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By Nicholas Vinen High-performance stereo valve preamplifier This stand-alone stereo valve preamplifier is based on the Currawong amplifier (November 2014-January 2015) but has a new power supply which runs off a low-voltage DC supply. It has very good performance, especially for a valve preamp, with low distortion and a very high signal-to-noise ratio of 105dB. It’s easy to build too, with the preamp and power supply all on one PCB. O UR FIRST VALVE preamplifiers were single-channel (mono) designs based on the 12AX7 twin triode (in the November 2003 and February 2004 issues). That design was also incorporated into the Currawong valve amplifier mentioned above. However, we have had a number of requests for a stereo version of the preamp and when we looked at the original mono design from 12 years ago, we realised 28  Silicon Chip that we could make a number of significant improvements. So for a start, this new design is stereo so you don’t need to build two separate units (which involved at least three PCBs). It also has a more compact and improved switchmode power supply which is on the same board as the rest of the components Also, the earlier design had exposed components on the top of the board which operated at 250V DC, necessitating the application of silicone sealant to render it safe – not a very attractive option. The new design still “shows off” its components but they are visible through a clear acrylic case, protecting the user from electric shocks. The overall performance is quite a lot better than the earlier design. Take a look at the graphs from our Audio Precision System Two, shown siliconchip.com.au 2x12AX7 Preamp THD vs frequency, 1.2V, 30kHz BW 07/12/15 13:32:23 10 5 5 2 2 Total Harmonic Distortion + Noise (%) Total Harmonic Distortion + Noise (%) 10 1 0.5 0.2 0.1 0.05 0.02 0.01 1 0.2 0.1 0.05 0.02 0.01 0.005 0.002 0.002 50 100 200 500 1k Frequency (Hertz) 2k 5k 10k Fig.1: total harmonic distortion plotted against frequency for an input of 300mV RMS and an output of 1.2V RMS (full power for a typical power amplifier). The measurement bandwidth is 30kHz in order to chop out any residual switching artefacts from the power supply while still measuring some of the harmonics of higher audio frequencies. The result is essentially flat with frequency. in Figs.1-4. If you compare these to the graphs for the mono preamp in the February 2004 issue (pages 32 & 33), you will see that this is a big improvement with lower distortion across the board and no high-frequency rise. The frequency response is pretty flat, with a very slight rise in response at both 20Hz and 20kHz, due to reduced feedback effectiveness at these extremes. One of the changes in our circuit is that we’ve put the volume control pot at the input end rather than the output end. This greatly reduces the chances of overload and gives lower output impedance and lower valve plate loading. In theory, it would increase the noise but in practice this design has ended up with a better signal-to-noise ratio. Besides stereo music, another application for a 2-channel valve preamp might be for use as a musical instrument preamplifier, either with two mics on one instrument or two separate instruments. For this application, we have provision for a mixed output with a pot that controls how the two inputs are mixed. This pot, and its associated RCA connector, can be left off for stereo applications. Since 12AX7 filaments are designed to run from 12.6V, the circuit has been designed to run off 15V DC, with an on-board regulator providing the correct filament voltage. However, we have tested the preamp with a 12V siliconchip.com.au 0.001 0.2 20k 07/12/15 13:42:03 0.5 0.005 0.001 20 2x12AX7 Preamp THD vs output, 1kHz, 20kHz BW 0.5 1 2 Output Level (Volts RMS) 5 10 Fig.2: distortion versus output amplitude. For signals below 1V (ie, <250mV RMS input), noise starts to affect the measurement while for signals above 3V RMS out, the intrinsic second harmonic distortion of the valve begins to dominate. Distortion rises dramatically for outputs above about 9V RMS as parasitic capacitances interact with the higher slew rate. Features & Specifications • • • • • • • • • • • • • • • Stereo preamplifier with volume control Uses two 12AX7 dual triodes (socketed) Variable gain: -100dB to +12dB Low distortion: <0.01% THD+N <at> 20Hz-20kHz, 1.2V output (see Figs.1 & 2) Flat frequency response: +1,-0dB 20Hz-20kHz (see Fig.3) Channel separation: >85dB <at> 1kHz, >60dB <at> 20kHz (see Fig.4) Signal-to-noise ratio: 105dB relative to 1V input (20Hz-20kHz bandwidth) Power & HT presence indicator LEDs RCA socket inputs & outputs Power supply: 13-15V DC <at> 650mA Power supply reverse polarity protection Onboard power switch No transformer winding necessary Optional mixed output for use with musical instruments. Fits in a custom-designed clear laser-cut acrylic case DC supply and it had little effect on performance so that is a valid option. A 12V automotive supply should be fine as it will normally be above 12.6V most of the time (assuming the battery charge state remains high). Circuit description The full circuit is shown in Fig.5. Both channels are shown in full, along with the power supply, although the op- eration of the two channels is identical. Looking at the left channel only, the signal comes in via RCA socket CON1 and passes through an RF-rejecting low-pass filter comprising a 100Ω resistor with a ferrite bead on one of its leads and a 100pF ceramic capacitor. The signal is then AC-coupled to 50kΩ volume control potentiometer VR1a via a 470nF MKT capacitor. The attenuated signal is then ACJanuary 2016  29 +3 12AX7 Stereo Preamp Frequency Response, 1.2V 07/12/15 13:46:25 0 +2 2x12AX7 Preamp Channel Separation, 1kHz, 20kHz BW 07/12/15 13:38:42 -10 +1 -20 -1 Relative Amplitude (dBr) Amplitude Variation (dBr) 0 -2 -3 -4 -5 -6 -30 -40 -50 -60 -70 -7 -80 -8 -90 -9 -10 10 20 50 100 200 500 1k 2k Frequency (Hertz) 5k 10k 20k 50k 100k Fig.3: the frequency response for the preamplifier is quite flat but there is a slight rise in the response below 50Hz due to the increasing impedance of the feedback circuit; feedback starts to drop off, allowing the gain to rise. There is a similar rise above 30kHz, however this is well above the audio band. A small bump is visible at 100Hz due to low levels of mains hum being picked up. coupled to the grid of triode V1a via another 470nF MKT capacitor and a 22kΩ RF stopper. This stopper is quite important. Without is, a fair bit of hash from the power supply can couple into the valve and then be amplified. A 1MΩ bias resistor shunts any grid leakage to ground and biases the grid to near-0V. V1a operates with a current of around 360µA, set by the combination of its 270kΩ anode resistor and 3.3kΩ cathode resistor. The amplified signal at its anode is coupled to the grid of V1b with a 220nF capacitor and the grid is biased with another 1MΩ resistor to ground. Since V1b needs to handle a higher signal voltage, it runs at around 1.5mA, set by its 68kΩ anode resistor and 680Ω cathode resistor. The output at its anode is coupled to output connector CON3 via another 220nF -100 20 50 200 500 1k Frequency (Hertz) 2k 5k 10k 20k Fig.4: channel separation is very good, being more than 90dB below 400Hz, rising to around -65dB at the upper end of the audio band. This was measured with the other channel input terminated with a low impedance. The signal coupled through from one channel to the other at higher frequencies is relatively undistorted so should not result in undesirable intermodulation. capacitor, with a 1MΩ resistor setting the DC level to 0V. AC-coupled negative feedback The same output signal is also fed back to V1a’s cathode via a pair of parallel 470nF capacitors and a 10kΩ resistor. The 10kΩ resistor forms a 4:1 voltage divider with V1a’s 3.3kΩ cathode resistor. Say a 100mV positive step is applied to V1a’s grid. This will turn V1a on harder, pulling its cathode negative and thus V1b’s grid will be pulled negative. That will cut off V1b in turn, causing its anode voltage to rise. Once its anode voltage has risen by 400mV, the 4:1 divider will have caused V1a’s cathode to increase by 100mV. Since it’s the grid-cathode voltage which determines how much current a valve conducts, the 100mV increase in V1a’s cathode voltage effectively cancels out the 100mV increase in its WARNING! HIGH VOLTAGES High DC voltages are present in this circuit. In particular, the power supply produces an HT voltage of up to 285V DC and this voltage and other high DC voltages derived from it are present on various parts of the circuit. Do not touch any part of the circuit when power is applied otherwise you could get a severe or even fatal electric shock. The red LED (LED2) in the circuit indicates when high voltages are present. If it is lit, the power supply and various parts on the PCB are potentially dangerous. Before applying power, the completed preamplifier must be mounted in a suitable case and fitted with a Perspex cover as described in Pt.2 next month to ensure safety. 30  Silicon Chip 100 grid, so it will be back to conducting roughly the same current it was initially. As its anode swing is a tiny fraction of the anode voltage of around 150V, it will therefore reach a steady state. Thus overall gain of the circuit is accurately set to 12dB by this negative feedback network. Mixed & panned outputs The preamp is intended to be used in stereo applications, with the two channels handling independent signals. However, it could be used as a musical instrument preamplifier. In this case, you can use it as two mono preamplifiers with the two outputs mixed together. For this configuration, VR2 and CON5 are installed and CON3/CON4 can be omitted. In this case, the output of each channel is mixed by VR2. VR1 still controls the overall output level and with VR2 at mid-setting, an equal amount of each input signal is mixed into the output. As VR2 is rotated clockwise, the output contains more of the amplified signal from CON2 and less of that from CON1 and the opposite is true if it’s rotated anti-clockwise. Basically, VR2 can be regarded as a pan control, panning from one channel to the other. Note that if VR2 is fitted, V1b and V2b are loaded with around 50kΩ and the output impedance is increased. Still, as long as the device being fed siliconchip.com.au siliconchip.com.au January 2016  31 FERRITE BEAD L3 100pF VR1b 50k 470nF MKT 100pF VR1a 50k 470nF MKT S K A 470nF MKT 470nF MKT G 1M 22k 630V 1W 3.3k 1W 10k 1W 3.3k 1W 10k 2x 470nF 1M 22k 630V 2x 470nF ZD1 15V 100 µF 25 V STEREO VALVE PREAMPLIFIER 100Ω 100Ω FERRITE BEAD L2 Q1 IRF540 OR IPA60R520E6 D 100k S1 1V 2 V2a 1W ~ 100V 3 4 1 ~ 100V ~150V 1W 680Ω 1V 7 V2b 5 8 6 1W 68k 1W 680Ω 1V 7 V1b 5 8 6 1W 68k 630V 630V +12 .6 V 1W 1M 220nF 1M 220nF 400V 39 µF 630V 1M ~ 25 0 V +12 .6 V 1W 630V 220nF 400V 39 µF ~ 25 0 V 100 µF 25 V 220nF 1M D2 1N4004 OUT GND ~150V 1 3 4 270k 1V 2 V1a 1W 270k IN REG1 LM2940CT-12 RIGHT OUTPUT CON4 VR2 100k (optional) MIXED OUTPUT CON5 10k 1W LEFT OUTPUT CON3 10k 1W +12 .6 V LEDS K 2.2 k 220k A K ZD3 15V S G D S IPA60R520E6, IRF5 40 0.5W 68Ω G D IN GND 1 50pF 100Ω 0.5W L1 10 0 µH 1 A Q2 D IPA60R520E6 A K A WARNING: VOLTAGES UP TO 300V DC ARE PRESENT WHEN THIS CIRCUIT IS POWERED. D1 UF4004 270k K λ HT A K A 1N4004, UF4004 K A TPG 39µF 40 0V TP1 ~265V LED2 ZD2 15V 1W 220k LED1 0.33Ω 100 µF 25 V +12 .6 V OUT LM2940 4 GND GND REG2 MC34063 VFB 3 Ct 5 SE 6 8 7 DRC Ips Vcc 1 SC 2.2k 2 K λ POWER A Fig.5: the complete stereo valve preamplifier circuit. Each channel uses a 12AX7 dual triode with an overall gain of four times (12dB). Amplification is done in two stages, with negative feedback around both to set the gain and also cancel distortion. The circuit runs off a nominal 15V power supply which is regulated to 12.6V for the filaments, while a ~265V HT rail is produced by switchmode regulator REG2 and high-voltage Mosfet Q2. SC 20 1 6 RIGHT INPUT CON2 LEFT INPUT CON1 13 -15 V DC POWER CON6 0V 15V GND 470nF 1M 1M 220k 1W 15V ZD2 ZD3 470nF S1 (under) C 2016 VR1 2x 50k log (under) 22k 68k 1W 1M ~1 V 470nF 630V 6 4 5 V2 12AX7 470nF 630V 1M 1W 10k 1W ~ 25 0 V 680 Ω 1W 7 10k 1W 10k 1W 220nF 630V SILICON CHIP 39 µF 400V 3.3k 1W 1M 10k 1W 1M 1W 22k 3.3k 1W ~ 25 0 V 8 3 + V1 12AX7 39 µF 400V ~150V 2 39 µF 400V 470nF 630V 9 + ~1 V 5 + 4 ~1 V ~100V 1 + 470nF 630V 6 3 TP1 + 01101161 RevB D2 TPG 7 2 Stereo Valve Preamp 68k 1W 270k 1W 220k 8 1 D1 UF4 220nF 630V 2.2k 680 Ω 1W 4004 100 µF 150pF + 9 ~150V L1 100 µH 270k ~1 V ~100V REG2 34063 12.6V 100 µF + 270k 1W 220nF 630V + REG1 LM2940 CT-12 0.33Ω Q2 IPA60R 520E6 100pF CON4 RIGHT OUTPUT (under) CON3 LEFT OUTPUT (under) 68Ω 100Ω 100 µF 100k L3 100pF 15V 100Ω L2 Q1 IRF540 + 100Ω ZD1 CON5 MIXED OUTPUT (optional, under) CON6 POWER (under) CON2 RIGHT INPUT (under) CON1 LEFT INPUT (under) ~ 26 5 V 220nF 630V 470nF 470nF LED1 A GND VR2 100k linear GND 2.2k LED2 A (optional, under) WARNING: HIGH DC VOLTAGES (UP TO 285V) ARE PRESENT DURING OPERATION CON6 POWER CON4 RIGHT OUTPUT CON5 MIXED OUTPUT (optional) CON3 LEFT OUTPUT CON2 RIGHT INPUT CON1 LEFT INPUT 9 9 1 8 7 7 2 3 6 5 GND 2 3 6 5 4 VR2 100k linear (optional) GND LED2 A 32  Silicon Chip 1 8 LED1 A VR1 2x 50k log 4 GND S1 siliconchip.com.au Fig.6: top and bottom PCB overlay diagrams. Use these as a guide when assembling the PCB. Start by fitting the components to the top side, which is everything except the connectors, power switch, pots and LEDs. Note the wires used to earth the pot bodies to the nearby GND pads. Leave VR2 and CON5 out if building a stereo preamplifier. CON3 and CON4 are optional if VR2 & CON5 are fitted. has a relatively high input impedance, this should not be a problem. Power supply A DC input of around 13-15V is required at CON6. As mentioned earlier, supply voltages down to 12V are acceptable however the filaments of V1/ V2 will run at lower power than they are designed for. Mosfet Q1 provides reverse polarity protection, with much lower voltage loss than a simple diode, even a Schottky type. If the supply polarity is correct, Q1’s gate is pulled positive with respect to its source and so ground current can flow back to CON6 normally. However, if the supply polarity is reversed, Q1’s gate is pulled negative and thus its channel will not conduct. Its body diode is also reverse biased in this condition so the only current that will flow is a few microamps through ZD1 and its series 100kΩ resistor. ZD1 protects Q1 in case the supply voltage spikes above 20V for more than a very brief period. Power switch S1 interrupts the supply to REG1, a low-dropout automotive 12V regulator. Its ground pin is “jacked up” by around 0.6V by diode D2, increasing its output to around 12.6V to suit the filament requirements of the 12AX7 valves. 100µF input bypass and output filter capacitors are provided and these should ideally be low-ESR types for supply stability. LED1 indicates the presence of the 12.6V rail. As well as running the filaments directly, this rail also supplies switchmode regulator REG2 which is configured as a boost regulator to produce the HT supply. When REG2’s internal transistor is switched on, current flows through the 0.33Ω shunt, into pin 1 (switch collector), out of pin 2 (switch emitter) and through a voltage divider formed by 100Ω and 68Ω resistors. The voltage produced by this divider drives siliconchip.com.au the gate of high-voltage logic-level Mosfet Q2. So when REG2’s internal switch is on, Q2 is biased into conduction and it pulls current through the 0.33Ω shunt and inductor L1 to ground. This charges up L1’s magnetic field. REG2 has an internal oscillator that we’ve set to around 100kHz using a 150pF capacitor from pin 3 (Ct) to ground. L1 continues to charge either until the ~7.5μs period set by this oscillator expires or the current builds to around 1A, at which point the voltage across the 0.33Ω shunt exceeds the ~300mV current trip level, as sensed by pin 7 (Ips). In either case, REG2’s internal transistor is switched off and the 68Ω resistor quickly pulls Q2’s gate to 0V, switching it off. This causes the magnetic field in L1 to begin collapsing, which continues to “push” current through the inductor in the same direction as it was flowing before it was interrupted. Since the “input” side of L1 is still connected to the 12.6V supply, the only way for current to continue to flow is for high-voltage ultrafast diode D1 to become forward biased. As a result, the voltage at D1’s anode increases dramatically. Before L1’s magnetic field can collapse completely, the oscillator in REG2 causes its internal transistor to switch back on, recharging it and repeating the cycle. When the circuit is first powered up, the voltage at D1’s cathode will start at around 12V but as the three 39µF 400V capacitors charge up, this voltage will continue to rise until it reaches nearly 300V. One of two things then happens. The voltage is either limited by the fact that the current limit enforced by REG2 prevents any more energy flowing into L1 in each cycle than is consumed by V1 and V2, or the voltage rises high enough that the voltage at the voltage feedback pin of REG2 (pin 5) rises above 1.25V. If this happens, REG2 will skip pulses until the output voltage drops, then it will switch back on to regulate said voltage to the set level. However, we have designed this circuit so that it can’t quite produce a high enough output voltage to regulate properly. This is because the pulse skipping that’s used to regulate the output voltage causes sub-harmonics of the 100kHz switching frequency to be M3 x 6mm SCREW Q1 PCB FLAT WASHER M3 NUT STAR WASHER M3 x 6mm SCREW Q2 PCB FLAT WASHER M3 NUT STAR WASHER M3 x 10mm SCREW FLAG HEATSINK REG1 FLAT WASHER PCB M3 NUT STAR WASHER Fig.7: mounting details for Q1 (top), Q2 (middle) and REG1 (bottom). Note that a longer machine screw is used for REG1 and that Q1 is in a fully insulated package with its centre lead bent over and soldered closer to the body than the other two. radiated and depending on how many pulses are skipped, these could be in the audio band (ie, below 20kHz) and could couple into the preamplifier, reducing its signal-to-noise ratio. This means that the HT voltage is not actually regulated but that isn’t much of an issue as the 12AX7s will run happily off quite a wide range of voltages; anywhere in the range of 250300V will do. The feedback divider really only exists to prevent damage in case one or both valves are removed, fails or becomes disconnected during operation. In this case, it will limit the HT rail to around 285V DC. The actual operating HT voltage will depend on a few factors but mainly on the exact value of L1, the 0.33Ω shunt, REG2’s current limit voltage sense threshold and the 150pF capacitor. These all affect how much energy L1 can store for each cycle, or in the case of the 150pF capacitor value, the maximum number of charge cycles per second. We’ve set the circuit up so that in most cases, the actual HT voltage produced should be high enough for January 2016  33 All the parts are mounted on a single PCB with the volume pot, power indicator LEDs and connectors on the underside. The board caters for various sizes of 630V capacitors. correct operation but not so high that pulse skipping is employed (ie, below the ~285V regulation target). In our prototype, it reaches 280V after about 30 seconds and eventually drops to about 265V once the valves have fully warmed up. Construction All parts are carried on the main PCB and assembly is quite straightforward. It should only take a couple of hours for experienced constructors. The board itself is coded 01101161 and measures 170 x 102mm. Referring to the PCB top side overlay diagram Fig.6, begin by fitting all the smaller resistors. It’s best to check all the resistor values with a DMM before fitting them. Don’t forget that the 68Ω and 100Ω resistors must be 0.5W types and that two of the other 100Ω resistors have ferrite beads slipped over their leads before they are soldered in place. The 0.33Ω resistor should also be fitted now, whether it’s a through-hole or SMD type. Follow with diodes D1 and D2 and zener diodes ZD1-ZD3. Don’t get the three different types mixed up and pay careful attention to polarity. This is indicated by the cathode stripes shown on Fig.6 and the PCB silkscreen. Having done that, solder inductor L1 in place. This is most easily done by first applying a little flux paste to the pads, then adding some solder to one of the pads – the right end if you 34  Silicon Chip are right-handed or left if you are lefthanded. Then place the inductor next to its final position, heat the solder on that pad and slide the component into place. You will find that once it contacts the solder, it will take a few seconds to heat the component up to the point where it will adhere and you can then move it into its final location. It’s then just a matter of adding solder to the opposite pad and continuing to heat it until it adheres to both the pad and component lead. Finally, go back to the other side, add some fresh solder and heat it further, again making sure it forms a good fillet. Next, solder REG2 to the board. Don’t use a socket and make sure its pin 1 dot is at upper-left as shown on Fig.6. Press it down flat on the PCB before soldering the pins. Follow with the larger (1W) resistors, using a similar procedure as before. Now bend the leads of Q1 and Q2 down through 90° about 5mm from the body of each component and attach them to the board using M3 x 6mm machine screws and nuts, with a shakeproof washer under each nut and a flat washer under the head. Don’t get these two components mixed up – Q2 should be encapsulated in black plastic while Q1 may have a metal tab (if you are using an IRF540) – see Fig.7 for details. Once the screws are done up tightly and the parts checked for proper alignment, solder and trim the leads. Having done that, solder the ceramic and MKT capacitors in place. These can all go in either way around, as they are non-polarised. Refer to Fig.6 to see which value goes where. Now fit regulator REG1. The procedure is the same as for Q1 and Q2 except that a flag heatsink is positioned under the regulator’s tab and an M3 x 10mm machine screw is used to secure it instead of an M3 x 6mm screw. Make sure that the regulator’s body and heatsink are square and that the screw is done up tightly before soldering the leads – see Fig.7. Fitting the valve sockets The valve sockets are retained mechanically, to avoid placing stress on the solder joints when inserting and removing the valves. Each is held in place with two M3 x 10mm machine screws, with a Nylon nut and two Nylon washers used to form a spacer. Fit a shakeproof washer under the nuts (see the photos for details). Basically, it’s just a matter of inserting an M3 x 10mm machine screw through the top of the two mounting holes on the valve socket and screwing a Nylon nut onto each thread. Do the nuts up tight, then slip pairs of Nylon washers over each screw shaft and feed these through the mounting holes on the PCB. You’ll need to coax the nine solder tabs into the slots on the PCB, then the whole thing should drop into place. Use the shakeproof washers and siliconchip.com.au nuts to fasten it in place, make sure the nuts are done up tightly, then solder and trim the nine tabs on each socket. You can now solder the three small and three large electrolytic capacitors in place (see Fig.6). In each case, make sure that the longer lead goes through the hole nearest the + symbol. Underside components Now it’s time to fit the components on the other side of the board – see Fig.6. The RCA connectors fitted are CON1-CON4 (for a stereo preamplifier) or CON1, CON2 and CON5 (mixed mono preamplifier for instruments). CON1 and CON3 are white, CON2 and CON4 are red and CON5 can be black. Unfortunately, white RCA sockets aren’t that easy to come by. We sell a set of four on our Online Shop, including red, white, black and yellow. These have a slightly different footprint to the types available from Jaycar and Altronics but as you can see from our prototype, the leads can be bent so that they fit. In fact, they are a little easier to fit than the other type and as a bonus, have a consistent mounting height, unlike some types which can vary between different colours. Whichever sockets you are fitting, make sure they are pressed down fully onto the PCB and are perpendicular to the board edge before soldering the three pins. You can also fit DC socket CON6 now, on the same side of the board, again making sure it’s nice and square before soldering. Before fitting the pot(s), you will need to use a file to scrape off a small area of the passivation on top of the body so that you can solder an earth wire in place. Basically, it’s just a matter of holding the body in a vice using a couple of scrap pieces of timber to prevent damage and then a few passes with a file should reveal a shiny surface. Don’t breathe in the dust produced; it may be toxic. If your pot(s) have long shafts, you will also want to cut them short now. Use a hacksaw and file to cut it/them to no more than 15mm. Then, referring to Fig.6, solder the pot or pots in place on the underside of the board. Solder some tinned copper wire between the provided GND pads, across the top of the pot body(s), then solder the wire to the pot(s) to “earth” them. Now fit power switch S1 in place, making sure it’s first pushed down fully onto the PCB. Finally, install LED1 siliconchip.com.au Parts List 1 double-sided PCB*, code 01101161, 170 x 102mm 1 set of clear acrylic laser-cut case pieces* 1 small tube acrylic adhesive 4 rubber feet 1 15V 1A plugpack 2 12AX7 dual triode valves 2 9-pin valve sockets (Jaycar PS2082) 1 100µH 12x12mm SMD inductor* (L1) (Murata 48101SC; element14 2112367) 1 50kΩ 16mm dual log pot (VR1) 1 100kΩ 16mm linear pot (VR2; optional, see text) 2 knobs, to suit VR1 & VR2 1 mini TO-220 flag heatsink, 6073B type 2 ferrite beads (L2,L3) 2 white switched RCA sockets (CON1,CON3)* 2 red switched RCA sockets (CON2,CON4)* 1 black switched RCA socket (CON5; optional, see text)* 1 PCB-mount DC socket to suit plugpack (CON6) 1 PCB-mount right-angle mini SPDT toggle switch (S1) (Altronics S1320) 2 M3 x 6mm machine screws 5 M3 x 10mm machine screws 4 M3 x 32mm machine screws 7 M3 shakeproof washers 3 flat washers, 3mm I.D. 7 M3 nuts 4 M3 Nylon nuts 8 Nylon washers, 3mm I.D. 4 M3 x 12mm Nylon machine screws 4 6.3mm M3 tapped Nylon spacers 4 12mm M3 tapped Nylon spacers 4 25mm M3 tapped metal spacers 1 200mm length 0.7mm diameter tinned copper wire Semiconductors 1 LM2940CT-12 12V 1A lowdropout regulator (REG1) 1 MC34063 switchmode regulator (REG2) 1 IRF540 or IPA60R520E6* N-channel Mosfet (Q1) 1 IPA60R520E6* 600V N-channel Mosfet or equivalent (Q2) 1 green 3mm LED (LED1) 1 red 3mm LED (LED2) 3 15V 1W zener diodes (ZD1-ZD3) 1 UF4004 ultrafast diode or equivalent (D1) 1 1N4004 1A diode (D2) and LED2. Check Fig.6 to determine the required orientation, then bend the LED leads through 90° 6mm from the base of the lenses. Solder the LEDs in place on the underside of the board, with the horizontal portion of the leads 13mm from the bottom of PCB. This may be easier to do if you cut a 13mm cardboard spacer first. four tapped spacers in each corner using an M3 machine screw. Test points are provided to monitor the HT voltage, near the centre of the PCB, however it’s easier and safer to use DMM alligator clip leads to connect to the anode of ZD3 (negative lead) and the right-hand end of the 220kΩ 1W resistor (positive lead) – see the 0V and ~265V markings on Fig.6. Set your DMM to a range which will read 300V DC and plug the power supply into the PCB but not the mains. . . . continued on page 96 Testing The first step is to check that the HT power supply is working but before doing this, temporarily attach the Capacitors 3 100µF 25V low-ESR electrolytic 3 39µF 400V low-profile snapin electrolytic (Nichicon LGJ2G390MELZ15* from Mouser) 4 470nF 63V MKT 4 470nF 630V metallised polyester 4 220nF 630V metallised polyester 1 150pF disc ceramic 2 100pF C0G/NP0 disc ceramic Resistors (1W, 5%) 2 1MΩ 2 10kΩ 2 270kΩ 2 3.3kΩ 1 220kΩ 2 680Ω 2 68kΩ Resistors (0.25W, 1%) 4 1MΩ 2 2.2kΩ 1 270kΩ 1 100Ω 0.5W 1 220kΩ 2 100Ω 1 100kΩ 1 68Ω 0.5W 2 22kΩ 1 0.33Ω through-hole or SMD 1206 resistor* * Available from the SILICON CHIP Online Shop; details in next month’s issue. January 2016  35 Valve Preamplifier . . . continued from page 35 Make sure nothing conductive is near the PCB and it isn’t close to the edge of your bench. Then, keeping clear of the assembly, plug the power supply into mains. Within about one second of power being applied, the HT voltage should reach 285V or thereabouts and stabilise, with the green and red LEDs lit. Either way, switch off power and wait for it to discharge to a safe level (below 40V) before continuing. If there’s a fault, once the HT rail has discharged, check component placement and orientation as well as solder joint integrity. Assuming all is well, connect regular probes to your DMM but leave it on the 300V (or higher) range. Power the board back up and measure the voltage between pins 4 and 5 on both valve sockets (see Fig.6). You should get a reading close to 12.6V. Now check the voltages at the other pins relative to GND. You should get ~285V for pins 1 and 6 and close to 0V for pins 2, 3, 7 and 8. Pin 9 is not connected to anything. KEEP YOUR COPIES OF SILICON CHIP AS GOOD AS THE DAY THEY WERE BORN! ONLY 95 $ 1P6LUS p&p A superb-looking SILICON CHIP binder will keep your magazines in pristine condition. * Holds up to 14 issues * Heavy duty vinyl * Easy wire inserts ORDER NOW AT www.siliconchip.com.au/shop You can now switch the power off and push the two valves into their sockets. They will be stiff, especially if this is the first time the sockets have been used. You may find it easier to gently rock them in. While you can in theory install the valves with HT voltage present, it’s much safer to wait for it to decay first. With the valves in place, power back up and check the HT voltage, using the test pads in the centre of the board. It should rise to around 270V at first and then slowly decay to around 250-260V as the valves warm up and their operating current builds. In the unlikely event that the HT supply remains above 280V and there are no board or valve faults, this may be because component variations are causing the supply to deliver more current than it’s designed to. The simple solution is to reduce the value of the 150pF capacitor to 120pF. This will increase the switchmode frequency and reduce the duty cycle and should bring the HT back in line. If you need to do this, don’t forget to wait for LED2 to go out before working on the board. Finally, perform a live signal test. Switch off, wait for LED2 to go out and connect a signal source to CON1/CON2 and an amplifier to CON3/CON4. Next, turn the volume right down, power on and wait 30 seconds or so for voltages to stabilise. Then press play on the signal source and slowly advance the volume until you hear clean, undistorted sound. If the sound is distorted or missing, switch off and carefully check the component values around each valve socket as well as the solder joints. Putting it in the case That’s all for this month. In the sec- Advertising Index Altronics.................................. 72-75 Digi-Key Electronics....................... 5 Emona Instruments...................... 65 Front Panel Express....................... 9 Hare & Forbes.......................... OBC Icom Australia.............................. 17 Jaycar .............................. IFC,45-52 KCS Trade Pty Ltd.......................... 3 Keith Rippon ................................ 95 LD Electronics.............................. 95 LEDsales...................................... 95 Master Instruments...................... 95 Ocean Controls.............................. 6 Radio & Hobbies DVD.................. 62 Sesame Electronics..................... 95 Silicon Chip Binders................ 64,96 Silicon Chip Online Shop............. 86 Silicon Chip Subscriptions......... IBC Silvertone Electronics.................... 7 Tendzone...................................... 11 Tronixlabs.................................. 8,95 ond and final article next month, we’ll go over the details of how to put together the custom laser-cut case and SC fit the PCB inside it. WARNING! SILICON CHIP magazine regularly describes projects which employ a mains power supply or produce high voltage. All such projects should be considered dangerous or even lethal if not used safely. Readers are warned that high voltage wiring should be carried out according to the instructions in the articles. When working on these projects use extreme care to ensure that you do not accidentally come into contact with mains AC voltages or high voltage DC. If you are not confident about working with projects employing mains voltages or other high voltages, you are advised not to attempt work on them. Silicon Chip Publications Pty Ltd disclaims any liability for damages should anyone be killed or injured while working on a project or circuit described in any issue of SILICON CHIP magazine. Devices or circuits described in SILICON CHIP may be covered by patents. SILICON CHIP disclaims any liability for the infringement of such patents by the manufacturing or selling of any such equipment. SILICON CHIP also disclaims any liability for projects which are used in such a way as to infringe relevant government regulations and by-laws. Advertisers are warned that they are responsible for the content of all advertisements and that they must conform to the Competition & Consumer Act 2010 or as subsequently amended and to any governmental regulations which are applicable. 96  Silicon Chip siliconchip.com.au