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A remote vol11me control for hifi systems Have you ever wanted to adjust the volume of your stereo system from the comfort of your armchair? Now you can with this high quality Remote Volume Control. It can control the balance setting too & has excellent distortion & noise specifications. By JOHN CLARKE How many times have you been listening to your favourite piece of music and wanted to adjust the volume? Many CD players now have remote volume control but other program sources don't. This Remote Volume Control overcomes that problem and can be added to just about any system. Apart from having infrared remote control, this project overcomes the limitations of the standard dual ganged potentiometers which are used for volume adjustment on almost every amplifier. All normal potentiometers become noisy with use and since the volume control is the one we use most, 32 SILICON CHIP it is the most likely control on your amplifier to become noisy. A second problem with volume control potentiometers is their poor "tracking" between the left and right channels, particularly at low volume settings. This means that as you turn the volume down, the sound tends to shift to one side of the room and then needs adjustment with the balance control. With the SILICON CHIP Remote Volume Control, there is no noise because there are no moving parts and the tracking between channels is superb - better than any dual-ganged potentiometer can ever be. There is also negligible distortion and excellent separation between channels and you can set the volume level far more precisely than with any normal volume control. The SILICON CHIP Remote Volume Control unit is housed in a one-unit high (ie, 44mm-high) rack-mounting case. It can be connected between your preamplifier and power amplifier if you have separate units. Alternatively, it can be connected into the Tape Monitor loop of an amplifier or stereo receiver. A separate Tape Monitor loop is built into the unit so that you don't lose this facility. As a matter of fact, if you normally use a CD player and tape deck as your only program sources, you could use this Remote Volume Control in front of a power amplifier in what would appeal to purists as "minimalist hifi". The system would have no input selector (apart from the Tape Monitm: switch), no tone controls and no knobs. In fact, one of our staff members has used just such a system in conjunction with the Studio 200 power amplifier described in the February 1988 issue of SILICON CHIP. The audio performance of the Remote Volume Control is genuinely hifi. As can be seen from the specifications, the noise and distortion are extremely low and separation between channels is excellent. What the figures mean is that the sound quality will not be degraded in the least. Block diagram Fig.1 shows the main features of the unit. The infrared remote control transmitter unit has five buttons: Up and Down volume, Left and Right balance, and Mute. The Up, Down and Mute buttons are duplicated on the Remote Volume Control unit itself. The volume setting is displayed on a digital readout which shows the attenuation in decibels from 0dB (maximum volume level) down to -88.5dB. The volume setting can be changed via the Up and Down buttons in 1.5dB steps. If you hold down the Up button, the volume setting will increase at a rate of about 6dB per second. Similarly, the Down button causes the volume setting to reduce by 6dB per second. This rate is about the same as that provided by the motor-driven remote controls on some commercial amplifiers. Balance display The balance display is a 9-LED bargraph which simulates the setting of a horizontal slider control. When the balance is centred, the centre LED lights. When the balance is shifted to the right, the LEDs to the right will be successively lit and vice versa. Balance adjustment is made in 1.5dB steps from 0dB to 9dB and then fully off. Three LEDs either side of centre indicate 3dB balance steps (ie, -3dB , -6dB and-9dB), while two LEDs lit at the same time indicate the inbetween steps (ie, -1.5dB, -4.5dB and -7.5dB). When the extreme left LED is on, the right channel is fully off. Similarly, when the extreme right LED is on, the left channel is off. The Mute facility enables you to reduce the gain by 21dB - similar to the mute control on normal amplifiers. The mute condition is indicated by seven of the nine balance LEDs lighting up. When the balance display is indicating the mute mode, balance adjustment is not possible. All nine LEDs of the balance display light in mute mode when the volume setting is -67.5dB or more. This simply indicates that both channels are muted beyond the normal -88.5dB range. The heart of the remote volume control circuit is a 68HC705C8P microprocessor. This is used to drive the digital readout and LED balance display, and to monitor the signal from the infrared remote control. However, its main function is to drive a dual logarithmic digital to analog (D-A) converter. It is this device which controls the level of the audio signal. ment is noise injection due to the high speed switching of its internal circuitry. This can be minimised by careful layout and shielding but the only really effective way to stop it is to shut down the microprocessor. To do this we use a method known as "static idle". This means that the microprocessor is only active when it is updating volume or balance settings and is static at other times. This facility is normally used in order to save power consumption in battery operated microprocessor applications. We're not concerned about power consumption here; just about noise. Static idle The companion remote control transmitter has five pushbuttons to control volume (Up & Down), balance (Left & Right) & muting. One problem that can occur when using a microprocessor in audio equip- LEFT v-----, DUAL LOG · D·A CONVERTER AUDIO INPUT RIGHT u - - - - - , BALANCE D D LEFT D DOWN AUDIO OUTPUT RIGHT D MUTE D MICROPROCESSOR UP INFRA RED TRANSMITTER c, ,_, c, ,_,c, ,_,. 00963036900 ATTENUATION DISPLAY (dB) BALANCE DISPLAY BALANCE A Fig.1: block diagram of the Remote Volume Control. The microprocessor decodes the incoming code picked up by the infrared receiver & controls an AID converter to vary the audio signal level. It also indirectly drives the LED displays to show the volume level & channel balance. MAY 1993 33 r--------------- -- --- -------1 I R R Vin VDD R -'WV\----, Vin A RFB A I I 2R 2R I I I I I RFB I I OUT I I I I I A GND ~----------------------~ ____ J Fig.2: the arrangement for a standard 17-bit R-2R DIA converter. In this application, the DIA converter is used as a programmable resistance to control the gain of an op amp & thus the audio level at the output. DAC A DAC B RFB B A dual logarithmic DI A converter is used to control the volume of the audio signal. However, analog to digital conversion and back again does not happen. All signals remain analog. How can this be? The answer is that we are not using the DI A converter to perform any signal conversion. Instead, we are using it as a programmable resistance. nal is supplied with a fixed reference voltage of typically +5V. When all switches (S1-S17) connect to the OUT position, the Wii Vin B D GND A GND 5V at Vin passes directly Fig.3: block diagram of the AD7112 DIA to the op amp output. If all converter IC. It has eight data inputs & these the switches are connected are buffered & decoded to control two 17-bit to ground, then the signal DIA converters (DACs), thus making it ideal is attenuated by a factor of for use in a stereo system. 217 , orto about 38µV. Other Basic concept settings of the switches provide voltages which are between bit code which provides logarithmic Fig.2 shows the concept. This diaattenuation. When the 8-bit code is gram depicts the arrangement for a these two values. standardR-ZRDIAconverter. ThevoltOur circuit does not use 5V DC but 0000 0000, the attenuation is 0dB. Each increase in count (0000 0001, age at Yin is applied to the inverting applies the audio signal to Vin. Hence, input of an op amp via a series string the signal at the output of the op amp 0000 0010, etc) increases the attenuaof resistors of value R which are can be directly controlled by the DI A tion by 0.375dB. shunted with resistors of value ZR. converter. Our circuit provides volume steps The ZR value resistors can be conThe DAC we have selected is the of 1.5dB, so we do not need 256 sepanected independently either to the AD7112 from Analog Devices. Its in- rate levels. Instead, only 60 steps are inverting input of the op amp or to ternal block diagram is shown in Fig.3. required. This is done by tying the DB0 and DB1 lines low (ie, to 0V) and ground via switches S1-S17. It has eight data inputs (DB0-DB7) Note that we are using a 17-bit DI A which are buffered and then decoded varying only the DB2-DB7 lines. converter (ie, with 17 switches) but with an 8-bit to 17-bit decoder. The 8Actually, the AD7112 provides two only four bits are shown. bit input provides 256 volume set- 17-bit DIA converters, one for each In normal DI A use, the Vin termi- tings and these are decoded to a 17- channel and both are controlled by the DB0 to DB7 inputs. This allows us to provide the balance facility so that the left and right channel gains can be Measured Performance individually adjusted. Signal-To-Noise Ratio ....... 110dB unweighted (20Hz-20kHz bandwidth) Circuit description Total Harmonic Distortion .................................. <0.005% (20Hz-20kHz) Now check out Fig.4, the main circuit diagram. This uses 11 ICs, several LED displays, three 3-terminal regulators, and various resistors, capacit-0rs and diodes. IC1 is the microprocessor, IC2 -IC6 are the LED display components, IC7-IC9 are the DI A converter and op amps, and IC10 and IC11 are for remote control reception and decoding. IC1 is the microprocessor which controls the rest of the circuit. Its clock circuit is based on a 3.579545MHz crystal connected to pins 38 & 39, Frequency Response ........................ ... .......... . DC to 50kHz (-3dB point) Separation Between Channels ... ........ -89dB at 20kHz; -90dB at 10kHz; -106dB at 1kHz; -11 0dB at 100Hz (measured with undriven channel. input loaded with 1kQ) Attenuation accuracy (1kHz, 25°C) ....... <1dB to -54dB; <2dB to -66dB; <2.5dB to -88.5dB Channel tracking accuracy .. .. ............... ....... ........ ................... <±0.25dB Maximum input signal ................................................................ 8V RMS Note: all performance measurements made with respect to 1V RMS input at 0dB attenuation 34 SILICON CHIP Most of the parts are mounted on two PC boards which are soldered together at right angles. The display board (top) carries three 7-segment LED displays for volume level indication, a LED bargraph to indicate channel balance, an acknowledge LED, & three pushbutton control switches (Mute, Down & Up). 10 goes low, pulling the IRQ input of although the microprocessor actually operates at half this frequency; ie, at IC1 low via diode D11. This awakens IC1 and the decoded outputs of IC11 1.78MHz. at pins 11-15 are now read by the The PA0-PA6 output lines of IC1 control ICZ, IC3 & IC4, the 4511 BCD microprocessor which acts accordto 7-segment display drivers. These ingly. Pin 10 ofIC11 also drives LED 1 (ACK) , which indicates when an indrive 7-segment LED displays via frared signal is received. 3300 limiting resistors . The display IC11 decodes the signal from IC10, drivers are accessed by IC1 only when the volume level is to be changed. a Plessey SL486 infrared receiver. This Outputs PA7 and PB0-PB7 of IC1 . IC has an infrared photodiode (IRD1) at its input to pick up signals from the control the balance display LEDs via remote control handpiece. The sigDarlington transistor drivers IC5 & IC6. nals are amplified and filtered before The Down, Up and Mute switches on the front panel are monitored by they appear at the output (pin 9). the PD0, PD1 and IRQ (interrupt reThe capacitors connected to pins 2, quest) lines of IC1. Normally, these inputs are tied high via 10kQ resistors to the +5V supply. When the Down Main Features switch is pressed, the PD0 input is pulled low and the IRQ input is also • lnfrared remote control of pulled low via diode D6. Similarly, volume , balance and mute the Up switch pulls PD1 low and pulls • 88.5dB volume control range the IRQ line low via D7. in 1.5dB steps The Mute switch pulls both PDQ • 3-digit display of volume and PD1 low via diodes D9 and D10 setting in dB and also pulls the IRQ line low via • 21dB mute DB. A low IRQ level tells the microprocessor to "wake up" from its static • Balance control in 1.5dB idle mode and check the PD inputs. steps to -9dB, then fully off Infrared signal decoding The PD2-PD5 and PD7 lines of IC1 monitor the data outputs of infrared decoder IC11. When valid data is received by IC11, its Data output at pin • Microprocessor uses "static idle" mode to minimise noise • Tape monitor loop • Initial volume setting -48dB 3, 5, 6 & 15 of IC1 roll off the frequency response to effectively filter out 100Hz signals. This is necessary to make the circuit immune to interference from 50Hz mains-powered lighting. IC11 operates at 614kHz, a frequency set by ceramic resonator XZ. This and the data rate set by links to pins 3 & 4 must match the settings inside the remote control handpiece (ie , the transmitter). Digital to analog converter IC7, the AD7211 dual D/A converter, is driven by the PC0-PC7 output lines of IC1. The PC0-PC5 lines provide the data to IC7, while the PC6 and PC7 lines drive the WR and A/B inputs of IC7 to provide balance control. As noted above, although IC7 has eight data inputs, we are using only inputs DB2-DB7. The DB0 and DB 1 inputs are simply tied to 0V. The left and right channel inputs are fed via the tape monitor switch Fig.4 (next page): the full circuit details for the Remote Volume Control. ICl is the microprocessor & this controls DIA converter IC7. This in turn controls the gain of op amps IC8 & IC9 to vary the volume. ICl also controls the LED displays via IC2-IC6. Instructions for the microprocessor are picked up infrared diode IRDl & processed by IClO & ICll. MAY 1993 35 ► +5V LEFT TAPE PLAYB ACK G LINE IN 17 LEFT RL1A +15V 220pF RFBA ;.n,m OUT A S2a 4 VIN A OAC A OUTPUT A GNO 1 LINE TO TAPE 13 14 I 7 TAPE MONITOR RIGHT I D81 RIGH RL1B OBO 15 CS 5 DGNO I TAPE PLA Y8ACK k 7 mEb S~b ~"''"' OUTPUT OUT B 20 18 VIN B OAC 8 082 083 0B4 DBS 0B6 087 WR A/8 12 11 10 8 9 ' 220pF LINE LINE IN 7 IC7 AD7112CN +15V 19 RF88 1200 0.5W 16 7 1200 0.5W - i 47 o+ 25VW 7 V NEG I 012 1N4004 RL1 7 7 470 +5V + + 6.8 47 10+ 0.22 22+ f:,; LK1 16 IRD1 BPW50 IC10 SL486 1- 4 15 0 8 IC11 MV601 C 8 A X2 614kHz 15 14 13 12 100pF I 100pF .015! K 14 13 12 11 011 4.7k _ DATA OE CLR I 10 1N914 7 0.15! 10k LE01 ACK A 7 10k 10k 16 MOM 7 D8 7 0.1+ V NEG. REG1 IN F1 500mA A 270 SW + 10 25VW+ 5x0.1 f +5V 7 REG2 +21V 240VAC IN +15V 1:1! ·::·11 + + 2x0.1 N- - - - - - - - - - E ~ CASE 36 S1ucoN . CHIP FLOAT 05 1N4004 EARTH -21V REG3 REMOTE VOLUME CONTROL - - oo LEFT - - 1 - - - -- - RIGHT-9 9 00 +5Y BALANCE DISP4 10k ,r :~ : ~ : ~ : ' :c; : c; : ). i, :~ ~ : 9x33011 10 11 12 13 14 10 11 12 13 1+ 8 + 40 7 '31 I3 PB0 PB1 PB2 22 23 24 25 26 27 28 6 8 + IC8 ULN2003 3 4 5 7 6 4 5 1 R PA7 21 IC5 ULN2003 PC7 PB3 PC6 PB4 PCS PBS PC4 PB6 PC3 PB7 4 12 13 14 15 ffi m .~. jK IGO 16 GIO 17 18 19 PC2 PC1 PCO IC1 MC68HC705C8P 38 4.7M l l X1 '"'"'I 'ij 39pf 39 ■ ■ 31 32 33 34 38 2 29 30 PD2 l'A0 PD3 PD4 PA1 PDS PA2 PA3 PD7 PA4 IRQ PAS PA6 PD0 PD1 11 10 9 8 7 6 5 I +SY -fo 5 LE --16 6 D 4 3 b a 2 d 13 12 11 A B I tlg 8 10 9 7 6 4 2 1 9 10 I,-:-,b e/ 'C OISP1 HDSP5303 -!-3,8 - - 3 D b a - - - - - -- 2 6 C C 13 12 11 d +5Y 7 1 A B I 8 + -- 8 4 2 9 10 I Dq g C d 8 10 9 15 14 +5Y • DISP2 HDSP5303 DP 33011 5 - - - - - ATTENUATION (dB) - : 7 I -!-3,8 A 7x33011 1 ,_, I IC4 4511 13 12 11 I : 7 b a 7 1 B C 4 15 14 10 9 LE 3 g 2 5 16 ,__ 8 IC3 4511 7x33011 a d 5 LE 4 15 14 : : 7x33011: --18 6 IC2 4511 C +SY 7 1 C ,_, ,_, 6 4 2 1 9 10 DISP3 HDSP5303 -!-3,8 - - -- --------- MAY 1993 37 PARTS LIST Receiver 1 1-unit high rack mounting case 1 dark red Perspex sheet, 150 x 20 x 2.5mm 1 screen printed front panel, 480 x 44mm 1 self-adhesive rear panel label, 180 x 34mm 1 PC board, code 01305931, 283 x 161mm 1 PC board, code 01305932, 283 x39mm 1 2 x 15VAC 20VA low profile transformer (Universal Electronics) 1 2AG panel mount fuse holder 1 500mA 2AG fuse 1 mains cord & plug 1 cord grip grommet 1 2-way mains terminal block 1 SPST mains switch (S1) 1 DPDT PC mount push on/off switch and knob (S2) 1 DPDT 12V relay (RL 1, Altronics S-4150) 2 4-way RCA socket panels 3 snap action PC-mount switches (S3-S5) 1 SPOT toggle switch (S6) 2 40-pin machine IC sockets 4 rubber feet 1 heatsink, 25 x 30 x 12mm 1 2.5-metre length of shielded cable 1 300mm-length of twin shielded cable 1 750mm-length 0.8mm tinned copper wire 5 cable ties 31 PC stakes 4 5mm standoffs 2 solder lugs 1 Murata CSB624P 614kHz ceramic resonator (X2) (S2a & S2b) to pins 4 & 18 of IC7 and its outputs at pins 2 & 20 connect to IC8 & IC9, respectively. The RFB connections from pins 3 & 19 of IC7 set the gain of each op amp to -1, while the 220pF capacitors provide high frequency roll off to prevent instability. Note that RFBa and RFBb inside IC7 each has a value of between 9.3kQ and 15kQ and these essentially match the series impedance of the lasertrimmed R-2R ladder in the DACs. 38 SILICON CHIP 1 3.579545MHz parallel resonant crystal (X1) Semiconductors 1 MC68HC705C8P programmed microprocessor (IC1 - available from SILICON CHIP) 3 4511 ?-segment display drivers (IC2-IC4) 2 ULN2003 7-way Darlington drivers (IC5-IC6) 1 AD7112CN dual log D/A converter (IC7) 2 OP27G op amps (IC8,IC9) 1 SL486 IR receiver (IC10) 1 MV601 IR decoder (IC11) 1 7805 5V 3-terminal regulator (REG1) 1 7815 15V 3-terminal regulator (REG2) 1 7915 -15V 3-terminal regulator (REG3) 6 1N4004 1A rectifier diodes (D1-D5,D12) 6 1N914, 1N4148 diodes (D6-D1) 1 10-segment LED bargraph (DISP4, Altronics Cat. Z-0180) 3 HOSP 5303 0.3-inch common cathode LED displays (DISP1 DISP3) 1 3mm red LED (LED1) 1 BPW50 IR diode (IRD1) Capacitors 1 4700µF 25VW PC electrolytic 1 4 70µF 25VW PC electrolytic 1 330µF 25VW PC electrolytic 1 47µF 16VW PC electrolytic 1 22µF 16VW PC electrolytic 6 10µF 25VW PC electrolytic 1 10µF 16VW PC electrolytic 1 6.8µF 16VW PC electrolytic 1 1µF 16VW PC electrolytic 1 0.22µF MKT polyester Thus, the input resistance seen by each op amp is more or less constant, regardless of the signal level. This is important otherwise the noise performance of the op amp would b£ prejudiced. In fact, the selection of the op amp in this ap plication is quite critical. We needed a low noise amplifier with good slew rate capabilities plus high gain. Also required are low input bias currents and low input offset voltage. 1 0.15µF MKT polyester 9 0.1 µF MKT polyester 1 0.015µF MKT polyester 1 .0047µF MKT polyester 1 .0047µF 250VAC polyester 2 220pF ceramic 2 100pF ceramic 2 39pF ceramic Resistors (0.25W, 1%) 1 4.7MQ 2 120Q 0.5W 610kQ 1 47Q 1 4.7k 1 27Q 5W 32 330Q Miscellaneous Heatshrink tubing, screws, nuts, solder, etc. Remote transmitter 1 remote control case (DSE Cat. ZA-4666) 10 chrome buttons for case (DSE Cat. ZA-3929) 1 PC board, code 01305933, 62 x 59mm 1 Dynamark front panel label, 73 x63mm 1 9V battery & battery clip 5 click action momentary switches (S1 -S5) 1 MV500 remote control transmitter (IC1) 1 MTP3055E N-channel Mosfet (01) 2 CQY89A IR LEDs (LED1 ,LED2) 1 Murata CSB614P 614kHz ceramic resonator 1 220µF 16VW PC electrolytic capacitor 2 100pF ceramic capacitors 1 10kQ 0.25W 1% resistor 1 10Q 0.25W 1% resistor 1 2.2Q 0.25W 1% resistor To meet these requirements, we have specified an OPZ 7GP for both IC8 and IC9. A,. relay is used to isolate the left and right channel outputs of the op amps at power on and power off. This prevents any turn-on thumps from being fed through to the external power amplifiers. Power supply Power for the Remote Volume Control is derived from a mains trans- former with two 15VAC windings connected in series to give 30VAC. This is rectified by diodes Dl -D4 plus D5 and filtered by a 4700µF capacitor in the positive supply line and a 330µF capacitor in the negative supply line. The resulting ±21 VDC rails are applied to 3-terminal regulators REGl, REG2 and REG3 to obtain +5V, +15V and -15V rails. The ±15V rails power the op amps, while the +5V rail powers the rest of the circuit. The relay coil is supplied from the negative rectified line at the junction of diodes D3, D4 & D5 via two 120Q 0.5W resistors connected in series. These resistors reduce the supply to a nominal -12V. xxxoo 10k - Below: the Remote Volume Control can be installed in the tape monitor loop of an integrated stereo amplifier or between the preamplifier & power amplifier where separate units are used. It includes its own tape monitor loop so that you don't have to sacrifice this facility. + T 13 UP 51 9V 1 I ..J.. mxx + A DOWN 52 100XX IC1 MV500 MUTE 53 011XX A BAL S4 - 110XX L BAL S5 101XX 16 .,. 18 17 X1 614kHz Transmitter circuit Fig.5 shows the circuit for the infrared remote control transmitter. It comprises a single IC, a ceramic resonator, two infrared LEDs, a Mosfet, several switches and a few resistors and capacitors. ICl is a Plessey MVS00 chip which provides PPM (pulse position modulation) signals suitable for driving a transistor and infrared LEDs. In stand- A 220 + 16VWi 100pF + + G05 100pF IR REMOTE CONTROL TRANSMITTER Fig.5: the transmitter circuit is based on an MV500 IC. Each time one of the switches is pressed, a unique code appears at the pin 1 output & this drives Ql & two infrared LEDs. by mode, the IC draws only 2µA which means that the circuit does not need an on/off switch. The MV500 operates with an oscillator frequency of 614kHz as set by its ceramic resonator, so that it matches the conditions of ICl 1 in the Remote Volume Control. Five pushbutton switches are connected between five of the row input pins (2 -6) and the +9V supply via a single lOkQ resistor. When a switch is pressed, a unique code for that switch is delivered from the output at pin 1 and this drives the gate of Mosfet Ql via a lOQ stopper resistor. Ql then drives two infrared LEDs (LED 1 & LED 2) via a series 2.2Q current limiting resistor. These two LEDs are driven very hard and are fed with 1.3A current pulses of 15µs duration (20% duty cycle). The 220µF capacitor across the 9Vbattery supplies the peak current to the LEDs. Next month we shall present the assembly details and complete the construction of the Remote Volume Control. SC MAY 1993 39