The Active Speakers/Amplifiers Selector (SAS-28) is an active matrix switch design that receives speaker outputs from up to eight stereo audio amplifiers for up to two pairs of loudspeakers, enabling simultaneous selection of two switched amplifiers, each one, with a pair of loudspeakers. Also is possible to associate two pairs of loudspeakers to a single amplifier.

The design conception allows expansion of the number of amplifiers and loudspeakers. This initial configuration was due to a particular demand, where two pairs of loudspeakers and at least eight amplifiers were available at the moment and I wished to use it always when wanted. As I wished at the musical auditions, in a moment listen to one of the amplifiers, and in a another time later, another one. The switch seems to me the most practical solution to not having to connecting wires whenever I need to use a loudspeaker in any of the amplifier.

Designing a switch to meet this goal is simple and relatively inexpensive if its architecture is passive, that is, using only rotary switches, wire interconnection and terminals. The trickiest thing these days is finding rotary switches, where its contacts support the high currents from amplifier outputs.

A second solution would be to continue using rotary switches, these for low current, but only to switch relays and these, would support the high switching currents. It is also a simple, inexpensive and easy to build solution.

The third solution that came in my mind, consists of an architecture with a elevated complexity and higher cost than previous solutions and with some perfumeries. In spite of having a high cost in relation to its functionality, this third one, whose project will be outlined below. It had as main motivation, the learning in the handling of the components to be used in the project. Basically, the project requirements are:

• Be 100% active (for the active term properly, the most correct would be to use semiconductors in the commutation, but this is not the case here)
• Use relays to compose amplifiers/loudspeakers switch matrix, and that support powers of up to 100 watts RMS
• Use control logic from a micro controller (like arduino); Possess visual indicators of operations and status, including something like VU Meter; Use chassis, panel, source and keys of a Tuner Model 9 of Gradiente brand (Brazilian manufacturing product of the 80s) to accommodate the components and controls of the switch.

Now, we will to presented the main features of the SAS-28. It will be always associated with the front panel functions of the chassis/panel of the above mentioned tuner which was renovated with another silkscreen. Figure 1 below (click on the image to see the high resolution version) represents a layout sketch that reflects the physical panel of the Gradiente M9 (renovated), whose the dial area were modified for the new product and that will be the basis of the explanation that follows. The source file in the format of Adobe Photoshop (each item is a layer) and Corel Draw (much used as art for serigraphy) for anyone wishing to venture into the project, therefore redo the screen printing using the M9 panel, whose files are found in the "files" folder at GitHub repository.

Figure 1: Black and natural aluminum layout of the SAS-28 panel using the Gradiente Model 9

The following features are associated with the buttons / controls and displays on the SAS-28 panel, and should therefore be related. Also no physical order of the commands/displays on the panel was followed.

Somewhat obvious, but this button has the function to turn on/off the SAS-28. For it, we will use the same used in the M9 which is a simple push button with retention.

The SAS-28 was specified to operate with until two pairs of loudspeaker, each par being able to be switched to one of eight separately available amplifier inputs.

This command is a "rotary encoder" control with a "push button", that is, it can be turned infinitely to the right, as to the left and, when pressed, triggers a contact. When turning this control, a pulse is generated at its terminals (a key open and close), so there is a central pin (connected to GND or VCC according to design) and two others that will receive the first pulse according to the direction of rotation.

In the case of the SAS-28, when turned to the left, a sequential value of 1 to 8 and the character "d" will appear on the seven-segment display called "SP / AMP SELECTOR", each value from 1 to 8 corresponding to one of the amplifiers connected to the SAS-28 and with direct association to the pair of loudspeakers "SP1", except for the position "d" (Deactivate) that will be explained at the end of this paragraph.

Thus, when the display is turned, the display will change its value sequentially from 1 to 8 and "d", which will be selected by pressing the push buttom, that is,  if the push bottom is pressed when the display is indicating the number 5 , this action switches the amplifier that is connected in the input 5 to the pair of loudspeakers "SP1".

The switching occurs for both the left and right amplifier channels and if an amplifier that is already switched to the other pair of loudspeakers is automatically selected, the adjacent pair is automatically switched off and the new switching is performed.

Analogous process occurs if the command is rotated to the right, however the association will occur with the pair of loudspeakers "SP2". Note that regardless of the last selection, switching will always occur for the two pairs of loudspeakers (SP1/SP2) as indicated by the seven-segment displays ("SP/AMP SELECTOR").

If the push bottom is pressed while the pairs of loudspeaker are selected for the same amplifier, it will cause the SP1 pair of loudspeakers to be in parallel with the SP2 pair by connecting them to the selected amplifier. Since the two pairs of loudspeakers now have a parallel-type connection, the impedance generated in this connection must be observed so that it is not inferior to that supported by the amplifier.

In case of pressing the letter "d", it will cause the switching to be undone for that pair of loudspeakers, that is, it is a way to disconnect the pair of loudspeakers from any amplifier.

The switching selected for the two pairs of loudspeakers and amplifiers are always recorded in non-volatile memory so that when the SAS-28 to power on switched, the last configuration in use is loaded.

This on/off button is intended to disable the "SP/AMP SELECTOR" command, avoiding undesirable switching. It was only to utilize one more remaining button of the M9.

This control consists of two buttons labeled "SP/AMP SEL" and "VU". When one of the buttons is pressed, the other is automatically released.

Two displays with 2 lines and 16 characters (black background with characters in red) were considered in the SAS-28, each one related to the functions associated with each pair of loudspeakers.

Each display has two functions, that is, if the "SP/AMP SEL" button is pressed, the information of the current switching between amplifiers and the pairs of loudspeakers will be displayed. The pair of loudspeakers "SP1" is for the left display and "SP2" for the right display. If you press the "VU" button, both displays will work with a VU Meter, each for a pair of loudspeakers. If current switching is to pair of loudspeaker to one amplifier, of course, both displays will show each channel of amplifier.

The figure below, the left display show the pair of loudspeaker SP1 connected to amplifier number 2 and right display show the pair SP2 connected to amplifier number 7.

This on/off button has the function of turning on/off all the display of the SAS-28. It can be useful in nightly auditions where the brightness of the lights may bother you. The exception is the left display that will have a point alight constantly, being a way to inform which equipment is in power on state (not in standby). It was also to use remaining available button of the M9.

This control consists of two low/high type functions buttons called "SP PAIR1" and "SP PAIR2". Since the displays will also have a VU Meter function, this control allows you to increase or decrease your sensitivity in two levels, where in the "LOW" position the VU will be less sensitive, while in the "HIGH" position, the latter being suitable for low power levels. It was not thinking of attenuation values ​​in dB or even WRMS/Peak power, but only one facility that might be useful.

Following the brief explanation of the features of the SAS-28, we now turn to the electronic part of the SAS-28, which includes the electronic diagram, printed circuit boards and components to be used.

The electronic circuit of the SAS-28 is composed of the following modules:

The Gradiente Model 9 tuner, whose chassis, panel, and other items will be used to accommodate the SAS-28, have a 13 VDC series regulator power supply. This voltage is ideal for supplying the switching module as the relays, whose coils are for 12 VDC, as for the arduino (control module) which also has as a recommendation to be between 7 and 15 volts. In true Arduino board had some regulators, including 5 VDC. This 5 volts provided by the Arduino will also be used to supply the voltage for the displays.

The power supply of the M9 is located in the main board that the same that accommodates all the switches of the product, therefore, will be 100% harnessed but only the power supply and the part of the printed circuit of connection of the switches. Some tracks of printed circuit board which connects switches to the rest of tuner circuit will be open to then connect to our switching module (relays) and control module (Arduino).

The switching module is the part of the SAS-28 that receives the loudspeakers outputs of the eight amplifiers (AMPxL/AMPxR) and switches them between the two pairs of speakers (SPxL/SPxR). The download of the schematic of this module can be downloaded in PDF format from the "files" folder at GitHub repository, as well as in the format of the Diptrace software.

According to the circuit, the switching module consists basically of relays, where 32 (U1 to U32) whose its coils are 12 VDC and contacts with a capacity of up to 10 amps were used. In this case, the current capacity of the contacts is directly associated with the power of the amplifiers. It was considered that a 10A hermetic relay could support amplifiers up to 100 watts RMS, which is the purpose of this project. In practice it supports even higher powers, but the useful life is reduced. Relays that support higher current can be used, thus allowing the use of amplifiers with higher powers than above.

Of the 32 relays used, two are used for each amplifier channel. Both relays allow a dual path of switching up to two pairs of loudspeakers, distinctly without risk of more than one amplifier applying signal in the same pair of loudspeakers (control module responsibility). Also, the relays are switched in pairs, that is, the command that switches the left channel relay is the one that switches from the right channel of a respective amplifier. Thus, only 16 relay commands were needed, which come from the control module (an Arduino Mega).

The digital outputs of the control module (Arduino) have low power, requiring the use of drivers (R1/Q1 to R16/Q16) and as the relays are energized in pairs, only one diode against reverse current was used aiming at the driver protection. In relation to other protections, such as fuses, coupling components against parasitic inductances and relieving relay contacts in the relay contacts, they were not foreseen, however, possible to be added.

To accommodate the components of this module, a double sided printed circuit board has been developed. The connections of the amplifier/loudspeakers from the plug/terminals installed in the rear panel of the M9 arrive through 1.5mm gauge wires to the board, whose connections type is "faston" . The control signals of the relays arrive via a HEADER connector (J1). Thus, it is observed that all connections with the switching board bring practicality to the connection/disconnection process, which is important in the case of maintenance.

The layout of the printed circuit board in PDF format and in Diptrace can be downloaded from the "files" folder at GitHub repository.

The SAS-28 control module is based on a microcontroller now provided by the Arduino Mega board (it is not the purpose of this document to address details of the Arduino, and you should consult the product website by clicking here).

In the SAS-28 the Arduino is responsible for interfacing the Rotary Encoder, read the status of the "LOCK SEL", "DISPLAY VIEW/VU" and "DISPLAY" buttons; control both LCD and seven-segment displays; activate all relays involved in the audio switching process between amplifiers from 1 through 8 and the two pairs of speakers (SP1/SP2) through their digital inputs / outputs.

Two analog inputs are responsible for the VU Meter which receives voltages ranging from 0 to 5 VDC and which correspond to the audio signal from the VU Meter Module.

Table 1: Mapping of the pins connections used in the Arduino to make the functionalities mentioned above.

Pin Number	Pin Mode	Function
2-8	Output	Display SP1, segments from "a" to "g" respectively.
9-16	Output	Display SP2, segments "a" to "g" and "." Respectively.
17-18	Input	Rotary Encoder Pins 3 and 1; pin 2 is connected to the GND.
19	Input	Rotary push button, pin 2 of Rotary Encoder; pin 1 is connected to the GND.
20	Input	To push button pin which make "Display View" function.
21	Input	Input of one of pin contacts from push button function "Display On/Off"; a second  contact is used to turn off the backlight power of the LCDs properly.
22-27	Output	LCD SP1 (Left) pins, RS, EN, DB0, DB1, DB2, DB3 signals respectively.
28-33	Output	LCD SP2 pins (Right), RS, EN, DB0, DB1, DB2, DB3 signals respectively.
36-51	Output	Relays controls of the switching module; the pair pins trigger relays of the SP1 related with the amplifiers and the odd ones with SP2 respectively.
78-79	Input	Analog inputs A0/A1 which will receive the audio signal for the left side VU Meter (SP1), respectively for the right and left channels.
80-81	Input	Analog inputs A2/A3 which will receive the audio signal for the right side VU Meter (SP2), respectively for the right and left channels.
82-83	Input	Analog inputs A4/A5 used as digital inputs which will receive the signal to control the sensitivity of the VU Meters from the push buttons to the left (SP1) and right (SP2) sides respectively.

The wired connections of displays, Rotary Encoder and Arduino can be viewed in PDF format in the files" folder at GitHub repository, as well as in the format of the Diptrace software. No printed circuit board was produced, and the assembly was done on pre-punched boards, that is, those that looked like a prototype board.

The "firmware" code that will control the SAS-28 can also be seen in the "files" folder. As the code has been commented on its functionalities, no comments were made in this text.

This project uses as drive to excites VU Meter (LCD 16x2), a China board based on the TA-7318P IC. Its design is like figure below.

Figure 2: VU meter drive schematic based on TA-7318P

Its output is wired direct to 22/27 and 28/33 analog input pins on the Arduino. This board have a control of output level regarding the input power which ensure any over voltage on the Arduino as soon as the analog inputs of the Arduino are limited to the range of 0 to 5 volts. Thus, in general, this circuit will pick up the entire audio spectrum, whose amplitude varies from negative to positive and will conform to the range of 0 to 5 volts.

The mechanical process for assembly of the SAS-28 has no difficulty and as already mentioned, was based on the chassis of a tuner Model 9, from brazilian brand called Gradiente.

So, the best way to present how the construction was done, is through photos. With this objective, some photos were taken contemplating the components and phases of the assembly and can be visualized HERE.