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Re s u r r ec t y our o l d L P s an d 7 8 s . . . Build this magnetic cartridge preamplifier an d d u b t h em on t o CD s or MP 3 f ile s Do you have an old turntable but no RIAA inputs on your preamplifier? If so, you need this preamplifier for playback and for converting them to CD or MP3 format. By JOHN CLARKE D O YOU HAVE a collection of old vinyl or 78 RPM records languishing in a cupboard? Perhaps you should resurrect them before they deteriorate further. To do this you need a computer with a CD or DVD burner, suitable software and a preamplifier. The preamplifier described here can be built to suit vinyl or 78 records and is self-contained. You can build it into a 48  Silicon Chip diecast metal case or underneath your turntable. Of course some people will just want to listen to their records, without the bother of feeding signals into a computer and so on. This preamplifier will suit those people too but ultimately, we think that anyone who has become used to the high-quality sound of compact discs or MP3 players will be disappointed with the clicks and pops and surface noise on LPs and 78s. So by all means build the preamplifier to play your old records but you will probably end up going the whole way and dubbing your records to CD. In the process, you can filter out most of the clicks and noise and once again enjoy those old favourites. If you only have vinyl records (LPs) you can build this project as a standard preamplifier with RIAA equalisation but if you want to play 78s, you will need to choose one of three equalisation curves which can also be built in. You will also need a turntable that can play at 78 RPM and a cartridge that accepts the correct stylus (more details on this in our feature article on transferring LPs to CD next month). siliconchip.com.au Fig.1: the preamplifier circuit is based on two LM833 dual op amps (one channel only shown). It includes three sets of feedback networks (R1-R3 & C1-C2) and the values are chosen to give the equalisation required (see tables). Ideally, the turntable should have a speed adjustment so that the pitch can be changed but this is a rare feature. Alternatively, commonly available recording software can adjust the pitch when you dub the records to CD. The SILICON CHIP RIAA Preamplifier is housed in a diecast box and has RCA sockets for the input and output connections. It has a control to set the output level and is powered using an AC plugpack. By the way, this preamplifier supersedes the preamplifiers described in March 2002 and April 1994. in Fig.1. This shows the left channel only; the right channel is identical. Some readers may wonder why we have used LM833 dual op amps instead of the newer high-performance OPA2134 devices featured in our recent Studio Series Preamplifier. In fact, they could be used but since the signal source is a magnetic cartridge playing vinyl or 78 RPM shellac records, any slight performance improvement will be negligible and unable to be discerned by listening. The input signal is fed through in- Specifications Signal-to-noise ratio: -84dB unweighted with respect to 10mV in and 560mV out (-89dB A-weighted) Total harmonic distortion at 1kHz 10mV in and 560mV out: 0.014% Crosstalk: -79dB at 100Hz, -80dB at 1kHz and -70dB at 10kHz Circuit description Signal handling: 140mV before clipping The preamp circuitry is based on two LM833 dual op amp ICs, as shown RIAA accuracy: typically within 1dB from 20Hz to 20kHz (see graph) siliconchip.com.au August 2006  49 Par t s Lis t 1 PC board, code 01108061, 102 x 81mm 1 blank PC board, 70 x 30mm 1 diecast box, 119 x 94 x 57mm 1 12VAC 250mA plugpack 1 SPST slimline toggle switch (S1) 2 dual RCA PC-mount sockets 1 5-pin DIN PC-mount socket (optional) 1 2.5mm PC-mount DC socket 1 16mm dual 10kW log PC-mount pot (VR1) 2 crimp eyelets 1 green banana socket 6 2-way 2.5mm pin headers (from a 12-way header strip) 2 2.5mm jumper plugs 2 ferrite beads 4mm OD x 1.5mm ID x 5mm (L1,L2) 4 10mm M3 tapped spacers 4 adhesive rubber feet 4 M3 x 6mm screws 2 M3 x 10mm screws 2 M3 nuts and star washers 2 No.8 self-tapping screws 7 PC stakes 1 150mm length of green hookup wire 1 150mm length of red hook-up wire 1 150mm length of 0.7mm tinned copper wire Semiconductors 2 LM833 dual op amps (IC1,IC2) 1 7812 12V regulator (REG1) 1 7912 -12V regulator (REG2) 2 1N4004 1A diodes (D1,D2) 1 5mm red LED (LED1) Capacitors 2 1000mF 35V PC electrolytic 6 47mF NP or BP non-polarised electrolytic 1 10mF 35V PC electrolytic 2 10mF 16V PC electrolytic 2 1mF NP or BP non polarised electrolytic 2 560pF ceramic 2 100pF ceramic Resistors (0.25W 1%) 2 47kW 4 150W 2 7.5kW 1 39W 4 1kW RIAA components 2 200kW resistors 2 16kW resistors 2 15nF capacitors 2 4.7nF capacitors 50  Silicon Chip Fig.2: the RIAA response curve. The bass and treble turnover frequencies are set by the RC components in the feedback network connected to IC1a in the preamplifier. ductor L1, a 150W resistor and a 47mF capacitor to pin 3 of IC1. The 47kW resistor and 100pF capacitor provide the loading for a typical magnetic cartridge. As well, the 100pF capacitor functions as an RF input filter, in conjunction with inductor L1 and the 150W resistor. IC1a’s gain is set by the feedback components between pin 1, pin 2 and ground. The 1kW resistor and 47mF non-polarised capacitor set the low-frequency roll-off for this stage at around 3Hz. We have shown three sets of feedback components (C1, C2, R1, R2 & R3) and these can be selected to provide the RIAA or other equalisation responses for older recordings. The three sets of feedback components are labelled SET1, SET2 and SET3 and the particular equalisation SET is selected using jumper plug LK1, LK2 or LK3. The feedback components for the various equalisation curves are shown in Tables 2 & 4. High-pass filter IC1a’s output appears at pin 1 and is fed to pin 3 of IC2a via a high-pass filter comprising a 1mF capacitor and a 7.5kW resistor. This filter rolls off Why So Many Choices For Equalisation? Readers may be surprised to see all the various equalisation curves for vinyl (45 & LP) and 78 RPM records. It is not well-known these days that before the recording industry standardised on the RIAA curves, the bigger recording companies had their own equalisation curves, hence there were curves such as Decca’s ffrr (it stood for “full frequency range recording”), EMI, NARTB and Columbia. The situation was even more chaotic before vinyl LPs came on the scene and there was more choice (and confusion) with 78 records. Hence, some of the equalisation curves used included Decca (EMI) 78, Westrex and so on. The reason for including these different sets of components in Tables 2 & 4 is so that if you can identify the company that made a particular recording, you can then select the appropriate equalisation characteristic. Note that we have also shown values for flat frequency response (ie, no equalisation) and tape head equalisation. siliconchip.com.au Table 1: Microgroove 45 & LP Frequencies Curve Treble Turnover Bass Turnover Lower Bass Turnover Cut at 10kHz Boost at 50Hz RIAA 2.1215kHz 500Hz 50.5Hz -13.6dB 17dB ffrr LP 3kHz 500Hz 100Hz -10.5dB 12.5dB EMI LP 2.5kHz 500Hz 70Hz -12dB 14.5dB NARTB 1.6kHz 500Hz – -16dB 16dB Columbia 1.59kHz 500Hz 100Hz -16dB 12.5dB Table 2: Components For Microgroove 45s & LPs Curve R1 R2 C1 C2 R3 RIAA 16kW 200kW 4.7nF 15nF – ffrr LP 220kW 18kW 15nF 3.3nF 270kW EMI LP 330kW 18kW 15nF 3.9nF 270kW NARTB 2.2nF 18kW 18nF 5.6nF//390pF 270kW Columbia 100kW 18kW 18nF//2.2nF 5.6nF//390pF 270kW Table 3: Coarse Groove 78 Frequencies Treble Turnover Bass Turnover Decca 78 3.4kHz 150Hz – -9dB 11dB ffrr 78 6.36kHz 250Hz 40Hz -5dB 12dB Westrex Flat 200Hz – – 15dB Blumlein Flat 250Hz 50Hz – 12dB BSI 78 3.18kHz 353Hz 50Hz -10.5dB 14dB Curve Lower Bass Turnover Cut at 10kHz Boost at 50Hz Table 4: Components For Coarse Groove 78s Curve R1 R2 C1 C2 R3 Decca 78 open 18kW 68nF 3.3nF 270kW ffrr 78 220kW 18kW 33nF 1.5nF 270kW Westrex 18nF 18kW 33nF – 270kW Blumlein 220kW 18kW 33nF – 270kW BSI 78 220kW 18kW 22nF 3.3nF 270kW Table 5: Components For A Flat Response Gain R1 R2 C1 C2 R3 x1 link 1kW – 4.7nF – x 11 link 10kW – 470pF – x 101 link 100kW – 47pF – Table 6: Components For Tape Head Equalisation NAB R1 R2 C1 C2 R3 – 3.6kW – 15nF 200kW These tables show the components necessary to achieve the various response curves required to play back from vinyl records and other recording surfaces, including Shellac 78s and tape heads. The parts necessary to achieve a flat response (with various gains) for general-purpose use are also shown. siliconchip.com.au August 2006  51 Table 7: Capacitor Codes Value 68nF 33nF 22nF 18nF 15nF 4.7nF 5.6nF 3.9nF 2.2nF 1.5nF 560pF 470pF 100pF 47pF μF Code .068mF .033mF .022mF .018mF .015mF .0047mF .0056mF .0039mF .0022mF .0015mF NA NA NA NA EIA Code   683   333   223   183   153   472   562   392   222   152    561    471    101    47 IEC Code   68n   33n   22n   18n   15n   4n7   5n6   3n9   2n2   1n5 560p 470p 100p 47p Above: this view shows the fully assembled PC board with all feedback component sets installed. In practice, you would normally only install one feedback set (typically for RIAA equalisation) and omit the parts for the other two sets. The PC board is installed in the case by angling it as shown here, so that the RCA sockets and the pot shaft go through their respective holes. It then sits on Nylon spacers that are pre-fastened to the bottom of the case and is secured using machine M3 x 6mm screws. 52  Silicon Chip signals below 20Hz to reduce rumble from the turntable. Signal above 20Hz is free to pass to the next stage of amplification within IC2a. The gain of op amp IC2a is adjustable using potentiometer VR1. When VR1 is set fully anticlockwise, its resistance is zero and IC2a has a gain of 1. Conversely, when the wiper is fully clockwise, VR1’s resistance is 10kW and so the gain is 11. The 560pF capacitor across VR1 rolls off the gain at higher frequencies to prevent oscillation. The outputs from IC2 are fed to the RCA sockets via 150W resistors, another measure to prevent oscillation because the of the capacitance of the screened signal leads. Power for the circuit comes from a 12VAC plugpack which is fed to two diodes and two 1000mF capacitors to produce positive and negative supply rails. These are fed to 3-terminal regulators to derive ±12V DC rails. Note that the plugpack feeds the two diodes via a 39W resistor to limit the siliconchip.com.au Fig.3: follow this parts layout and wiring diagram to build the preamplifier. Note that you can select only one set of feedback components at a time using either links LK1 or LK2 or LK3. Table 8: Resistor Colour Codes o o o o o o o o o o o o o o   No. 1 1 1 2 1 2 1 2 1 2 4 4 1 Value 330kW 270kW 220kW 200kW 100kW 47kW 18kW 16kW 10kW 7.5kW 1kW 150W 39W peak current into the 1000uF capacitors. This minimises any tendency for 100Hz rectifier buzz to become audible in the preamp’s output signal. The case of the preamp may be earthed should this be necessary to siliconchip.com.au 4-Band Code (1%) orange orange yellow brown red violet yellow brown red red yellow brown red black yellow brown brown black yellow brown yellow violet orange brown brown grey orange brown brown blue orange brown brown black orange brown violet green red brown brown black red brown brown green brown brown orange white black brown avoid mains hum in the signal. In most cases, this will not be required. Building the preamplifier The new preamplifier is built on a PC board coded 01108061 and measur- 5-Band Code (1%) orange orange black orange brown red violet black orange brown red red black orange brown red black black orange brown brown black black orange brown yellow violet black red brown brown grey black red brown brown blue black red brown brown black black red brown violet green black brown brown brown black black brown brown brown green black black brown orange white black gold brown ing 102 x 81mm. It fits into a diecast box measuring 119 x 94 x 57mm. Fig.2 shows the wiring details. It’s a good idea to first check the PC board for any defects such as shorts between tracks or for any breaks in the August 2006  53 earth available (eg, an earthed metal case) to connect to the green banana socket. The shield consists of a piece of PC board 70 x 30mm and is soldered to the PC stakes in the position shown. LED1 mounts high on its leads so it can be bent over and inserted into a hole in the side of the case. Case work The metal case can be earthed (via the banana socket) if mains hum is a problem. This usually won’t be necessary, however. copper areas. Repair these if necessary and also check that the board has the correct hole sizes for the components such as the RCA sockets, DC power socket and the dual ganged potentiometer. Now begin the assembly by installing all the links, PC stakes and 2-pin headers. Before you insert the resistors, decide on the value of components you are going to use for each of the equalisation sets. In most cases you would only use one set for the RIAA equalisation (the others can be left out). Note that you need to place the Fig.4: use the 5-pin DIN plug only if you need an earthed supply. The Jaycar MP-3022 earthed 17VAC plugpack can supply the required AC power and earth. 54  Silicon Chip same components in both the left and right channels for each set. Install the resistors using the colour code table (Table 8) as a guide to finding the correct values. It’s also a good idea to use a digital multimeter to make sure they are correct, as some of the colours can be difficult to decipher. The two ferrite beads are mounted with short lengths of tinned copper wire passed through them. The ICs can go in next, taking care to orient them correctly. Install the two diodes and the two regulators and make sure the 7812 and the 7912 types are placed in the correct positions. Next up are the capacitors. The polarised electrolytic types must be mounted with the correct polarity, as shown on the overlay. Also, make sure you use the 35V 10mF capacitor adjacent to IC2. The NP (non-polarised) or BP (bipolar) electrolytic capacitors can be inserted either way around. Use Table 7 as a guide to selecting the non-electrolytic capacitors. Next, mount the two RCA sockets, the potentiometer and the DC socket. The 5-pin DIN socket can be installed later if you find that you need an earthed supply and there’s no mains The metal case will require drilling out to accommodate the two stereo RCA sockets, the potentiometer and the LED on the front face of the case. On one side, holes are required for the earth screw, the power switch and the DIN socket if used. At the rear, holes are required for the DC supply socket and the banana socket. Mark and drill these holes out. The slot required for the switch is best made by drilling about three holes within the cutout area and then filing it to shape. Four holes also need to be drilled in the base for the plastic spacers for the PC board. That done, attach the four rubber feet to the base of the case and then wire up the switches and earth connections as shown in Fig.3. Testing Connect power to the preamplifier and check that the LED lights when power is switched on. If it does not light, then perhaps the LED is installed the wrong way around. Next, measure the voltage between pins 4 & 8 of IC1 and IC2. It should be close to 24V DC in both cases. If this is correct, you are ready to connect a turntable and test the preamplifier. Select RIAA equalisation for both the left and right channels using the jumper links, then connect the RCA leads from the turntable to the input sockets on the preamplifier. The RCA outputs on the preamplifier go to either a power amplifier or the line input of a computer using a “Y” lead. The “Y” lead consists of a shielded stereo lead with RCA plugs at one end and a stereo 2.5mm jack plug at the other end. If you are connecting the preamp to an amplifier, then plug in headphones or use loudspeakers. If you are playing to a computer, make sure the line input level is turned up. You can set this in Windows XP via Start/Settings/ Control Panel/Sounds and Audio Devices/Audio, then selecting Volume in SC the Sound Recording section. siliconchip.com.au