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Stereo preamplifier with infrared remote control This new stereo preamplifier incorporates the very latest trends in audio design technology. It has excellent speci­fications for noise & distortion & includes infrared remote control for input & mode selection, volume & balance. All control settings are indicated on LED displays. By JOHN CLARKE Sit back and relax with your Studio Remote Control Pream­plifier. You can adjust the volume and balance from your armchair or select the program from six signal sources (Phono, CD, Tuner, VCR, Aux 1 and Aux 2) plus a Tape deck (Tape Mon). The green LED display on the front panel shows the settings made via the infrared remote control. Volume level is displayed directly in dBs, while the balance setting is indicated with discrete LEDs as a bargraph. Separate 24  Silicon Chip green LEDs show the select­ed program source. We know that you will be impressed with the action of the remote volume control. It provides volume changes in steps of 1.5dB over a huge 88.5dB range with perfect tracking between channels. 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. The three LEDs either side of centre indicate 3dB balance steps (-3, -6 and -9dB), while when two adjacent LEDs are lit they indicate the in-between settings (-1.5, -4.5 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. For temporary interruptions such as phone calls you can instantly reduce the volume setting by 21dB using the Mute con­trol. This is indicated by seven of the nine LEDs being on. Mono and stereo selection can also be made via the remote control. Knobs are provided on the front panel for the bass and treble controls and there is a tone defeat switch which can be used to bypass the tone circuitry for a ruler-flat frequency response. The front panel also carries a headphone socket for private listening and duplicate volume control switches so that you can change the volume setting without having to use the remote control. While the remote volume control is very convenient, it also solves the limitations found on conventional dual-ganged volume controls. All normal potentiometers become noisy with use and since the volume control is the one we use the most it is the first control to have problems. A second problem with volume control potentiometers is their poor tracking between channels, particularly at low volume settings. This means that as you turn the volume down, the bal­ance between channels shifts and requires adjustment with the balance control. With this new remote control preamplifier, no noise can develop because there are no moving parts in the volume control and the channel tracking is excellent, even at low volume settings. The new Studio Remote Control Preamplifier is housed in a black 1-unit high rack case with a screen printed front panel. The volume LED and balance LED displays are located behind a neutral Perspex filter in the front panel and there are nine green LEDs for program and mode selection. The front panel is relatively uncluttered, with only a few controls. This has been made possible because most functions are accessed via the remote control which has 15 pushbuttons. Inside the unit there is a large single PC board which accommodates most of the components, including the tone control potentiometers, the tone defeat switch and the headphone socket. A small front board is used for the front Most of the parts are mounted on a large PC board, while a second smaller board accommodates the LED displays & three click-action pushbutton switches (Volume Up, Volume Down & Mute). panel displays and switches. Inputs and outputs As noted above, the Studio Remote Control Preamplifier caters for six pairs of inputs and has a tape monitor loop. This means that you can connect up to seven stereo program sources, all of which can be selected via the remote control. When select­ing Tape Monitor or Source via the remote control, you have the choice of either mono or stereo modes. Having a mono tape monitor mode means that a mono tape deck can • • • • • • • • • • • • • drive both channels or alternatively, the stereo program being fed through the preamplifier will be converted to a mono signal if you wish to make a monaural tape recording. When listening via headphones, the preamplifier’s output signal to the power amplifier is disconnected. This prevents you from inadvertently overdriving your loudspeakers when listening with headphones. The headphone amplifier has the potential to deliver more than adequate drive for even insensitive headphones. This will allow listening Main Features Infrared remote control of all functions except power on/off, tone controls & tone defeat switch Very low noise on phono & line inputs Very low harmonic & intermodulation distortion Up to seven program sources can be connected Tape monitor loop Separate high quality headphone amplifier Headphone socket disables output signal to power amplifier Tone defeat switch 88.5dB volume control range in 1.5dB steps with 3-digit display 21dB mute Balance control in 1.5dB steps to -9dB then fully off Initial settings of -48dB volume and CD stereo signal source Excellent left and right channel tracking for volume setting September 1993  25 26  Silicon Chip CMOS SWITCH IC11 VCR AUX1 AUX2 PHONO CD TUNER TAPE IN x1 LATCH IC10 5 CONTROL INPUTS AUX2 TAPE OUT 4 AUX1 IC8 3 VCR CMOS SWITCH IC2 2 OUT TUNER 1 0 IC1 CD PHONO RIAA PREAMPLIFIER A ACK. MONO TAPE MON. DECODER AND LATCH IC12 CONTROL INPUTS By Bx C MONO Cy CMOS SWITCH IC3 TAPE Ay MON. Ax INFRARED RECEIVER AND DECODERS IC22, IC23 DOWN DUAL LOG D-A CONVERTER IC15 MUTE MON. UP RIGHT MONO STEREO SOURCE TAPE IC16 x2.5 IC5 INFRARED TRANSMITTER BALANCE DISPLAY (dB) h 9 6 3 0 3 6 9 h L BALANCE R MICROPROCESSOR IC14 ATTENUATION DISPLAY (dB) BALANCE AUX2 TUNER LEFT AUX1 VCR x1 IC4 CD PHONO TO RIGHT CHANNEL UP, DOWN MUTE SWITCHES TO RIGHT CHANNEL 330k 4.7k BASS AND TREBLE CONTROLS IC6 IN TONE S5 HEADPHONE OPERATED S6 OUT x4.7 IC7 TO RIGHT CHANNEL RELAY PHONES OUTPUT VDD R Vin 2R R 2R S1 2R S2 Vin A R RFB A 2R S3 2R OUT A 17 BIT DAC A S17 RFB OUT A GND OPAMP 17 BIT LATCH Vout DB0 Fig.2: the arrangement for a standard 17-bit R-2R D/A converter. In this application, the D/A converter is used as a programmable resistance to control the gain of an op amp & thus the audio level at the output. at ear-deafening levels should the need arise. When the preamplifier is turned on, it always has the CD source selected, the volume set at -48dB and the Mute on (-21dB) This prevents the speakers from blasting if the CD player goes straight into play at switch-on. Omissions To keep the unit simple, we have omitted some features that are found on some stereo amplifiers. First, there is no loud­ speaker switching which is rather unwieldy when you have a sepa­rate control unit. Second, we have not provided for moving coil cartridges in the RIAA phono preamplifier. And third, there is no dubbing and monitoring facility between two tape decks. Dubbing is possible however, if the outputs of one deck are fed into a pair of auxiliary inputs. Block diagram ▲ Fig.1 shows the main features of the unit. To keep the block diagram simple, we have shown only one channel. The second channel has identical circuit functions. The six inputs (Phono, CD, Tuner, VCR, Aux 1 & Aux 2) are selected using CMOS analog switch IC2. It Fig.1 (left): this block diagram shows the general layout of the Remote Control Preamplifier. Incoming signals are routed via CMOS switches IC2 & IC3 & fed to a D-A converter (IC15). This D-A converter is controlled by microprocessor IC14 & in turn controls the gain of op amp stage IC16. The signals from IC16 are then further amplified & fed to the tone control stage. 8 BIT BUFFER DB7 DAC A DAC B CONTROL LOGIC DECODE LOGIC 17 BIT LATCH RFB B operates as a single-pole OUT B 6-way switch. For stereo 17 BIT DAC B operation, a second IC is required. The input selected depends on the code at the CS Vin B D GND A GND WR control inputs. Fig.3: block diagram of the AD7112 D/A Note that the Phono input converter IC. It has eight data inputs & these are buffered & decoded to control two 17-bit is fed via RIAA pre­amplifier D/A converters (DACs), thus making it ideal stage IC1 before passing to for use in a stereo system. IC2. The output of IC2 connects to the Ax input of IC3 and is muting when the preamplifier is powalso fed to amplifier IC8. IC8 provides ered up and down. a buffered signal for the tape monitor Microprocessor control output. IC3 provides for tape monitoring The heart of the preamplifier is a and mono/stereo mode selection. This Motorola 68HC705C8P microprocesIC contains three separate single-pole sor. This is used to drive the digital double-throw switches. The “A” readout and the LED balance display, switch provides switching between and to monitor the signal from the the tape monitor or source signals from infrared remote control receiver. It IC2. The “B” switch provides identical also controls the dual D-A converter, switching for the other channel. IC15, which in turn controls the volThe A output of IC3 is fed via a ume level. 4.7kΩ resistor to amplifi­er IC4. The Control signals from infrared rereason for the 4.7kΩ resistor is to avoid ceiver IC22 and decoder IC23 are undue signal loading when the “C” monitored by the microprocessor, switch in IC3 is turned on to mix the decoder and latch stage IC12, and by signal with that from the other channel latch IC10. IC10’s logic outputs confor mono listening. trol IC2 while logic data from IC12 IC4’s output connects to a dual log- controls IC3. IC12 also drives the tape arithmic D-A converter. This device, monitor and mono LEDs, as well as the in conjunction with op amp IC16, acknowledge LED which lights whe­n controls the level of the audio signal. a valid transmission from the remote The signal then passes on to op amp control transmitter is detected. IC5 which has a gain of 2.5. From IC11 is a CMOS switch identical there, the signal goes to the unity gain to IC2 and it decodes and drives the feedback tone control stage IC6 which source display LEDs. can be bypassed using the tone defeat One problem that can occur when switch S5. using a microprocessor in audio A jack-operated switch diverts the equipment is noise injection due to signal to amplifier IC7 when head- the high speed switch­ing of its internal phones are in use. When headphones circuitry. This can be minimised by are not in use, the signal passes careful circuit board layout but the through the relay contact and then to only really effective solution is to shut the output. The relay provides signal down the microprocessor whenever it September 1993  27 Specifications Frequency response Phono inputs: RIAA/IEC ±0.3dB from 20Hz to 20kHz High level inputs: -0.2dB at 20Hz, -0.2dB at 20kHz Total Harmonic Distortion Better than .005%, 20Hz-20kHz with respect to 1V output and 0dB volume setting. Signal-to-Noise Ratio Phono (moving magnet): 92dB unweighted (20Hz-20kHz) with respect to 10mV input signal at 1kHz and rated output with 1kΩ resistive input termination; 97dB A-weighted with respect to 10mV input signal at 1kHz and rated output with 1kΩ resistive termination. High level inputs (CD, Tuner, VCR and AUX1 & 2): 100dB unweighted (20Hz-20kHz) with respect to rated output (volume at maximum) with Tone Defeat switch in or out; 102dB A-weighted with respect to rated output (with volume at maximum) with Tone Defeat switch in or out. Separation Between Channels -67dB at 10kHz; -82dB at 1kHz and -88dB at 10Hz with respect to rated output and with undriven channel input loaded with a 1kΩ resistor. Crosstalk (between input sources) -93dB at 10kHz; -100dB at 1kHz and -100dB at 10Hz with respect to rated output and undriven inputs loaded with 1kΩ resistors. Input Sensitivity Phono inputs at 1kHz: 9mV High level inputs: 400mV Input impedance (phono): 50kΩ shunted by 100pF Input impedance (CD, etc): 47kΩ Overload capacity (phono) 300mV at 1kHz Output Level Rated output, 1VRMS; maximum output, 8V RMS; output impedance, 600Ω Tone Controls Bass: ±11dB at 100Hz; Treble: ±12.5dB at 10kHz Attenuation Accuracy (1kHz, <at> 25°C) <1dB to -54dB; <2dB to -66dB; <2.5dB to -88.5dB Channel Tracking within ±0.25dB Phase Non-inverting (ie, zero phase shift) from Phono to output and from high level inputs to output. Non-inverting from all inputs to Tape Out. With tone controls defeated: inverting (ie, 180° phase shift) from phono and high level inputs to output. is not needed and that is most of the time. This technique is called “static idle” and it means that the microprocessor only becomes active when a signal from either the remote control 28  Silicon Chip or a front-panel volume control switch is received. Volume control system As previously mentioned, a dual logarithmic D-A converter (IC15) is used to control the volume of the audio signal. Howev­er, analog to digital conversion and back again does not occur. All audio signals remain in analog form. Instead, IC15 is used as a programmable resistance to change the audio signal level ap­plied to op amp IC16. Fig.2 shows the concept. This diagram depicts the arrange­ ment for a standard R-2R D-A converter. The voltage at Vin is applied to the inverting input of an op amp via a series string of resistors of value R which are shunted with resistors of value 2R. The 2R value resistors can be connected independently either to the inverting input of the op amp or to ground via switches S1-S17. Note that we are using a 17-bit D-A converter (ie, with 17 switches) but only four of these are shown here. When all switches (S1-S17) connect to the OUT position, the signal at Vin passes directly to the op amp output with no atten­uation. If all the switches are connected to ground, then the signal is attenuated by a factor of 217. Other settings of the switches provide attenuation levels which are between these two values. The D-A converter we have selected is the AD7112 from Analog Devices. Its internal block diagram is shown in Fig.3. It has eight data inputs (DB0DB7) which are buffered and then decoded with an 8-bit to 17-bit decoder. The 8-bit inputs provide 256 volume settings in 0.375dB steps. Our circuit only requires volume setting steps of 1.5dB, so we only need to use the most significant 6-bits (DB2DB7). For this reason, the DB0 and DB1 inputs are permanently tied low. Actually, the AD7112 provides two 17-bit D-A converters, one for each channel, and both are controlled by the DB0-DB7 inputs. This facility allows us to provide the balance facility whereby the left and right channels can be individually adjusted. Transmitter Circuit Fig.4 shows the circuit for the infrared remote control transmitter. It comprises a single IC, a ceramic resonator, two infrared LEDs, a Mosfet and several resistors and capacitors. IC1 is a Plessey MV500 IC which provides PPM (pulse position modulation) signals suitable for driving a transistor and infrared LEDs. In stand- XXX00 ▲ Fig.4 (right): 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 Q1 & two infrared LEDs. 10k XXX10 9V A UP S1 SOURCE STEREO S2 11 2 BAL-R S3 VCR S4 12 LED1 13 111XX A 3 BAL-L S5 AUX2 S6 ON S7 PHONO S8 4 SOURCE MONO S9 5 TAPE MON STEREO S11 MUT1 S10 6 TUNER S12 7 AUX1 S13 8 CD S14  K 2x CQY89A A 15  LED2 B by mode the IC draws 2µA and so the circuit does not require an on/off switch. The MV500 operates with an oscillator frequency of 500kHz as set by its ceramic resonator. This matches the receiver frequency of IC23. Fifteen switches are connected between the row pins (pins 2-9) and the column pins (pins 11-13). Note that the connection to pin 13, which is actually the positive supply pin, is via a 10kΩ 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 Q1 via a 10Ω stopper resistor. Q1 then drives two infrared LEDs (LED 1 and LED 2) via a 2.2Ω current limiting resistor. The LEDs are driven by 15µs duration 1.3A pulses at a 20% duty cycle in order to obtain a good range from the remote control. The 220µF capacitor across the battery supplies the peak current required for the LEDs. Next month we will describe the full circuit of the pream­plifier and present SC the parts list. 220 16VW 0.1 XXX01 TAPE MON. MONO S15 9 14 K 110XX 101XX 2.2 Q1 MTP3055E IC1 MV500 OUT 10  1 D G S 100XX 011XX 010XX 001XX GDS A K 000XX 16 100pF X1 500kHz 17 18 100pF IR REMOTE CONTROL FOR PREAMPLIFIER September 1993  29