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Stereo preamplifier with IR remote control Last month, we gave the block diagram of the Studio Remote Control Preamplifier & also described the transmitter circuit. In Pt.2 this month, we give the full circuit details of the main preamplifier unit. PART 2: By JOHN CLARKE Because of its size, the circuit has been split into two separate diagrams. The first diagram is designated Fig.5 and this shows the input selection circuitry, the phono preamplifier stage and the associated control circuitry and LED displays. The second diagram, Fig.6, shows the digital volume control and its asso­ciated LED displays, the remote control receiver stages, the tone control stage, and the headphone amplifier stage. For the sake of clarity, only the left channel of the stereo circuitry 40  Silicon Chip is shown on each diagram. The ICs for the left channel are numbered as shown on Fig.5 and Fig.6, while the right channel ICs are numbered by adding 100 to the equivalent left channel number; eg, IC1 in the left channel is equivalent to IC101 in the right channel. Note that we have mainly used lownoise NE5534AN op amps to buffer or amplify the audio signal, the one exception being an OP27GP in each channel for the volume control. The 5534 op amp is amongst the best available for low distortion and noise, while the OP27 also has low noise and distortion plus extra low input offset voltage. This latter specification is necessary to allow the op amp to be connected to the D-A converter. Phono amplifier We’ll begin the circuit description by looking at Fig.5. IC1 is the phono preamplifier and RIAA/IEC equalisation stage. It takes the low level signal from a moving magnet cartridge and amplifies this by 56 at the mid-band frequency of 1kHz. The equalisation network ensures that we get less gain at frequencies above 1kHz and more gain below 1kHz. More specifically, a 100Hz signal is boosted by 13.11dB while a 10kHz signal is cut by 13.75dB. The phono signal is fed directly from the input socket via a small inductor (L1), a 150Ω resistor and 47µF bipolar capacitor to the non-inverting Left: the 68HC705C8P microprocessor (IC14) is mounted in a socket near the centre of the main PC board. This IC sets the volume by providing control signals to a dual D-A converter (IC15) & drives the digital readout & the balance display LEDs. more than one, a situation that could otherwise lead to non-symmetrical clipping and premature overload in the preamplifier. Source selection input (pin 3) of IC1. The inductor, series resistor and 100pF shunt capacitor form a filter to remove any RF signals which might be picked up by the phono leads. The 100pF capacitor is also necessary to provide correct loading for the magnetic cartridge. Most cartridges need to be loaded with a capacitance of 200-400pF for best results. When combined with the usual 200pF or so of cable capacitance (from the phono leads), this 100pF capacitor will ensure optimum load­ing. The RIAA/IEC equalisation is provided by the feedback components between pins 2 and 6 of IC1. These components provide the standard time constants of 3180µs (50Hz), 318µs (500Hz) and 75µs (2122Hz), as required for RIAA equalisation. The IEC recom­mendations also include a roll-off below 20Hz (7950µs). This is provided by the .068µF output coupling capacitor, the 1MΩ resis­tor and the 330kΩ resistors following IC2 and IC3, and other low frequency roll-offs in the circuit. One of these roll-offs (at about 4Hz) is provided by the 100µF capacitor and its series 390Ω resistor on pin 2 of IC1. The 390Ω resistor sets the gain for AC signals above 4Hz, while the 100µF capacitor ensures unity DC gain. This unity DC gain ensures that any input offset voltages are not amplified by IC2 is a CMOS analog switch which provides source selection for the PHONO, TUNER, CD, VCR and AUX inputs. Each input, except for the phono input on pin 14, is loaded with a 47kΩ resistor to protect the IC from damage due to electrostatic charges, as could occur if the inputs are left unconnected. The A, B and C control inputs at pins 9, 10 and 11 are used to select which source is switched through to the output at pin 3 (more on this later). The signal from pin 3 of IC2 is now fed via two paths. First, it is fed directly to the pin 12 (ax) input of IC3, anoth­er CMOS analog switch. Second, it is fed via a 100Ω resistor to the pin 3 input of unity gain buffer stage IC8. The output from IC8 appears at pin 6 and provides the TAPE OUT signal via another 100Ω resistor. This resistor provides short circuit protection for the op amp and also isolates the output of the op amp from the signal leads to prevent RF feedthrough. IC3 is used to select either the source signal from IC2 or the TAPE IN input for tape monitoring. This IC also provides for mono/stereo switching. Just how this is achieved is best under­stood by first noting that IC3 is essentially a 3-pole 2-position switch. The three poles are designated “a”, “b” and “c” and each pole can select either its corresponding “x” input or its corresponding “y” input, depending on the status of the A, B and C control inputs at pins 9, 10 and 11. In other words, pole “a” can select ax or ay, pole “b” can select bx or by, and pole “c” can select cx or cy. As shown on Fig.5, the left channel program and tape moni­ tor inputs are applied to the ax and ay inputs respectively (note: the right channel inputs are applied to bx and by, although this is not shown here). Thus, depending on the status of the A, B and C control lines, either the selected program signal on the ax input or the TAPE IN signal on the ay input will be switched through to the “a” output at pin 14. The “c” pole is used to provide stereo/mono switching. This pole is connected to the left channel signal path via a 4.7kΩ resis­tor, while the cy terminal is connected to the right channel via another 4.7kΩ resistor. In stereo mode, the “c” pole selects the cx terminal (which is not connected to a signal), while in mono mode, the “c” pole selects the cy terminal so that left and right channel signals are mixed together. Op amp IC4 is used to buffer the left channel signal. Its input (pin 3) is fitted with a 1kΩ “stopper” resistor to prevent the possibility of RF breakthrough from mobile phones and 2-way radios. The buffering provided by IC8 and IC4 at the outputs of CMOS switches IC2 and IC3 is vital in order to obtain very low levels of distortion. The distortion from these switches is typically .04% for a 1kHz 5V p-p signal when driving a 10kΩ load. However, if the load is greater than 220kΩ, as provided by the op amps, the distortion is less than .005%. To obtain maximum signal handling capability, the two CMOS switch ICs are powered from ±7.5V rails. These supply rails are derived from ±15V rails via 1kΩ limiting resistors and zener diodes ZD1 and ZD2. The ±15V rails are in turn derived from regulators in the main power supply circuit. Control circuitry IC9, IC10, IC12 and IC13 make up the program selection control circuitry. IC9, a 7-stage Darlington transistor driver, is used to convert the 0-5V signals from the IR remote control decoder chip (IC23) to 0-7.5V signals, as required by the CMOS switches. The A-E inputs at pins 1-7 of IC9 each connect to the base of a Darlington transistor via an internal 10kΩ resistor. These Darlington transistors have open collector outputs at pins 1016 and these are all tied to the +7.5V rail via 10kΩ pull-up resis­tors. The emitters all connect to pin 8 which goes to ground. October 1993  41 42  Silicon Chip DATA A B C D E FROM IC23 TAPE IN TAPE OUT AUX2 AUX1 VCR TUNER CD PHONO 4 16 1 8 15 14 2 3 6 IC13a 11 2 IC13b .01 14 14 5 3 1 12 15 13 14 5 7 4 10k 1M 100  13 13 5 .068 12 4011 10k D3 1N4148 100  6 11 10k .015 5% 100  .0047 5% 200k 8 4 -15V 16k IC1 5534 6 10k 100 BP 390  2 5 10k 100pF 5 4 10k 100k 3 10pF 10 10k 100k 150  7 +15V 7 IC9 ULN2003 5-7.5V CONVERTER 47k 47k 47k 47k 47k 47k L1 47 BP L D3 D2 D1 D4 5 4 3 2 0 1 A B C 11 10 9 8 IC10 4042 16 +7.5V Q3 Q2 Q1 POL 12 9 3 6 TO IC102 (OTHER CHANNEL) 8 IC2 4051 16 330k 100k 10 7 6 4 2 3 -7.5V 2 100  3 7 5 100  4 8 6 -15V 10pF IC8 5534 +15V 13 12 ay ax A 11 B 10 IC3 4053 16 C 100k 3 7 4 330k 8 14 6 9 10 10 11 11 16 +7.5V C B A 6 2 330W -7.5V 4.7k 7 IC11 4051 3 +5V 4 1k +7.5V POWER-ON MUTE TO IC104, PIN3 (OTHER CHANNEL) 9 cy c a INH 10 8 2 3 5 A 1 A 12 A 15 A 13 A      K K K K K LED1-LED6  K 14 A 100  4 8 -15V 10pF 5 IC4 5534 7 +15V 6 AUX2 AUX1 VCR TUNER CD PHONO TO IC15 PIN4 10 16VW ZD2 7.5V 400mW 1k 0.1 STUDIO REMOTE CONTROL PREAMPLIFIER (1) -7.5V +7.5V 2x 10 16VW ZD1 7.5V 400mW 1k K 330   3 CK2 S 11 CK1 D2 9 IC13d 9 IC13c 10 12 8 13 +7.5V 11 .01 D4 1N4148 10k 5 D1 8 S 6 7 Q2 Q1 12 2 330  LED7 TAPE MON. A K IC12 13 4013 Q2 Q1 1 100k 10 R 4 R  LED8 MONO A +5V -15V +15V POWER-ON RESET +7.5V 10 14 Fig.5 (left): this diagram shows the phono preamplifier stage (IC1), the input selection circuitry (IC2 & IC3), & the associated control circuitry (IC9-IC12) & LED displays. Fig.6 on the following pages shows the digital volume control circuit (IC14 & IC15), the remote control receiver & decoder stages (IC22 & IC23), the tone control stage (IC6), & the headphone amplifier stage (IC7, Q1 & Q2). As well as providing level translation, the Darlington transistors inside IC9 also function as inverter stages; ie, they invert the signals from IC23. Note that we have not used the Darlington transistor which connects to pins 4 and 13. IC10, IC12 and IC13 monitor the outputs of IC9. These outputs are all normally high except for the pin 16 output which is normally low. When a valid infrared transmission is decoded, pin 16 goes high while the other outputs variously go low or stay high depending upon the transmitted code. Note that pin 15 will always switch low if an input source is being selected. Similarly, pin 14 always switches low for Tape/Mode selection. IC10 is a 4042 quad latch and initially, at power up, its D1-D3 data inputs (pins 4, 7 & 13) are high, while its polarity input (POL) at pin 6 is pulled high by a 10µF capacitor. When pin 6 is high, the levels at the Data inputs are inverted and fed to the Q-bar outputs (pins 3, 9 & 12), provided the latch (L) input at pin 5 is also high. This latch input is initially high, since pin 1 of NAND gate IC13b is pulled low (by pin 16 of IC9) and thus the output at pin 3 is forced high. As a result, IC10’s Q-bar outputs are all initially set low, since they invert the Data inputs. This means that the A, B and C control inputs of IC2 and IC11 are also all low and so IC2 selects the CD input (ie, input 0 at pin 13). At the same time, IC11 switches its pin 13 terminal to pin 3 to light the CD LED (LED 2). The 330Ω resistor in series with pin 3 limits the cur­rent through the LED to about 10mA. Thus, each time the preamplifier is switched on, the CD input is selected by default. Following switch-on, the 10µF capacitor on pin 6 of IC10 charges via a 100kΩ resistor until pin 6 is at 0V (ground). This means that the signals on the data inputs are now inverted and transferred to the Q-bar outputs when the latch input at pin 5 is low. When a decoded signal is received by IC9, its pin 16 output goes high (this is the data acknowledge signal). If an input is being selected, then pin 15 goes low and this low is inverted by IC13a and applied to pin 2 of IC13b. Thus, pin 3 of IC13b switch­es low and momentarily pulls pin 5 of IC10 low to latch the new signal at the data inputs to the Q-bar outputs. As a result, a new code is applied to the A, B and C inputs of IC2 and IC11 and so a new source is selected and the appro­priate indicator LED is lit. Note that IC10 can only latch through the signal at its Data inputs when its Latch input (pin 5) goes low. This only occurs when pin 15 of IC9 goes low. In practice, this means that the pin 10-12 outputs of IC9 can be used to control other parts of the circuit without affecting IC10 (and thus the program selection), simply by keeping pin 15 of IC9 high. Tape/source selection The Tape/Source/Mode selection circuitry functions in simi­lar fashion to the program selection circuit. In this case, however, the data signals from pins 10 & 11 of IC9 are controlled by IC12, a 4013 dual-D flipflop. Its Q outputs in turn control CMOS switch IC3 (to select between source and tape) while its Q-bar outputs switch the Tape Monitor and Mono indicator LEDs (LEDs 7 & 8). When the appropriate button on the transmitter is pressed, pin 14 of IC9 goes low and pin 16 goes high. These outputs are decoded by IC13c and IC13d to provide a clock pulse to IC12. Each time a clock pulse is received, the data levels on pins 5 and 9 are clocked through to the Q outputs and applied to IC3. IC12 is reset at power-on to force the Q1 and Q2 outputs low. This corresponds to a stereo source selection. The power-on reset circuit consists of the 10µF capacitor and the 100kΩ resis­tor on pins 4 & 10. Volume control Let’s now take a look at the volume control stage – see Fig.6. The audio October 1993  43 .01 +5V 17 3 RFBA OUT A 4 VIN A FROM IC4 13 13 14 14 15 15 55 DAC A A GND 2 2 1 7 10 9 8 2 20 WR 7 A/B 16 RFBB 4 6 TO IC105 -15V .0047 4.7k 4.7k 7 1k +15V 19 120  0.5W 6 120  0.5W D10 1N4004 470 25VW RL1 47  6.8 47 2 22 5 4 330 5 7 4 6 IC22 SL486  D B IC23 MV601 1 C B 6 X2 500kHz 4.7k A 7 CLR 8 0.15 15 14 13 12 100pF 15 E 100pF OE 2 9 DATA 10k E V NEG. F1 500mA POWER A S1 T1 20VA D5-D8 4x1N4004 30V 27  5W +21V 15V 240VAC 0V N D9 1N4004 E CASE 44  Silicon Chip 10 25VW 4700 25VW 10 25VW 4700 25VW 10 25VW -21V 7805 GND 13 12 11 D11 10 1N914 8 D12 IN REG2 7815 GND GND IN 7915 REG3 10 25VW +15V 10 25VW 2x0.1 10 25VW 2x0.1 D15 D16 5x1N914 5x0.1 OUT OUT D13 +5V OUT 10 25VW A DATA K REG1 IN TO IC9 C B 14 D14 .0047 240VAC D  0.1 .015 10k 10k A LED9 ACK A 3 9 11 16 MOM .0047 16 16 IRD1 BPW50 +5V 0.22 3 .0047 TREBLE VR2a 25k LIN 1.5k V NEG 10 22k 22k 8 6 330pF IC116 OP27GP 100pF DB2 DB3 DB4 DB5 DB6 DB7 11 5 IC5 5534 4 2 DAC B 12 7 3 4 3 DGND 18 18 VIN B 1k 6 -15V IC15 AD7112CN CS BASS VR1a 100k LIN 22k 10pF IC16 OP27GP 3 OUT B FROM IC104 100pF DB1 DB0 +15V +15V DOWN S2 UP S3 MUTE S4 9x0.1 -15V STUDIO REMOTE CONTROL PREAMPLIFIER (2) +15V Q1 BC338 B 10k 10pF +15V 10pF 7 5 2 8 IC6 5534 3 TONE CONTROLS S5a 10pF IN 6 7 3 S6a OUT 10k HEADPHONES AMPLIFIER 4 RL1a -15V E B 6.8 BP OUTPUT 47k 22pF FROM OTHER CHANNEL C Q2 BC328 10k -15V 10k HEADPHONES 33  D2 1N4148 4 100 BP 100  33  8 6 IC7 5534 2 E D1 1N4148 5 C 10k LEFT 10k 40 21 21 22 23 24 25 26 27 28 37 1 PC5 29 30 11 8 6 12 13 14 10 IC17 ULN2003 PC4 9 RIGHT h +5V 11 12 C VIEWED FROM BELOW 13 8 IC18 ULN2003 PC3 PC2 7 PC1 PCO PA7 PB0 PB5 2 3 E PC6 PB3 36 0 9x 330  10 PB4 33 3  PC7 PB2 34 6 B 1 R PB1 32 9 BALANCE LED10-18 3 IC14 MC68HC705C8P 31 h PB6 PD2 PB7 PD3 PD4 PA0 PD5 PA1 PD7 PA2 PA3 IRQ PA4 PD0 PA5 PD1 PA6 4.7M X1 3.58MHz 5 39pF 3 7 6 5 78-- 7915 I GO G IO 4 12 13 14 15 16 17 K A 18 K A 19 11 10 9 8 7 6 5 5 20 16 6 LE D 2 C 4 +5V 7 1 B A a b c d 5 16 8 IC19 4511 3 6 LE D e f 4 g 15 14 +5V 7 1 B A b c d 8 5 4 2 3 2 LE f 13 12 11 10 9 g a 15 14 B A IC21 4511 3 e 7 1 C 4 8 7x 330  10 9 5 16 IC20 4511 a 7x 330  2 C 3 13 12 11 10 9 39pF 4 4 +5V 38 39 6 b c d D e f 6 8 g 13 12 11 10 9 15 14 10 9 2 7x 330  10 9 8 5 4 2 3 8 5 4 3 a f e +5V b g c d DISP1 HDSP7803 DISP2 HDSP7803 DP 1,6 1,6 7 330  DISP3 HDSP7803 1,6 ATTENUATION (dB) October 1993  45 PARTS LIST FOR REMOTE CONTROL STEREO PREAMPLIFIER Main preamplifier 1 1-unit high rack mounting case 1 screen printed front panel to suit case 1 rear panel self adhesive label 1 PC board, code 01308931, 350 x 230mm 1 PC board, code 01308932, 243 x 25mm 1 neutral Perspex® sheet, 130 x 20 x 3mm 1 plastic film mask for front panel LED displays 1 2 x 15VAC 20VA low profile transformer (Transcap) plus four screws & nuts to suit 1 240VAC panel-mount mains switch (S1) 1 mains cord & plug 1 cord grip grommet 1 3-way mains terminal strip 1 M205 panel-mount fuse holder (F1) 1 500mA 2AG fuse 2 micro U heatsinks, 18 x 19 x 10mm (Altronics H 0504 or equival­ent), plus screws & nuts 1 TO-220 heatsink, 30 x 25 x 12mm (Jaycar HH-8504 or equivalent) plus screw & nut 2 16mm brushed black aluminium knobs 1 6.35mm stereo DPDT switched insulated phones socket (Altronics P 0076 or equivalent) 1 micro PC-mount 12V DPDT relay (Altronics S 4150 or equivalent) 1 16mm 100kΩ linear dual- output from IC4 (Fig.5) is fed into pin 4 of IC15, an AD7112 dual logarithmic D/A converter (DAC). As stated in Pt.1, this device is used as a programmable resistance to control the gain of op amp stage IC16 and thus the level of the audio signal. The way in which this works was described in detail in Pt.1. An internal resistor inside IC15, designated RFBA (at pin 3), sets the maximum gain of IC16 to -1, while the 100pF feedback capacitor ensures high 46  Silicon Chip ganged pot (DSE R-7661 or equivalent) 1 16mm 25kΩ linear dualganged pot (DSE R-7657 or equivalent) 1 PC-mount DPDT push-on/ push-off switch plus a black knob (S5) 3 snap-action PC-mount switches (S2-S4) 1 black panel-mount banana socket 18 panel-mount insulated RCA sockets (Arista RCA31 or equivalent), or use an insulated sub-panel plus screws & nuts 2 2µH wideband chokes (Philips 4330 030 3896) 1 40-pin IC socket 45 PC stakes 5 rubber feet 6 6mm standoffs plus screws & nuts 10 cable ties 1 4-metre length of 0.8mm tinned copper wire 1 2.5-metre length of shielded audio cable 1 120mm length of twin shielded audio cable 1 400mm length of green hookup wire 1 400mm length of green/yellow mains rated wire 2 solder lugs 1 screw, nut & star washer 1 Murata CSB500E 500kHz ceramic resonator 1 3.579545MHz parallel resonant crystal Semiconductors 12 NE5534AN low noise op amps (IC1, IC4, IC5, IC6, IC7, IC8, IC101, IC104, IC105, IC106, IC107, IC108) 3 4051 8-channel analog multiplexers (IC2, IC102, IC11) 1 4053 triple 2-channel multiplexer (IC3) 3 ULN2003 7-way Darlington drivers (IC9, IC17, IC18) 1 4042 quad latch (IC10) 1 4013 dual D-flipflop (IC12) 1 4011 quad NAND gate (IC13) 1 MC68HC705C8P programmed microprocessor (IC14) – see footnote 1 AD7112CN dual log D/A converter (IC15) – NSD Aust. 2 OP27GP op amps (IC16, IC116) 3 4511 BCD to 7-segment LED display drivers (IC19-IC21) 1 SL486 IR receiver (IC22) 1 MV601 IR decoder (IC23) 1 7805 5V 3-terminal regulator (REG1) 1 7815 15V 3-terminal regulator (REG2) 1 7915 -15V 3-terminal regulator (REG3) 2 BC338 NPN transistors (Q1, Q101) 2 BC328 PNP transistors (Q2, Q102) 12 1N914, 1N4148 diodes (D1, D2, D101, D102, D12-D15) 6 1N4004 1A diodes (D5-D10) 2 7.5V 400mW zener diodes (ZD1, ZD2) 3 HDSP7803 0.3-inch green LED displays (Disp1-Disp3) frequency stability. Both DACs inside IC15 are individually controlled by the DB2-DB7 inputs and these in turn are controlled by microprocessor IC14. This allows the left and right channel gains to be adjusted separately (in 1.5dB steps) to provide the volume and balance functions. resistor to prevent RF breakthrough. This stage has a gain of 2.5, as set by the 1.5kΩ and 1kΩ feedback resistors, while the 330pF feedback ca­pacitor rolls off the high-frequency response to ensure low RF sensitivity and to provide stability. IC5 in turn drives the tone control stage which is based on IC6. This arrangement has the tone controls connected in the feedback network. When the bass and treble controls are centred, the gain of the stage is -1. Tone controls The audio output from IC16 is coupled to non-inverting amplifier stage IC5, again via a 1kΩ stopper 9 3mm green LEDs (LED1-9) 9 rectangular green LEDs (LED10-LED18) 1 BPW50 IR diode (IRD1) Capacitors 2 4700µF 25VW PC electrolytic 1 470µF 25VW PC electrolytic 4 100µF 50VW bipolar electrolytic 2 47µF 50VW bipolar electrolytic 1 47µF 16VW PC electrolytic 1 22µF 16VW PC electrolytic 14 10µF 25VW PC electrolytic 2 6.8µF 50VW bipolar electrolytic 1 6.8µF 16VW PC electrolytic 1 1µF 16VW PC electrolytic 1 0.22µF MKT polyester 1 0.15µF MKT polyester 20 0.1µF MKT polyester 2 .068µF MKT polyester 2 .015µF MKT polyester (5%) 1 .015µF MKT polyester 4 .01µF MKT polyester 2 .0047µF MKT polyester (5%) 5 .0047µF MKT polyester 1 .0047µF 240VAC polyester 2 330pF ceramic 4 100pF ceramic 2 39pF ceramic 2 22pF ceramic 13 10pF ceramic Resistors (0.25W, 1%) 1 4.7MΩ 2 1.5kΩ 2 1MΩ 8 1kΩ 4 330kΩ 35 330Ω 2 200kΩ 2 150Ω 7 100kΩ 2 120Ω 0.5W 14 47kΩ 10 100Ω 6 22kΩ 1 47Ω 2 16kΩ 4 33Ω 22 10kΩ 1 27Ω 5W 7 4.7kΩ Winding the bass or treble controls towards the input side of IC6 (ie, the output of IC5) increases the gain for frequencies above 2kHz for the treble control and below 300Hz for the bass control. The reverse happens when the tone controls are rotated in the opposite direction. This has the effect of increasing the negative feedback at bass and/or treble frequencies to provide bass or treble cut. The amount of treble boost or cut provided by IC6 is limit­ ed by the Remote transmitter 1 remote control case (DSE ZA4666) 15 chrome buttons to suit case 1 switch membrane to suit case 1 PC board, code 01308933, 59 x 62mm 1 PC board, code 01308934, 57 x 72mm 1 Dynamark front panel label, 73 x 63mm 1 9V battery & clip 1 Murata CSB500E 500kHz ceramic resonator 1 100mm length of 11-way rainbow cable 1 250mm length of 0.8mm tinned copper wire Semiconductors 1 MV500 remote control IC (IC1) 1 MTP3055E or MTP3055A N-channel Mosfet (Q1) 2 CQY89A IR LEDs (LED1, LED2) Capacitors 1 220µF 16VW PC electrolytic 2 100pF ceramic Resistors (0.25W, 1%) 1 10kΩ 1 2.2Ω 1 10Ω Footnote: the coded 68HC705C8P microprocessor is available from Silicon Chip Publications Pty Ltd & is priced at $45 plus $6 p&p any­ where in Australia (price includes sales tax). Payment may be made via cheque, postal order or credit card authorisation (Bankcard, Visa & Mastercard. 4.7kΩ resistors on either side of the treble pot. Similarly, the amount of bass boost and cut is limited by the 22kΩ resistors on either side of the bass control pot. Tone bypass Switch S5 bypasses the tone control circuitry when switched to the OUT position, or selects the output from the tone control circuitry in the IN position. From there, the signal passes via headphone-operated switch S6a, relay contacts RLY1a and a 6.8µF bipolar capacitor to the output terminal. The 6.8µF capacitor prevents any DC offset that may appear at the output of IC6 from being fed to the input of the stereo power amplifier. Relay RL1 is used to isolate the outputs from S6a and S6b at switch on and switch off. This is mainly to prevent a chirp from the preamplifier circuitry from being fed through to the exter­ nal power amplifier after switch off. If a set of headphones is plugged in, S6a diverts the audio signal from S5a to the headphone amplifier. This consists of IC7 and transistors Q1 and Q2. The two transistors boost the output current capability of the NE5534 op amp and are slightly forward biased (to keep crossover distortion to a mini­ mum) by diodes D1 and D2. IC7 functions with an overall gain of 5.7, as set by the 47kΩ and 10kΩ feedback resistors. The 22pF capacitor in the feedback path reduces the high frequency gain above 150kHz, while the two 33Ω emitter degeneration resistors provide local negative feedback to reduce distortion and improve the temperature stabil­ ity of the output stage. The output of the headphone amplifier is coupled to the headphone socket via a 100µF bipolar capacitor and series 100Ω resistor. This provides short-circuit protection for the op amp and protects the headphones from damage if one (or both) of the output transistors fails. Infrared receiver IC22 and IC23 form the heart of the infrared receiver cir­cuit. The incoming IR signals from the transmitter are picked up by photodiode IRD1 and the resulting current pulses applied to differential inputs at pins 1 & 16 of IC22, an SL486 infrared preamplifier IC. The received pulses are then amplified and filtered before appearing at the output (pin 9). The capacitors at pins 2, 3, 5, 6 & 15 of IC22 roll off the frequency response of the internal gain stages to filter out any 100Hz signals. This ensures that the circuit is immune to mains lighting interference. One important feature of the SL486 is an automatic gain control circuit and this is provided by an internal peak detector which measures the output signal on pin 9. The 0.15µF October 1993  47 Despite the complicated circuit, the IR Remote Control Preamplifier is easy to build. That’s because many of the control functions are taken care of by the microprocessor (IC14), while two CMOS switch ICs take care of the input selection. The microprocessor automatically switches to static idle mode when no IR signals are being received, to ensure excellent noise specifications. capacitor on pin 8 filters the output of the peak detector and the result­ing signal is used to control the internal amplifier stages. IC23, an MV601 remote control receiver, decodes the pulse signal from IC22. This device operates at 500kHz, as set by ceramic resonator X2, and provides five BCD outputs (A, B, C, D & E), the exact code depending on which transmitter button is pressed. In this application, momentary operation of the BCD outputs has been selected by tying pin 5 of IC23 high. In addition to the five BCD outputs, IC23 provides a Data-bar signal (pin 10) which goes low whenever a valid code is received. The five BCD outputs and the Data-bar output are ap­plied to microprocessor IC14 and also to IC9, the 5-7.5V convert­er (see Fig.5). The Data-bar output of IC23 also drives an AC­ Knowledge LED (LED 9), which indicates that an infrared signal is being received. Microprocessor control IC14 controls the digital portion of the circuit. It oper­ates from a clock based on a 3.579545MHz crystal connected bet­ween pins 38 and 39. This clock frequency is internally divided 48  Silicon Chip by two, so that the microprocessor actually runs at 1.78MHz. The microprocessor decodes the BCD signals from IC23 on its PD2-PD7 lines and uses its PA0-PA7, PB0-PB7 and PC0-PB7 lines to control the LED displays and the D/A converter (IC15) according­ly. In greater detail, the PA0-PA6 output lines control IC19-IC21 which are 4511 BCD to 7-segment display drivers. These drive the 7-segment LED displays via 330Ω current limiting resistors to in­dicate the attenuation level. The display drivers are only ac­ cessed by IC14 when the volume level is to be changed. Outputs PA7 and PB0-PB7 of IC14 control the balance display LEDs via Darlington transistor drivers IC17 and IC18, while outputs PC0-PC7 control the D/A converter (IC15) to set the volume level. The Down, UP and Mute switches on the front panel are monitored by the PD0, PD1 and IRQ (interrupt request) lines of IC14. Normally, these lines are tied high via 10kΩ resistors. When the Down switch is pressed, the PD0 input is pulled low and the IRQ input is also pulled low via D13. Similarly, the Up switch pulls PD1 low and pulls the IRQ line low via D14. The Mute switch pulls both PD0 and PD1 low via diodes D15 and D16 and pulls the IRQ line low via D12. In each case, a low IRQ level tells the microprocessor to “wake up” from its idle state, check its PD inputs and act accordingly. Power Power for the Remote Control Preamplifier is derived from a mains transformer with two separate 15VAC windings which are series connected to provide 30VAC. This is rectified by diodes D5-D8 and D9 and filtered by two 4700µF capacitors. The resulting ±21VDC rails are applied to 3-terminal regulators REG1, REG2 and REG3 to obtain +5V and ±15V rails. The ±15V rails power the op amps, while the +5V rail powers the microprocessor, LED dis­plays and associated ICs. The relay coil (RLY1) is supplied from the negative recti­ fied line via two series 120Ω 0.5W resistors. These resistors reduce the supply to a nominal -12V. Diode D9 isolates the relay supply from the 4700µF filter capacitor in the negative rail so that the relay switches off quickly when the power is switched off. That’s all we have space for this month. Next month, we shall present the assembly details for the IR Remote SC Control Preamplifier.