HOW IT WORKS: TEARDOWN OF TWO AUDIO RECEIVERS, BUILT 40 YEARS APART

In this blog post we will explore two audio receivers, which are a stereo system’s control center for audio and video sources. Specifically, we will compare a Beomaster 1900 receiver, designed and built by Bang and Olufsen circa 1978, with a Denon AVR-S540BT receiver manufactured 40 years later. B&O, and indeed the country of Denmark, is known for its devotion to high quality and aesthetics. Denon originated in Japan and is presently one of several major consumer brands, along with Marantz and Polk Audio, owned by parent company Sound United.

Photo 1: Bang and Olufsen Beomaster 1900 receiver

Photo 2: A look at the controls under the lid of the Beomaster 1900

Not surprisingly, the outward appearance of the two products is strikingly different. In the iconic Beomaster 1900, B&O produced a sleek, radically different receiver to be admired from all angles. It is intended to be set on a table-top with all controls accessible from the top, clearly labeled, and easy to reach. All ports are hidden underneath, and all switches and knobs are nestled under a long aluminum hinged panel. Frequently accessed controls are touch-sensitive buttons that are tightly integrated with the outward industrial design, while display elements are backlit, flush, and blacked out when off.   

Photo 3: Denon AVR-S540BT receiver

In comparison, the Denon receiver follows a much more traditional design. Intended to be stuffed into a cabinet or mounted on a shelf, only one side of the rectangular box is meant to be within view of the user. All knobs and buttons are labeled, but the predominant feature is the fluorescent dot matrix display centered on the top half of the front surface. This display informs the user of all functions and important aspects of its operation and changes on-the-fly. As expected, it also includes a remote control.

Photo 4: A look inside the Beomaster 1900

The development of integrated circuits in the 1970s and 1980s meant that analog circuits with numerous passive components could be replaced with digital circuitry on a chip. Although the Beomaster 1900 is blissfully analog in its audio handling, it’s evident that its designers went to great lengths to achieve what was at the time a modern and futuristic design by making the user interface look digital. For example, the setting of each of the manual sliders that control treble, bass, and balance is visible, without opening the lid, as one of 9 segments illuminated on the control panel light displays. This is especially intriguing, as balance and tone adjustments are fundamentally analog. When we remove the cover, we will see how this effect is achieved using no digital circuitry.

Photo 5: Modular design of the Denon receiver

Opening the cover of the Denon reveals other priorities, such as component modularity. The AVR-S540BT contains several discrete printed circuit assemblies (PCAs), which can be loosely split into functional blocks. There are several PCAs mounted directly on the front panel, providing a mounting for the user controls and displays, an audio power amplifier board attached to a large heatsink, two power supply boards, a digital board, a tuner board and an audio connector board. Additional features expected in a modern connected consumer device – including options such as Ethernet, WiFi, and Bluetooth – can seemingly be added by plugging in additional PCAs. This modularity is especially relevant since Denon is one of several brands owned by Sound United. It’s possible that several modules could be fitted depending on the brand and feature set, thus easing manufacturability and achieving cost savings.

Another priority in the AVR-S540BT is the separation of the digital and analog PCAs, in that the power amplifier has a traditional linear power supply using a heavy iron-cored transformer and other circuits are powered from a switch-mode power supply. This is presumably done to avoid any possibility of switching noise contaminating the final output.

Photo 6: Denon display

Now let’s take a closer look at the user interfaces. In the AVR-S540BT, all controls are accomplished digitally. The digital board contains a multi-channel codec which can digitize the analog inputs and interface with the SP/DIF optical input. It has multiple output channels, capable of driving amplifiers for a full surround sound speaker installation. There is an HDMI receiver chip that handles all the HDMI encryption. All these signals can be selected digitally, have volume scaling applied, as well as frequency-based filtering to compensate for the room in which it is installed. The setting of each control – such as bass, treble, and balance – is momentarily displayed during adjustment, and is represented as a number, such as a volume of 12.0.

Without the benefit of such a display, the Beomaster 1900 relies on trickery. Let’s return to the “digital” balance, treble and bass indicators. We see now that they are controlled with linear potentiometers (pots). These pots consist of an aluminum guide rail and a plastic carrier. They are of exceptional feel and robustness more resembling a linear guide rail than a manufactured component. Even more unique to the design are the indicators for these controls. To appear more digital, as the user adjusts the control, a printed mylar film moves with the control beneath a cylindrical lens and is backlit with an incandescent lamp through a colored plastic film. To the user, the indicator appears as a red bar with a brighter “pip” segment that moves left and right depending on the slide position.

Video 1: Adjusting bass

Video 2: “Digital” bass, treble, and balance indicators

Below the two-position switches and sliders under the lid are the touch-sensitive buttons that B&O branded “sensi-touch.” Standby, volume, input, and FM presets are controlled with by touching sensi-touch buttons on the front panel. Each button is a capacitive sensor with a copper contact behind the touch panel. This copper contact has a finger that touches the one of the legs of a TO-92 transistor on the main PCA.

Video 3: Internal view of FM tuning using rotary knob

The main FM tuning is controlled with a rotary knob and displayed to the user with a typical linear gauge. The tuning pot, the knob and the slide indicator are coupled together using a serpentine twine routed around pulleys and a spring-loaded tensioner. Thanks to this mechanism, the knob feels sturdy and smooth. Presets 1 thru 4 are activated through sensi-touch buttons and each have their own tuning control and have a much simpler and smaller, directly coupled linear screw pot driven by a thumbwheel below its indicator.

Photo 7: Replaceable module for volume detection (center)

Video 4: Bowtie-shaped volume indication

Two more sensi-touch buttons are for volume up and down. Had the Beomaster 1900 used a traditional rotary knob or slider, the volume could have been read directly using a potentiometer, as elsewhere. The use of the sensi-touch buttons, however, necessitated a unique volume sensor. Inside a replaceable module is an incandescent lamp placed between two photoresistors, and the touch sensors provide the bulb with 16 different brightness levels. Each brightness level changes the resistance of the photoresistors. Since the volume is inversely proportional to the brightness, the default state of the bulb is to be lit brightly at low volume. Both stereo channels are controlled simultaneously with the single light source. Interestingly, this inverse relationship would imply that the bulb burning out would result in maximum volume. Since this is undesirable, there is a small circuit constantly monitoring the current flow through the lamp, making the mute signal if the current is detected to be zero.

Photo 8: Incandescent bulbs used for volume indication

Also noteworthy is the volume indicator, which appears as a red bowtie-shaped display. The bowtie is lit with just two stationary incandescent lamps in a molded housing. The brightness of each lamp is modulated such that the light appears to move left to right with each volume step. That is, at low volume, the left side of the bowtie is brightly lit, and at high volume the right side is brightly lit. At mid-volume, the light is at about 75% of full brightness. This pattern does not follow basic modern user interface guidelines of increasing brightness proportionally to volume, but it was continued on several B&O models.

In conclusion, we see that the two products compared, the 1978 Bang and Olufsen Beomaster 1900 and the 2018 Denon AVR-S540BT receivers, accomplish many of the same function in vastly different fashion. The Beomaster 1900 was radical and avant-garde, intended to sit on a tabletop and be viewed from all sides. Its sliders and buttons were a joy to control, and it delighted (and perhaps at times perplexed) users with its futuristic glowing red “digital” displays. In contrast, the AVR-S540BT is austerely functional, its design emphasizing a modularity that allows the manufacturer to quickly and cost-effectively sell products under a variety of brand names and with a range of feature sets. The modular design also acknowledges the rapid pace of change, allowing for new technologies and connectivity options to be incorporated with the swap of a PCA. While it is a little unfair to compare display styles between the AVR-S540BT and the Beomaster 1900 because of the significant increase of information available on the Denon receiver, it is interesting to note that the controls and display are purely functional. The fluorescent dot matrix display simply provides information, the designers having made no attempt to delight the user by visually showing increasing volume or shifting balance in a creative way. This unimaginative interface is in sharp contrast to that of the Beomaster 1900, where the designers made so much out of very limited technology available to them at the time.