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This universal preamplifier can be easily constructed for use with a magnetic cartridge, cassette deck or a dynamic microphone. It uses a single dual op amp IC & has very low dis­tortion. By DARREN YATES Low-noise universal stereo preamplifier T HIS PROJECT WAS borne out of the recent news that National Semiconductor has discontinued its LM380 series of stereo pream­ plifier ICs. These have been around since the early 1970s and have been popular with enthusiasts for all sorts of projects. In fact, these devices were not all that good by today’s standards which is another reason to produce an up-to-date design. Our little universal preamp uses the industry standard LM833 dual op amp IC which has very low noise and distortion. 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 or tape preamp which could be improved. And that applies to the phono preamps in a great many amplifiers. They weren’t designed to 32  Silicon Chip 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 many 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 true nonetheless. How do you decide whether it would be worthwhile to upgrade your amplifier’s preamplifier. That is fairly easy to determine, pro­viding you still listen to vinyl records. Just set your amplifier’s controls to their normal settings and listen for hiss with no record playing. Can you hear hiss from the loudspeaker (or headphones) at your normal listening position? If so, does this 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 likely that your existing preamplifier produces more than its fair share of noise. This new design is extremely quiet 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 the line inputs of the stereo amplifier. Alternatively, you may have an audio mixer which does not have a phono preamplifier or you may wish LEFT INPUT +15V L1 150  100k 47 BP 3(5) 100pF 100k 2(6) 8 1(7) IC1a LM833 0.33 1M 4 L1 : 4T, ENCU WIRE ON PHILIPS 4330 030 3218 FERRITE BEAD 100  LEFT OUTPUT -15V IC PIN NUMBERS IN BRACKETS ARE FOR RIGHT CHANNEL R1 16k R2 200k C1 .0047 C2 .015 R4 390  47 BP R1 0W +15V +15V R3 3.6k C3 .015 R1 0W R2 200k R4 200  47 BP 0.1 GND 0V 0.1 -15V R2 200k -15V R4 390  C2 22pF 47 BP UNIVERSAL PREAMPLIFIER Fig.1: the circuit is shown with three different feedback networks: one for a magnetic cartridge (top); one for tape or cassette decks (centre); & a third for microphones (bottom). The inductor, series resis­tor & 100pF shunt capacitor at the input form a filter circuit to remove RF interference signals. to incorporate it into a public address system. 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’re presenting this universal preamp as a PC board only, leaving you with the opportunity to install it anywhere you have space for it. The major rule is to keep it away from any mains wiring or transformers. This will reduce any hum pickup. The circuit The circuit shown in Fig.1 looks a little odd but we’ve presented it this way to avoid having to show three completely separate versions. So we have shown just one channel of the preamplifier with three different feedback networks: one for magnetic cartridge, another for tape or cassette decks and a third for microphone. For the magnetic cartridge function, IC1a not only has to amplify the signal but must also apply RIAA equalisation. It takes the low level signal from the moving magnet cartridge (typically, a few millivolts) and applies a gain of 56, at the median frequency of 1kHz. 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 100Hz signal has a boost of 13.11dB while a 10kHz signal has a cut of 13.75dB. The phono signal is fed directly April 1994  33 from the input socket via inductor L1, a 150Ω resistor and a 47µF bipolar capacitor to the non-inverting input (pin 3) of IC1a. The inductor, series resis­ tor and shunt 100pF capacitor form a filter circuit to remove RF interference signals which might be picked up by the phono leads. The 100pF capacitor is also important in capacitive loading of the mag- PARTS LIST (Magnetic cartridge version) 1 PC board, 01106941, 80 x 78mm 8 PC stakes 2 Philips ferrite beads 4330 030 3218 Semiconductors 1 LM833 dual op amp (IC1) Capacitors 4 47µF 50VW bipolar electrolytic 2 0.33µF 63VW MKT polyester 2 .015µF 63VW MKT polyester 2 .0047µF 63VW MKT polyester 2 100pF ceramic Resistors (0.25W, 1%) 2 1MΩ 2 390Ω 2 200kΩ 2 150Ω 4 100kΩ 2 100Ω 2 16kΩ Miscellaneous Shielded cable, screws, nuts, tinned copper wire. +20 20Hz (7950uS) netic cartridge. Most moving magnet (MM) cartridges operate best with about 200 to 400pF of shunt capacitance. The 100pF capacitance in the preamp input circuit plus the usual 200pF or so of cable capacitance for the pickup leads will therefore provide about the right shunt capacitance. For its part, the 47µF 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 would have an impedance of around 15kΩ which is considerably less than the nominal 50kΩ 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, R1, C1, R2 and C2, between pins 1 and 2 of IC1a (or pins 6 and 7 of IC1b, 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 pream­ plifier also adds in the IEC recom- mendation for a rolloff below 20Hz (7950us). 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 50kΩ). There is also a further low frequency rolloff, at around 9Hz, caused by the 47µF capacitor in series with the 390Ω resis­tor. The 390Ω resistor sets the maximum AC gain at very low frequencies while the 47µF 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 Control Unit, published in the June and July 1988 issues. 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. Tape equalisation In the tape equalisation version, the value of R2 is identi­cal to that of the phono preamplifier but R4 is changed to 200Ω and R1 is replaced by a wire 50Hz (3180uS) DECIBELS +10 2.12kHz (75uS) 0 500Hz (318uS) -10 -20 2 10 20 100 HERTZ 1k 10k Fig.2: this graph shows the RIAA/IEC equalisation characteristics provided by the feedback components for the magnetic cartridge preamplifier version. 34  Silicon Chip 20k 0.33 47uF 1M R2 R1 47uF 47uF 0.1 IC1 LM833 1 C1 L1 100k LEFT OUTPUT 100pF 1M R2 R4 GND 100  100k 0.33 0V -15V R1 150  GND +15V C1 0.1 L1 R3 150  RIGHT INPUT C2 R4 100pF 100k GND 100  GND LEFT INPUT C3 100k R3 RIGHT OUTPUT C2 C3 47uF Fig.3: refer to the main circuit diagram for the values of R1-R4 & C1-C4 & install these parts to suit your application. link. C1 and C2 are omitted and replaced by R3 and C3. Microphone version In the microphone version, R2 and R4 are the same as in the phono preamp while R1 is a short circuit and C1 is omitted alto­gether. The microphone preamp 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 R2 to R4. For exam­ple, if you want a gain of 100 times, make R4 470Ω and R2 47kΩ. Power supply The required power supply is a regulated source of ±15V DC at around 10mA. This could come from 7815 and 7915 3-terminal regulators or derived from supply rails in your existing equipment. If you want a PC board for this job, refer to the “Universal Power Fig.4: check your PC board before installing the parts by comparing it with this full-size etching pattern. Supply Board for Op Amp Circuits” published in the August 1988 issue of SILICON CHIP. (This issue is now out of print but we can supply photostat copies of the article for $6. Alternatively, you could use the discrete regulator design published elsewhere in this issue. Construction All the input circuitry for the universal preamp goes onto a small PC board measuring 80 x 78mm and coded 01106941. Before you begin construction, check the PC board carefully for any shorts or breaks in the tracks. If you find any, correct the problem before installing any parts. When you’re happy that the board is OK, you must decide which version you are going to construct. In each case, make sure you know which resistors and capacitors numbers must be included and which must be left out or replaced with wire links. In any case, use the component wiring diagram of Fig.3 to carefully check the position of all components. Begin by installing the wire links, followed by the resistors and the MKT capacitors. This done, solder in the IC and then continue with the electrolytic capacitors. Once the board is fully assembled, check it for correct installation of all the components. You can now connect the ±15V supplies and check the DC voltages with respect to one of the PC stakes which is connected to 0V to GND. You should have +15V at pin 8 and -15V at pin 4 of the IC. You can also check the offset voltages at the outputs of IC1, pins 1 & 7. The voltage at these pins should be within ±100mV of 0V but will most likely be a lot less than this. If that is the case, the PC board is ready to be installed SC into your equipment. RESISTOR COLOUR CODES ❏ No. Value 4-Band Code (1%) 5-Band Code (1%) ❏ 2 1MΩ brown black green brown brown black black yellow brown ❏ 2 200kΩ red black yellow brown red black black orange brown ❏ 4 100kΩ brown black yellow brown brown black black orange brown ❏ 2 16kΩ brown blue orange brown brown blue black red brown ❏ 2 390Ω orange white brown brown orange white black black brown ❏ 2 150Ω brown green brown brown brown green black black brown ❏ 2 100Ω brown black brown brown brown black black black brown April 1994  35