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Studio 200 Stereo Control Unit Ever since we published the Studio 200 stereo power amplifier in our February 1988 issue, there has been a constant stream of requests from readers for a matching stereo control unit. Well here is it is: low profile, reasonable cost, excellent performance, plenty of control features and easy to build. By LEO SIMPSON & BOB FLYNN Designers have all sorts of different philosophies when they sit down to produce a stereo control unit. Some take the purist approach: no tone controls, a minimum of switching, very few control facilities and so on. Then they charge a mint for it on the assumption that nobody would buy such a spartan unit unless it had extraordinary performance (and it must have exotic performance otherwise price would not be so high, eh?). At SILICON CHIP we have to take the constructors and the high fidelity enthusiasts into account. They all want high performance but they don't want a unit which is too complicated or expensive to build. And we think that most people want a 28 SILICON CHIP good range of controls. Most people want tone controls and a balance control, for example, although they may seldom use them. They also want plenty of inputs and outputs and tape monitoring facilities. Most people also prefer to see a stereo/mono switch on the control panel, even though, again, it may seldom be used. And finally, a headphone socket is required, although this facility is often omitted on many separate control unit/power amp combinations. Of course, we were not developing this control unit just to suit the Studio 200 power amplifier. It had to match any commercial stereo power amplifier with an input sensitivity (for full power) of 1V RMS. So these were some of the considerations we had in mind as we set about developing this control unit. Actually, it has been under development almost since the ink was dry on our February issue but our readers were not to know that. Features The new Studio 200 control unit is housed in a one unit high standard rack-mounting case. It does not need to be mounted in a rack though; it can situated above or below its accompanying power amplifier or it can be positioned up to three metres away from the power amplifier. Overall dimensions of the chassis are 483mm wide x 44mm high x 290mm deep, including knobs and rear projections. As already intimated, the Studio 200 stereo control unit has the usual line-up of controls: bass, treble, balance, input selector, tape monitor switch, stereo/mono switch and volume control. It also has a tone defeat switch and a headphone socket. The internal circuitry has been greatly simplified by the use of a new low cost, low noise dual op * Main Features Very low noise on phono and line level inputs - better than many CD players. Very low harmonic and intArmodulation distortion. Up to seven stereo program sources can be connected. Tape monitor loop. Separate headphone amplifier giving very high quality. * * * * amp, the LM833 made by National Semiconductor, Inc of the USA. In fact, the total semiconductor count is quite small. Apart from the power supply which is quite standard with two 3-terminal regulators, the entire active circuitry of the control unit uses just four LM833 dual op amps, four transistors and four diodes. Years ago, op amps were just not good enough to be used in high performance low noise circuitry by themselves. They had to be preceded by low noise discrete transistors in order to get the best performance. But this new dual op amp from National Semiconductor is just one of a new breed which has been released in the last year or so. And it really does perform, as the spec panel accompanying this article shows. So good is the performance that it is as good as or better than the majority of commercial stereo control units, even those which retail for thousands of dollars. In fact, when * Headphone socket disables output signal to power amp. Stereo/mono switch Tone and balance controls with centre detent. * * * Tone defeat switch. * Easy-to-build contruction using three separate PCB assemblies. * Estimated cost: $230.00 you think about it, the designers of many of today's so-called state-ofthe-art control units must be pretty slack. We think that any commercial stereo control unit, integrated amplifier or receiver which offers a phono signal-to-noise ratio of worse than B0dB and a high level SIN ratio worse than 95dB unweighted is poor. Most manufacturers tend to quote A-weighted figures too, which show their equipment in a better light. Some readers may think that calling for such high signal-to-noise ratios is unnecessary but it isn't, if the signal from CD players is not to be degraded. The Studio 200 is phenomenally quiet: on the phono input, with a typical magnetic cartridge fitted, the signal-to-noise ratio is - 86dB with respect to an input signal of 10mV at lkHz, with a noise bandwidth from 20Hz to 20kHz (ie, an unweighted figure]. On the high level inputs, the signal-to-noise ratio is 104dB or better, with the same noise bandwidth. A-weighting improves these results by about 2 or 3dB. Interestingly, the total dynamic range available via the phono inputs of the Studio 200 is in excess of 109dB. This is the ratio of the phono input overload capability (150mV at lkHz] to its equivalent input noise voltage (0.5 microvolts]. So as far as dynamic range is concerned, the phono input is also better than that available from compact discs. Unfortunately, neither vinyl discs or phono cartridges have this capability. Inputs and outputs The Studio 200 caters for no less than six pairs of inputs and it also has a tape monitor for connection of inputs and outputs from a cassette recorder. This means you can connect up to seven stereo program sources. The Selector switch is labelled as follows: Phono, CD, Tuner, VCR, Aux 1 and Aux 2. The rear panel has an array of 18 RCA sockets to provide for all the inputs and outputs. Push-on push-off switches are used for the tape monitor, tone defeat and stereo/mono switches. These are accompanied by symbols on the front panel which indicate their settings. The balance, treble and bass controls each have a detent to indicate their centre settings. The volume control has multiple detents. The headphone socket has an inbuilt switch to disconnect the control unit's output signal to the JUNE 1988 29 RIAA/lEC PREAMPLIAER SOURCE__. TAPE .------,TONE-OEFEAT TUNER eJ S1a INPUT SELECTOR AUX. 2 lj'" TAPE OUT OTHER CHANNEL VCR AUX. 1 BASS AND VOLUME 50k LOG. MONO TREBLE CONTROLS S4a .,. BALANCE~~-<r 10k M/N S5a .,. ::::::;v.,. HEADPHONE AMPLIFIER TAPE IN .,. Q POWER ":r' AMPLIFIER T I ~ HEADPHONES OTHER CHANNEL Fig.1: all the features of the new stereo preamplifier are illustrated in this block diagram. To keep things simple, only one channel is shown. The second channel uses exactly the same circuitry. power amplifier when headphones are in use. This stops you from unwittingly blasting your loudspeakers when you are using headphones. It also allows you to turn off the power amplifier when you are using headphones. This last point is important because when a power amplifier is off, the control unit's output signal is loaded by the unenergised input transistors (in the power amplifier). This could cause serious distortion. The headphone amplifier has the potential to deliver more than adequate drive so that even insensitive headphones can be driven to painful levels (not that you should da this if you value your ears). With 32-ohm phones, the drive can exceed 100 milliwatts while for 8-ohm phones the drive capability is more than 40 milliwatts. This means that if you are the audio equivalent of a "petrol head" you will be able to listen at headsplitting sound levels without disturbing your neighbours in the slightest. Curiously, listening via a set of closed headphones is the only way that most hifi enthusiasts will ever be able to experience the full dynamic range of compact discs. Most listening rooms have quite a lot of background noise and this tends to mask the really quiet signals on compact discs. Signal to Noise Ratio Readers will notice that we have quoted two separate figures for the various signal-to-noise ratios of the Studio 200 : (1) Flat, with a noise bandwidth (-3dB points) of 20Hz to 20kHz; and (2) A· weighted , according to the CCIR characteristic. To our knowledge, there is no self-contained commercial test equipment which will perform such measurements. To enable us to measure the signal-to-noise ratios of today's extremely quiet high fidelity equipment, we have designed a state30 SILICON CHIP of-the-art AC millivoltmeter using the best available low noise op amps. This instrument enables noise measurements to be made down to better than - 60dB with respect to one millivolt RMS (that's less than one microvolt) . This means that it • can make noise measurements in excess of 1 20dB A-weighted or flat (with respect to 1V RMS) which is far better than the best CD players. The new AC millivoltmeter will be described in a coming issue of SILICON CHIP. Unfortunately though, the headphone outputs on some commercial amplifiers and receivers are not as quiet as the main amplifier outputs (some have quite high hum levels via the headphone outputs). So not only are the amplifier outputs not as quiet as they should be, as we noted above, but the headphone outputs are even worse. By contrast, with the Studio 200 control unit and power amplifier combination you really do get phenomenally low background noise, on both the headphone and the main amplifier outputs. Omissions To keep the unit reasonably simple and inexpensive, we have omitted a couple of features that are found on some amplifiers and control units. First, there is no speaker switching, which is quite unwieldy when you have a separate control unit. Second, we have not provided for moving coil cartridges; there are relatively few in use and the trend is away from turntables anyway. Third, we have not provided for dubbing and monitoring between two cassette decks. Dubbing is possible though, if the outputs of one deck are fed into a pair of the auxiliary inputs. Inside the chassis, the circuitry is accommodated on three printed cir- cuit board assemblies. We did consider designing one large printed board to accommodate all the circuitry but that would have caused problems. First, we wanted to have the control knob centres below the centreline of the front panel (so the control legends could be above the knobs). This meant that the board for the tone controls has to be upside down in the chassis. Second, we wanted to keep the power supply components and the phono preamplifier as far away from each other as possible and we wanted the phono preamp right at the relevant input sockets. When these and other conflicting requirements were taken into account, we could not justify having one large printed board - it would have had a lot of wasted space and it probably would have made the overall assembly harder to work on. Ergo, there are three board assemblies; one for the power supply, near the transformer, one for the phono preamp and input selector and one for the four dual op amps comprising the volume, bass, treble and balance controls and the headphone amplifiers. Circuitry Now let's have look at the circuit features which are illustrated in the block diagram of Fig.1. This shows one channel only, to keep matters simple. Remember that all circuit functions are duplicated in the second channel. The same goes for the main circuit diagram of Fig.2. S1 is the 6-position selector switch. It selects the input signal and its wiper feeds the tape output as well as the following Tape Monitor switch S2. This selects the signal from S1 or from the cassette deck connected to the Tape In inputs. The signal then goes to S3, the stereo/mono switch, which shorts the two channel signals together to obtain the mono function. When the mono function is switched in by S3, the left and right · channels of the selected program source will load each other (as far as the difference signals are concerned). To avoid any degradation in signal quality due to this effect, Specifications Frequency Response Phono inputs: RIAA/IEC ±0.3dB from 20Hz to 20kHz High level inputs: within ± 1dB from 1 OHz to 1 OOkHz Total Harmonic Distortion Less than .005%, 20Hz to 20kHz, at rated output level for any input or output. Signal-to-Noise Ratio Phono (moving magnet): 86dB unweighted (20Hz to 20kHz) with respect to 1 OmV input signal at 1 kHz and rated output with 1 kO resistive input termination; 90dB A-weighted with respect to 1 OmV input signal at 1 kHz and rated output, with 1 kn resistive input terminal. High level inputs (CD, Tuner, etc): 104dB unweighted (20Hz to 20kHz) or better, with respect to rated output (with volume at maximum) with Tone Defeat switch in or out; 107dB A-weighted, with respect to rated output (with volume at maximum) with Tone Defeat switch in or out. Separation Between Channels -62dB at 1 OkHz; -81 dB at 1 kHz; and -93dB at 1 OOHz with respect to rated output and with undriven channel input loaded with a 1 kn resistor. Input Sensitivity Phono inputs at 1 kHz 4.3mV High level inputs 240mV Input impedance (phono) 50k0 shunted by 1 OOpF Input impedance (CD, etc) 50k0 Overload capacity (phono) 1 50mV at 1 kHz Output Level Rated output, 1.25V; maximum output, 8.4V RMS; output impedance, 4700; headphone output, 40 milliwatts into 80 phones, 1 00 milliwatts into 320 phones. Tone Controls Bass, ± 1 OdB at 1 OOHz; treble, ± 12dB at 1 OkHz Phase With tone controls defeated, non-inverting (ie, zero phase shift) from phono to main output; non-inverting from high level inputs to main outputs. Non-inverting from all inputs to Tape Out. With tone controls in circuit, inverting from phono and high level inputs to main outputs (ie, 180° phase shift). there are tkn resistors between S1 and S2 and in the Tape In signal path to S2. After S3 the signal is fed to the volume control and then to a noninverting op amp stage with a gain of 5.7. From there, the signal goes to the unity gain feedback tone control stage, which can be taken out of the circuit by the Tone Defeat switch, S4. After S4, the signal goes to the balance control and then to S5 which is integral with the headphone socket. It normally feeds the control unit's signal to output sockets and thence to the following stereo power amplifier. When a headphone jack plug is inserted into the socket, S5 switches the signal to the headphone amplifier. This has a gain of two. Circuit description The complete circuit diagram (for one channel) is shown in Fig.2. The four op amps are depicted as ICla, IC2a, IC3a and IC4a. The pin numbers for op amps IC2a, 2b, 2c and 2d in the second channel are shown in brackets on the circuit. ICla is the phono preamplifier JUNE 1988 31 The circuitry for the new control unit is accommodated on three PCB assemblies: one for the tone controls, one for the phono preamplifier PCB, and one for the power supply. The tone control board is mounted upside down in the chassis. and equalisation stage. It takes the low level signal from a moving magnet cartridge (typically a signal of a few millivolts) and applies a gain of 56, at the median frequency of lkHz. Higher frequencies . get less gain while lower frequencies get considerably more, as shown in the accompanying equalisation curve of Fig.3. To be specific, a 100Hz signal has a boost of 13.lldB while a 10kHz signal has a cut of 13.75dB. The phono signal is fed directly from the input socket via a small inductor, a 1500 resistor and a 47µF bipolar capacitor to the noninverting input, pin 3 of ICla. The inductor, series resistor and shunt lOOpF capacitor form a filter circuit to remove RF interference signals which might be picked up by the phono leads. The lOOpF capacitor is also important in capacitive loading of the magnetic cartridge. Most moving magnet (MM) cartridges operate best with about 200 to 400pF of shunt capacitance. The lOOpF capacitance in the control unit plus the usual 200pF or so of cable capacitance for the pickup leads will therefore provide an optimum shunt capacitance. For its part, the 47 µF bipolar capacitor is far larger than it needs 32 SILICON CHIP to be, as far as bass signal coupling is concerned. But having a large capacitor means that the op amp "sees" a very low impedance source at low frequencies and this helps keep low frequency noisEl, ganerated by the input loading resistors, to a minimum. RIAA/IEC equalisation The RIAA equalisation is provided by the feedback components between pins 1 and 2. These equalisation components provide the standard time constants of 3180µs (50Hz), 318µs (500Hz) and 75µs (2122Hz). The phono preamplifier also adds in the IEC recommendation for a rolloff below 20Hz (7950µs}. This is provided by the 0.33µF output coupling capacitor in conjunction with the load represented by the 50k0 volume control, together with other low frequency rolloffs in the circuit. One of these rolloffs (at around 4Hz) is caused by the l00µF capacitor in series with the 3900 resistor. The 3900 resistor sets the maximum AC gain at very low frequencies while the l00µF capacitor ensures the gain for DC is unity. This means that any input offset voltages are not amplified (by more than one) which would inevitably cause trouble with unsymmetrical clipping and premature overload in the preamplifier. As noted above, the signal from the phono preamplifier is coupled via S1 and S2 to the volume control. From there the signal goes via a lµF capacitor to non-inverting op amp IC2a. The feedback around this stage is set by the 4.7k0 and tkO resistors to give a gain of 5.7. The .formula for this calculation is given by: Av = (4.7k0 + lkO) + lkO = 5.7. The input (pin 3) of IC2a has a series tkO resistor acting as an "RF stopper" to prevent the possibility of strong RF signals (from local radio or TV transmitters) being detected inside the op amp. The 180pF capacitor across the 4. 7k0 resistor also provides a high frequency rolloff (above 180kHz) to ensure low sensitivity to RF signals and ensure stability of the stage. Tone controls Besides providing gain, IC2a acts as a low impedance source to dri~e Fig.2 (right): the circuit is based on ► the new low-noise LM833 dual op amp. Note that one channel only is shown; the figures in brackets refer to IC connections in the second channel. The circuit is powered from regulated ± 15V rails. r:r: L1 PHONO +15V 47 * .,. 100pF OOk .,.. 1 +15V .,. - 15V * 16k * 200k VOLUME VR1a 50k LOG. * 390() *1 % METAL FILM -15V 4.7k 1001 BP MONO ·plm• co 180pF 1k STEREO TUNER VCR 22 BP .,. OTHER CHANNEL AUX 1 AUX 2 TAPE OUT TAPE IN .,. .01 BASS VR2a 22k +15V 22k +15V 8211 HEADPHONES 10k j +:/ OTHER / CHANNEL / HEADPHONES S5a - 15V I 10k .,. AEER I I I I [ I AMPLIFIER I .01 250VAC I 10k (6 ') S6 I .,. __'T- __ __ ____ _ ___ _ __ D3-D6 4x1N4002 I / I __} m ECB "o..,u.,_ T_._ _....._ _ _~ - - - - - - - - - - - - - , 1 1 - - - + 1 s v 240VAC .ffi_., N-'---------' GND E • .,. 1000 25VW STUDIO 200 STEREO CONTROL UNIT SC01 -1-688 ]UNE 1988 33 PARTS LIST Hardware 1 rack-mounting case (Altronics Cat. No. H-0411) 1 30V 150mA centre-tapped transformer (Altronics Cat. 3 1-metre lengths of hook-up wire 3 plastic cable ties 1 60mm length of heatshrink tubing No. M-2855) 1 push on/push off SPST mains switch with black button {Altronics Cat. No. S-1090) 1 2-pole 6-position rotary switch (input selector) 3 2-pole push on/push off PCB mount switches (Lorlin or similar) with black knobs 5 22mm-dia. black anodised aluminium knobs (with index mark) 1 cord-grip grommet 1 3-way insulated terminal block 1 black binding post terminal 18 insulated panel-mounting RCA sockets 1 PCB-mount 6. 5mm headphone socket with integral switch (Jaycar Cat No PS-0180) 1 1 /4-inch solid shaft coupler 1 72mm long 1/4-inch shaft 1 1 /4-inch ID x 3/8-inch OD bush for switch shaft 1 mounting bracket (for selector switch) 4 rubber feet 2 solder lugs 2. FX111 5 ferrite beads Printed circuit boards 1 phono preamplifier board, code 01106881, 180 x 125mm 1 switch mounting board, code 01106882, 54 x 32mm 1 tone control board, code 01106883, 300 x 94mm 1 power supply board, code 04106881, 71 x 52mm 40 1 mm-dia. PC pins 4 6mm spacers Cable 1 3-core mains cord and moulded 3-pin plug 1 metre of figure-a shielded audio cable the tone control stage IC3a. This has the tone controls connected in the negative feedback network. When the bass and treble controls are centred (ie, in their "flat" settings), the gain of the stage is one, over the frequency range up to at least lOOkHz. Winding the bass or treble control towards the input side of IC3a [ie, setting the controls for boost) increases the gain for frequencies above ZkHz 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 34 SILICON CHIP Semiconductors 4 LM833 low noise op amps 2 BD1 39 NPN transistors 2 80140 PNP transistors 8 1 N4002 rectifier diodes 1 7815 3-terminal regulator (NB: 78L 15 not suitable) 1 7915 3-terminal regulator (NB: 79L 15 not suitable) 1 5mm red LED and bezel Capacitors 2 1 OOOµF 25VW PC electrolytics 4 100µF 16VW PC electrolytics 2 1 OOµF bipolar electrolytics 2 4 7 µF bipolar electrolytics 2 22µF bipolar electrolytics 2 6.8µF bipolar electrolytics 2 1µF bipolar electrolytics or miniature metallised polyester (Wima MKS 2) at treble and/or bass frequencies and the effect is treble or bass "cut". The amount of treble boost and cut provided by IC3a is limited by the 3.9k0 resistors on either side of the 25k0 treble pot. Similarly, the amount of bass boost and cut is limited by the 22k0 resistors on either side of the lOOkO bass control pot. (Yes, we can immediately see those bass hungry readers will want to fiddle the circuit to obtain more bass boost. Don't do it. Too much boost can lead to instability, increased tendency to overload, and general loss of moral fibre in 2 0.33µF metallised polyester (greencap or miniature) 4 0. 1µF metallised polyester (greencap, monolithic or miniature) 2 .015µF metallised polyester (5% tolerance preferred RIAA preamp) 2 .01 µF metallised polyester 4 .004 7 µF metallised polyester (tone controls) 2 .004 7 µF metallised polyester (5 % tolerance preferred RIAA preamp) 2 180pF ceramic 2 1OOpF ceramic 2 1OpF ceramic Potentiometers 1 dual gang 1 OOkO linear potentiometer, PCB-mounting (bass) 1 dual gang 50k0 log potentiometer, PCB-mounting (volume: must not have loudness taps; it won't fit) 1 dual gang 25k0 linear potentiometer, PCB-mounting (treble) 1 dual gang 1 OkO M/N potentiometer (balance) Resistors (0.25W, 5%) 2 x 1 MO, 2 x 1 OOkO, 6 x 22k0, 6 x 1 OkO, 4 x 5.6k0, 2 x 4. 7k0, 4 X 3.9k0, 1 X 1.5k0, 8 x 1 kO, 2 x 4 700, 2 X 1000, 2 X 820 Resistors (0.25W, 1 %) 2 x 200k0. 4 x 1 OOkO, 2 x 16k0, 2 X 3900, 2 X 1500 today's youth.) Note how S4a, the Tone Defeat switch, bypasses the circuitry associated with IC3a. Its output feeds the balance control via a 4700 resistor and 6.BµF capacitor. The 4700 resistor is there to provide short-circuit protection to IC3a, in the event of the output being short-circuited. It also lets IC3a drive long cables [well, not too long) without cable capacitance causing stability problems. The 6.BµF capacitor is there to block any DC offset voltage at the output of IC3a from being fed to the input of the stereo power amplifier. It also stops DC from appearing To make construction really easy, the control pots, pushbutton switches and headphone socket are soldered directly to the tone control board. Full wiring diagrams of all the PC assemblies will be published next month. across the balance control pot, which could otherwise become noisy. + 1,- 20 20Hz (7950µs)---;" / ~ 50Hz (3180µs) '- I;/ I'--, Headphone amplifier I +10 I/ '' -~ I/ PROPOSED IEC ~ Following the balance control is / S5, the power amp mute switch 2.120kHz (75•s) / which is inside the headphone r-, ' I / socket. This diverts the output ............ ) signal of IC3a to the headphone 500Hz (31B•s) ;-.,_, amplifier which consists of IC4a combined with transistors Ql and Q2. The two transistors are there to _10 ~ boost the output current capability of the LM833 op amp. They are '\. slightly forward-biased (to keep "\. cross-over distortion to a minimum) I "\ I I I by the two diodes connected bet- -20 2 10 100 HERTZ 1k 10k 20k ween the bases. Fig.3: this is the RIAA compensation curve of the new preamplifier showing This is a version of the '' current the IEC modification to roll off the response below 20Hz. This has the effect mirror" circuit employed in many of removing low frequency noise as well as the :rumble on all records. op amps. It works by balancing the voltage drop across each of the The output current of the head- transformer feeding a bridge recdiodes with the base-emitter phone amplifier is limited by the tifier and two 1000µF capacitors. voltage of the associated transistor. 820 resistor. This provides short- This produces unregulated supplies Provided the diodes are similar in circuit protection and protects the of. about ± 22 volts which are then characteristics to the base-emitter headphones against damage in the fed to 3-terminal regulators to projunctions of the transistors it works unlikely event of the amplifier being duce balanced supply rails of ± 15 well as a low-power class-B damaged (which would otherwise volts. amplifier circuit without the need cause ± 15 volts to be applied That's all we have space for this to manually adjust the quiescent across the headphones). month. Next month we'll present current (ie, the current drawn with Power for the circuit is provided the construction and troubleno signal present). by a 30 volt centre-tapped mains shooting details. ~ '" ', ,, ~- ' JUNE 1988 35