----------
Class G Amplification: What Is It?
To understand the operation of a Class G amplifier, it is advantageous to compare it with the two "standard" circuit configurations for audio amplifiers: Class A and Class B (described in detail in "How to Trans late Amplifier Jargon," HF, March 1975). Class A out put devices conduct current at all times. This means that the stage dissipates power continuously; more over, the highest dissipation is under no-signal conditions. Consequently, while a Class A amp is capable of very linear (distortion-free) performance it is inherently inefficient, particularly when it idles for a large part of the time. This is not a drawback for low level amplification stages, where output power is low in any case. But in a power amp's output stage it makes severe demands on the power supply and in particular on the heat sinking.
In a Class B stage both output devices of the push pull pair are cut off when there is no signal, and no power at all is dissipated. When signal is applied, one output device of the pair handles the positive swings of the waveform, the other the negative swings. In practice, since solid-state devices are markedly non linear at very low current levels, a small current flows at times of zero signal (technically, a Class AB configuration) to prevent crossover distortion. The efficiency of a Class B stage approaches 67%, but only when it operates near full output. (The maximum efficiency of a Class A stage is 50% under the same conditions.) In a Class G stage, there are four output devices: two to handle the positive swings, two for the negative swings. The circuit is arranged so that one pair of devices is fed from a low-voltage power supply and the second pair from a higher voltage. When a positive signal is applied, the low-voltage transistor on that side begins to conduct while the high-voltage transistor remains cut off. When the input voltage exceeds the supply to the low-voltage device, it cuts off and the high-voltage section takes over, remaining on until the voltage falls into the range of the low-volt age section once again. The action is exactly sym metrical in the negative portion of the waveform.
In effect, the low-voltage pair acts as a Class B stage, but since it is designed for relatively low power, its efficiency is of small concern. Moreover, in practice it is driven fairly close to its limits most of the time and is thus near its maximum efficiency. The high-voltage pair is also essentially of Class B, but since it acts only to "rescue" the low-voltage stage from clipping, it never works at low levels. In this way it is spared operation in the inefficient part of its range. The key to the optimization of a Class G stage is the choice of the ratio between the two supply volt ages.
------------
The Equipment:
Hitachi SR-903, a stereo FM/AM receiver in wood -veneer case.
Dimensions: 19 1/4 by 5 5/8 inches (front panel), 15 3/4 inches deep plus clearance for controls and connections. Price: $500.
Warranty: "limited," three years parts and labor.
Manufacturer: Hitachi, Ltd., Japan; U.S. distributor: Hitachi Sales Corp. of America, 401 West Artesia Blvd., Compton, Calif. 90220.
Comment:
The first clue that there is something unusual about the Hitachi SR-903 receiver comes when you pick it up. Can this, one wonders, be an 183A-dBW (or 75-watt) receiver? Well, it is: and the reason it weighs so little is that it contains a novel output circuit (called Class G by Hitachi, though the components incorporating it are called Series E) that is a good deal more efficient than a standard Class B stage and thus requires less heat sinking. In addition, the new circuit has an unusually large amount of short term headroom: It can produce a little more than 3 dB above rated power-that is, twice its continuous wattage rating-on transient peaks without clipping, rivaling in this case the performance of many a super-amp.
Data from CBS labs verify that the SR-903 meets its full-power distortion specs with a little to spare. Both total harmonic and intermodulation distortion fall to a roughly constant value near half power, remaining there to levels below 0 dBW, after which there is only an inconsequential rise. Testing with tone bursts (as an approximation of musical peaks) confirms that the headroom claimed does exist. The largest deviation from flat frequency response (20 Hz to 20 kHz) is just 3/4 dB, and that is at an extreme of the band. Signal-to-noise ratios are very good through all in puts, and that of the phono stage is equivalent to 76 1/2 dB when referenced to the conventional 10-millivolt input.
In our listening test we operated the Hitachi with two pairs of 8-ohm loudspeakers connected to its spring loaded terminals and were surprised at how cool the back panel (which acts as heat sink for the output transistors) remained. It is possible with this nominal 4-ohm load to make the protection circuitry intervene, but that requires turning the volume to levels that are uncomfortably loud, in our listening room at least. The protection system, incidentally, acts cleanly and decisively, keeping the amplifier section off long enough to allow due contemplation of the misdeed that caused the shutdown.
The FM section contributes solidly to overall performance as well. Stereo reception is automatically suppressed until the available signal-to-noise ratio exceeds 38 dB, and 50 dB of quieting is achieved with an RF input just below 38 dBf, which is a healthy par for the course. The ultimate stereo S/N ratio is reached at 65 dBf and is, at 66 1/2 dB, substantially in agreement with the manufacturer's spec.
Capture ratio, frequency response, and stereo separation are very good, and distortion is properly minimal under all test conditions. The alternate -channel selectivity graph in Hitachi's spec sheet agrees reasonably well with the 60 dB measured by CBS, representing good selectivity--though not as good as is suggested by the 80 dB that Hitachi (using a different measuring level) specifies for the SR-903.
One nice feature is the Auto-lock tuning system. Automatic control is disabled when you touch the tuning knob and select a station: when you let go, the system locks to the center of the channel once again. The Autolock can be switched out via a control that disables muting (which is nonadjustable and very effective) as well. The tuning dial is highly legible and offers more than adequate resolution.
A midrange tone control, which is not a common feature in this class of receiver, is included. Although its scale is identical to those of the bass and treble controls -which offer, respectively, ± 15 and I 10 dB of boost and cut -its range is limited (perhaps wisely) to ±6 dB.
Provisions are made for two tape recorders. You can dub from one to the other while you listen to a different pro gram source. The ADAPTER connections can be used for a third deck (you can dub to this deck, but not while listening to another source) or for signal -processing equipment -an equalizer, a Dolby adapter (for FM), etc. The high and low filters have break points of 4.5 kHz and 120 Hz, respectively, and their gentle slopes (nominally 6 dB per octave) prevent them from being more than moderately effective.
The phono stage has adequate gain and overload and, like the high-level stages, a very good S/N ratio. Its sound seems accurate, and it interfaces well with pickups, even ones that we would consider "difficult."
--------- HARMONIC DISTORTION CURVES
----------INTERMODULATION CURVES
----------Square -wave response
---------INPUT IN MICROVOLTS
Hitachi SR-903 Receiver--Additional Data (not shown)
Clearly, the principal strong point of the Hitachi is its amplifier section. Performance far exceeds what can be rightfully expected of a conventional 75-watt amp. The generous headroom contributes to a sense of ease, clarity, and crispness. The standard set by the amplifier outpaces the other sections to a degree, but they are in fact much better than adequate. The 903 may not be the ultimate choice for someone whose primary music source is FM. But if discs are your bag and you like concert hall levels, this receiver will really show you something.
-------------
(High Fidelity, Apr. 1977)
Also see:
Sony STR-5800SD stereo FM/AM receiver (Equip. Report, Nov. 1977)
Empire 698 turntable (Equip. Profile, Apr. 1977)
ADC--Accutrac turntable (ad, Apr. 1977)