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Low noise universal stereo preamplifier Do you have a need for a low noise stereo preamplifier for your magnetic cartridge, tape player or perhaps for microphone use? If so, this unit is the answer. It is based on the economical LM833 dual low noise op amp IC. By JOHN CLARKE & LEO SIMPSON This universal stereo preamplifier will have quite a few uses for audio enthusiasts. Perhaps the prime use will be for those people who have an integrated stereo amplifier which they are quite keen on but which has a phono preamplifier stage which could be improved. That applies to the phono preamplifiers in a great many amplifiers. They weren't designed to give the minimum noise, minimum distortion and the greatest overload margin. In fact, about the best thing you can say about the preamplifier stages in most older amplifiers is that they are still working. By comparison, the performance of the design presented here is far better than most preamplifiers in most stereo amplifiers - that's a pretty ambitious statement but it is nonetheless true. How do you decide whether it would be worthwhile to upgrade your amplifier's preamplifier. That is fairly easy to determine. Just set your amplifier's controls to their normal settings and listen for hiss with no record playing. The prototype preamplifier was housed in a metal case hut could be built into an existing amplifier chassis or installed underneath a turntable. Keep the circuit away from power transformer hum fields. 40 SILICON CHIP Can you hear hiss from the loudspeakers (or headphones) at your normal listening position? If so, does the hiss greatly reduce or disappear when you rotate the volume control to its minimum setting. If the answer to both questions is yes, then it is highly likely that your existing preamplifier produces more than its fair share of noise. This new design is one of the quietest ever published so you are sure to hear a reduction in hiss. Even if you don't need to upgrade your existing amplifier's preamplifier, you may still have an application for the design presented here. For example, you may want to run two turntables. If your amplifier only has one pair of phono inputs, you could use this external preamplifier for the additional turntable and then feed its outputs to one pair of line inputs of the stereo amplifier. We have also shown how this preamp could be used with a tape deck which does not have its own playback electronics or where the existing tape preamp is unduly noisy. Finally, this design can function as a high quality microphone preamplifier for use with cassette decks (which normally don't have microphone inputs) or in a public address system. We've housed the universal preamp in a standard metal case but it could be built into an existing amplifier's chassis or installed underneath a turntable. Either way, you'd have to make sure it was well out of the way of the hum field of any power transformer. As it stands, our prototype preamplifier has a power switch and LED power indicator, and four RCA phono sockets, two for the inputs and two for the outputs. The circuit The circuit shown in Fig.1 looks a +15V ,~cr-T' .;,;.i '.i: -. l1 150!l 4T. ENCU WIRE ON FX1115 BEAD 47 lOOpFI. .,. Rl 16k 1¾ R4 390!l 1% R2 200k 1¾ IC PIN NUMBERS IN BRACKETS ARE FOR RIGHT CHANNEL MAGNETIC CARTRIDGE 100! BP .,. Rl on R2 200k 1¾ R4 2DOn 1% NAB Rl on R2 200k 1% R4 390n 1°/, 100.I BP .,. Sl POWER MICROPHONE ~K D1 1N4002 0-0 +15V 12V AC FROM PLUGPACK 1 • 1 16VW • _ ';:{ SENSE - 6 3.3k 0.5W A X LEDl K 7 -15V VO-t-- - - - - - - - - - - UNIVERSAL PREAMPLIFIER Fig.1: the circuit is based on the LM833 low-noise op amp. By changing the feedback network, it can he wired as a magnetic cartridge preamplifier for your hifi system, as a tape head amplifier or as a microphone preamplifier. little odd but we have presented it this way to avoid having to show three completely separate versions of the circuit. So in effect, we have shown just one channel of the preamplifier with three alternative feedback networks: for magnetic cartridge, for tape or cassette deck (NAB), and for microphone. For the magnetic cartridge function, ICla not only has to amplify the signal but must also apply equalisation. 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.2 . To be specific, a lOOHz signal has a boost of 13.lldB while a lOkHz 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 4 7µF 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 preamp input circuit 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 to be, as far as bass signal coupling is concerned. If we were merely concerned with maximising the bass signal from the cartridge, then an input coupling capacitor of 0.47µF would be quite adequate. At 20Hz, a capacitor of this value has an impedance of around 15k0 which is considerably less than the nominal 50k0 input impedance of the preamp. But having a large capacitor means that the op amp "sees" a very low impedance source (ie, the DC resistance of the cartridge) at low frequencies and this helps keep low frequency noise, generated by the input loading resistors, to a minimum. RIAA/IEC equalisation The RIAA equalisation is provided by the feedback components, Rl, Cl, R2 and C2, between pins 1 and 2 of ICla (or pins 7 and 6 of IClb, in the other channel, which is not shown). 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 following amplifier's volume control and input circuitry (which is likely to be around 50k0). There is also a further low freJUNE 1990 41 The PC board is installed in the case on four 6mmlong standoffs. It is shown here wired as a magnetic cartridge preamplifier but you can wire it as a tape head or microphone preamp simply by changing the feedback components depicted in Fig.1 (see also Fig.2). Note the use of shielded cable for the connections between the board and the RCA sockets. r quency rolloff, at around 4Hz, caused by the lO0µF capacitor in series with the 3900 resistor. The 3900 resistor sets the maximum AC gain at very low frequencies while the lO0µF capacitor ensures the gain for DC is unity. This means that any input offset voltages are not amplified, which would inevitably cause trouble with asymmetrical clipping and premature overload in the preamplifier. Actually, the magnetic cartridge version of the circuit just described is identical to the phono preamplifier of the Studio 200 stereo control unit, published in the June and July 1988 issues of SILICON CHIP. Incidentally, the mention of RIAA/IEC equalisation above refers to two different disc recording standards. The RIAA standard was originally set by the Record Industry Association of America in 1953. The later IEC variation was recommended by the International Electrotechnical Commission in the 1970s. Specifications Frequency Response Phono version: RIAA/IEC ±0.3dB from 20Hz to 20kHz Mic version: within ± 1 dB from 1 OHz to 20kHz Less than .005%, 20Hz to 20kHz, at 1V RMS Signal-to-Noise Ratio • Phono: 83d8 unweighted (20Hz to 20kHz) with respect to 1 OmV input signal at 1 kHz with 1 kO resistive input termination; 89.5dB Aweighted with respect to 1 OmV input signal and with 1 kO resistive input termination. Separation Between Channels -82dB at 1 OkHz, -88dB at 1 kHz and -80dB at 1 OOHz, with undriven channel input loaded with a 1 kO resistor. Gain Maximum Output Signal 8.4 volts RMS at 1 kHz 42 SILICON CHIP In the tape equalisation version, the value of RZ is identical to that of the phono preamplifier but R4 is now 2000 and Rl is replaced by a wire link. Cl & CZ are omitted and replaced by R3 & C3. Microphone version In the microphone version, RZ and R4 are the same as in the phono preamp while Rl is a short circuit and Cl is omitted altogether. The microphone has a gain of 513, making it suitable for low impedance microphones. If less gain is required, it is simply a matter of changing the ratio of RZ to R4. For example, if you want a gain of 100 times, make R4 4700 and RZ 47k0. Power supply Total Harmonic Distortion Phono: 56 at 1 kHz Tape: 53 at 1 kHz Tape equalisation Microphone: 513 Input impedance: 50k0 Phono Input Overload 1 50mV RMS at 1 kHz The power supply may look a little unconventional but is quite straightforward. It is fed from an external 1 ZV AC plugpack which enables us to keep the power transformer well away from the preamp circuitry. The 1 ZV AC input is rectified by two diodes, Dl and DZ, and filtered by two 470µF electrolytic capacitors to give positive and negative unregulated DC rails of around 21 volts. These rails feed an LM325 voltage regulator IC. This is an economical substitute for a pair of 7815 and 7915 15V 3-terminal regulators. Apart from ,w.4 A 12V AC FROM PLUGPACK S1 RIGHT INPUT Fig.2: refer to the main circuit diagram for the values of R1-R4 and C1-C3, and use the values to suit your application. Take care with component orientation and use your multimeter to check resistor values before installing them on the board. TABLE 1: RESISTORS D D D D D D D D D D No. Value 4-Band Code (5%) 5-Band Code (1%) 2 2 1MO 200k0 100k0 16k0 3.6k0 3 .3k0 3900 2000 1500 1000 brown black green gold not applicable not applicable not applicable not applicable orange orange red gold not applicable not applicable not applicable brown black brown gold brown black black yellow brown red black black orange brown brown black black orange brown brown blue black red brown orange blue black brown brown orange orange black brown brown orange white black black brown red black black black brown brown green black black brown brown black black black brown 4 2 2 1 2 2 2 2 taking up less board space than separate regulators, the LM325 gives better tracking between the positive and negative rails, within ± 300mV. Its output current capacity is lO0mA which is well in excess of the 5mA or so needed by the preamplifier. Construction All the circuitry for the universal preamp goes onto a small printed circuit board measuring 75 x 95mm and coded SC0l 106901. It has the power supply connections at one end and the signal input and output connections at the other. For our prototype, we mounted the printed board in a standard metal case measuring 105mm wide, 65mm high and 150mm deep (Jaycar Cat. HB-5442 , DSE Cat. H-2743). This case has ari aluminium base fitted with four rubber feet and a black crackle enamel TABLE 2: CAPACITOR CODES Value D D D D D 22pF 100pF .0047µF .0 15µF 0 .33µF Alt Value IEC Code EIA Code (10%) 4.7nF 15nF 330nF 22p 100p 4n7 15n 330n 22K 101K 472K 153K 334K finished steel top cover. Before commencing assembly, carefully check the PCB pattern fo r any shorts or breaks in the copper tracks, which should be corrected at this stage. Fig.3 shows the wiring details. Start by installing the 12 PC stakes on the PC board. Once this has been done, you can install the wire links and the resistors. We suggest you use a digital multimeter to check each resistor value as it is installed. The two inductors (Ll in each channel) are made by winding 4 turns of 0.6mm enamelled copper wire on an FXl 115 ferrite bead. Scrape the enamel off the ends of the inductor leads and tin them with solder before soldering them into the printed board. Note that the feedback components in each channel are shown on the boa rd as Rl, R2, Cl , C2, etc. You will have to look at the circuit diagram to see the value for each of these components, depending on which version you are building. JUN E 1990 43 Don't m iss a single issue IF YOU'RE THE KIND OF READER that can't wait to get the next copy of SILICON CHIP, then why not have the magazine delivered direct to your door? Each month, we'll bring you the best and brightest electronics magazine in the business, put together by Australia's most experienced team. 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Detach Coupon and Mail to: FREEPOST 25 SILICON CHIP PUBLICATIONS PO BOX 139 COLLAROY BEACH NSW 2097 *No postage stamp required in Australia. 44 SILICON CHIP Street _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ __ Suburb/town _ _ _ _ _ _ _ _ __ 1 year ( 12 issues) □ $A42 Australia _ Postcode_ _ __ 2 years (24 issues) □ $A84 NZ & PNG (airmail) □ $A65 □ Overseas surface mail □ $A62 □ $A130 Overseas airmail □ $A 1 20 □ $A130 $A240 Enclosed is my cheque/money order tor $ _ _ _ or please debit my □ Bankcard Card No. □ Visa Card □ MasterCard I I I I 11 I I I 11 I I I 11 I I I I Signature _ _ _ _ _ _ _ _ Card expiry date_ _ /_ L _______________________ J r• 7 UNIVERSAL PREAMPLIFIER ~IH/ll- POWER L OUT L IN R IN R OUT • • • • • L ~ _J I PARTS LIST 1 PCB, code SC01106901, 75 x 95mm 1 metal case , 150 x 61 x 102mm 1 Scotchcal panel, 100 x 52mm 1 1 2V 5VA AC plugpack 1 SPOT toggle switch 1 5mm LED bezel 4 panel mount RCA sockets 4 6mm PCB standoffs 12 PC stakes 4 screws and nuts to suit standoffs 2 FX 11 1 5 ferrite beads 1 small rubber grommet 1 100mm length of 0 .8mm enamelled copper wire 1 1 50mm length of shielded audio cable 1 200mm length of light duty hookup wire Semiconductors 1 LM833 dual low noise op amp (IC1) 1 LM325 ± 15V regulator (IC2) 2 1N4002 diodes (01 ,02) Fig.3: here are actual size artworks for the front panel & the PC board. Be sure that the polarised components are correctly oriented on the PCB. These parts include the electrolytic capacitors, diodes and the two ICs. Mount the ICs on the board last of all. Once the board assembly has been completed, check it for correct installation of all the components. You can now connect the 12V AC plugpack to the circuit. Check the DC voltages around the circuit with respect to one of the PC stakes which is at 0V. You should find + 15V present at pin 8 of ICl and pins 1, 13 and 14 of ICZ. For the negative rail, - 15V should be present at pin 4 of ICl and pins 6 and 7 of ICZ. Actually, due to the production spreads in the LM325, you will find that the + 15V rail can be anywhere between + 14.5V and + 15.5V. Similarly, the negative rails should be anywhere between - 14.5V and - 15.5V but the absolute values of both rails should be within ± 300mV. You can also check the offset voltages at the outputs of ICl, pins 1 and 7. The voltage at these pins should be within ± lO0mV of 0V. It will most likely be quite a lot less than this figure. The PC board is now ready to be installed in the case, if that is the way you intend to use it. Before you can do that, you will need to drill the holes for the front panel hardware, the grommet in the rear panel and the four PC pillars. If you are using a Scotchcal front panel, you can use it as a marking template for the front panel holes. Capacitors 2 470µF 25VW PC electrolytic 2 1 OOµF bipolar PC electrolytic 2 4 7 µF bipolar PC electrolytic 2 1µF 16VW PC electrolytic 2 0.33µF metallised polyester 2 .015µF metallised polyester 2 .0047µF metallised polyester 2 1OOpF ceramic 2 22pF ceramic Resistors (0.25W, 5%) 2 1MO 2 200k0 1 % metal film 4 1OOkO 1 % metal film 2 1 6k0 1 % metal film 2 3 .6k0 1 % metal film 1 3.3k0 0 .5W 2 3900 1 % metal film 2 1 500 1 % metal film 2 1000 Note: depending on the version built, resistor values for R1 and R4 will change. The same applies to C1 and C2 (see text) . When all the holes are drilled and the front panel artwork fitted, you can mount the PC board in the case and complete the wiring. When all the wiring is complete, you can switch on and check the voltages again. ~ JUNE 1990 45