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Pt.2 By Nicholas Vinen High-performance stereo valve preamplifier Having described how our new stereo valve preamp works and how to put the PCB together, now it’s time to build the case. This houses the PCB so that you can still see all the components but can’t touch the high-voltage sections. It’s custom designed and made from clear acrylic, glued and screwed together. T HE STEREO Valve Preamplifier PCB was originally designed to fit inside a UB2 “jiffy” box. However, because it has connectors on the back and controls on the front (including a volume knob), it’s pretty much impossible to actually get it inside such a box even after drilling the required holes. 74  Silicon Chip Rather than compromise the practicality of the device to allow it to fit in that box, by doing something like putting the inputs on the front or side, we have designed a custom case instead. This is made in three sections which screw together so you can get it apart if necessary. We think the result is much more attractive – hopefully you’ll agree after seeing the photos of the prototype. We’re using clear 3mm acrylic to show off the components and the cutting pattern is shown in Fig.8. It can be downloaded from the SILICON CHIP website (free for subscribers) in a varisiliconchip.com.au ety of formats to suit typical laser cutter software, including AutoCad DXF. We used a 60W CO2 laser operating at full power and at a speed of 8mm/s (10mm/s also works but we’re being a little conservative to ensure it cuts reliably). The parts are cut from a sheet of acrylic measuring at least 230 x 315mm. The cuts shown in red are done first, in case the sheet isn’t perfectly flat. In this case, if the outer sections were cut first they could shift slightly and then the inner cuts would be less accurately placed. We have optimised the cutting path to eliminate the duplication of overlapping cuts, although typical laser cutters have an extremely high degree of repeatability so this will generally not affect the quality of the end result. The case pieces incorporate holes in the top for the valves and some cooling slots over the power supply. The front panel has holes for the power switch, volume pot, its anti-rotation tab and the two power LEDs, while the rear panel has holes for the input and output RCA sockets and the DC power connector. There are also four screw holes in the top and bottom panels corresponding to holes on the PCB, to hold the whole thing together. If you’ve building the musical instrument version of the PCB with the extra panpot (VR2) and mixed output (CON5), we’ll supply a modified version of the cutting diagram with these extra holes. Their locations are shown in Fig.9, relative to the pre-cut holes. Putting it together You will now have the completed PCB, six case pieces (with protective film on both sides) and an assortment of machine screws, tapped spacers and feet (as specified in he parts list last month). Start by removing the two 12AX7 valves (or “vacuum tubes”, if you prefer) and setting these aside so they won’t get damaged. Don’t put them on a surface they might roll off! The bottom, rear and side panels of the case are glued together into one assembly which screws to the PCB. The top panel is also screwed to the PCB while the front panel is held on by the potentiometer nut(s). The PCB is sandwiched between the top panel and the bottom panel assembly and held in place with screws and spacers. It’s a good idea to check that everything will fit before gluing. Leave siliconchip.com.au Note: diagram not to scale Fig.8: the laser cutting pattern used to produce the six pieces for the Stereo Valve Preamplifier case, from a 230 x 315mm piece of 3mm-thick clear acrylic (Perspex). The sections shown in red are cut first to maximise precision of the hole placement. Note the cooling slots in the top cover, which go over the power supply circuitry. The prototype lacked an onboard power switch so a hole has been added to the front panel for the final version (note: diagram not to scale). the protective film on the case pieces for now, so they don’t get scuffed or dirty – clear acrylic shows fingerprints quite well unfortunately, so you want to avoid getting these on the inside of the case if possible, where they’re hard to clean off. Using Fig.10 as a guide, pass an M3 x 32mm machine screw through a hole in the top panel and screw on one of the shorter spacers. Do it up tight against the underside of the lid, then thread the other spacer on and repeat the procedure for the remaining corners. Next, orientate the lid so that the valve socket holes are above the sockets, then push the protruding screw threads through the PCB mounting holes and attach the M3 x 25mm tapped February 2016  75 M3 x 32mm MACHINE SCREW Rear panel CASE TOP PANEL 25.4mm ( 8.25mm) 25.4mm 2 x 9mm or 1 x 6.3mm & 1 x 12mm TAPPED NYLON SPACERS 50.8mm PCB 25mm TAPPED SPACER Front panel 50.8mm 20.32mm CASE BOTTOM PANEL 22.86mm RUBBER FOOT Note: diagram not to scale 7.62mm ( 8.25mm) Fig.9: here’s where to drill the extra holes in the rear panel (for the mixed output RCA socket, CON5) and in the front panel (for panpot VR2) if required spacers to hold the PCB in place. You can then screw the bottom panel on using the remaining 10mm machine screws (no need to fit feet just yet). With the top and bottom panels held rigidly in place, check that the rear panel fits. The RCA sockets will be a relatively tight fit through the holes but assuming they have been soldered evenly, it should slide into place. Otherwise, you may need to use a tapered reamer to open the holes up slightly. RCA sockets Note that, on our prototype, we used the RCA sockets which are available from our online shop, because it’s hard to find white sockets of this type at the usual retailers. They have a slightly different footprint but will fit on the board with some slight bending of the pins. We mention this because this is likely to affect the height that the sock- ets sit at, so if you use a different type, they may not line up with the holes in the case. In that case, you would either need to adjust the RCA socket height by melting the solder joints (tricky) or simply enlarge the panel holes until they fit through. You may find, depending on the exact height of the spacers you have used, that the rear panel will be slightly too tall to fit between the top and bottom panels. In our prototype, it was an almost an exact fit but spacer lengths can vary slightly. In this case, you may need to add some sort of a shim (eg, a washer or two) somewhere in the spacer stack to increase the gap enough for the panel to fit correctly. Assuming it fits OK, remove the nut from the pot(s) and fit the front panel. This can be held in place temporarily with the potentiometer nut(s). If necessary, reach behind the front panel using small pliers to push the LEDs Changing The Preamp’s Gain The circuit as presented last month has a maximum gain of four times (12dB). While the output swing is limited by the valve operating conditions, if you have a situation with low-level input signals, you may wish to increase this. This can be achieved by increasing the value of the 10kΩ 1W resistors to the lower left of each valve socket. For example, using a 15kΩ resistor will give a gain of 15kΩ ÷ 3.3kΩ + 1 = 5.5 times or 15dB. A 22kΩ resistor will give 22kΩ ÷ 3.3kΩ + 1 = 7.5 times or 17.5dB. And a 30kΩ resistor gives a gain of 10 times or 20dB. Note though that increasing the gain will slightly prejudice the performance by increasing the distortion and reducing the signal-to-noise ratio. Having said that, the performance as specified is sufficiently good that you probably won’t notice the difference. 76  Silicon Chip M3 x 10mm MACHINE SCREW Fig.10: the top panel, bottom panel and PCB are sandwiched together using four sets of machine screws and tapped spacers. This spaces the top and bottom panels the correct distance apart for the front, back and side panels to fit. If they’re too close together, add 3mm inner diameter washers in each stack as shims. through their respective holes. With the front and rear panels in place, it should be possible to slide the side panels into place. Gluing the case Once you’ve confirmed that everything fits, disassemble it and peel the protective film off the pieces. Reassemble the top and bottom halves and the PCB as before, using the screws and tapped spacers, to form a rigid assembly. The parts are glued together using a specialised, solvent-type plastic adhesive formula. We used SciGrip “Weld On” 16 fast set clear, medium-bodied solvent cement. This is available from Plastix [Sydney (02) 9599 2499, Northern Beaches (02) 9939 0555]. With a clean cloth at hand to wipe up any excess, the next step is to glue two pieces of the case together (see below) by applying a moderate amount of the adhesive to all the mating surfaces and then pressing and holding them together. Try to avoid getting any of the adhesive on the faces, especially via your fingers. Start by gluing the rear panel to the base. Make sure it’s pressed in fully until the adhesive sets (this takes a few minutes). You may find that friction holds it in for you, otherwise you may siliconchip.com.au The rear panel of the case has holes for the RCA stereo input and output sockets and a square cut-out to provide access to the DC power socket. Note the ventilation slots in the top cover above regulator REG1 and Mosfet Q2. Warning Improving The Bass Response Voltages of up to 285V DC are present on the PCB when power is applied and whenever the red LED is lit. Do not operate this unit without the top cover in place. A reader has brought to our attention the fact that the circuit as presented last month could have a significant amount of bass attenuation when driving a fairly typical power amplifier load impedance of around 10kΩ. have to hold it. Once it’s nice and rigid, carefully unscrew the base and remove the two panels which are now joined. You can then glue the two side panels on, again making sure they are pressed in fully before it sets. Don’t use great dollops of glue but don’t be too stingy either. If you’re quick, you can wipe off any excess from the outside with a cloth. Leave this assembly aside for some time (ideally, overnight) before re-fitting it using the four screws. If you have stick-on rubber feet, stick them on now, otherwise attach screw-on feet using the four mounting screws. That’s what we did on the prototype (see photos). Fitting the front panel All that’s left is to push the front panel in place, ensuring the LEDs pass through the two holes, attach the potentiometer nut and fit the knob. If you find the front panel won’t sit flat, it may be that the LEDs are protruding too far and pushing on it. Pressing them siliconchip.com.au This is because the 220nF output coupling capacitor is not quite large enough. Our Audio Precision test equipment has a 100kΩ input impedance and in combination with the 220nF coupling capacitor and 1MΩ onboard bias resistor, this results in a -3dB point of around 8Hz. However, with a 10kΩ load impedance, the -3dB frequency increases by nearly a factor of 10, to 72Hz. We’ve confirmed this by simulating the full preamp circuit (see the graph in Ask SILICON CHIP). This is not an unrealistic load impedance; for example, our Ultra-LD Mk.2, Mk.3 & Mk.4 power amplifiers all have an input impedance of around 11.8kΩ. The solution is simple: increase the coupling capacitor value. At the very least, use 470nF 630V capacitors (one in each channel) for a -3dB point of 34Hz for a 10kΩ load. Ideally, though, they should be at least 1μF. The simplest way to achieve this is to use pairs of parallel 470nF capacitors, one on either side of the PCB for each channel. This will yield a -3dB point below 20Hz. carefully back into the case should fix this. You will need to do this to both LEDs or the result could look strange. It’s now ready to use. Remember that it takes 10-15 seconds each time you power it up before the HT rail rises to its normal level and all the bias voltages stabilise. Until then you aren’t likely to get much output. Ideally, wait 30 seconds or so after powering up for it to achieve a reasonable level of performance. The valves will continue to warm up for some minutes and this may affect performance slightly. Note: Altronics have announced that they are in the process of preparing a kit for this project, Cat. K5192. The plan is for it to be a complete kit, including the parts required to make the case. However, the case may not be identical to the one described here. Instructions regarding the case assembly will be supplied with the kit. SC February 2016  77