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AUDIO OUT AUDIO OUT L R By Jake Rothman Analogue Vocoder – Part 7: Assembly and testing Mains DPDT switch Mains power in 315mA A/S L Filtered IEC socket E V N Mains earth chassis Direct synth level CW Fig.1. A finished Vocoder – with all the bells and whistles – what we’ve been aiming for all these months! A t last! – we’ve reached the stage where we can ‘box-up’ the Vocoder. Putting electronics in boxes or housings can be quite fraught for those who’ve only ever assembled PCBs. The construction techniques described here are applicable to all professional audio projects. Little gremlins First, a little housekeeping. In a project this complex a few errors can creep in, so please accept my apologies for the following corrections and amendments. In last months’ power supply (April 2022, Fig.10), the PCB overlay shows two D19s. The one on the right next to C24 should be D18 (a 10V Zener diode). Since this was a transcribing error from the Eagle CAD PCB file to the drawing, the silkscreen on the PCBs supplied by PE is correct. There were also two errors in the component list. D18 (see above) was incorrectly listed as D19. In the list, the 5.6V Zener diode was given as D18 – it should be D13. I also have a few images that we couldn’t squeeze into last month’s PSU article. Fig.2 shows the final version of the PSU PCB (the one available from the PE PCB Service) Signal Synth R input Mic (mono) Synth L input R output bus Vox in To Aux input Molex on motherboard 0V 3x TRS jack sockets bottom view CW R Synth input (carrier) 0V VR1a/b 22kΩ Lin Dual Screen 0V – + L Synth input (carrier) (Also mono balanced) Balanced input Driver Amplifiers Screen 0V – To star earth tag + Mod input Mic gain 3 – Switch contact + 2 42 1 0V Mic preamp Aux H P/LP Mix resistors: 4 x 2.2kΩ 1/2 Board top 3-way Molex Pin 1 Synth in L output bus Fig.2. Updated power supply board from the April 2022 issue. This one uses only six wet Nichicon electrolytic capacitors. All the other smaller polarised capacitors are solid types. This reduced the residual ripple in the dual rail by 50%, and the phantom power by 75%. The capacitor life expectancy will now be 40,000 hours at 75ºC. The initial cost of this upgrade is about £15, but the labour cost and downtime for a capacitor change would be much more. Mic XLR input 0V to cable screens Pin 10 Plug in board with trimmers pointing up Practical Electronics | May | 2022 3/4 To tra 15 30 n Fig.5. (Left) It’s well worth filing IC2’s heatsink to clear the mounting hole. (Or, use a plastic bolt or drill another hole.) Fig.3. The final test of the power supply: loading the output rails with high-power wire-wound resistors. Always ‘cook’ boards before any in-service burn-in. This weeds out prematurely failing components and/or bad heatsink clamping. 0V 4 Phantom power switch Primaries LED + +48V Toroidal transformer 15-0-15V 30VA De-thump relay output (via Molex) PSU board Secondaries Motherboard power in Molex +15V Pin 3 –15V Pin 6 Pin 7-10 0V Synth input (pin 4) Output Driver Amplifiers Sum bus R output (pin 1) 24V de-thump relay (mount on stripboard) 2x TRS jack sockets bottom view + NO R output – NC er s Mic LR nput 1. Direct Synth input to Motherboard Aux 2. Microphone preamplifier to Mod input 3. DIrect Mic signal and L/R Synth input to Motherboard Aux input Molex 4. Phantom power to Mic preamplifier To mic preamp VDR Fig.8. Motherboard showing cut track and how extra mix input resistors can be added to the mix busses. Optional items Mains power A A/S nput n oard Fig.4. Tape should be placed under the heatsink to insulate it from the track. I’m not sure how tough the solder resist is. Mod/Mic/Vox input (pin 2) 0V 4x 2.2kΩ mixing resistors Motherboard with seven filter boards (All inputs and outputs are via ‘Signal’ Molex) Sum bus L output (pin 5) NO 0V + L output – NC Direct Mic level Aux input Molex VR2 22kΩ Lin 0V 5/6 1 2 R Synth input 0V 3 L Synth input Virtual earth summing inputs on the output Driver Amplifiers 0V Denotes transformer positive phase Chassis earth point Fig.6. Block diagram for Vocoder showing basic carrier and modulator version, and also options for mixing in direct synth and microphone signals with the Vocoder output. 7/8 9 /10 11/12 PSU Fig.7, Vocoder motherboard layout Pin 1 V+ V– 0V (parallel wires for low impedance) Pin 10 Practical Electronics | May | 2022 with a few changes to heatsink-mounting holes. It supersedes the image in Fig.11 in last month’s column. Fig.3 shows the load-test resistors connected. Fig.4 shows tape fitted to insulate track under the heatsink. Fig.5 shows a PCB mounting hole that is just a little too close to the heatsink – just file away some of the heatsink above the PCB to allow the fixing to sit properly. Motherboard I have used a motherboard (Fig.6 and Fig.7) to bring the assorted Vocoder sub-systems together. All the filter boards are connected in parallel on the motherboard, which is essentially a bespoke stripboard. Seven straight double-row sockets (Rapid 22-5140) are inserted for the filter boards. Take care to insert the filter cards the right way. On the left (looking at the motherboard component side) is a 10-way Molex connector for the audio signals, the two inputs and the left/right stereo outputs. These should be connected via screened cable. The screen being connected to the 0V pins of the signal Molex. Power for the motherboard is supplied via another 10-way Molex on the opposite (right) end of the motherboard. Note that multiple 0V connections should be used to achieve the lowest possible ground impedance. 43 Fig.9. 19-inch rack case – expensive to buy new, but do look for second-hand bargains. The audio signals are connected to four driver boards (two input and two output). The output boards are configured as virtual-earth summing boards, while the input driver boards are configured for a balanced input. (See Audio Out, January and February 2022 on how to do this). Optional inputs for direct synth and mic can be fed into the output mixing busses on the motherboard by using a 3-pin Molex connector. There is a stereo left/right input and a mono input. It is necessary to insert the four mixing resistors of 2.2kΩ shown. To avoid noise pick-up it’s a good idea to break the three tracks to the right of the resistors. This can be seen in Fig.8 The motherboard uses plated-through holes so both sides have to be cut. The direct synch and direct mic levels feeding these resistors are set by the 22kΩ linear pots, VR1 and VR2. Vocoder block diagram Fig.10. Reusing an old hard drive case (it was 44Mb and £400 in the old days). Fig.11. Front panel layout (left-to-right control knobs: synth/carrier direct level; modulator/mic direct level and mic gain.) Fig.11. Front panel sketch. Fig.6 shows how the multiple sub-system boards are connected together to form a fully functioning Vocoder. See the note (top-right) about which parts are optional, extra features that can be added now or later. To fully use a Vocoder it is often necessary to mix its output with other signals. In the recording studio this is usually accomplished with a mixing desk and its computer-based equivalents. In such a studio situation only a basic Vocoder structure is required. This will have a carrier (or synth) input and a modulator (mic) input. There will also be the stereo outputs. All these connections will be balanced to wire up to a patch-bay where the Vocoder can be connected in any configuration with other studio equipment. For a stand-alone Vocoder with an integral microphone preamplifier a bit of internal mixing is required. Since we already have a couple of mixing/summing amplifiers in the Vocoder it is a simple matter to mix direct mic and stereo synth signals into the output. Doing this makes the sound clearer and thus performance easier. ‘Just-in-case’ junkies Fig.12. Enclosure ready for drilling. 44 When I was a young constructor, I had a junk box. To my wife’s horror I now have three junk rooms. On a square footage basis, these rooms are worth around £100,000, so I suppose I ought to start using up some of the £5000 worth of junk stored within. I’ve started the decluttering process by using an old hard drive 2U 19-inch rack case for the Vocoder, as shown in Fig.9. If you can’t find an old case, you might have to cough up £82.20 for a new one, such as the RS 801-434 from Schroff (but you’d probably get a Practical Electronics | May | 2022 Fig.13. A carpenter’s pen is ideal for poking through board mounting holes for marking drill holes. much cheaper one on eBay). I did have to buy a new front panel (Hammond PBPA19003GY2, Rapid 30-7201) because of the disfiguring slots and awful 1980s graphics (Fig.10). I was able to reuse the metal handles though, which also act as knob protectors. If you don’t fancy eBay, a great many older electronics constructors are downsizing to bungalows – so now’s the time to pick up 19-inch rack gear from radio rallies for a tenner! The 19-inch standard might be regarded as excessively large for today’s constructor, but in the 1950s it was perfect for valve equipment. It is still the standard today for analogue recording studio gear and lab instrumentation. The large size is beneficial for development work since many modifications can be easily incorporated and this Vocoder design is a long way from being a small surface-mount device, plastic-cased commercial product! Fig.14. (Top) Drill ‘wandering’ is avoided by centre punching. Fig.15. (Bottom) Automatic centre punch – a worthwhile investment for drilling holes. Fig.16. Nylon brass-insert spacers are ideal for PCB mounting. Stick-down ones don’t stay stuck! construction it is cutting rectangular holes in steel panels. It’s a quick way to A&E if you don’t have the right tools.) Front-panel design A whole article could be written on this aspect of audio (and one day I will). Normally, I like the signal flow to be from left to right, but in this case, due to the position of the mains wiring I had to put the input socket on the right. There is no general standard for power-switch positioning, but in audio it’s always ‘down for on’, the opposite to normal electronic practice. This is because it’s unforgivable to accidentality turn something off and end up with ‘dead-air’ in broadcasting. The basic layout is shown in Fig.11. Drilling The mic input and the mains transformer should be as far apart as possible to minimise hum. This normally means they’re placed in opposite corners. Also, the lowest-frequency filter cards should be the furthest away from the transformer. I had to put the mains wiring on the left-hand side of the box because the hole for the IEC mains connector was already there. (If there’s one thing I hate in electronic Once the subsystem layout for your particular box has been finalised, usually after 30 minutes of shuffling things around (Fig.12), it’s time for the lovely job of drilling mounting holes. First, these have to be marked out by positioning the boards and poking through the mounting holes with a long-nibbed felt pen. The Edding 8850 carpenter pen is ideal for this job (Fig.13). The next job is to centre-punch each hole to stop the drill wandering (Fig.14). If you are stuck, a hammer and nail will do the job, but if you are going to do a lot of metal-bashing it’s wise to invest Fig.17. Small holes can be deburred with a larger diameter twist drill. Fig.18. Use of proper de-burring tool for large holes. Layout Practical Electronics | May | 2022 in an automatic punch (Fig.15) where a spring delivers a defined blow as it is depressed. If you aspire to high quality audio, remember to protect your most important asset while doing this kind of high-impact noise-generating work – wear ear protectors. Usually, PCBs are mounted with M3 screws and spacers. I also like nylon brass-insert spacers (RS PRO 601-136, see Fig.16). At £1.60 each, they’re expensive, but they have a little ‘give’ in them to accommodate slight drilling tolerances. A 3mm ‘jobber’ high-speed steel (HSS) bit is used to drill the holes. Sometimes it’s not impossible to drill holes located near the corners from the inside of an enclosure; just turn the box over and drill from underneath. I pilot drill all holes at 3mm and then work up to larger holes – for example, 4-5mm for toroidal transformers and 12.5mm for jack sockets. Bigger holes, such as for XLR and IEC sockets are best done with dedicated hole cutters supplied by ‘Q’ Max and RS. However, these are expensive and the hand-operated ones require considerable strength, but ‘chain drilling’ with small drill bits and finishing with a file also works. It’s a good idea to chamfer all small holes with a larger 10mm drill bit (Fig.17). For large holes and apertures, a round file or proper de-burring tool is required (Fig.18). Remember, always clean up all swarf after metal work with a brush and big magnet. If these particles enter the magnetic systems of loudspeakers, microphones or motors, they can ruin them! The main Vocoder filter card assembly is mounted with angle brackets (Fig.19). PCB guide slots are needed to stop the filter cards wobbling about. I had none, so I used red felt strip from an old loudspeaker as a stop-gap. A spring-loaded clamp to hold them all down is also recommended for on-the-road use. Another useful 45 Fig.19. Mounting the motherboard with angle brackets. anti-vibration measure is to use spring or shakeproof washers under all the screw heads. I like to use a 6mm-long bolt with an integral lock washer, such as RS 278-641 to mount the spacers (Fig.20). The front panel components are shown in Fig.21. Mains wiring In commercial equipment it is now a legal requirement to use a double-pole mains switch, which switches both mains conductors (live and neutral) rather than just the live. This is because residual current devices (RCDs) in buildings can be tripped by an earth-to-neutral leakage current as well as the more important live-to-earth short. The last thing you want in a studio is sudden power loss. (There is also the possibility some incompetent DIYer has wired up a mains lead backwards.) The IEC socket should be of the filtered variety, and it is also good practice to follow this with a transient suppressor or voltage-dependent resistor (VDR), often called a ‘varistor’. The best place to put the VDR is right on the transformer’s primary winding, as shown in Fig.22. A suitable device should have a 250V AC rating and a peak energy dissipation of over 100J (joules), such as the Epcos B72220S0251K101 (Rapid order code 51-4590). Generally, the bigger it is, the more energy can be absorbed, this Epcos device is 20mm in diameter. Fig.21. Front panel components rear view. metalwork. I use a ‘star-tag’ with a solid M4 fastening with shakeproof washers on all surfaces. Remember to scrape off any paint or anodising, as shown in Fig.23. Remember that these earthing requirements also apply to the front-panel mounting arrangements. Fuse holders and sockets I’m sure many an older electronic boffin has had a shock undoing a fuse on a back panel. This can be avoided by wiring it the correct way, with the end tip going to the live input and the tag nearest the rear panel connected to the transformer. The fuse should also be after the mains switch, as shown in Fig.24. Alternatively, an IEC socket with a built-in fuse holder reduces wiring. Make sure all mains wiring is properly insulated with rubber boots and sleeving (Fig.25). Balanced connections The safety of any metal equipment enclosure is dependent on the integrity of its earthing. To be legal, it is essential the mains earth has a resistance of <0.2Ω between the earth pin of the plug and the A female XLR is used for the balanced microphone input since this is the standard. The line level input and output connections are jack sockets, commonly referred to as ‘tip, ring, sleeve’ or ‘TRS jacks’ in audio circles. These connectors have the advantage of being able to accept unbalanced (or mono jacks) as well as balanced. This is useful, since most synthesiser outputs are unbalanced. A benefit in construction is that the 12.5mm mounting holes are much easier to drill than XLRs. This was a major advantage with the hard steel case used. I didn’t even have to drill these. I mounted the jacks in the old ‘D’ connector slots using M12 fibre washers (Fig.26). The aluminium front panel made drilling the XLR hole relatively Fig.20. Bolts with integral lock washers are quicker to install and more secure. Fig.22. A zinc-oxide varistor is wired across the transformer primary to absorb mains transients. Earthing 46 easy. I used a standard 25mm serrated hole-cutter, as shown in Fig.27. Muting relay Extraneous powering up/down noises can ruin recordings and ears, so the outputs of the unit are muted until the circuitry has settled. This is best done with a relay. The change-over contacts are used to ground the output transformer winding when the unit is off, and to connect to the audio outputs when on. We don’t want the transformer to act like a hum pick-up coil. Unfortunately, the relay originally used in this Vocoder was discontinued long ago and to compound the situation I found most of my stock in the ‘24V relay drawer’ turned out to have 48V coils. These are much better employed in power-amps for speaker muting. Instead, I have opted to use a much smaller signal relay, the gold-plated contact Hongfa HFD2/024-S-D (Rapid order code 71-4713). This is small enough to be mounted on strip board (Fig.28) and will happily fit into a 16-pin turned-pin DIL socket. Output transformers Low harmonic distortion is not required for musical instruments and the ‘soft’ distortion of iron-cored output transformers is regarded as ‘musically enhancing’. Thus, the Hi-Fi distortion-cancelling circuitry on the driver boards is not needed for the Vocoder. In my circuit I used cheap output balancing transformers (New old stock (NOS) Gardeners surplus transformers – see Fig.29. I sell these from the AO Shop.) For stability, do remember to take the earth return from the transformer directly Fig.23. Main earthing stud for a metal box. Note how the paint has been removed to obtain good contact. Practical Electronics | May | 2022 Fig.24. The fuse should come after the switch. The fuse here (315mA 20mm time-delay) protects the transformer. The mains wiring is protected by the 3A fuse in the mains plug. Fig.28. A good modern relay for muting. For safety, don’t lace mains wiring with low-voltage wiring. For lowest hum, keep all audio leads well away from the transformer or any AC wiring. All audio cables should be screened. Don’t use the braided type since this is difficult to crimp to the connectors. Use Belden studio cable or similar with a twisted drain wire and foil screen. The earth connections on the balanced jacks all go to the chassis. The final wired unit is shown in Fig.30. Testing It is assumed at this stage that all boards have already been tested to avoid pulling the system down with power supply shorts, instability and other problems. Shut the lid and make music Fig.25. Mains wiring – note extensive insulation sleeves/boots to prevent shocks. to the power supply’s 0V. Alternatively, you can use any other 600Ω output balancing transformer, for example one from Vigortronix. Wiring With large systems like this, it’s best to lay down the power supply and earthing Fig.29. New-old-stock audio output balancing transformers (available from the AO Shop – see page 41). first. Get that going properly, before doing the signal wiring. Since all the boards have pluggable power connectors, power can then be applied to the boards in easy testable steps. In a large multi-board project such as this, do not daisy-chain supplies; it’s best to ‘star’ (bring to a central point) all the V+, V– and 0V connections to the power supply board to minimise interactions. These cables can be seen laced together going to the power supply. Vocoders are one of those pieces of electronic equipment that enable technicians to appear like musicians. Every ‘Eurorack man’ aspires to one since it enables them to move on from instrumental bleep and boop sequences. Feed the Vocoder with a piano soundtrack from a YouTube video of your favourite tune and just talk into the microphone and away you go. I did a good rendition of 10cc’s ‘I’m Not In Love’ that way. Surprisingly, talking into a Vocoder, rather than singing the tune can give smoother results. This is because if the frequencies of carrier and modulator exactly coincide, nasty feedback-inducing peaks can occur. Have fun! Sourcing PCBs All the PCBs for the PE Analogue Vocoder are now available – see the May 2022 page of the PE PCB Service. Fig.26. Jack sockets adapted to fit into ready cut slots with M12 fibre washers. Fig.27. Cut large circular holes, such as those for XLR connectors, with a toothed hole-cutter. Practical Electronics | May | 2022 Fig.30. Completion of the Vocoder wiring. 47