is set to mono operation, the stereo indicator light will never light. In these circumstances, a listener who is unaware of this design format may well tune across the entire dial and wonder why his set was not receiving any stereo broadcasts! Finally, some less sophisticated stereo indicator arrangements will often become illuminated in the presence of interstation noise-leading the inexperienced listener to conclude that the interstation noise is being "transmitted" in "stereo"!
Tape Monitor (P). Many tape recorders (particularly the better ones) are equipped with multiple tape heads ( separate heads are used for record and playback). The "Tape Monitor" feature present on most receivers enables the user to take full advantage of these tape recorder designs. Actually, the tape-monitor switch does nothing more than cause an interruption in the audio circuitry. The amplifier is, in effect, disconnected from the early preamplifier stages, internally. The recorder output jacks can then be connected to the tape recorder input, for making whatever recordings you desire, while the output of the extra head ( suitably amplified by a preamp contained in the recorder) can be fed to the amplifier, via the tape-monitor jacks. In this way, the recordist hears not the original program material, but his own recording of it, played back a fraction of a second later. By monitoring in this way, any defects noted in the resulting tape can be immediately corrected without having to wait to listen to the entire tape--which might have been inadvertently ruined. Now, it follows that if the receiver owner is not equipped with a multiple headed tape recorder which lends itself to this application, he should never actuate the tape monitor switch-since to do so causes a "break" in the circuit which is not restored by the presence of a tape recorder connection. Many, many service calls could have been avoided if more receiver owners checked the setting of their tape monitor switches before calling to report a receiver that "doesn't play."
THD (TOTAL HARMONIC DISTORTION), AMPLIFIERS (P,A). This form of distortion arises from the production of harmonics, or multiples, of the desired fundamental tone. Thus, a 1000-Hz tone may be fed into an amplifier and the amplifier may produce, in addition to 1000 Hz, small amounts of 2000 Hz ( called the second harmonic), 3000 Hz (third harmonic), etc. The sum of all these harmonic overtones not present in the original signal is added up and expressed as a percentage of the total signal present in the output. In the case of most amplifiers, harmonic distortion is very low at all but full power output and beyond. For this reason, most manufacturers state THD for full output only, allowing us to assume that distortion figures at lower than full output power will be much lower. However, such is not always the case and a meticulous manufacturer will often give additional distortion figures for lower power outputs. Our qualitative curves, shown in Fig. 18, are general in nature and apply regardless of power level. Distortion is unpleasant at both low and high listening levels and large amounts of it in reproduced music in the home lead to a vague malady often called "listener fatigue." Interestingly, many people who have not joined the high fidelity component adherents often question our loud levels of playing. The reason they are not accustomed to lifelike levels is that if they were to attempt to play music at such levels via their "table model" radios or portable phonos, the distortion would be intolerable. It is, therefore, the distortion and not the loud level which most disturbs these people. Because high fidelity receivers have such low distortion (at all playing levels ), one can turn up the volume with no aural discomfort.
Fig. 18--THD, amplifier ratings
THD (TOTAL HARMONIC DISTORTION), MONO FM (T). The meaning of THD, as applied to the monophonic tuner section of a receiver is exactly the same as its meaning applied to amplifier performance. The causes of the distortion are quite different, however, and a statement as to the level of THD in the tuner section gives us an idea of how well that section (as distinct from the amplifier or preamplifier) has been engineered and produced. Too, the distortion created in the tuner detection process is independent of the level at which the volume control of the amplifier is set and for this reason, is perhaps more significant even than amplifier distortion. Our qualitative recommendations are shown in Fig. 19.
Fig. 19--THD, mono FM ratings
THD (TOTAL HARMONIC DISTORTION) STEREO FM (T). The remarks applicable to THD Mono FM ( see above) apply here, except that few manufacturers publish this specification. There are two reasons for this: first, the IHF Tuner Specifications ( written in 1958, before stereo came upon the scene) only call for a disclosure of mono FM distortion and, secondly, THD in Stereo FM tends to be a bit higher than in mono, owing to the extra decoder circuitry which is required. Still, this need not be regarded as a general rule and some designers have been able to achieve low orders of distortion in the stereo FM mode which are as good or nearly as good as their equivalent monophonic distortion specifications. As of this writing, however, we shall be a bit more liberal in setting up the quality criteria shown in Fig. 20.
Tone Controls (P). The presence of wide range bass and treble tone controls on high fidelity stereophonic receivers often puzzles newcomers to the field. "Why," they say, "must a receiver have tone controls if the objective is to have 'flat frequency response' throughout the audio ranger' This would be so if all other elements of the listening system were "fiat," and by "all other elements" we include the room acoustics in the listening area as well as the loudspeakers, phono cartridge, and even the hearing characteristics of the listener. Properly ( and moderately ) used, bass and treble controls can correct for deficiencies in other parts of the system. Used to excess, the tone controls can totally distort the tonal relationships of the music we wish to hear. Virtually all receivers are equipped with separate bass and treble controls. In some cases, each of these controls affects both channels simultaneously, while in more elaborate designs, concentrically mounted knobs can be turned independently, thus enabling tonal compensation of left or right channel separately. There is also a growing tendency to divide the total audio spectrum into more parts than just "bass" and "treble." Thus, certain newer receivers sport five or more tone/equalizer controls, each responsible for compensating or modifying just a small segment of the audio spectrum. Such subdivision obviously affords more precise control over total tonal response, if that is what is required in a given installation.
Volume and Balance Controls (P). Both of these controls are involved in the level or loudness-setting process. The volume control adjusts overall loudness on both channels, while the balance control sets up equality of levels between left and right channels. Often, it becomes necessary to readjust the balance control when the volume control setting has been altered substantially. This is because the volume control really consists of two controls actuated by a single shaft and the mechanical rotation of both controls does not always correspond exactly to equal electrical sampling of the audio voltages present at the take-off point of each control. Manufacturers of quality receivers take great pains to select the volume control pairs in such a way that this discrepancy is kept to an absolute minimum and, though it is certainly not a vital specification, some manufacturers will note that they achieve good tracking ( the term used to describe this dual-control effect) down to 60 or more dB below top setting of the control.
Fig. 20--THD, stereo FM ratings
As for balance controls, most of them are constructed in such a way that only a very small amount of shift of left and right levels takes place near the center of the control rotation. In this way, it is possible to balance your stereo speakers with a high degree of accuracy. The best way to do so, by the way, is with the receiver set to monophonic operation. The balance control is then adjusted until the sound seems to be coming from a point exactly mid-way between the two loudspeaker systems.
Obviously, a brief glossary such as this could not possibly cover every aspect of stereo receiver technology and design--nor was that the intent. Whether you've read it "cover-to-cover" or found a single clarifying explanation of something that's been puzzling you, we hope it has been helpful to present and future stereo receiver owners.
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(Audio magazine, Oct. 1970)
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