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Studio Twin Fifty Stereo Amplifier Looking for a new compact stereo amplifier to replace your ageing clunker? Or perhaps you are looking to upgrade the amplifier on your midi system. Either way, this new stereo amplifier will deliver the goods and give you sound quality equivalent to amplifiers costing hundreds of dollars more. By LEO SIMPSON & BOB FLYNN As the name suggests, this new amplifier gives a high standard of performance, especially so since it is quite a compact design. It uses the new "Plastic Power" stereo power amplifier module described in the February 1992 issue, together with a stereo preamplifier which is very similar in features to the Studio 200 Stereo Control Unit published in the June and July 1988 issues of SILICON CHIP. Our approach to the design and 22 SILICON CHIP presentation of this new amplifier has been to take the middle road. It is not a spartan, no-frills, no-expense spared design but nor is it laden with every feature you could possibly want and most likely never use. Hence, the new Studio Twin Fifty has all the features that most users will want but still includes some design features that dyed-in-the-wool audiophiles would regard as mandatory. For example, while everyone would like to see a headphone socket on an amplifier (some no-compromise designs lack this feature), many hifi enthusiasts would not be keen on the idea of having the speakers switched when the headphones are in use. Relays or headphone socket switching are regarded as undesirable because it can introduce unpredictable contact resistances into the amplifier/loudspeaker circuit. Our approach has been to provide a separate ultra-low distortion amplifier just to drive the headphones. This has the benefit of eliminating relays and any need to switch the speaker signals. But we're getting a little ahead of ourselves in telling the story of this new amplifier which has been under development for the previous six months. Features The new Studio Twin Fifty Amplifier is housed in a midi-sized case. It could be mounted under a midi-sized rack hifi system or together with other free-standing units such as a compact disc player and cassette deck. Overall dimensions of the amplifier are 360mm wide x 95mm high x 360mm deep, including knobs, rubber feet and rear projections. As can be seen from the photos, the Studio Twin Fifty Amplifier has the usual line-up of controls found on most mid to high-priced amplifiers : bass, treble, balance, input selector, tape monitor switch, tone defeat switch and volume control. It also has a stereo/mono switch, headphone socket and power switch. The circuitry of the preamplifier section has been greatly simplified compared to amplifiers produced a few years ago and uses just four low cost dual op amps (this includes those used in the stereo headphone amplifier). The overall performance of the preamplifier section of the new amplifier is superior to some really highcost stereo control units which can cost thousands of dollars. The Studio Twin Fifty is particularly quiet on both the high inputs and phono inputs. On the phono inputs, with a typical magnetic cartridge fitted, the signal-to-noise ratio is -83dB with respect to an input signal of lOmV at lkHz, with a noise bandwidth from 20Hz to 20kHz (ie, an unweighted measurement). This improves to -88dB with A-weighting. On the high-level inputs, the signal to noise ratio is 99dB with the same noise bandwidth. This is as good as many CD players. A-weighting, as ' .. - .. · . .. , -· ., - - .., . Main Features * 50 watts per channel with both channel driven into 8-ohm loads. * Very low noise on phono and line level inputs - comparable with many CO players. * Up to seven stereo program sources can be connected. * Tape monitor loop. * Separate ultra-low distortion stereo headphone amplifier. * Headphone socket disables signals to stereo power amplifier. * Stereo/mono switch. * Tone defeat switch. * Easy to build construction using three separate printed circuit board assemblies. * Estimated cost: $350. used by most hifi manufacturers, improves the figures by about 2 or 3dB. Inputs & outputs The Studio Twin Fifty caters for six pairs of inputs and it also has a tape monitor loop for the connection of inputs and outputs from a cassette recorder. This means that you can connect up to seven 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 16 RCA sockets to provide for all these inputs and outputs. There is also a set of four spring-loaded terminals for the loudspeaker connections. Push-on push-off switches are used for the tape monitor, tone defeat and stereo/mono switches. These have accompanying symbols on the front panel which show their settings. As already noted, the headphone socket has an inbuilt switch to disconnect the signals to the power amplifiers when headphones are in use. This stops you from unwittingly blasting your speakers when you are wearing headphones. 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 do this if you value your ears). With 32-ohm phones, the drive can exceed lO0mW while for 8-ohm phones the drive capability is more than 40m W. The Studio Twin 50 Amplifier uses this 50-watt/channel stereo power module, as described in the previous issue. It uses four big plastic Darlington transistors in each channel to give a rugged, compact design that incorporates full protection. MARCH 1992 23 RIAA PREAMPLIFIER 'PH0N0 u-----t MONITOR S2 SOURCE-TAPE CDO>-------...'t) TUNERO------on VCR0>-------0 n AUX1O,___ _ _ _ _,,,- J SOURCE S1 VOLUME VR2 TONE CONTROLS SPEAKER MONO AUX20>--------'- 1AJlu-----------J TW:Eu-------------J ~tMgfE t -;EREO OTHER CHANNEL Fig.1: the circuit features of the new stereo amplifier are illustrated in this block diagram. To keep things simple, only one channel is shown. The second channel uses exactly the same circuitry. Note the use of a separate headphone amplifier to drive the headphones. Interestingly, 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. Unfortunately though, the headphone outputs on some commercial amplifiers 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 Twin Fifty Amplifier, you really do get phenomenally low background noise, on both the headphone and the main amplifier outputs. of the auxiliary inputs. Inside the chassis, the circuitry is accommodated on three printed circuit board assemblies and the power transformer is a toroidal type, giving low hum radiation. Omissions Block diagram 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, as already mentioned, there is no speaker switching. Second, we have not provided for moving coil cartridges. Indeed, some readers may regard the phono inputs as unnecessary since vinyl records are now virtually unobtainable except from some specialist stores. Third, we have not provided for dubbing and monitoring between two cassette decks. Tape dubbing in one direction is possible though, if the outputs of one deck are fed into a pair 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 SZ. 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 The rear panel carries eight pairs of RCA sockets. These accept up to seven program sources as well as providing stereo output signals for a cassette recorder. The loudspeaker terminals are located to the left of the fuseholder. 24 SILICON CHIP 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, there are 1kn resistors between S1 and S2 and in the "Tape In" signal path to S2. Following S3, the signal is fed to the volume control and then to a noninverting op amp stage with a gain of 4.1. 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 tone control stage's signal to the following stereo power amplifier. When a headphone jack plug is inserted into the socket, S5 switches the signal to the headphone amplifier instead. Circuit description The complete circuit diagram (for one channel) is shown in Fig.2. The four op amps are depicted as ICla, ICZa, IC3a & IC4a. The pin numbers for op amps IClb, ICZb, IC3b & IC4b in the second channel are shown in brackets on the circuit. ICla is the phono preamplifier 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 13.1 ldB 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 1son resistor and a 47µF bipolar capacitor to the non-inverting input, pin 3, of ICla. The inductor, series resistor and shunt 150pF capacitor form a filter circuit to remove RF interference signals which might be picked up by the phono leads. The 150pF capacitor is also important because it provides capacitive loading of the magnetic cartridge. Most moving magnet (MM) cartridges operate best with about 200-400pF of shunt capacitance. The 150pF capacitance Specifications Output Power 47 watts into 8-ohm loads, both channels driven; 70 watts per channel into 4-ohm loads, both channels driven. 55 watts per channel into 8-ohm loads or 80 watts per channel into 4-ohm loads, with one channel driven. Frequency Response Phono inputs: RIAA/IEC ±0.5dB from 20Hz to 20kHz High level inputs: within ±1dB from 10Hz to 20kHz; -2.4dB at 50kHz (measured at one watt into 8 ohms). Total Harmonic Distortion Typically less than .05%, 20Hz to 20kHz, at rated output level for any input or output. Signal-to-Noise Ratio Phono (moving magnet): 83dB unweighted (20Hz to 20kHz) with respect to 10mV input signal at 1kHz and rated output with 1kn resistive input termination; 88dB A-weighted with respect to 10mV input signal at 1kHz and rated output, with 1kn resistive input termination. High level inputs (CD, Tuner, etc): 99dB unweighted (20Hz to 20kHz) or better, with respect to rated output (with volume at maximum) with Tone Defeat switch in or out; 101 dB A-weighted, with respect to rated output (with volume at maximum) with Tone Defeat switch in or out. Separation Between Channels -51 dB at 10kHz; -67dB at 1kHz; and -82dB at 100Hz with respect to rated output and with undriven channel input loaded with a 1kn resistor. Input Sensitivity Phono inputs at 1kHz: High level inputs: Input impedance (phono): Input impedance (CD, etc): Overload capacity (phono): 4.3mV 235mV 50kn shunted by 150pF 50kn 1S0mV at 1kHz Headphone Output 40 milliwatts into an headphones; 100 milliwatts into 32n headphones. Tone Controls Bass: ±1 0dB at 100Hz Treble: ±12dB at 10kHz 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. Noninverting 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). Protection SA fuses plus RDE245A Polyswitches Damping Factor >50 (for an loads) in the control unit, plus the usual 200pF or so of cable capacitance for the pickup leads will therefore provide an optimum shunt capacitance. Stability Unconditional For its part, the 47µF bipolar capacitor is far larger than it needs to be, as far as bass signal coupling is concerned. But having a large capacitor MARCH 1992 25 N ;; ::r:: n :z: 0 0 c=::: (f) Cl) CD PHONO r L2 100k 10k - 15V +15V ' * PHILIPS 4312-020-367<at> 100k 47 *' ,,.,,, \i,i)(I I..I ' o- ·1 ' 100:, BP'+ 47k ' I +15V I I BCOB • E • • C VIEWED FROM BELOW ECB BF-.80139 PLASTIC1 SIDE OTHER CHANNEL I I I I BCE TIP- 01 POWER AMPLIFIER ~ ~ JHEADPHONES I S5a HEAOPHONEO / AMPLIFIER / AUX 2 k ~ t 47 .,: 16VW+ 1k .033 04 50011 SET CURRENT VR1 22k 4 + l i ~ TJ 100ll$ 1.6V ! +15V STUDIO TWIN 50 AMPLIFIER I t 1 C 2xBC556 , 03 BC556 4. 7k;f 2.2V 4.7kJ: 2.2V C ·c 1' 02 STEREO OTHER CHANNEL ~ S3a M°f~ODE r1t MONITOR S2a SOURCE-TAPE 68pF l ? + 0.22! 0.1 } 37.2V ♦ h'f + .... 37.2V ~ u B .01 t 1 0.22l F2 SA SA Fl 0.22:r 0.22,I OUT-IN 33pF O -38 5V 6.BuH Ll 0+3B 5V -15V +15V + OUTPUT Fig.2 (left): this diagram shows the circuitry for one channel of the new Studio Twin Fifty Amplifier. The total semiconductor complement is four low noise dual op amps, 26 transistors, 10 diodes, one bridge rectifier and two 3-terminal regulators. means that the op amp "sees" a very low impedance source at low frequencies (ie, the DC resistance of the magnetic cartridge coils) and this helps keep low frequency noise, generated by the input loading resistors, to a minimum. RIAAIIEC equalisation The RIAA equalisation is provided by the feedback components between pins 6 and 7. 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 roll-off below Z0Hz (7950µs) . This is provided by the 0.33µF output coupling capacitor in conjunction with the load represented by the 50kQ volume control, together with other low frequency rolloffs in the circuit. One of these roll-offs (at around 4Hz) is caused by the lO0µF capacitor in series with the 390Q resistor. The 390Q resistor sets the maximum AC gain at very low frequencies while the lO0µF capacitor ensures that the DC gain is unity. This means that any +20 i-,;--- 20Hz (795Dµs)~ ...... / 1/1/ ~ +10 ' / I/ - PROPOSED IEC ,.... -..' input offset voltages are not amplified (by more than one) which would inevitably cause trouble with unsymmetrical clipping and premature overload in the preamplifier. By the way, the feedback capacitors (.0047µF and .015µF) in the RIAA network should be 2.5% or 5% tolerance if you are to achieve equalisation within ±0.5dB of the RIAA/IEC standard. 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 1µF capacitor to non-inverting op amp ICZa. The feedback around this stage is set by the 4.7kQ and 1.5kn resistors to give a gain of 4.1 times. The input (pin 5) of ICZa has a series 1kQ 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 330pF capacitor across the 4. 7kQ resistor also provides high frequency roll-off above 100kHz to ensure low sensitivity to RF signals and to ensure stability of the stage. towards the input side of IC3a (ie, setting the controls for boost) 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 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 3.9kQ resistors on either side of the 25kQ treble pot. Similarly, the amount of bass boost and cut is limited by the 22kQ resistors on either side of the 100kQ bass control pot. Note how S4a, the Tone Defeat switch, bypasses the circuitry associated with IC3a. Its output feeds the balance control via a 6.8µF capacitor. The 6.8µF 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 across the balance control pot, which could otherwise become noisy. Tone controls Headphone amplifier Besides providing gain, ICZa acts as a low impedance source to drive 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 unity over the frequency range up to at least 50kHz. Winding the bass or treble controls Following the Balance control is S5a which is inside the headphone socket. This diverts the signal from the Balance control to the headphone amplifier which consists ofIC4a combined with transistors Q12 and Q13. The two transistors are there to boost the output current capability of the TL072 op amp. They are slightly forward-biased (to keep cross-over disFig.3: the RIAA compensation curve for the new preamplifier incorporates the IEC modification to roll off the response below 20Hz. This has the effect of removing low frequency noise, as well as the rumble on records. 50Hz (318Dµs) . "'" / ' I").. t--... '\I'. / I'\ I/ I) 500Hz (318µs) 2.12DkHz (75µs) "i,.. "-- ,,,I r---....' r-...' ~ N -10 "' ~ '\. -20 I I 2 10 100 1k " I "\ I HERTZ 10k 20k MARCH 1992 27 4 CHANNEL LIGHT CHASER DIMMER A superb 4-channel lighting controller. It's designed to take the rigours of commercial use in theatres, discos, stage shows, etc. Powered by a 240VAC 15amp outlet with a total lamp load of 2400 or 3600 watts. The chaser can be operated automatically or manually. Complete with a fully pre-punched black case, silk screened front panel and all components. Cat K-3165 Now Only s279 DUAL TRACKING +/- 50V POWER SUPPLY Save $50 with this quality switch mode power supply. It has a variable output from 0 to 100 volts DC and a current output of 1. 7 A <at> 0-87V and 1A <at> 100V. With short circuit protection and a LED indicator for ripple exceeding 5mV p-p. Comes with a pre-punched, silkscreened, front panel. Reduced! s199 Cat K-3465 DOWN, DOWN, DOWN! Compare our original kit prices to our current kit prices and save! DESCRIPTION ORIGINAL PRICE (& YEAR) CURRENT PRICE Touch Lamp Dimmer Dimmer Extension (suit K-3001) Stereo Control Unit Transistor Tester Telephone Bell/Monitor Battery Power Supply Fast NiCad Charger Sprinkler Timer Lotto Selector Book Shelf Speaker Enclosures Car Interior Light Delay 120-150 WATT Speaker Protector Car Tune Up Adaptor Brake Lamp Flasher Vehicle Exhaust Monitor RF Attenuater Box 200 Watt HF Linear Amp HF Upgrade 40 mtrs (Suit K-6330) HF Upgrade 20 mtrs (Suit K-6330) HF Upgrade 10 mtrs (Suit K-6330) Morse RTTY Decoder Transistor Beta Tester Beepo Continuity Tester 30 Watt Stereo Amp S/Form 30 Volt Variable power supply Egg Timer 12V-100 WATT DC Converter Telephone Amp Portable AM Stereo Radi o Gel Cell Battery Charger (S/ Form) Hands Free Phone Adaptor Hot Canaries 4 Input Mixer Pre-Amp 6 mtr FM Receiver 420-450 MHZ Converter (suit K-6005) 144-148M HZ Converter $ 34.95 $ 14.95 $ 99.00 $ 46. 95 $ 19.95 $ 49.95 $ 99.95 $ 99.00 $ 59.50 $212.50 $ 24.95 $ 39.95 $ 29.95 $ 29.95 $ 59.50 $ 79.95 $379.00 $ 39.95 $ 39.95 $ 39. 95 $169.00 $ 19.95 $ 29.95 $ 99.00 $ 99.00 $ 19.95 $129.00 $ 39.95 $ 79.95 $ 69.95 $ 89.95 $ 24.95 $ 69.95 $139.00 $ 39.95 $ 49.95 19.95 8.95 49.95 $ 29.95 $ 14.95 $ 32.95 $ 49.95 $ 59.95 $ 39.95 $ 99.00 $ 14.95 $ 19.95 $ 19.95 $ 16.95 $ 39.95 $ 49.95 $199.00 $ 9.95 $ 9.95 $ 9.95 $ 79.95 $ 12.95 $ 16.95 $ 69.95 $ 59.95 $ 12.95 $ 69.95 $ 24.95 $ 59.95 $ 49.95 $ 69.95 $ 14.95 $ 49.95 $ 99.00 $ 19.95 $ 32.95 (1990) (1990) (1990) ( 1990) (1990) (1990) (1988) (1990) (1990) (1989) (1990) (1989) (1990) (1990) (1990) (1988) (1988) (1988) (1988) ( 1988) (1989) (1990) (1990) (1990) (1989) (1991) (1991) (1991) (1991) (1991) (1991) (1991) {1991) (1991) (1991) (1991) CAT NO. $ $ $ K-3001 K-3002 K-3045 K-3052 K-3102 K-3460 K-3476 K-3588 K-3700 K-4000 K-4002 K-4008 K-4400 K-4500 K-4610 K-6323 K-6331 K-6332 K-6333 K-6337 K-6339 K-7200 K-7700 K-5100 K-3475 K-3590 K-3235 K-3104 K-5200 K-3220 K-3100 K-3530 K-3036 K-6005 K-6008 K-6006 STORE LOCATIONS: If the kit you want is out of stock, ask our staff to check on it's storewide availability. In most cases our mail order service can get it for you within days. NSW • Albury 21 8399 • Bankstown Square 70l 4888 • Blacktown 671 7722 • Brookvale 905 0441 • Bondi 3871444 • CampbelltowP 27 2199 • Chatswood Chase 4111955 • Chullora 642 8922 • Gore Hill 439 5311 • Gosford 25 0235 • Hornsby 477 6633 • Hurstville 580 8622 • Kotara 56 2092 • Liverpool 600 9888 • Maitland 33 7866 • Miranda 525 2722 • Newcastle 611896 • North Ryde 878 3855 • Orange 618 400 • Parramatta 689 2188 • Penrith 32 3400 • Railway Square 211 3777 • Sydney City 267 9111 • Tamworth 66 1711 • Wollongong 28 3800ACT• Belconnen (06) 2531785 • Fyshwick 80 4944 VIC • Ballarat 31 5433 • Bendigo 43 0388 • Box Hill 890 0699 • Coburg 383 4455 • Dandenong 794 9377 • East Brighton 592 2366 • Essendon 379 7444 • Footscray 689 2055 • Frankston 783 9144 • Geelong 232 711 • Melbourne City 399 Elizabeth St 326 6088 & 246 Bourke St 639 0396 • Richmond 4281614 • Ringwood 879 5338 • Springvale 547 0522 QLD • Brisbane City 229 9377 •Buranda 391 6233 • Cairns 311 515 • Capalaba 245 2870 • Chermside 359 6255 • Redbank 288 5599 • Rockhampton 27 9644 • Southport 32 9033• Toowoomba 38 4300 • Townsville 72 5722 • Underwood 341 0844 • SA • Adelaide City 223 4122 • Beverley 3471900 • Elizabeth 255 6099 • Enfield 260 6088 • St. Marys 277 8977 WA • Cannington 451 8666 • Fremantle 335 9733 • Perth City 481 3261 • Midland 250 1460 • Northbridge 328 6944 TAS • Hobart 31 0800 NT• Stuart Park 81 1977 ORDER BY PHONE: - SYDNEY AREA 888 22 6610 OUTSIDE SYDNEY !FREE CAUi 008 22 6610 81293/PB The Go-Anywhere Receiver It's Portable! 16 BAND SHORTWAVE RECEIVER 10 BAND PORTABLE SHORTWAVE RECEIVER The Sangean SG -789 is a neat and compact receiver with the power to pull in radio broadcasts from around the world! •Local AM/FM bands (FM stereo with optional headphones) •Receives the 8 most popular international shortwave bands •Bandspread tuning and LED tuning indicator •Superb sound reprod uction •Sockets for headphones & AC adaptor $ Cat D-2834 Listen to the world direct... Europe, Asia, America and Australia. Also covers local AM / FM stations, longwave and 13 international Shortwave bands. •Incredibly compact & portable •Large LCD screen showing major city times •With dual alarms, sleep timer & tuning indicator •Electronic band selection & volume contro ls • Pop out stand for desk use Cat D-2839 A World Of Listen,ing 45 MEMORY SHORTWAVE RECEIVER $169 7995 MULTI BAND SHORTWAVE RECEIVER Tune into the world via shortwave with the superb ATS-808 from Sangean . With 45 memories, stereo headphones and more! •External antenna socket & •AM & FM-stereo bands •16 band divisions with built-in whip antenna • Longwave: 150-519k Hz direct access buttons •Mediumwave: 520-1720kHz •Keypad, auto-scanning, •Shortwave: 1.711-29.999MHz or rotary tuning •Alarm, sleep timer & dual continuous or in 13 bands time settings •FM: 87.5-108MHz •LCD signal strength meter cat D-2829 Hear the world news as it happens via shortwave, or your local AM/FM stations. Listen to Commercial comm unications, Amateur, Marine, HF Aircraft and more. •Covers 150kHz-30MHz, 87 .5-108MHz •9 pre-set station memories •5 tuning functions including direct keypad entry •Backlit LCD frequency screen & 24hr clock •RF gain control, plus BFO for SSB reception •Battery powered or optional AC adaptor Cat D-2831 $269 - - - - - - - AUDIO CABLES - - - - - - Extremely high quality audio cables. OFC Spkr, 60 x 0.12mm conductors, Figure 8. Cat W-2013 $0.95/m OFC Spkr, 315 x 0.12mm conductors, Figure 8. Cat W-2017 $3.95/m Audio, 'MUSIFLEX' Twin Shield Pro. Cat W-2044 $2.45/ffl Audio, Super OFC Single Shielded, Cat W-2032 $1.95/m Audio, Super OFC Figure 8 Twin Shield . Cat W-2033 $3.95/m - - - - - - SURGE PROTECTORS Protect your valuable electronic equipment from dangerous voltage surges and lightning spikes with our great range of surge protectors. llf~ HPM DOUBLE ADAPTOR & SURGE PROTECTOR 111 A quality double adaptor with a bui lt in surge protector. The red indicator light tells you surge protection is activated. Simply plugs into any powerpomt. Cat M-7153 s29ss SURGE SPIKE SAFETY PLUG Protect your computer, fax machine, hi-fi and other valuable eq uipment from voltage surges and lightn ing spikes. It simply plugs into any powerpoint and automatical ly absorbs surges and spikes. With a green LED 'on' light. Cat M-7157 I 1 , l1 . \ I CENSOR PASSIVE POWER FILTER A quality multi-stage L-C filter. Designed to give affordable protection from transients for microprocessor equipment. With an attenuation bandwidth from 100KHz to 4MHz and suppression of 40dB over this range. Cat M-7185 $2995 IEC SURGE/SPIKE SAFETY CORD A cord set which continuously monitors and auto matically absorbs voltage surges and spikes. With a green LED 'on' light at both the plug and socket. Cat M-7159 s39s5 B1293/PB .01 250VAC ffi ffi 5 F3 A 1A r powEii7 I 1' 240VAC I I I I BRIDGE RECTIFIER 12 IN +3B.5V S6 5 OUT I IN +15V '--- - N 47 35VW + 100 + 16VW OV -::- CHASSIS 47 35VW - GNO 100 16VW r••15V 100 "H'tt-r 100 0.1 OUT n -·1 10 0.1 E OUT \ I I GNO + GNO + 15vwI 01 - + 1 J3:I o.1I I -15V 15V PREAMPLIFIER AND CONTROL UNIT -38.SV Fig.4: the power supply for the new Studio Twin 50 Amplifier delivers unregulated ±38.5V rails for the power amplifiers and regulated ±15V rails (via the 3-terminal regulators) for the preamplifiers and stereo headphone amplifier. tortion to a minimum) by the two diodes connected between the bases. The output current of the headphone amplifier is limited by the 15Q emitter resistors and 68Q output resistor. This provides short-circuit protection and protects the headphones against damage in the unlikely event of the amplifier being damaged (which would otherwise cause ±15V to be applied across the headphones). By the way, this headphone amplifier circuit is very similar to that used in ~4.e 16-Channel Mixer described in the Feb-May 1990 issues of SILICON CHIP. The only significant difference is that we have specified a TL072 dual op amp instead of the LM833 used elsewhere in this circuit. The reason for not using the LM833 in this circuit is that we have found it has a tendency to oscillate at around 60MHz; the TL072 does not do this. Power amplifiers As noted above, the power amplifiers are the same as featured in the stereo power module described last month. For the sake of those readers who have not seen last month's article, we repeat the circuit description here. The input signal is coupled via a lµF metallised polyester capacitor and 1.8kQ resistor to the base of Ql which together with Q2 makes up a differential pair. Q3 is a "constant current tail" which sets the current though Ql and Q2 and thus renders the amplifier largely insensitive to variations in its supply rails (otherwise known 30 SILICON CHIP as power supply rejection). Diodes Dl and D2 provide a voltage reference of about 1.2V for Q3 so that it applies a constant voltage to its 680Q emitter resistor. This sets the current through Q3 at close to lmA. This means that Ql and Q2 each operate with a collector current of about 0.5 milliamps. Signals from the collectors of Ql and Q2 drive another differential pair, Q4 and Q5, which have a "current mirror" as their load. The main advantage of the current mirror, D3 and Q6, is that it makes the second differential pair highly linear and therefore low in distortion. The output of Q5 drives a class-AB output stage consisting of Darlington transistors Q8-Q11. By class-AB we mean an amplifier which is essentially class-B (ie, each half of the output stage conducts for only half the signal) but which has a small current bias to minimise crossover distortion. The current bias in the output stage is controlled by transistor Q7 and trimpot VRl. Q7 is a Vbe multiplier, so called because the voltage between its base and emitter is multiplied by the ratio of the resistors between base and collector and base and emitter, respectively. VRl adjusts this voltage to give a voltage between the collector and emitter of Q7 of about 2 volts. In practice, it is adjusted to give an output stage current of 40 milliamps. Q7 is a BD139, a transistor normally used in amplifier driver stages and video circuits. It has a dissipation rating of 8 watts and so is only doing light duty. However, it is specified here because it gives better Vbe tracking with the output stage transistors and therefore better stability for the quiescent current. The Darlington transistors -Q8 and Ql0 and Q9 and Ql 1 - are connected as parallel pairs to share the output current. Each Darlington has a 0.47Q emitter resistor which helps ensure equal current sharing. As well, the emitter resistors improve the output stage bandwidth and the stability of the quiescent current. The value is a compromise though; bigger emitter resistors would give better stability and improved current sharing but would reduce the maximum output power capability. The lO0Q base resistors for the Darlingtons serve a number of functions. First, they reduce any tendency for the output stage to oscillate; always a possibility with emitter follower stages. Second, they limit the base current in the event of a short circuit in the output and thereby reduce the possibility of damage to the Darlingtons. Negative feedback is applied from the output stage back to the base ofQ2 · via a 22kQ resistor. This resistor, and the lkQ resistor also connected to the base of Q2 , sets the voltage gain to 23. The low frequency roll-off of the voltage gain is set by the 47µF capacitor in series with the lkQ resistor. This sets the -3dB point at about 3Hz. However, the lµF input capacitor is the main factor in the low frequency response of the amplifier and sets a -3dB point at 7Hz. The overall effect of the two time constants is a -3dB point at lOHz. The 820pF capacitor and the 1.8kQ PARTS LIST 1 steel case with 2.5mm anodised black aluminium front panel 1 60 x 60 x 290mm 3mm-thick angle aluminium heatsink 1 power transformer, 2 x 25V, 160VA (available from Harbuch, Altronics or Torrtech) 8 SOT-93 (TO-218) transistor mounting kits 2 SOT-32 (TO-126) transistor mounting kits 1 3AG panel-mounting safety fuse holder 1 1A 3AG fuse 8 20mm fuse clips 4 5A M205 20mm fuses 2 Philips 4322-021-30330 or Neosid 60-601-72 coil formers 2 Philips 4312-020-36760 ferrite wideband chokes 2 RDE245A polyswitches 1 rocker type SPST mains switch with inbuilt neon 1 2-pole 6-position rotary switch (S1) 3 2-pole push on/push off PCB mount switches (Lorlin or similar) with black knobs (S2,S3,S4) 5 22mm-dia. black anodised aluminium knobs (with index mark) 1 cordgrip grommet 1 3-way insulated terminal block 1 black binding post terminal 3 3 x 2-way RCA socket panels (Altronics Cat. No. P-0123) 1 PCB-mount 6.5mm headphone socket with integral switch (Jaycar Cat. No. PS-0180) 1 1/4-inch solid shaft coupler 1 140mm long 1/4-inch shaft (for selector switch) 4 rubber feet 2 solder lugs Cable & ties 1 3-core mains cord and moulded 3-pin plug 1 150mm length of black insulated 1mm diameter single strand cable 1 300mm length of red insulated 1mm diameter single strand cable 1 500mm length of figure-8 shielded audio cable 3 1-metre lengths of hook-up wire 1 2.5-metre length of 0.8mm enamelled copper wire 1 plastic cable tie 1 160mm length of heatshrink tubing (to fit over mains switch & wiring) PC boards & hardware 1 phono preamplifier board, code SC01103921 , 186 x 135mm 1 switch mounting board, code SC01103922, 54 x 38mm 1 tone control board, code SC01103923, 252 x 85mm 1 stereo amplifier board, code SC01102921, 80 x 233mm 43 1mm dia. PC pins 3 15mm threaded spacers 1 10mm threaded spacer 2 6mm spacers Semiconductors 3 LM833 low noise dual op amps (IC1 ,IC2,IC3) 1 TL072 FET-input op amp (IC4) 6 BC556 PNP transistors (01-03) 4 BF469 NPN transistors (04,05) 2 BF470 PNP transistors (06) 2 BD139 NPN transistors (07) 4 TIP142 NPN Darlington transistors (08,010) 4 TIP147 PNP Darlington transistors (09,011) 2 BC337 NPN transistors (012) 2 BC327 PNP transistors (013) 8 1N4148 signal diodes (D1 -DS) 1 PO4 6A bridge rectifier 1 7815 3-terminal regulator (NB: 78L 15 not suitable) 1 7915 3-terminal regulator (NB: 79L 15 not suitable) Capacitors 2 4700µF S0VW electrolytics 6 100µF 16VW PC electrolytics 2 100µF bipolar electrolytics (phono preamps) 2 47µF 35VW electrolytics (power supply) 2 47µF 16VW electrolytics (power amps) 2 47µF bipolar electrolytics (phono preamps) 2 22µF bipolar electrolytics 2 6.8µF bipolar electrolytics 4 1µF 63VW MKT polyester 2 0.33µF MKT polyester 8 0.22µF 63VW MKT polyester 2 0.15µF 10% 100VDC 10mmpitch metallised polycarbonate (Philips 2222 344 21154) 8 0.1 µF 63VW MKT polyester 2 .033µF 1O0VW MKT polyester 2 .015 MKT polyester (2.5% or 5% tolerance preferred) 2 .01 µF MKT polyester 1 .01µF or .0047µF 250VAC metallised paper 4 .0047µF MKT polyester (tone controls) 2 .0047µF MKT polyester (2.5% or 5% tolerance preferred - phone preamps) 2 820pF S0V ceramic 2 470pF S0V ceramic 2 330pF S0V ceramic 2 150pF S0V ceramic 2 68pF 100V ceramic 2 33pF S0V ceramic 2 2.2pF S0V ceramic Potentiometers 1 dual gang 100k.Q linear potentiometer, PC-mounting (VR3, bass) 1 dual gang 50k.Q log potentiometer, PC-mounting (VR2, volume) 1 dual gang 25k.Q linear potentiometer, PC-mounting (VR4, treble) 1 dual gang 10k.Q M/N potentiometer, PC-mounting (VRS, balance) 2 500.Q horizontal mount trimpots (VR1) Resistors (0.25W, 1%, 7mm body) 21M.Q 4 3.9k.Q 2 200k.Q 2 1.8k.Q 6 100k.Q 2 1.Sk.Q 2 47k.Q 101k.Q 12 22k.Q 4 680.Q 2 16k.Q 4 180.Q 2 15k.Q 2 150.Q 4 10k.Q 12 100.Q 4 5.6k.Q 2 68.Q 6 4.7k.Q 4 15.Q 2 4.7k.Q 1W 26.8.Q1W Wirewound resistors 2 270.Q 3W 10% (Philips AC03 or similar) 8 0.47.Q 3W 10% (Philips AC03) 2 560.Q SW resistors (for setting quiescent current) M ARCH 1992 31 The circuitry for the new amplifier unit is accommodated on three PC boards: one for the tone controls, one for the phono preamplifier and high level inputs, and one for the power amplifiers. To make construction easy, the control pots, pushbutton switches and headphone socket are soldered directly to the tone control board. input resistor feeding Ql form a low pass filter which rolls off frequencies above lO0kHz. The 68pF capacitor between base and collector of Q5 and the 2.ZpF capacitor between base and collector ofQZ are used to roll off the open loop gain to ensure stability with feedback applied. We have also used our standard RLC network in the output stage. A configuration originally proposed by Australian engineer Neville Thiele, it uses 6.8µH air-cored choke, a 6.8Q resistor and a 0.15µF capacitor. 32 SILICON CHIP The beauty of this network is that it effectively isolates the amplifier output stage from any nasty impedance dips which may occur at high frequencies and which could cause the amplifier to be unstable. It also has another favourable effect because it kills any RF signal pickup by long speaker leads. Power supply The power supply, as shown in Fig.4, uses a 160VA transformer with a centre-tapped 50V winding feeding a 6-amp bridge rectifier and two 4700µF 50VW electrolytic capacitors. These provide unregulated supplies of about ±38.5V (at no signal and with 240VAC mains input). The unregulated supply rails feed the two power amplifiers directly. For the small signal circuitry, the unregulated ±38.5V rails are fed via 270Q 3W resistors to 3-terminal regulators to produce balanced supply rails of ±15V. The 270£2 dropping resistors reduce the input voltage to the regulators and thereby reduce their power dissipation. That's all we have space for this month. Next month, we 'll present the construction and troubleshooting details. SC