1978 HITACHI SR-2004 MONSTER RECEIVER
Featuring 200 wRMS/ch "Class G" Amplifier
The 1978 Hitachi SR-2004 Class G Receiver, retailed for $1,195.00 and has the distinction of being the fifth most powerful "all in one" Monster Receiver made during the "Power Wars" of the late 1970's and one of the least well known. In fact, with its dynamic headroom specification of +3db, it just might have been the most powerful of all. And, it WAS the most powerful Class "G" Receiver ever made.
Hitachi, which manufactured OEM electronics for most of the major audio manufacturers, was a relative late-comer to the Receiver "Power Wars". However, when Hitachi joined the fray with the SR-2004, they put their best foot forward. Almost all the internal components in the SR-2004 were made by Hitachi, making this receiver a real rarity and somewhat of a "purebread". And, outside the box, it has real presence. As you can see above, the SR-2004 is simply HUGE. In the photo above, that's a Concept 16.5 in the back, to give you an idea of the size of this Monster.
Double click the thumbnails for the "big picture":
The Front
Inside
The SR-2004 had most of the "bells and whistles" of the other Monster Receivers it was designed to compete against and a few unexpected extras like switchable IF Bandwidth (wide/narrow), "AutoLock" FM Tuning (triggered by changes in the capacitance on the tuning knob) to prevent FM drift, and a SAW Filter. In fact the 5-gang front end on the SR-2004 was one of the best analog receiver tuners ever. Be sure to take a look at the spec. pages below.
The SR-2004's deluxe features also included two (2) different Audio Muting levels, a front panel Mic Input w/ level control and a very comprehensive Tone Control section with bass, mid-range and treble controls with switchable frequency settings for the bass and treble controls. The images below are from the Owner's Manual and the left most describes all the front panel controls.
Front Panel Controls
Specifications Pg. 1
Specifications Pg. 2
But, what really made the SR-2004 stand out was its "Class G" Amplifier, with a prodigious 200 watt RMS per channel power output, placing it among the most powerful receivers ever made. In fact, according to a review from the November 1978 issue of "High Fidelity" Magazine, it was capable of dynamic peaks of 400 watts/ch "that gives it a shot a first place in the receiver power race". When High Fidelity tested the SR-2004, it found that the power output was actually more like 240 watt/ch RMS. And, better yet, the Hitachi stayed relatively cool, a good indicator for long transistor life. That's why the SR-2004 was popular most among sound reinforcement professionals and DJs. Roger Russell, former loudspeaker Guru for McIntosh Labs was kind enough to provide me with a copy of this review (for a measly $5.00) from his collection of audio magazines:
Review Page 1
Review Page 2
Review Page 3
So, now you know a few of the reasons why the HitachiSR-2004 is one of the greatest receivers ever made. The power supply was dominated by a huge Toroidal Power Transformer surrounded by four (4) huge capacitors, 2 per channel. All of the internal electronics were shielded by metal enclosures so that barely any wiring is visible. And, of course, all eight (8) discrete output transistors on the massive heatsinks were proudly marked "Hitachi". There's nothing like being a purebred!
Unfortunately, Hitachi was never able to establish an audio brand identity in the American market, so the Hitachi line essentially disappeared after 1982 and the SR-2004 is now just a vague memory in the minds of many enthusiasts.
For the technically inclined, what follows is a description of "Class G" amplification from a forum at audiokarma.org:
Class G, Class H Amplifiers Explained:
Soundcraftsmen made some EXCELLENT sounding Class H amps. The Hitachi Class G amplifier allowed them to make a 200 wpc unit that could double that output as required for short periods. These are popular where high-power and cool-running are needed, such as in pro applications and in mobile (cars) systems due to their efficiency. I have four of the big Soundcraftsmen amps, and let me say again, they are GREAT sounding amps (or they have no sound of their own, I guess is the point of my comment).
Class G
Class G improves efficiency in another way: an ordinary class AB amplifier is driven by a multi-rail power supply. A 500 watt amplifier might have three positive rails and three negative rails. The rail voltages might be 70 volts, 50 volts, and 25 volts. As the output of the amplifier moves close to 25 volts, the supply is switched the 50 volt rail. As the output moves close to the 50 volt rail, the supply is switched to the 70 volt rail. These designs are sometimes called "Rail Switchers". This design improves efficiency by reducing the "wasted" voltage on the output transistors. This voltage is the difference between the positive (red) supply and the audio output (blue). Class G can be as efficient as class D or T. While a class G design is more complex, it is based on a class AB amplifier and can have the same clean characteristics as well.
Class H
Class H is similar to class G, except the rail voltage is modulated by the input signal. The power supply rail is always just a bit higher than the output signal, keeping the voltage across the transistors small and the output transistors cool. The modulating power supply rail voltage is created by similar circuitry that you would find in a class D amplifier. In terms of complexity, this type of amplifier could be thought of as a class D amplifier driving a class AB amplifier and is therefore fairly complex.Here's a different point of view from an audio technician, "Ron", who e-mailed me in response to this page:
"My other comment has to do with your definition of Class G, where you state: "an ordinary class AB amplifier is driven by a multi-rail power supply". This is incorrect. Classes A to D are defined by conduction angle of the output devices, regardless of number or level of supply rails. (In Class A, output devices conduct at all times, through 360 degrees). In Class AB, between 180 to 360 degrees, depending on input signal level. In Class B, 180 degrees. In Class C (never used for audio), less than 180 degrees. In Class D, alternating conduction through a small angle.) In contrast, Classes G and up (coined in Advertising, rather than Engineering) define the power supply and not the output device operation. IOW, a Class G supply can still feed a Class A, AB, B (or even C or D) output stage."
Thanks for the input, Ron!
Written by Chris Opfell
