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R ecording I ndustry A ssociation of A merica PREAMPLI FOR TURNT Did you want to build the Ultra-LD Amplifier described recently in SILICON CHIP? Are you put off because it does not have a preamplifier for a magnetic cartridge? Then don’t be because this self-contained preamplifier has been designed to install under your turntable and give better performance than the RIAA preamp in most stereo amplifiers and receivers. By LEO SIMPSON & ROSS TESTER S eemingly, no sooner than the print was dry on the December 2001 issue which featured the second article on the Ultra-LD Amplifier, than we had several requests for a magnetic cartridge preamp to go with it. What to do? This had not been part of the game plan: there is really not enough space inside the amplifier for a preamplifier. We were also concerned about being able to obtain a sufficiently good signal-to noise-`ratio, if it was somehow packed into the rear compartment of the amplifier. Of course, one neat solution would be to build the LP Doctor featured in the January & February 2001 issues of SILICON CHIP. This not only features a good preamp but it also electronically 68  Silicon Chip removes the clicks and pops on vinyl records. Kits are still available from Dick Smith Electronics. However, as good and effective as the LP Doctor is, many people want just a simple preamp – and that is what we are presenting here. It comes on a single PC board together with its own power supply, including an encapsulated 20VA toroidal power transformer. It is intended to be mounted inside a standard turntable plinth, either on the underside of the turntable chassis or the Masonite (hardboard) cover underneath the plinth. The beauty of having the preamp directly underneath the turntable is that the input signal leads are kept to an absolute minimum and that should mean minimum hum and noise. On the other hand, care has to be taken to ensure that any hum and noise from the turntable motor and mains wiring is minimised. Circuit description Fig.1 shows the circuit. It uses a LM833 dual low-noise op amp, with one op amp being used in each channel. The circuit is practically identical to the preamp stage used in the above-mentioned LP Doctor except that it has higher supply rails and higher overall gain. The higher gain is necessary to raise the nominal 5mV to 10mV signal of a magnetic phono cartridge to the nominal input sensitivity of 500mV rewww.siliconchip.com.au The RIAA Preamplifier and ±15V power supply can be built as one board, as shown here, or divided in two for a separate power supply and preamplifer. The reasons for the “split” are explained in the text. IFIER TABLES quired for full power from the Ultra-LD power amplifier or, indeed, from the “aux” input of most amplifiers. At the same time, the preamplifier needs higher supply rails in order to provide good overload capability for high level signals from the magnetic cartridge. For those who have not seen the previous articles on this preamplifier, we will now describe how the circuit works. Both channels are identical so we will just refer to the left channel. Accordingly, the left channel phono cartridge signal is fed from the input socket via inductor L1, a 150Ω resistor and a 47µF bipolar capacitor to the non-inverting input of IC1a, which is one half of an LM833 dual low-noise op amp. The inductor L1, 150Ω series resistor and the shunt 100pF form a lowpass filter to remove RF interference signals which might be picked up by the phono leads. There is also a 270pF capacitor connected directly across the input terminals and this, added to the 100pF filter network capacitor, provides a capacitive load across each channel of the cartridge. This is done because most magnetic phone cartridges operate best with about 200-400pF of shunt capacitance. Normally, most of the capacitance www.siliconchip.com.au will be present in the pickup leads from the turntable to the preamplifier input but since this preamplifier is intended to be built into the turntable itself the long connecting leads will not be present. In fact, if you know the cartridge manufacturer’s recommended shunt capacitance you can connect the appropriate capacitor on the board instead of the 270pF we have shown on the circuit. Just subtract 100pF from the recommended capacitance and use that instead of the 270pF capacitors. While we’re still at the input circuit, the 47µF bipolar capacitor looks to be far larger than it needs to be in terms of bass signal coupling from the cartridge and so it is. In fact, as far as bass signal coupling is concerned, the capacitor only needs to be about 0.47µF. The reason we have specified such a large input capacitor is so that the op amp “sees” the very low source impedance of the cartridge, especially at low frequencies, and this helps to minimise any low frequency noise generated by the input loading resistors. RIAA equalisation Due to the way in which vinyl discs are recorded and the characteristics of Underneath the old Apan beltdrive turntable which had sat, unused and unloved, under the house since CDs came into vogue. The mains terminal strip can clearly be seen centre of pic; the output cables and wires from the cartridge terminate on a tagstrip hidden under the metal shield immed-iately to the right. March 2002  69 LEFT INPUT L1 (L2) 270pF 150 100k 100k LEFT CHANNEL ONLY SHOWN FOR CLARITY RIGHT CHANNEL IS IDENTICAL (IC PIN NOS IN BRACKETS ARE RIGHT CHANNEL) L1, L2: LINK THROUGH 4330-030-3218 BEAD SC  2002 +15V 47F BP RIAA PREAMP 3 (5) 8 1 (7) LM833 100pF 2 (6) LEFT OUTPUT IC1a (IC1b) 0.33F 4 100 1M -15V 390 F BP 16k 200k .0047F .015F 8 4 1 Fig 1: one channel of the RIAA preamp. Gain is set at 56, making it ideal for use with most amplifier “aux” inputs. typical magnetic cartridges, the input signal to the preamplifier will be very strong at the treble end and becomes progressively less as the frequency drops. To compensate for this, all magnetic cartridge preamplifiers need to substantially boost the bass and cut the treble (ie, equalisation) according to the well-known RIAA characteristic curve. In our circuit, the RIAA equalisation is provided by the RC feedback components between pins 1 & 2 of IC1a (and between pins 6 & 7 of IC1b, the right channel). In fact, there are three capacitors and three resistors which vary the feedback to provide the correct equalisation. For example, the .0047µF and .015µF capacitors provide the required treble rolloff, in conjunction with the 16kΩ and 200kΩ resistors which help determine the gain. This feedback network provides the standard equalisation time constants of 3180µs (50Hz), 318µs (500Hz) and 75µs (2122Hz). The circuit also includes the IEC recommendation for rolloff below 20Hz. This is provided by the 390Ω resistor in series with the 33µF capacitor and the 0.33µF output coupling capacitor. The overall gain of the circuit is set by the 390Ω resistor in series with the 33µF capacitor. This is a compromise between overall gain (set at 56 at 1kHz) and input overload margin. The latter is important because while the nominal signal level from a typical magnetic cartridge may only be about 5mV or 10mV, the level can rise to well over 70mV (or much higher for high- output cartridges). The frequency response is shown in the graph of Fig.3. Signal-to-noise ratio is about as good as you can get with a typical magnetic cartridge, around -83dB with respect to an input signal of 10mV at 1kHz. The actual signal-to70  Silicon Chip noise ratio you achieve depends more on your turntable, its rumble performance, shielding from hum fields and so on. Power supply To make the circuit self-contained, we have included the power supply. It employs a potted 10VA toroidal transformer with two 15V secondary windings feeding a bridge rectifier, two 470µF capacitors and two 3-terminal regulators to produce balanced supply rails of ±15V. Both the power supply and the stereo preamplifier have been combined onto one PC board measuring 210 x 65mm and coded 01103021. This has can be used in one piece or split, to separate the power supply and the preamplifier. This has been done to ensure maximum flexibility. While we intended it to be installed underneath a standard turntable, this may not be practical in some cases where space is ALTRONICS M4330 120V A POWER 15V very limited. In that case, you would need to house the whole PC board in its own earthed metal case. If you do install the preamplifier underneath your turntable, you will need to take great care in order to get hum-free performance. Generally, that means installing the power supply section of the board as far away as possible from the tone arm and its “arc” across the turntable platter. Typically, this means installing it near the motor or in the opposite corner to the tonearm pivot. The preamplifier section of the board should then been installed as close as possible to the tonearm shielded cable terminations, typically near the back of the turntable base. Use very short shielded input leads to the preamp and then terminate the turntable’s existing shielded output leads to the preamp-lifier outputs. Construction Before starting construction, you will need to make that decision about whether to use the PC board intact, or to split it. While you can saw the board apart after assembly (we did!), it is easier to do before it is populated. And before you do anything, inspect the board for any manufacturing defects. We’ll assume you are going to use it intact but if you aren’t, the sequence of construction is much the same. Because the heavier components are mounted at the power supply end, we’ll start with the preamp. Start with the resistors – and as you D1 - D4 4 x 1N4004 F1 1A 240VAC IN 0V 120V 15V 0V N REG1 7815 0V T1 20VA 0V IN OUT +15V GND 470F 25V 100F 16V 0V E EARTH LINK (SEE TEXT) A E N TURNTABLE 7815 IN GND GND IN GND IN OUT -15V REG2 7915 7915 OUT 100F 16V 470F 25V OUT SC 2002 RIAA PREAMP POWER SUPPLY Fig.2: there’s nothing particularly unusual about the ±15V power supply . . . except perhaps that “earth link” which is explained in the text. www.siliconchip.com.au Here’s what the intact board looks like, fully assembled (except for the PC stakes not yet fitted to the preamp inputs and outputs.) The board can be divided where indicated by the dotted line and joined together by 3 wires (+15V, 0V and -15V). Note that there are some minor changes to component values since this photo was taken: the overlay is the latest version. solder them in and crop their leads, use some of those leads for the four links on the board. The accompanying table shows the colour code for the resistors – some are perhaps unusual, being 1% values (the 200kΩ and 16kΩ, for example). If in any doubt, use a digital multimeter to check them before soldering them in. Resistor pigtails can also be used for the “inductors”, which are simply one pass of wire through the ferrite beads. Now move on to the non-electrolytic capacitors, which of course are not polarised (note our earlier comments regarding the 270pF input shunt capacitor). Most of the electrolytic capacitors in the preamp (all bar one, in fact) are “bipolar” types which once again means you won’t be caught out with polarity problems. The one exception is the 10µF right across the supply rails which must be installed as shown on the overlay diagram. There are eight PC stakes to solder in at the inputs and outputs. Originally we had planned to use RCA sockets to make the board as flexible as possible but in the end settled for plain old stakes. Of course, there is nothing to stop you soldering suitable RCA sockets to the stakes, if you wish. The final component at this end of the board is the LM833 op amp which must mount the way indicated. Solder the pins carefully – it’s very easy to get a solder bridge between IC pins. At best, the preamp won’t work. At worst, this could destroy the op amp. Power supply Just about everything in the power supply is polarised. Start with the four rectifier diodes – they mount in pairs with the two in each pair facing the same direction but the other pair mount in the opposite direction. The direction AUDIO PRECISION SCTHD-HZ AMPL(dBr) vs FREQ(Hz) 11 FEB 102 08:40:04 of these diodes 20.000 determines which track 15.000 becomes the 10.000 positive rail and which be5.0000 comes the negative rail. 0.0 Next, solder in the four -5.000 elec-trolytics -10.00 with the orientation shown. -15.00 The larger (input) capacitors -20.00 face in oppo20 100 1k 10k 20k site directions; Fig.3: the preamp equalisation matches the RIAA specificathe two smalltion very closely. www.siliconchip.com.au er (output) capacitors face the same direction. The two voltage regulators are different – one is a positive regulator (7815) while the other is a negative regulator (7915). Don’t mix them up! These are mounted flat on the board with the ends of their leads bent down 90° to pass through their respective PC board holes. To get the bend in the right position, loosely mount the regulators to the PC board with M3 screws and grip the legs with fine (needle nose) pliers where the bends should go. Still holding the legs in the pliers, remove the screw and bend the leads down 90°. The regulators are photographed and drawn with small U-shaped heatsinks fitted. In fact, on the board there is room for much larger heatsinks. Due to the minimal current drain of this circuit, heatsinks are not really necessary – but we have in mind another task for this power supply and preamp later on which will need heatsinks. So we’ve fitted them. No insulation is necessary between regulator and heatsink. What is that task? That would be telling! (Incidentally, if you need a general purpose ±15V supply for other projects, this part of the board is perfect. Is that a clue?) You will note that there is an earth link shown on the circuit and provision made for it on the PC board. In the vast majority of cases, the turntable itself will be earthed to the amplifier (in turn, earthing the preamp via the input leads), so this link will not be necessary. In fact, it may increase hum levels due to the earth loop formed. March 2002  71 Parts List – RIAA Preamp 1 PC board, 215 x 67mm, code 01303021 1 4-way, PC-mounting mains terminal block, (Altronics P2103 or similar) 2 U-shaped heatsinks 8 PC stakes (14 if separated boards) 2 Ferrite beads, Philips 4330-030-3218 or similar 2 25mm lengths 0.25mm enamelled copper wire 1 240V to 30VCT PC-mounting transformer, Altronics M4330 or similar 1 SPST mains rated switch to suit – neon indicator optional 1 M203 or 3AG safety fuseholder, with 1A fuse. 4 (or 7 for split boards) mounting screws – type to suit use (see text) Optional: 1 metal case, minimum 75 x 75 x 25mm inside (see text) Semiconductors 4 IN4004 or similar 1A rectifier diodes (D1-D4) 1 7815 15V 1A positive regulator (REG1) 1 7915 15V 1A negative regulator (REG2) 1 LM833N twin low-noise op amp (IC1) Capacitors 2 470µF, 25VW PC electrolytics 2 100µF, 16VW PC electrolytics 2 47µF, 16VW PC bipolar electrolytics 2 33µF, 16VW PC bipolar electrolytics 1 10µF, 35VW PC electrolytic 2 0.33µF MKT polyester 1 0.1µF MKT polyester 2 .015µF MKT polyester 2 .0047µF MKT polyester 2 470pF MKT polyester 2 100pF MKT polyester Resistors (0.25W, 1%) 2 1MΩ 2 200kΩ 4 100kΩ Hum and noise minimisation 2 16kΩ 2 390Ω We used two resistor pigtails soldered “end on” into the link holes. To prove whether we needed the link, it was simply a matter of shorting them together. Hum levels increased significantly, so we left them open circuit. They won’t do any harm sitting there like that. If your hum level drops when you check the preamp out later, you need the link soldered. The final two “components” to be mounted are the mains input terminal block and the transformer. First the mains terminal block: it is a special noise) into the preamplifier. Second, it is designed to be mounted directly onto the PC board. The only problem with this transformer is that the pins for input and output appear identical so you could solder it in back to front. Again, if you get this wrong, the result will be briefly spectacular (and expensive). Read the label on the transformer and check it twice before soldering it in! The transformer in fact has two primaries and two secondaries, each of which the PC board connects in series. The two 120V primaries therefore become a 240V primary; the junction of the two 15V secondaries is connected to 0V. While the four pins will hold the transformer in place, a self-tapping screw should be used from under the board to hold it secure, relieving the strain on the soldered joints. Check your component placement, polarity (particularly that transformer!) and soldering before moving on to the next phase of the project. 2 150Ω 2 100Ω type designed for mains use with barriers between each screw terminal. Don’t substitute a block without this insulation. One strand of wire astray and bang . . . It’s also important which wire of the three in the mains lead goes to which terminal, but we’ll cover this shortly. The power transformer The transformer for this project was specially chosen for two reasons. First, it is a toroidal type which should mean minimum radiation (and therefore We’re going to put the cart before the horse, so to speak, and briefly mention the physical layout before looking at the wiring. Earlier, we talked about the physical location of the preamp and power supply. To reiterate, you need to get the power supply (transformer, especially) as far away from the pickup as possible (including the complete arc of the tone arm). Initially, we left the PC board in one piece and mounted it so that the transformer was very close to the motor. While performance was good we thought it could be better. So we separated the two parts of the board and mounted the preamp right up close to where the wires emerge from the tone arm. And that dramatically improved the signal-to- Resistor Colour Codes        No. Value 2 1MΩ 2 200kΩ 4 100kΩ 2 16kΩ 2 390Ω 2 150Ω 2 100Ω 72  Silicon Chip 4-Band Code (1%) brown black green brown red black yellow brown brown black yellow brown brown blue orange brown orange white brown brown brown green brown brown brown black brown brown 5-Band Code (1%) brown black black yellow brown red black black orange brown brown black black orange brown brown blue black red brown orange white black black brown brown green black black brown brown black black black brown Capacitor Codes         Value    IEC code    EIA code 0.33uF 330n 334 0.1uF 100n 104 .015uF 15n 153 .0047uF 4n7 472 470pF 470 471 100pF 100 101 www.siliconchip.com.au IN RIGHT www.siliconchip.com.au L NI 100pF 1 100k .0047 16k 150 100k 390 0.1 16k .0047 200k L OUT .015 0.33 10F 12030110 100 2002 )C( 100 L R OUT R 1M 0.33 E E 100pF 100k 200k .015 33F BP 47F IC1 LM833 47F 100k 150 L2 L1 270pF 270pF 390 33F BP 1M E Wiring it up –15V E +15V DIVIDE BOARDS DOWN THIS LINE (IF REQUIRED) –15V E +15V 120OPTIONAL 3 0110 HEATSINK OPTIONAL HEATSINK 2002 )C( REG2 7915 REG1 7815 5197 5187 470F 470F 15V 1N4004 100F 1N4004 1N4004 1N4004 100F OPTIONAL EARTH LINK (SEE TEXT) OK, back to the wiring. Fairly obviously, we needed to unscrew the plywood, masonite or other sheet which covers the bottom of the turntable plinth. Most turntables will have similar plinths. Ours happened to be an Apan belt-drive turntable which had sat unused and unloved under the house since CDs came into vogue. But its belt was still perfect (they made ’em good in those days!), while the cartridge and more importantly the stylus were in great shape. (You should check out the stylus under a powerful magnifying glass or loupe to make sure its point isn’t chipped or worn). In some turntables, the very thin wiring emerging from the tone arm under the turntable will be terminated directly to RCA or similar output sockets. But the majority will terminate to a tagstrip or block, with flying leads going out through the back of the plinth. You have to connect the tone arm wiring to the inputs of the preamp. This might be by resoldering them direct, or perhaps using a short length of stereo shielded lead. Every turntable will be different; you have to make the decision according to yours. In our case, it was easiest to unsolder the flying leads from a tagstrip, solder those directly to the output pins on our preamplifier and use short lengths of shielded cable to connect from the tagstrip to the input pins. On the mains side, first check and double check that the plug is removed from the wall socket! (Don’t laugh, it happens. . .). Now identify where the wiring comes in and how it connects to the motor. Almost invariably, there is some form of switch (often a microswitch) mechanically linked to the tone arm to turn the motor on and off. You need to cut in before that switch – you don’t want the preamp turning on and off with the motor. There are very few turntables with either a power switch or a fuse, yet we wanted to add both of these so the preamp wasn’t permanently left on. So we traced the mains wiring from the power lead and IN LEFT E E R NI E noise ratio. While we used our Audio Precision test gear to confirm the fact, you could actually hear the improvement. Even more improvement was achieved mounting the preamplifier in a metal box – we used a small folded aluminium type which itself was earthed to the 0V rail. We didn’t mount the power supply in a metal box (though theoretically this could be the ultimate!) for two reasons. First, we tried various methods of shielding the power supply and couldn’t measure any improvement but second, and most importantly, parts of the underside of the PC board are at mains potential and we would have had to take extra precautions in the mounting. As it was, we were able screw the power supply board directly to one of the plywood surfaces underneath the turntable, completely covering and insulating the “bitey” bits. Note that the mounting screws are all well away from PC board tracks, which also assists in this regard. Please, be careful when selecting the location for the power supply. If necessary, use some insulated pillars or stand-offs to mount the board. 15V T1 20VA TRANSFORMER SELF-TAPPING SCREW UNDER BOARD TO SECURE TRANSFORMER 120V 120V MAINS RATED TERM BLOCK E N E A Use this diagram in conjunction with the photographs to construct your RIAA Preamplifier. March 2002  73 IN R IN L E R 01103021 01303021 (C) 2002 L E 01303021 01103021 (C) 2002 7915 7815 A E unsoldered the active (brown or perhaps red wire) and took this directly to a single pole mains switch. From there, we connected another length of cable to a fuseholder and yet another length back to original “active” termination on the tagstrip. We used a neon-indicator mains switch which necessitated connecting a length of blue mains-rated cable from the neutral terminal on the tagstrip to the neon terminal on the switch. So far, all we’ve effectively done is include a mains switch and fuse into the existing turntable wiring. To connect the preamplifier, we soldered a length of three-core mains lead to the (now switched and fused) active terminal on the tagstrip, the neutral terminal and the earth terminal. This we ran around the inside perimeter of the plinth, secured with cable clamps, until we got to the power supply board. The outer insulation was removed to a length of about 30mm and the three wires were bared back about 5mm or so. Each was connected to its appropriate terminal on the PC board. You will note that we have used a four-terminal block for the three wires (Active, Neutral and Earth). This is just a bit of added safety: the Neutral and Earth wires (blue and green/yellow) sit side-by-side – these are nominally at the same potential anyway because they are linked back at your switchboard. The Active wire (brown) is separated from the others by an empty terminal. Connecting the split boards Because we (eventually!) decided to separate our two boards, we had to connect them together again. Provision is made for this on the boards. Solder another six PC stakes into the –15V, 0V and +15V positions on both boards and use a suitable length of cable, secured around the inside perimeter of the plinth, to connect them. Colours of the cables are not particularly important as long as the same colour connects the same points on each board! And there you have it. All of those black vinyl things (which many children today don’t even recognise!) which have been sitting in your cupboards can now be enjoyed once again. Oh, by the way – the outputs of the preamp simply plug in to one of your unused “aux” or similar inputs on your stereo amplifier. Sometimes they are labelled “line in”. And if at some time you want to use your turntable with an amplifier that does have “phono” inputs, don’t be tempted to use them with this preamp in circuit. Apart from probable massive overload, you’ll end up with double RIAA equalisation. Not a good sound at all. . . SC Enjoy! N Above is the full-size pattern for the PC board. At right, this is what your RIAA preamplifier should look like if separated from the power supply. At this stage, we hadn’t soldered in any PC stakes. 74  Silicon Chip www.siliconchip.com.au