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2-channel guitar preamplifier Pt.3: Building the preamps and reverb module into a metal rack case The 2-Channel Guitar Preamplifier and Digital Reverberation Module can be coupled together to produce a classy unit. This article describes how they are inter­ connected and installed into a rack case. By JOHN CLARKE As shown in the photos, the completed pream­ plifier boards and the reverberation module fit neatly into a 2-unit rack case. The 2-unit high case is required to allow room to mount the second channel above the first channel. However, if you intend to build a single-channel version only, it could be housed with the reverb unit in a 1-unit case. Before putting the case together, you have to first drill the front and rear 66  Silicon Chip panels. Use the wiring diagram (Fig.1) and the front panel artwork (Fig.2) as a guide to positioning the holes. Starting with the front panel, you will have to drill holes for mains switch S1, the 10 potentiometers and the three 6.35mm jack sockets. The hole for switch S1 can be made by first drilling a series of small holes around the inside perimeter, then knocking out the centre piece and carefully filing to shape. Don’t make the hole too big – the mains switch must be a tight fit so that it is properly secured by its retaining tabs. The rear panel requires holes for the fuseholder and mains lead cord­grip grommet at one end and the XLR panel plug and two 6.35mm jack sockets at the other end. Take care with the hole for the cordgrip grommet. This hole is not round – instead it must be carefully profiled to match the shape of the grommet, so that the grommet can not later be pulled out. Case assembly Once all these holes have been drilled, assemble the case without the lid, using the machine screws supplied. Important: be sure to scape away the paint at the countersunk screw points, so that each section of the case makes good metal-to-metal contact. This ensures that each section is properly earthed (important for safety reasons) and stops hum problems. This done, fit the channel 1 preamp­ lifier board to the front panel and secure it using the potentiometer nuts (these should all be tight). Now mark the locations for the three standoff mounting holes (the board mounts on 10mm-long threaded standoffs). You will also need to mark out mounting holes for the reverberation module, the mains transformer (4mm), the earth lug (4mm), the 3-way terminal strip and the cable tie mount – see Fig.1. Next, remove the preamp board and drill all the marked holes in the base of the case. While you’re at it, scrape away the paint or anodising from the area around the earth screw hole. This is necessary to ensure a good earth contact with the bare metal. You are now ready to install the various hardware items in the case. First, cut the pot shafts on the two preamplifier boards to a length suitable for the knobs, then install the board in the case on 10mm standoffs. The reverb module also mounts on 10mm standoffs, while the channel 2 preamplifier board is secured to the front panel solely via its pots and jack socket. This done, mount the remaining hardware items as shown in Fig.1. Note that both the transformer and earth lug are secured using M4 x 10mm screws, nuts and star washers. Do not attach the earth lug to one of the transformer mounting screws – it must be separately bolted to the case as shown in Fig.1. We recommend that you use a second “lock” nut to secure the earth lug, so that it cannot possibly come loose later on. Once it’s fitted, use your multimeter to confirm a good earth contact between the earth lug and case. Final wiring Now for the final wiring. Begin by installing the shielded cable signal wiring – see Fig.1. You also need to connect a 0.47µF MKT capacitor between pin 1 of the XLR socket and an earth solder lug secured by one of the socket’s mounting screws. Once again, be sure to scrape away the paint around the mounting hole to ensure a good earth contact. Be sure to secure the signal wiring with cable ties and to the cable tie mount, as shown in the photos. This will prevent undue stress from being placed on any one connection. Exercise extreme caution with the mains wiring – your safety depends BELOW: this is the view inside the completed prototype. The two preamplifier boards are stacked one above the other at top, while the reverberation unit is in the bottom righthand corner. January 2001  67 68  Silicon Chip Fig.1: here’s how to install the modules into the chassis and complete the wiring. Be sure to use mains-rated cable for all mains wiring and check that the earth lug makes good contact with the chassis. Note that all exposed mains terminations must be sleeved with heatshrink tubing and the wires should be laced together using cable ties – see text. January 2000  69 2001  69 If you intend moving the unit about a lot, it would be a good idea to make up some metal brackets to support the rear of the channel 2 preamp board. This will prevent the tracks from cracking around the solder joints for the pots and headphone socket. on it. First, strip back 380mm of the outer sheath on the mains cord, then clamp the cord into position using the cordgrip grommet. Check carefully to ensure that the cord is properly secured by the grommet; you must NOT be able to pull it back out. The Active (brown) mains lead goes to the centre terminal of the fusehold- er (which must be a safety type) and the excess lead then run between the out­side terminal and switch S1. Slip a 40mm length of 15mm-dia. heatshrink tubing over the two leads before soldering them to the fuseholder. Once the connections have been made, push the tubing over the fuseholder and shrink it down using a hot-air gun. Parts List 1 2-unit (2U) rack metal case; Altronics H-5036 or equiv. 1 2855 30V centre tapped 5VA transformer (T1) 2 6.35mm mono jack sockets 1 XLR panel plug 1 3AG panel-mount safety fuseholder; Jaycar SZ-2025 or equiv. 1 500mA 3AG fuse 1 SPST mains rocker switch with integral neon (S1) 1 mains cable cord grip grommet 1 mains cord and plug 1 3-way 5A terminal strip 1 cable tie mount 2 crimp eyelets 3 fully insulated 6.4mm female spade connectors 70  Silicon Chip 3 M4 x 10mm screws, nuts and star washers 14 M3 x 6mm screws 2 M3 x 10mm screws and nuts 1 M3 x 15mm screw and nut 7 10mm brass tapped standoffs 8 M3 star washers 8 100mm cable ties 1 40mm length of 15mm diameter heatshrink tubing 1 600mm length of red hookup wire 1 400mm length of black hookup wire 1 200mm length of green hookup wire 1 1.5m length of single core shielded cable 1 0.47µF MKT polyester capacitor The Neutral (blue) mains lead is run directly to the mains switch and is wired in parallel with the blue primary lead from the power transformer. The other transformer primary lead (brown) goes to the remaining terminal on the mains switch. Note that all the connections to the power switch are made using fully insulated female spade terminals. Make sure that the various leads are all securely crimped to these terminals before installing them (use the correct crimping tool for the job). The Earth (green/yellow) lead from the mains cord is sol­dered directly to the earth lug. This should be left long enough so that it will be the last connection to break if the mains cord is “reefed” out. Finally, use four cable ties to lace the mains wiring to­gether, as shown in the photo. That way, if a lead does come adrift, it will be secured to the other leads and the “live” end cannot make contact with the case. The transformer secondary wiring is run to the 3-way termi­nal block in the centre of the case and from there to the 15V AC and 0V terminals on the channel 1 preamp board. The DC supply wiring to the other two PC boards can then be run using medium-duty hookup wire. Use red wire for the +15V DC connections, black for the -15V DC connections and green for the 0V (ground) connec­tions. SMART FASTCHARGERS® 2 NEW MODELS WITH OPTIONS TO SUIT YOUR NEEDS & BUDGET Testing Now with 240V AC + 12V DC operation PLUS fully automatic voltage detection Before applying power, go over your work and check your wiring carefully, In particular, make sure that all the mains wiring is correct before installing the fuse in the fuseholder. Now apply power and check the supply rails on both the channel 1 and channel 2 preamp boards. First, check that there is +15V on pin 8 of IC1, pin 7 of IC2, IC3 & IC5 and pin 4 of IC4. Similar­ly, there should be -15V on pin 4 of IC1, IC2, IC3 and IC5 and on pin 11 of IC4. Now connect a multimeter between TP1 in the centre of the channel 1 preamp board and the 0V supply pin. This done, switch the multimeter to the mV range and adjust trimpot VR7 for a reading of 0V, or as close to this as the potentiometer will allow. Repeat this procedure for the channel 2 board. Now check the supply rails on the reverberation board. There should be +15V on pin 8 of IC1 and pin 7 of IC3; -15V on pin 4 of IC1 & IC3; and +5V on pins 1 & 24 of IC2 & IC4. If the supply voltages are all OK, the preamplifier can be connected to a suitable music source and an amplifier while you check out the various controls. Check that VR5 (Volume) adjusts the overall volume and that the tone controls all produce the expected results. Similarly, the Effects control should increase the amount of reverberation as it is wound up. Finally, the Level control (VR1) should adjust the signal level from each channel. Corrections: the circuit diagram on pages 34-35 of the November 2000 issue incorrectly shows S1 as a 2-pole (DPDT) power switch. It should be a SPDT type and it switches the Active mains lead only – just follow the wiring diagram in this article. In addition, the parts list for the main PC board contains some errors. First, there should be 6 x 2.2µF NP PC electrolytic capacitors (not five) and a 1 x 1µF NP PC electrolytic capacitor should be added to the list. Second, there should be 15 x 10kΩ resistors and 4 x 150Ω (not 14 & 3). Finally, the 4.7kΩ resistor connecting to pin 3 of IC3 on the overlay (Fig.4) SC should be 27kΩ. Use these REFLEX® chargers for all your Nicads and NIMH batteries: Power tools 4 Torches 4 Radio equip. 4 Mobile phones 4 Video cameras 4 Field test instruments 4 RC models incl. indoor flight 4 Laptops 4 Photographic equip. 4 Toys 4 Others 4 Rugged, compact and very portable. Designed for maximum battery capacity and longest battery life. AVOIDS THE WELL KNOWN MEMORY EFFECT. SAVES MONEY & TIME: Restore most Nicads with memory effect to capacity. Recover batteries with very low remaining voltage. CHARGES VERY FAST plus ELIMINATES THE NEED TO DISCHARGE: charge standard batteries in minimum 3 min., max. 1 to 4 hrs, depending on mA/h rating. Partially empty batteries are just topped up. Batteries always remain cool; this increases the total battery life and also the battery’s reliability. DESIGNED AND MADE IN AUSTRALIA For a FREE, detailed technical description please Ph (03) 6492 1368; Fax (03) 6492 1329; or email smartfastchargers<at>bigpond.com 2567 Wilmot Rd., Devonport, TAS 7310 Truscott’s •  RESELLER FOR MAJOR KIT RETAILERS •  PROTOTYPING EQUIPMENT •  COMPLETE CB RADIO SUPPLY HOUSE •  TV ANTENNA ON SPECIAL (DIGITAL READY) •  LARGE RANGE OF ELECTRONIC COMPONENTS Professional Mail Order Service Truscott’s Amidon Stockist ELECTRONIC WORLD Pty Ltd Fig.2: this is the front panel artwork, reproduced here 50% of full-size. It can be enlarged on a photostat machine for use as a drilling template, or you can download the full-size artwork from www.siliconchip.com.au ACN 069 935 397 Ph (03) 9723 3860 Fax (03) 9725 9443 27 The Mall, South Croydon, Vic 3136 (Melway Map 50 G7) email: truscott<at>acepia.net.au www.electronicworld.aus.as January 2001  71