Fullscreen HTML5Med Res (??MB)HTML4

Precision 230V/115V, 50/60Hz Turntable Driver by John Clarke This Precision Turntable Driver will power belt-drive or idler driven turntables with 230VAC at 50Hz or 115VAC at 60Hz. As a bonus, the turntable pitch is capable of being adjusted over a range of ±12%, which is great for music teaching applications. It also enables you to adjust the music speed to obtain the correct number of beats to the minute for dancing applications. OK so it does all the above but why would you want it? The most obvious reason is if you have an imported American turntable which needs to run from a 115VAC 60Hz supply. That’s a real problem in Australia and New Zealand where we have a 230VAC 50Hz mains supply. 64  Silicon Chip Sure, you could get a 230VAC to 115VAC step-down transformer to provide the correct drive voltage but at 50Hz the turntable would run almost 17% slow and the motor would tend to overheat, making it unusable. So that’s reason number 1. The second reason to consider building this Precision Turntable Driver is if you travel around the country- side and want to play records in your caravan or motor home when you are away from the 230VAC 50Hz supply. Sure, you may have a 12V to 230VAC inverter but there is no guarantee that its frequency will be reasonably close to 50Hz, which can mean that the turntable may run noticeably fast or slow. Realistically, most people are relasiliconchip.com.au Features tively insensitive to pitch errors but small inverters can not only be • incorrect in their frequency but can also change frequency according to the load. The same effect can occur • with portable petrol generators. Sinewave and modified square wave inverters can also have quite • a high proportion of buzz and hash • in their outputs – and this can be picked up by the very sensitive • preamplifier needed for a magnetic cartridge. Thirdly, while it is fairly common for “better” CD players to have a pitch control, which is useful for music and singing teachers, a variable speed facility on a turntable is (was) generally only available on expensive directdrive models. Now, with our Precision Turntable Driver, you can have this facility on any belt-drive or idler drive turntable. And if you are the disc jockey running dances, having a variable speed facility on a turntable is also very useful to obtain the correct beats per minute. For example, music for a Viennese Waltz should be at around 160 beats per minute; quite fast. Finally, we should note that if you have an old beltdrive or idler-drive turntable, its speed may not be exactly correct, when checked with 12V DC, supply, the output will be less (at around 180VAC) Can be used with a 12 to 15V DC but most turntable motors seem supply or a 12V battery to run quite well at this lower voltage. 230VAC or 115VAC (nominal) sineThe Precision Turntable Drivwave output er is housed in a rugged diecast aluminium case with a DC sock50Hz or 60Hz output frequency et, On/Off toggle switch and a Crystal accuracy red power LED at one end. At the other end is the 3-pin mains Pitch control (frequency adjustment) output socket. If you want to use the speed a strobe disc and our Turntable Strobe adjust facility (without opening the (see SILICON CHIP, December 2015). case), you will also need to mount Unless you can perceive perfect three small momentary-contact butpitch, any small error in speed is tons on the lid. simply academic but you do have the Block Diagram ability to make it precisely correct via the fine speed adjustments available Fig.1 shows the block diagram for on the Turntable Driver. the Turntable Motor Driver. A 5-bit The Turntable Driver can be run digital to analog (D-to-A) converter is from a 15V 2A DC supply or from a used to generate a 32-step sine wave 12V battery. The battery options means signal. This is shown as the yellow that you can use it anywhere where trace in the screen grab, Scope1, and you don’t have a mains supply. it is close to 5V peak-to-peak. With a 15V DC supply, the output The waveform is then amplified and will be close to 220VAC, or if you are filtered by op amp IC2a and the result operating a 110-120VAC powered is the green trace, with an amplitude turntable, the output will be of just over 14V peak-to-peak. close to 110V. The signal is then fed to op amp IC2b If you run which functions as a 12dB/octave lowf r o m a pass filter to remove the 32 steps and produce a smooth sinewave. This can be seen in screen grab Scope2 as the yellow trace. The The Precision Turntable Driver is housed in a diecast case, with a flush-mount mains outlet accepting a standard 250VAC mains plug. Note the way the transformer is mounted at 45° to the PCB to allow it to fit in the case. siliconchip.com.au May 2016  65 HALF SUPPLY Vs/2 5-BIT DIGITAL TO ANALOG CONVERTER R C IC2a (IC1, X1, 150k & 75k RESISTORS) AC COUPLING AMPLIFIER AND FILTER A IC2b INVERTER LOW PASS FILTER IC2c B Vs Vs Q1 TRANSFORMER DRIVER STAGES (IC3b, Q5, Q6) Q3 Q2 DRIVER STAGES D C 0 9V 0 0 9V Q4 (IC3a, Q7, Q8) 230V AC (115V AC) Circuit details GPO Fig.1: the Precision Turntable Driver circuit generates a clean sinewave signal and feeds it to a Class B amplifier driving a step-up transformer to produce 220VAC or 115VAC. waveform is then inverted in op amp IC2c, as shown in green trace. The two signals are then buffered by complementary Mosfet driver stages which provide the high current drive for the transformer. The resultant drive signals are depicted in the yellow and green traces in screen grab Scope3. The drive signal across the transformer primary windings is shown in the mauve trace, which is the difference (MATH function) of the two buffered drive signals. Its amplitude is a little less than twice that of the drive signals. The 9V primary windings of the Scope1: The stepped sinewave from the 5-bit DAC is shown in the yellow trace. The steps are smoothed out by the first op amp low-pass filter and the result is the green trace. 66  Silicon Chip transformer are connected in parallel to be driven by the Mosfet buffer stages. The transformer steps up the output to drive the turntable. The secondary of the transformer has two 115V windings, which are connected in series for a nominal 230VAC output (for turntables which require a 50Hz mains supply) and in parallel for a nominal 115VAC output (for turntables which require a 60Hz mains supply). Losses in the transformer mean that it does not deliver the full 115VAC output across each winding, even though the 9V windings are driven at more than 10VAC. The (unloaded) output waveform from the transformer is shown in Scope4. The actual voltage delivered to the turntable motor will depend on the DC supply voltage fed to the circuit and the loading of the motor. The complete circuit of the Precision Turntable Driver is shown overleaf (Fig.2). It is split into two sections: on the left (top) is the Power Supply and Signal Generator while the Mosfet output stages and step-up transformer are on the right (bottom). Referring now to the power supply section, IC1 (a PIC16F88 microcontroller) is used to generate the 50Hz or 60Hz sinewave. IC1 uses a 20MHz crystal for its timebase and the internal software program operates at a 5MHz rate. Jumper Scope2: The smooth waveform from the low pass filter (yellow trace) is inverted by op amp IC2c to produce the complementary waveform shown in the green trace. siliconchip.com.au link JP1 sets the output frequency: with the jumper open it’s 50Hz and with a jumper shunt inserted it’s 60Hz. Momentary-contact switches S3 and S4 provide the up and down frequency adjustment. The frequency is changed in 0.02Hz steps at a rate of four per second while a switch is held pressed. Pressing default switch (S2) returns the frequency to the precise 50Hz or 60Hz setting (depending on the state of JP1). All the inputs associated with JP1, S2, S3 and S4 include separate internal pull-ups that keep the inputs high (at 5V). The inputs are pulled low (0V) when there is a shorting link in JP1 or when a switch is pressed. S2-S4 can be mounted on the lid via flying leads from CON2 if you wish to be able to adjust the speed without opening the box. In this case, the internal S2-S4 can be omitted. Digital-to-analog conversion Five outputs from IC1, RA0 to RA5, produce square wave signals at multiples of the desired output frequency. These outputs are fed to the 5-bit digital to analog converter (DAC) which is actually a standard R/2R resistor ladder (comprising 150kand 75k resistors). This generates the 32-step sinewave signal. The R/2R resistor ladder makes a cheap DAC and five bits is about the maximum number that can be used when only 1% resistors are used. It is quite sufficient for this application. The DAC output is filtered with a 1nF capacitor to remove switching glitches. The resulting 5V sine wave is amplified using IC2a which is one quarter of an LMC6484AIN quad railto-rail op amp. Trimpot VR1 is used to adjust the output level to produce the maximum undistorted sinewave signal. Following this, the signal from IC2a is AC-coupled with a 10F capacitor and is now referenced via 10k resistor to half supply (Vcc/2) which is derived with two 10k resistors connected across the Vcc supply and then buffered using op amp IC2d. Note that the supply to IC2 (and IC3) comes via a 10 resistor and is clamped to 15V using Zener diode ZD1. This is to protect the op amps that are rated for a maximum rail voltage of 16V. This clamped rail is marked Vcc’. IC2b and its associated resistors and capacitors form a 2-pole SallenKey low-pass filter that rolls off above 160Hz at 12dB/octave. The filter only affects the high frequency components of the 32-step generated sinewave which are multiples of 32 x 50Hz or 1.6kHz (or 32 x 60Hz or 1.92kHz). IC2b’s output is then inverted by op amp IC2c to produce a complementary waveform (ie, 180° out of phase). Transformer drive circuit We need complementary drive signals for the transformer drive circuit which are shown in Fig.2 (labelled “A” Scope3: the complementary sinewave signals from the Mosfet Class-B amplifier stages are effectively added to drive the transformer primaries. Its amplitude is the sum of the two signals (mauve trace). siliconchip.com.au Specifications • Rating: 20W <at> 220VAC or 110VAC (nominal) • Input Supply: 12-15VDC at 2A • Output waveform: Sinewave • Output Voltage: 220VAC or 110VAC with a 15V supply • Output regulation: 9% from no load to 15W • Frequency accuracy: ±50 ppm (ie, ±0.005%) • Pitch control: ±12% • Frequency adjustment for 50Hz: 41.5Hz to 56Hz in approximately 0.02Hz steps • Frequency adjustment for 60Hz: 50Hz to 67.2Hz in approximately 0.02Hz steps • Default button: Restores the output frequency (JP1) setting to 50Hz or 60Hz and “B”). These signals are fed to noninverting buffer stages IC3a and IC3b which in turn are connected to the complementary output stages. Let’s just describe the lefthand side of the power supply circuit whereby IC3b drives complementary transistors Scope4: the unloaded output from the two 115VAC transformer windings connected in series. It is 222.6VAC or 656V peak-to peak. Note that this signal is much cleaner than the normal mains supply in homes, offices and factories. May 2016  67 68  Silicon Chip 11 IC2c 100k 3.3nF +VCC /2 INVERTER 10 100k 9 LOW-PASS FILTER 7 ZD1 15V IC2: LMC6484AIN 10F 10k K A 75k 100nF +VCC/2 120k 16V 10F 10k 10k 6 5 33nF 16V IC2b 10F 12 13 680pF IC2d 4 8 14 B A TO DRIVER STAGES +VCC/2 +VCC’ +VCC 15nF 22k ADJUST OUTPUT VOLTAGE Vss 5–BIT DAC (DIGITAL TO ANALOG CONVERTER) 12 RB7 RB6 13 2x 33pF 15 OSC2 OSC1 16 X1 20MHz S4 FASTER S3 SLOWER S2 DEFAULT OUT = 50Hz IN = 60Hz 5 150k 75k 3 RB5 11 8 RB2 RA2 RA4 IC1 PIC16F88 6F88-2 RA3 I/P RB1 7 RB0 6 RB4 10 150k 75k 150k 75k 150k 1 18 RA1 RA0 VR1 20k 10k 2 150k 150k JP1 CHASSIS D1 1N5404 RA5/MCLR 9 RB3 14 A 100nF 16V LOW ESR S1 4700F K 2A SLO-BLOW + CON1 Vdd 10k 16V 10F K A +15V FUSE F1 4 17 K A 2.2k POWER  LED1 75k IN GND OUT 1nF 3 IC2a AMPLIFIER 16V 10F +5V 1 10 REG1 7805 D2 1N4004 POWER 12–15V DC IN Q5 & Q6, followed by complementary Mosfets Q1 & Q2. Q1 is an IRF9540 Pchannel Mosfet while Q2 is an IRF540 N-channel type. The righthand side of the circuit is identical. These stages operate in the same way as a Class-B audio amplifier and can simply be regarded as a unitygain buffer. More particularly, just as in a ClassB output stage, there is no quiescent current which means that it does produce some crossover distortion and while no-one will ever hear that crossover distortion we have incorporated biasing to minimise it. The biasing is provided by the two diodes (D3 & D4) between the bases of transistors Q5 and Q6, with the current through the diodes provide by the 22kresistor from Vcc. This current is about 300A or so and the resultant bias voltage between the bases of Q5 & Q6 is insufficient (at around 1.1V in total) to cause them to conduct. However, that small amount of bias is enough to provide a significant reduction in crossover distortion but not enough to eliminate it. Why do we care? It is simply because the crossover distortion was causing significant inflections in the output waveform on the secondary side of the transformer and we judged it worthwhile to minimise it. A 1.5nF capacitor between ground and the tied together transistor bases is included to reduce the rate at which the transistors can switch on and off. This prevents high frequency instability in the stage. So we have the complementary signals at A & B being fed through the Class-B output stages and then connected to the paralleled 9V windings of the transformer. The actual maximum undistorted drive voltage from each Class-B output stage, assuming Vcc is 15V DC, is close to 5.1V RMS (14.43V peak-to-peak) and that gives a total AC voltage across the 9V windings of 10.2V. The two 115V windings of the transformer are connected in series to provide a nominal 230VAC output (for turntables that require a 50Hz supply). For turntables that require a 60Hz 110VAC supply, the two 115V windings are connected in parallel rather than in series. Because the transformer is a highly inductive load, especially when it is unloaded, its primary current lags the siliconchip.com.au The Precision Turntable Driver is built on a PCB (coded 04104161) measuring 84.5 x 112mm. It is housed in a diecast box that measures 171 x 121 x 55mm. The board is designed to be mounted on two of the integral mounts within the box and the outline of the PCB is shaped so that it fits neatly inside it. Fig.3 shows the parts layout on the PCB. Assembly can begin with installation of the resistors, using the resistor colour code table as a guide. It is a good idea to also use a DMM to check GPO 115VAC VERSION A E 2.2F X2 N 115V 0 115V 0 0 9V 0 9V D C E OUT A 50/60H z PRECISION TURNTABLE DRIVER SC K A 1N4148 B 2016 K A ZD1 K DRIVER STAGE 1k 4 6 IC3b 33nF 1.5nF 1k 7 8 A 5 +VCC’ 100nF 10k C K D4 1N4148 K A D3 1N4148 A B B E E Q6 BC327 1k G S A 1N4004 Q2 IRF540 D D Q5 BC337 C Assembly procedure Fig.3: here’s the wiring to drive a 115VAC, 60Hz turntable with the two windings connected in parallel. Note the change in capacitor across the windings; also note JP1 will need to be set for 60Hz operation. C S D G B IN K A 1N5404 GPO C 0 230VAC VERSION 0 A E 470nF X2 0 115V 9V 0 N 9V 115V T1 D K LED Q4 IRF540 GND D S G 7805 GND 1k 10k C BC327, BC337 COMPLEMENTARY DRIVER STAGE 33nF 1k 1.5nF 1 K B E Q8 BC327 D Q3 IRF9540 Q1 IRF9540 S G 10k 22k 100 +VCC/2 Fig.2: the top section of the circuit shows the micro and 5-bit DAC, low pass filter and inverter stage. Its complementary sinewave signals are fed to the Class-B output stages shown immediately above. Note the back-to-front transformer. D IRF540, IRF9540 2 IC3a 3 IC3: LMC6482AIN LMC6 482AIN D6 1N4148 1k C Q7 BC337 E B K A A D5 1N4148 100 22k 10k G S +VCC/2 +VCC +VCC +VCC’ siliconchip.com.au voltage by almost 90° and this would cause substantial heating of the Mosfets. This “power factor” problem is corrected by the 470nF (or 2.2F for 115VAC) capacitor corrected across the transformer output. each value as it is installed, as the colours can sometimes be hard to read or close to each other. Follow this by installing diodes D1 to D6 and Zener diode ZD1. These must be mounted with the orientation shown. Install IC1’s socket next, followed by IC2 & IC3. Check that the orientation is correct before soldering each in place. Then install 20k trimpot VR1 (it may be marked as 203). Switches S1 to S4 can be installed now along with the DC socket (CON1). Note our earlier comments about S2-S4. Install Q5, Q6, Q7 and Q8, making sure Q5 and Q7 are BC337s and Q6 and Q8 BC327s. Leave Mosfets Q1 -Q4 off for the moment. When installing the fuse clips, they must go in with their retaining tabs toward the outside ends, otherwise you will not be able to fit the fuse later on. May 2016  69 Use the drilling template in Fig.5 to mark out and drill the holes for the power switch S1, LED1, the DC socket hole and earth screw at one end of the case and the surface mounted mains AC socket at the opposite end. The larger hole for the mains socket can be drilled out using a series of small holes around the perimeter and then after knocking the inside piece out, filing to shape. Place the PCB inside the case inserting the switch(es) and LED into their holes. Mark out the hole positions for the two PCB mounting positions that require 9mm stand-offs and orient the transformer diagonally as shown in the photos and diagram and mark out the mounting hole positions. Drill out the holes for the stand-offs to 3mm in diameter and drill out the transformer mounting holes at 4mm in diameter. Place the PCB in position, temporarily mounting this on the integral stand-offs in the box and on the 9mm spacers. Then mark the positions for Q1, Q2, Q3 and Q4 by marking where the metal tab holes of each are located when held against the side of the box. Remove the board and drill these mounting holes to 3mm, then use an oversize drill to remove any metal swarf so that the area around each hole is perfectly smooth. 70  Silicon Chip 22k 1k 75k 22k Q7 10F 4148 10k IRF9540 Q3 0V 1k 33nF 4148 10k 1k S4 D S F G 0V JP1 100 S3 9V TO TRANSFORMER T1 150k 75k IC3 LMC6482 ADJUST Faster Slower 1 10F BC337 Fig.4 (above): the PCB component overlay, with a matching same-size photo below. The photo is actually of an earlier prototype board so there could be minor component differences compared to the PCB above. If in doubt, use the PCB component overlay! 10k IC1 PIC16F88-I/P 10k 150k 150k 150k 150k Default S2 2.2k 9V 33pF X1 20MHz 33pF 1 100nF REG1 7805 GND 150k 75k 75k K S1 1k 10 10k 1nF 75k CON1 A 10F 15nF D2 1N4004 + 33nF 33nF 3.3nF VR1 20k 4700F 10F 1 Transformer 22k IC2 LMC6484AIN OUTPUT 12–15V DC INPUT 1.5nF 100k 100nF ZD1 15V 1W 1N5404 LED1 10k 1k 2A Rev.B 04104161 100k 100 10k 120k F1 D1 Q6 10F 100nF C 2016 BC327 680pF Q5 CON2 Q8 BC327 In: 60Hz Out: 50Hz 1.5nF 1k Case drilling Q2 Q1 IRF540 IRF9540 Turntable Motor Driver 10k 10k 4148 4148 BC337 5404 Better still, clip the fuse into the fuse clips first before installing the clips into the PCB holes. Next, install the capacitors, ensuring the electrolytic types are placed with the correct polarity. Now place Mosfets Q1, Q2, Q3 & Q4, taking care to fit the correct Mosfet in each location. These are positioned so that the mounting hole centre in each tab is about 22mm above the PCB. In every case, the metal tab must go towards the outside edge of the board. The LED is mounted so that it can protrude through a hole in the end of the box. The leads are inserted with the longer anode (A) lead oriented as shown. If bent over, the LED can be set about 5mm above the PC board. Connect a short (20mm) length of wire to the GND terminal on the PCB and terminate the other end to a solder lug. Also solder two 50mm lengths of wire to the 0V and 9V pads ready for connection to the transformer. IRF540 Q4 siliconchip.com.au Parts List – Precision Turntable Driver 1 PCB coded 04104161, 84.5 x 112mm 1 panel label 158 x 95mm (download from siliconchip.com.au) 1 diecast box 171 x 121 x 55 (Jaycar HB-5046) 1 20VA mains transformer 50/60Hz 2 x 115VAC, 2 x 9VAC (RS Components 504-274) 1 transformer terminal shroud (RS Components 504-004) 1 250VAC mains panel socket, flush-mounting (Altronics P 8241 or P 8243, Jaycar PS-4094) 1 15VDC 2A supply (preferably a linear supply) or plugpack (or 12VDC with reduced output) or suitable 12V battery 1 DC socket, PC mount with 2.1 or 2.5mm centre pin to suit plugpack or supply lead DC plug 3 SPST micro switches (~3-4mm actuator) [for internal mounting] (Jaycar SP-0602, Altronics S 1120) (S1-S3) OR 3 SPST momentary contact switches [for on-lid mounting] (Jaycar SP-0710/0711; Altronics S1060/1071A or similar) 1 SPDT PCB mount toggle switch (Altronics S 1421 or similar; S1) 1 2-pin header with jumper shunt 1 20MHz 50ppm (or less) crystal (X1) 1 M205 2A slow blow fuse (F1) 2 M205 PCB mount fuse clips 1 18-pin IC socket 4 rubber feet 2 M3 tapped x 9mm spacers 2 M4 nuts 2 M4 x 10mm machine screws (countersunk or pan head) 2 M3 x 6mm machine screws (countersunk or pan head) 2 M3 x 6mm machine screws 5 M3 x 10mm machine screws 5 M3 nuts 1 star washer for M3 screw 1 large solder lug 4 TO-220 insulating bushes 4 TO-220 silicone washers 1 20mm diameter x 50mm heatshrink tubing 1 20mm length of green or green/yellow 7.5A mains rated wire This is necessary to prevent punchthough of the insulating washer. Reinsert the board into the case. Mount the transformer using M4 screws and nuts and mount the PCB using the screws supplied with the enclosure for the two mounting points in the corners. The stand-offs are secured to the base of the case using countersunk 1 500mm length of brown 7.5A mains rated wire 1 100mm length of blue 7.5A mains rated wire 4 100mm cable ties Semiconductors 1 PIC16F88-I/P microcontroller programmed with 0410416A.HEX (IC1) 1 LMC6484AIN quad rail to rail op amp (IC2) 1 LMC6482AIN dual rail to rail op amp (IC3) 1 7805 5V regulator (REG1) 2 IRF9540 P channel Mosfets (Q1,Q3) 2 IRF540 N channel Mosfets (Q2,Q4) 2 BC337 NPN transistors (Q5,Q7) 2 BC327 PNP transistors (Q6,Q8) 1 3mm red LED (LED1) 1 15V 1W zener diode (ZD1) 1 1N5404 3A diode (D1) 1 1N4004 1A diode (D2) 4 1N4148 diodes (D3 - D6) Capacitors 1 4700F 16V low ESR electrolytic 5 10F 16V electrolytic 1 470nF 275VAC X2 class MKP (polypropylene) for 230VAC output (1 x 2.2F X2 class polypropylene for 115VAC output) 3 100nF MKT polyester 3 33nF MKT polyester 1 15nF MKT polyester 2 1.5nF MKT polyester 1 3.3nF MKT polyester 1 1nF MKT polyester 1 680pF ceramic 2 33pF ceramic Resistors (1%, 0.5W; # = metal film) 6 150k# 1 120k 2 100k 5 75k#   3 22k 9 10k    1 2.2k 6 1k   2 100    1 10 1 20k miniature horizontal mount trimpot (VR1) M3 x 6mm screws (or machine screws) and machine screws (M3 x 6mm) to secure the PCB. Then attach the TO-220 devices to the sides of the case as shown in Fig.7, using the M3 x 10mm screws. Note that it is necessary to isolate each device tab from the case using an insulating washer and insulating bush. Once they have been installed, use a digital multimeter on a low Ohms range to confirm that the metal tabs are indeed isolated from the metal case. If a low resistance reading is measured, check that the silicone washer for that particular TO-220 device has not been punctured and that the insulation bush is not damaged. Also connect the earth lug to the case using an M3 x 10mm screw, star 4.5mm dia 3mm dia 9mm 11mm 19mm 10mm dia 21mm CUT OUT 35mm 21mm 16 m m 27mm 66mm 19mm 3mm 5.5mm dia dia Fig.5: drilling details for the two ends of the case; the left is for the flush-mounting 250VAC mains socket and the right is for the switch, LED and DC socket. Additional holes will need to be drilled in the lid to accommodate the three speed switches. siliconchip.com.au May 2016  71 WIRING SHOWN FOR 230VAC VERSION. FOR 115VAC VERSION, THESE WINDINGS SHOULD BE IN PARALLEL (NOT SERIES) Turntable Motor Driver 4148 4148 C 2016 0 Rev.B 04104161 TRANSFORMER T1 115 0 115 Transformer 5404 CABLE TIE 1 TRANSFORMER MOUNTS AT 45o ANGLE TO PCB 0 9 0 9 9V + CABLE TIES 1 USE MAINS RATED 7.5A CABLE THROUGHOUT 0V N 1 Default ADJUST Faster Slower D S F G GND 4148 4148 JP1 *0.47F 275V AC X2 CAPACITOR CON2 In: 60Hz Out: 50Hz A (SHEATH CAPACITOR IN HEATSHRINK) *FOR 115V VERSION CAPACITOR IS 2.2F X2 CLASS SOLDER LUG THREE NORMALLY OPEN MOMENTARY PUSHBUTTON SWITCHES (OPTIONAL – ONLY REQUIRED TO ADJUST SPEED WITHOUT OPENING CASE) Fig.6 (above) shows the wiring required for the unit, while Fig.7 (right) shows the mounting of the four Mosfets. After mounting, check with a multimeter (on low Ohms range) to ensure the tabs are isolated. Slower Default (REAR OF GPO) Faster 10mm M3 SCREW M3 NUT INSULATING BUSH TO-220 DEVICE (Q1 – Q4) SIDE OF CASE SILICONE WASHER Here’s a close-up of the mains output socket, connections to the transformer and to the capacitor across the transformer secondary. This must be sheathed in heatshrink, as shown. Resistor Colour Codes o o o o o o o o o o Qty. 6 1 2 5 3 9 1 6 2 1 72  Silicon Chip Value 150kΩ 120kΩ 100kΩ 75kΩ 22kΩ 10kΩ 2.2kΩ 1kΩ 100Ω 10Ω 4-Band Code (1%) brown green yellow brown brown red yellow brown brown black yellow brown violet green orange brown red red orange brown brown black orange brown red red red brown brown black red brown brown black brown brown brown black black brown 5-Band Code (1%) brown green black orange brown brown red black orange brown brown black black orange brown violet green black red brown red red black red brown brown black black red brown red red black brown brown brown black black brown brown brown black black black brown brown black black gold brown siliconchip.com.au washer and nut. The adhesive rubber feet can now be attached. Wiring it up Follow the wiring diagram of Fig.6 to connect up the transformer and GPO. Make sure 250VAC mains-rated wire is used and you must use the terminal shroud for the high voltage terminals (at the two 0-115V winding connections) so these are covered over. Wire the 115V windings in series for 230VAC and in parallel for 115V. Note the different capacitor value connected across the mains for the 230VAC (470nF X2) and 115V (2.2F X2) versions. Two wires each connect to the Active and Neutral GPO terminals. One is from the transformer and the other from the mains-rated capacitor. The capacitor and wire connections are encased in some heatshrink tubing for insulation. Note also that the Earth terminal on the GPO is left disconnected. The 230VAC wiring is anchored with cable ties as shown. Set-up procedure Ideally, the unit should be powered using a linear 15V DC 2A supply rather than a switchmode type. A linear supply is one where the mains voltage is stepped down using a 50Hz transformer and is then rectified, filtered and regulated to 15V. This type of supply will avoid the injection of switching hash into sensitive magnetic cartridge signal leads. Insert the slow-blow 2A fuse and apply power. Check that the voltage between pins 5 and 14 of the IC1 socket is close to 5V. Anywhere between 4.85 and 5.15V is OK. Switch off power and wait for the voltage on the IC socket to drop to below 1V and insert the pro- The Turntable Strobe, featured in the December 2015 issue, is perfect for ensuring accurate speed with the Precision Turntable Driver. If your platter doesn’t have strobe markings (as this one does) a Strobe Disc, suitable for both 50Hz and 60Hz, is available from the SILICON CHIP Online Shop for $10.00 (see below) (www.siliconchip.com.au/shop/19/3273) grammed PIC16F88 into the socket, taking care to orient it correctly. Trimpot VR1 needs to be adjusted to give the maximum undistorted sinewave output. The ideal way to do this is to use an oscilloscope to monitor the waveform. If you do not have access to a scope and assuming that the DC supply is 15V, adjust trimpot VR1 to deliver 10.2V AC across the primary windings of the transformer. If you are using a 12V battery, adjust trimpot VR1 to deliver 8.2V to the transformer primary. When connecting up the turntable to the Turntable Motor Driver use the normal practice of connecting the turntable Earth wire back to the amplifier Earth terminal to minimise hum and noise. The case of the Turntable Motor Driver does not need to be connected to mains Earth. Setting the turntable speed As mentioned, you can adjust the turntable speed using the faster or slower switches. For an exact speed setting, you will need to monitor the turntable speed using the turntable strobe and a strobe disc (see the article published in December 2015 entitled Check Turntable Speed With This White LED Strobe). Adjust the speed so the strobe markings appear stationary. Note that, depending on the power rating of the turntable, the DC input voltage and the ambient temperature, the case of Precision Turntable Driver will become warm after a few hours use. This is normal and to be expected since the drive circuitry is linear and its efficiency is only about 50%. SC Want to upgrade your turntable? We’ve got what you need. Decibel Hi Fi is your best source for the supply of a wide range of products that can improve the quality of music you enjoy from your vinyl collection. Visit decibelhifi.com.au We sell Origin Live DC motor kits, Jelco tonearms, Graham Slee phono preamps, Audio Technica cartridges, Garrott cartridge repair and retipping, Soundring replacement styli (soundring.com.au), turntable belts, platter mats, vinyl related tools and accessories, record cleaning machines and products, record sleeves and more. FREE: Vinyl Replay System Philosophy and Upgrading Guide email enquiry<at>decibelhifi.com.au to request a copy. siliconchip.com.au Phone: 07 3344 5756 PO Box 55, Coopers Plains QLD 4108 May 2016  73