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Surround Sound MIXER & DECODER PART 2 – By JOHN CLARKE Last month, we described the circuit details for the Surround Sound Mixer & Decoder and gave the parts layout for the main PC board. This month, we complete the construction and give the test procedure. We’ll begin this month with the display driver PC board assembly – see Fig.5(a). Among other things, this board carries the four display driver ICs (IC12-IC15) plus four 11-way pin header sockets which make the connections to the display board. Begin by installing PC stakes at the external wiring points. This done, 60  Silicon Chip install the wire links, resistors, capacitors and diodes D1-D4, taking care with the diode orienta­tion. The four ICs (IC12-IC15) can now be installed, followed by transistors Q1-Q4. Trimpot VR1 (shown dotted) must be mounted on the underside of the board – see also Fig.6. Do not insert the pin headers yet; that step comes later when the display board is mounted. Fig.5(a) also shows the LED display board. Begin by soldering in the four 150Ω resistors. This done, the 40 LEDs can all be mounted, taking care to ensure that they are correct­ly oriented (the anode lead is the longer of the two – see Fig.3). Do not solder the LED leads yet, since the LEDs must all be later adjusted for height when the board is fitted to the front panel. Preparing the case The Surround Sound Mixer and Decoder is built into a cabi­net with a sloping front panel. This cabinet measures 170 x 213 x 31 x 82mm and is fitted with two self-adhesive labels. Begin by affixing the appropriate label to the rear panel, then drill pilot holes for the DC power socket, the RCA sockets and the 6.35mm stereo jack sockets. These holes can then all be Fig.5(a): install the parts on the display driver board and the LED display board as shown here. Do not solder the LED leads until after the board has been fitted to the front panel. carefully reamed to size. This done, mount the DC socket and the three top 6.35mm sockets in position. Once the rear panel is completed, the front panel can be prepared in similar fashion. Note that the holes for the LEDs and the display board Fig.5(b): these are the full size etching patterns for the display driver and LED display PC boards. Check the boards carefully for etching defects before installing any of the parts, as this can eliminate a lot of hassles later on. mounting holes should all be drilled to 3mm. The four mounting holes for the lid can be cut out using a knife and lightly finished using a reamer. Final assembly The major hardware items can now be fitted to the recommended case. Attach the small plastic feet to the underside of the case first, then install the main PC board. This board is simply slid into the bottom of the case so that the input and output sockets protrude through their matching holes February 1996  61 62  Silicon Chip Fig.6: shielded audio cable is used for most of the wiring from the main PC board to the front panel controls and the input sockets. Check the wiring carefully as it is installed, as it is easy to make a mistake which would be difficult to trace afterwards. in the rear panel. Attach the nuts to the 6.35mm sockets, then secure the board to the integral standoffs in the base using the self-tapping screws that come with the case. Cut each pot shaft to length before mounting all the pots on the front panel. The pots should all be oriented so that the markers on the knobs line up correctly with the front-panel markings. The toggle switches all mount towards the rear of the front panel. Once all the hardware items are in position, the LED dis­play board can be completed. First, mount the board to the underside of the front panel using four untapped 6mm spacers and 12mm-long screws, with the four 9mm tapped spacers used as nuts to hold the board in position. This done, push each LED into its front panel hole, check that the top surfaces are all aligned and solder the leads. Next, solder the 11-way pin headers to the underside of the display board, adjacent to each row of LEDs. Now plug the display driver board into these pin headers and secure this board to the tapped spacers using 6mm-long screws. Complete the assembly by soldering the pin headers to the driver board – see photo. Fig.6 shows the internal wiring details for the unit. Use medium-duty hook-up wire for the wiring to the display driver board and for the power supply connections. It’s a good idea to use red wire for the +12V wiring, green for the GND wiring, blue for the 0V wiring, and yellow for the DC socket and power switch (S7) wiring. The remaining wiring must all be run using shielded audio cable. This includes the wiring between the main board and all the pots, the top row of 6.35mm sockets and the remaining toggle switches. Keep these leads as short as possible while still allowing sufficient length for the lid to be opened comfortably and use cable ties to bundle them into neat looms – see photo in Pt.1. Test & adjustment Before applying power, check thoroughly for possi­ble wiring errors. This done, apply power and check the voltages on the main PC board. If all is well, there should be 12V between the +12V and GND terminals, while the GND terminal should be at about 5.45V with respect to the 0V rail. If these are incorrect, switch off power February 1996  63 immediately and locate the fault before proceeding. Assuming that all is OK, check that pin 8 of each LM833 IC (IC1-IC10) is at +12V with respect to 0V. You can also check the supply to the ICs on the display driver board (IC12-IC15). In each case, pin 3 should be at +12V. All other voltages are measured with respect to GND. Check the pin 1 and pin 7 outputs of IC1-IC11. These should all be within 100mV of the GND voltage. If all these voltage checks are OK, set VR1 to midway, then rotate it clockwise so that all the LEDs in each display are just extinguished. The mixer can now be tested with a signal. However, before doing this, it may be necessary to make up some adaptor leads; eg, leads with an RCA to mono 6.35mm plug and/or leads with an XLR socket to stereo plug. This latter adaptor is depicted in Fig.9. The rear panel carries the six phono input sockets, four RCA output sockets (L, R, C & S) and a power socket. This view shows the completed display board assembly, prior to installation on the front panel. Make sure that the LEDs are all correctly oriented. The LED display board in mounted on the driver board using 12mm spacers and the connections made via four pin header sockets. Fig.7: this full-size artwork can be used as a template when drilling the rear panel. + 12VAC IN A INPUT L INPUT C INPUT + + + L OUT + + C OUT B INPUT R INPUT S INPUT R OUT + + S OUT + + + 64  Silicon Chip Fig.8: this is the full-size etching pattern for the main PC board. Check the board carefully for etching defects by comparing it against this pattern before installing any of the parts. To test the unit, first secure the lid and rotate all level controls fully anticlockwise. This done, connect a signal to the Left input and adjust the output and left level pots so that a reading appears on the left bargraph display. Check that there is a 20dB change in level when switching between the LINE and MIC inputs. If the Left channel checks out OK, Fig.9: this diagram shows how to wire an adaptor cable with an XLR socket on one end and a stereo phono plug on the other. Be sure to use 2-core shielded audio cable. February 1996  65 4-CHANNEL + L + C MIC LINE + A-CHANNEL MIC + LINE SURROUND SOUND MIXER & DECODER B-CHANNEL MIC + LINE + R POWER S + OUTPUT INPUTS + C C + + 3 0 L LEFT PAN R L PAN R -3 -6 -9 + -12 + + -15 -18 CENTRE C PAN S C PAN -21 S -24 SIGNAL LEVEL (dB) L + C R S RIGHT + + + + SURR LEVEL LEVEL OUTPUT LEVEL Fig.10: this full-size artwork can be used as a drilling template for the front panel. you can test the Centre, Right and Surround channels in exactly the same manner. Now feed the signal source into the A channel input and check the opera66  Silicon Chip tion of the Pan and Level pots. Note the interac­tion between each channel as the knobs are rotated. This done, test the B channel in the same way. Assuming that everything checks out, you are now ready for some surround sound recording. You will probably need some prac­tice to get everything just right but the results SC will be well worth the effort.