The signal that appears at the output of the demodulator or a first audio amplifier in either a tuner or receiver is like a new born babe-in no position to be disputatious, but nevertheless full of promise. At this point in the electronic circuitry, the audio signal has left the womb of its surrounding carrier and now is naked and unsupported. What it needs is substantial nourishment in the form of voltage and current to enable it to grow to cope with the realities of an outside and demanding world. In the case of the audio signal, that outside world is a speaker system.
There are two basic components that will help the audio signal achieve maturity. One of these is the pre-amplifier, more familiarly known as the pre-amp, while the other is the main or power amplifier. At its entrance to the pre amp, the signal is still in its electrical form, whether the signal comes from the tuner, a cartridge, or a tape unit, and so can be modified by all the electronic circuits we have at our disposal. In the case of the tuner, it would be nice to assume that the signal at this stage is a perfect replica of the original audio modulating signal used in the broadcast studio. However, in the process of getting to your tuner, the signal was forced to undergo a rather horrendous experience.
At the station, it was converted from sound energy to electrical energy with the help of a microphone, unceremoniously loaded onto a carrier wave, and then literally thrown out to fend for itself in space. In its travels it was unmercifully harassed by noise signals, and all other kinds of signals as well, finally arriving emaciated and almost non-existent at your receiving antenna. Here it was picked up, given an invigorating dose of amplification, cleansed of miscellaneous unwanted voltages that clung to it like so many burrs, finally arriving at the output of the tuner. At this juncture it is strong enough to produce sound in a pair of headphones, but that is about all. At this point also the signal should be a faithful replica of the original energy conversion process-that is, the change of sound energy into electrical energy by the microphone. Whether it really is depends on more than just the electronic ability of your tuner or receiver.
In FM broadcasting, high fidelity starts at the broadcasting station-or it should. It is entirely possible that your hi fi components might be superior, electronically, to those at the station, since such is the advanced state of the art in the hi-fi components industry. And you may also not know whether the signal followed a direct path from the station to your receiving antenna, or used a multipath, a situation in which the signal competes with itself-the direct signal and that portion that is bounced off buildings or other structures. And so the assumption of an FM signal input to a pre-amp is high-fidelity isn't always justified.
Sound Sources
The FM signal is just one of several that can be fed into the pre-amp. You might opt for records, or for signals supplied by a cassette deck, a tape deck or a microphone. All of these sound sources require additional amplification-amplification that can be supplied by the pre and main amps. Note the economy of this arrangement. The same amps can be used for a number of different sound sources.
Terminology
The pre-amp and power amp can exist as separate components. The pre-amp then becomes the intermediary link between the tuner and the main amp. An integrated amp (Fig. 1) consists of a combination of pre- and main-amplifiers in a single enclosure. The advantage is that the pre amp does not require any connecting cable to the main amp. There is also some economy since both units are housed in one box. However, this joining of the pre- and main amplifiers is at the expense of component flexibility, and so some integrated units have a provision to let the owner use either the pre or main amps separately, or as an integrated unit. This gives you the best of both worlds. Of course, if you have a receiver instead of a tuner, the problem of separated or integrated amps does not arise, since the receiver is equipped with both amplifiers. Originally, amplifiers used in hi-fi receivers were rather low-power types, making separated or integrated amps preferable. However, modern receivers now have amps whose power ratings are substantial. The disadvantage, of course, is that if you want to modify or upgrade a system that has a receiver, then, in effect, you must dispose of a tuner cum pre-amp cum amp, for that is what a receiver is.
Fig. 1-Integrated stereo amplifier. (not shown)
The receiver with its built-in amps has powerful arguments in its favor. One of these is economy. A combined component such as a receiver costs less to manufacture. The receiver needs less room than a tuner-pre-amp-power-amp combination. And, finally, there is no need to make connections. But in the face of all these considerations, the separate tuner, separate pre-amp, separate power amp (or integrated amp that has provision for operating the pre and main amps separately) supplies flexibility. It facilitates moving up to a multiple-amp system or to a quadraphonic setup without sacrificing equipment. Individual components also supply the satisfaction of having a custom-built, rather than an "off the rack" hi-fi system.
Position of the Amp
The position of the amplifier with reference to the other components in the hi-fi system depends on whether we are talking of a receiver or a tuner. Figure 2 shows a receiver (A). Since both pre and main amps are contained within the receiver, the question of position does not arise. Connect an FM antenna to the receiver "ant" terminals, a pair of speakers to the speaker terminals, and the hi-fi system is ready to go. With a tuner, though, we have more choices: an integrated amplifier (B) or separate pre and main amps (C).
Fig. 2-Some possible hi-fi system arrangements.
These are all fundamental hi-fi systems and do not take advantage of the fact that other input signal sources can also be used. Figure 3 shows a sophisticated arrangement using a pair of record players, two tape decks (one of which could be a cassette type and the other open reel), stereo headphones which can plug into the front of the integrated amp, one or more microphones, and a pair of speaker systems.
Still another arrangement, known as a multi-amp system, is shown in Fig. 4. In this setup, there is one pre-amp contained within the integrated amplifier. The integrated amplifier also has a main amp, a power amp that is used solely for the bass tones. There are two additional power amps, both driven by the pre-amp-that is, they receive their input signal voltage from the one pre-amp in the integrated amplifier. One of these power amplifiers handles all the mid-range tones while the remaining power amp takes care of all the high range. An electronic crossover network is used to supply each of the power amps with signals in the mid and high ranges.
Fig. 3-A pre-amp or integrated amp can act as the control center for a number
of hi-fi components.
Fig. 4-The integrated amp can serve a multi-amp installation more easily
when its pre and main amp sections can be worked independently.
Some Basic Differences
The pre-amp and the main amp are both audio amplifiers. The basic difference lies in their responsibilities. The pre-amp is a voltage amplifier; its function is to strengthen the signal voltage, to increase its amplitude without modifying the input signal waveform in any way. The main amplifier is also a voltage amplifier but since the speakers will be connected to its output, must deliver current in addition to voltage. But a combination of voltage and current means power. With this as its primary function, the main amp is sometimes called a power amp. The pre-amp must strengthen the audio signal to the point where the output amplitude is strong enough to satisfy the signal input voltage requirements of the main amp. In turn, the main amp must furnish enough signal power-that is, both voltage and current-to operate the speaker system (or systems) to your listening satisfaction. Thus, the output of a pre-amp is measured in volts; that of a power amp in watts.
The voltage output of a pre-amp, since it is a signal voltage, will be a.c. Its amplitude will be somewhere in the region of 1 to 5 volts, depending on its overall amplification. Some pre-amps do deliver more voltage than this; others less.
Sometimes a manufacturer's spec sheets will indicate two different voltages for the pre-amp's output. One will be rated output; the other maximum. A pre-amp might supply 3 volts rated output; 5 volts maximum.
Input Sensitivity
The amount of input is sometimes designated as the pre amp's input sensitivity and in manufacturer's spec sheets is given in terms of voltage at a specific frequency, quite often 1 kHz. A turner, for example, might supply 200 millivolts to the pre-amp's input to produce 3 volts rated output.
However, even though the pre and main amps can be, and sometimes are, separate entities, they are related in the sense they must function as a team.
The Pre-amp as a ControlCenter
The pre-amp does more than get the signal ready for introduction to the power amp for it works as a control center for the hi-fi system whether it exists alone as an individual component, as a combined component with a power amp, or set up as an integrated amp. If a receiver is used instead of a tuner, then the only difference is that the pre and main amps are contained within the receiver, and the concept of the component as a control center is still applicable. And so, front panel controls on a receiver, or on a pre-amp, or on an integrated amp (Fig. 5) can include bass and treble controls, a loudness switch, volume control, speaker balance control, a mode selector switch, 'phone and microphone jacks, tape duplicate and tape monitor switches, various noise filters, a speaker selector control, etc. Naturally, not all pre-amps are so thoroughly equipped since the degree of sophistication of the pre-amp depends fairly directly on what you are willing to pay.
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Fig. 5-Front panel controls of an integrated amp.
Fig. 6-Rear panel connections for an integrated amp
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The Rear Panel
Modern hi-fi components such as receivers and amps are distinguished by the fact that the rear apron or rear panel (Fig. 6) is as busy an area as the front. Again, what you have on the rear panel depends on the degree of sophistication you want. Here you will find convenience a.c. outlets, since the average home is simply not equipped with enough outlets for a multi-component hi-fi system. Some of the outlets are unswitched, meaning that any component plugged into such outlets must be provided with an on-off switch. You will also find one or more `switched' outlets-those which are activated by the power on-off switch on the front panel of the amp. These outlets have power limitations-that is, they are limited in the amount of a.c. line power they can safely handle. 100 watts maximum is about what you can expect from either switched or unswitched outlets.
On the rear panel you will also find input connections for one or more turntables, the tuner, one or more tape decks, cassette deck and possibly a cartridge deck as well, and inputs for one or more stereo speaker systems. A well-designed pre-amp will have different phono inputs-magnetic or ceramic.
Program Selector
The program selector, sometimes called the function selector, chooses the program source that is to be fed to the input of the pre-amp. These inputs can be one or more mikes, phono, tuner or tape units such as cassette or open reel. There may also be one or more auxiliary (labeled "aux") inputs for other audio signal producing components.
Tape Monitoring
Tape sound can be switched on and off with the tape monitor switch, a useful control when making a recording on a cassette or open reel deck. If the deck has independent recording and playback heads-that is, if it is a 3-head deck this switch supplies a double method of monitoring recording quality. When this control is in its "source" position, the original "before it gets on tape" sound is heard from the speakers; in the "play" position, the sound is that recorded on tape. By switching back and forth between the two switch positions, the source sound can be compared with the tape sound supplying a clue as to the proper setting of the "record level" control on the cassette or open reel deck.
Tape Duplicating
You can use the amp to duplicate tapes. If you tape record FM broadcasts, for example, it is difficult to eliminate unwanted sounds, such as an announcer's comments or advertising break-in. The tape can be edited by cutting and splicing, but an easier method is to use the technique shown in Fig. 7. This requires a pair of tape deck input terminals on the rear apron of the amp. First, tape a complete stereo FM program, including commercials and announcements, and then later re-record with another tape deck, wiping out the unwanted material.
Volume, Balance and Tone Controls
These controls on the amp front panel are rather obvious.
The balance control, for example, when turned to the right does not increase right channel sound since it cannot do so.
Instead, it reduces left channel volume and so the right channel sounds louder. Also, the volume and balance controls do not affect the signal supplied to the tape deck for recording.
This does not mean you have no bass or treble control on tape, for the controls are effective during tape playback through the pre-amp.
Depending on the amp, some volume controls have a greater or lesser amount of electronic sophistication. Thus, some volume controls are assisted by a level-set control.
The level-set control could be a click-stop device reducing amplifier output by a predetermined amount, such as 15 dB or 30 dB. The level-set control can then be regarded as a "coarse" setting, and the volume control as a fine or vernier adjustment. When a powerful integrated amp is used for listening at quiet sound levels, the volume control must be turned down almost to its minimum point, making fine adjustments for low volume listening levels sometimes difficult. However, the level-set control can push the volume down to the required level, thus letting the volume control have a greater arc of swing. This makes the volume control setting much less critical.
Another advantage of a level-set control combined with a volume control is that limiting power output also protects speakers, especially those of small input power handling ability from accidental damage that might occur when you turn up the volume by mistake. Using speakers having small power-handling capacity with a relatively high-power integrated amplifier may seem anomalous, but the situation can exist. With speakers of small power-handling capabilities it is advisable to have the level-set control positioned at-15 dB or -30 dB at all times.
Frequency Response
The frequency response of an amp is the range of audio signals the unit will reproduce when an audio voltage is connected to its input terminals to yield 1 watt of output signal power. By itself, frequency response is meaningless unless some information is supplied about the amount of deviation, plus and minus, generally given in dB. A spot frequency can be selected as the reference. Thus, we might consider the output level of 1 watt at 1 kHz as indicative of 0 dB. The smaller the deviation in dB from the reference, the better.
An amplifier might have several frequency response figures. The same amp could be described as having a frequency response of 20 Hz to 18 kHz, ± 1 dB, or 18 Hz to 25 KHz, ± 3 dB. The deviation in dB is an indication of how far the amp wanders from fidelity.
Speaker Selection
Another highly useful, but possibly seldom used control on the front panel of the amp is the speaker switch (Fig. 8) often combined with the power on-off control. Aside from the power off position, this control could have three speaker selection settings. If you designate your left speaker as A and the right speaker as B, then this control can let sound come through the A speaker only, or the B speaker, or a combined output, that is, both A and B speakers functioning.
There may also be a "speaker off" position, helpful when listening through headphones and you want the speakers silenced.
Fig. 7-Technique for tape duplicating.
Fig. 8-Amp may be equipped with a speaker switch.
On to the Power Amp
The pre-amp in its dual role of amplifier and control center is one of the most important links in the hi-fi system.
We've been following the audio signal since its inception at the source and so the speaker system is our logical destination. To get there we must go through the main or power amp-and that's exactly what we will do in the next installment.
(Source: Audio magazine, Dec. 1974; Martin Clifford)
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