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AUDIO OUT AUDIO OUT L R By Jake Rothman Transformers in audio – Part 3 Fig.25. An early prototype of the Steve Dove microphone preamplifier made by Student George Perakis. Fig.23. The inside of a 1970 Rogers Ravensbrook amplifier. Note how the cores of the driver transformers (centre) are mounted at right angles to the mains transformer core (top left) to prevent hum pick-up. I n Part 2, last month, we looked at some of the limitations of audio transformers. This month we’ll cover techniques manufacturers use to improve performance in real transformers. Fig.24. PL84 valve guitar practice amp (see Fig.17, Audio Out, September 2022). The output transformer used was the B18A011F from Carnhill Transformers. 54 Loose windings First though, it seems the level of technical scrutiny is highest in Surrey, and I will pass on details of a few errors that slipped through proofing in recent articles – all reported to me by readers from Surrey! Fig.6 from Transformers in Audio – Part 1 (PE, July 2022) shows the Roger’s Ravensbrook audio amplifier; note that the phase-splitter transformer allows output transistors of the same polarity to be used – not driver transistors – as stated in the caption. Also, the unmarked lower preset is 100Ω and the resistor above is 2.2kΩ. This type of circuit is a good introduction to power amp building because if you make a mistake in the driver stage you don’t blow-up your output transistors – the transformer isolates the two sections. The interior of the Ravensbrook is shown in Fig.23. In Transformers Part 2 (PE, September 2022), Fig.14a, sharp-eyed reader Peter Brearey has pointed out that the upper output transistor is inadvertently shorted out by a line. This should have taken an 820Ω 1W resistor to the junction of the decoupling network of the 47µF capacitor and unmarked (47Ω) resistor, not the positive rail. It’s the same Rogers configuration shown in Fig.6 from Part 1. This circuit prevents current being drawn out of the output stage which would unbalance the quiescent current passing through the output transistors. Also, the speaker impedance of the Mullard Class A amplifier in Fig.18a should be ‘3Ω’, not ‘3W’ and the transformer part number is PT1 (possibly a Partridge design). My apologies for the errata, and the absence of photos for some of the circuits. (Note Fig.26. Pye Vanguard police radio monitoring unit. Built in 1965, this ran well into the 1980s. Practical Electronics | October | 2022 Primary Primary section 1 section 2 Join centre tap 1500 turns 1500 turns F S Secondary to loudspeaker 150 turns 22 SWG F n From output valve 3000 turns 34 SWG S Core section, 32mm2 Fig.29. Sectioning of windings improves high-frequency response. This was a simple valve radio output transformer. at 50Hz or 60Hz. Power transformer laminations are Fig.27. Large, gapped output transformer used in the Vanguard typically around police radio monitoring unit. It delivered a couple of class-A half a millimetre germanium watts into a Celestion 5-inch speaker. in thickness, while audio transformers typically have lamthat I do aim to physically build and inations of 0.35mm. Very expensive test all my circuits since paper calcuaudio transformers may have lamilation can only get you in the ballpark nations just 0.1mm thick. Sometimes with audio design.) Fig.24 shows the imperial sizes are used, typically 15 mils PL84 valve amplifier given in Fig.17 and (1 mil = 0.001 inch or 0.0254mm) which the transformer microphone pre-amp is 0.38mm. Audio cores are run at a circuit from Fig.20 is shown in Fig.25. much lower flux level than power transFinally, a bit of history, the 1964 Pye formers to reduce distortion by avoiding Vanguard police radio amp (Fig.19a) is saturation. What matters is obtaining shown in Fig.26 and Fig.27. maximum permeability at high frequenNow we’ll get back to the special cies so that inductance is maximised for construction techniques used in audio a given number of turns. This then mintransformers to improve their frequenimises winding capacitances, extending cy response and distortion. the response. Mumetal laminations have about 20-times the permeability of standard M6 FeSi transformer steel. Laminations However, it can only support about a A good rule of thumb for audio transquarter of the maximum flux density, formers is the thinner the laminations, which means it can only be used for the lower the losses at high frequencies. relatively low powers. A common audio This is because the flux tends to concenapproach is to use 49% nickel or ‘Ratrate on the surface of the laminations. diometal’ cores which provide a good The result is that audio transformers compromise between cost, distortion need to have much thinner laminations and power handling. This is what is than mains transformers which operate used in most of the medium-cost audio transformers, such as the Vigortronix VTX-A series. Brian Sowter, who at the fine age of 83 still consults and deals with technical support at Sowters Transformers, told me that the Sowter 5069 transformer used both Mumetal and M6 laminations interleaved. This gives the advantages of both. As the Mumetal saturates, the M6 laminations take over, giving a seamless transition. They used to call it the ‘Dolby mix’. Fig.28. CineMag transformer used in Some small transformers use ‘T’ and a condenser microphone. It only uses ‘U’-shaped laminations and occasion‘U-shaped’ laminations. The alternate ally on some microphone transformers interleaving gives the misleading just ‘Es’ are used. These lamination impression of a magnetic gap. Notice the shapes give higher permeability, but twin bobbins/coils. Practical Electronics | October | 2022 at the expense of maximum saturation level. This can be seen in the CiniMag microphone transformer in Fig.28. Some microphone transformers have U-shaped laminations with windings either side. This arrangement reduces hum pickup by cancelling out the voltage induced by the hum in the two arms of the core; similar to a hum-bucking guitar pickup. It is rare for audio transformers to have bolt holes punched in the laminations. These discontinuities in the magnetic circuit can cause flux concentrations and raise distortion. To avoid this, the cores are generally clamped or potted. (Note, however, that low distortion is not necessary for guitar amp transformers, it’s part of the guitar’s ‘sound’ or tone.) Sectionalised windings If the windings are just two separate coils, such as in a split-bobbin mains transformer, the leakage inductance is high because the magnetic coupling is low. This is one reason mains transformers give very poor high-frequency response when used as audio transformers. An effective technique with valve output transformers is to split the high-impedance primary winding into two layers with the low impedance secondary winding in-between, as shown in Fig.29. This improves the frequency response at 10kHz by around 9dB relative to 1kHz. Fig.30 shows a Sowter 5069 line output transformer with sectioning. This splitting up of primary and secondary windings can be extended to up to 14 sections, as in Sowter’s 50W KT88 ultra-linear output transformer for the GEC amp design. This greatly increases the cost, but it does enable response up to 40kHz to be achieved. It is essential to have a wide flat frequency response (with its accompanying low phase shift) if a large amount of negative feedback is used, otherwise oscillation could occur. 55 Fig.30. If you look down into the bobbin the three sections of this 1:1 ratio Sowter 5069 output transformer can be seen, along with the black screen connection. There is a yellow wire joining the two halves of the outer and inner sections. Generally, the bigger the turns ratio, physical size and impedance, the worse the high frequency response. It is much more difficult to extend the response of a 20W 8000Ω:4Ω valve output transformer than a 50mW 600Ω:600Ω line-output balancing transformer. Interlayer insulation Good insulation is required between the primary and secondary windings in valve output and power transformers – nobody wants +400V HT on their loudspeaker terminals. This is achieved by wrapping polyester tape around each winding. Wax impregnated paper was once used, but it suffered from leakage current and moisture retention, just like old waxed-paper capacitors. Another approach was a special varnish-impregnated glass fibre tape called ‘Empire Cloth’, which was quite good. The Empire name is still around – Empire Tapes make the standard yellow Mylar heat-resisting transformer sticky tape used today (an alternative is 3M type 56). For prototype low-voltage audio transformers, I have used gas-fitters PTFE tape, which is good for pulling everything tight. Bifilar Bifilar winding is where two lengths of wire (ie, the transformer’s primary and secondary) are wound on the core together at the same time. The windings thus occupy almost identical magnetic space and have the closest possible magnetic coupling. This technique can be scaled up to trifilar and quadfilar, as in the Ravensbrook and Ravensbourne amplifiers. It is better to buy special multifilar wire ready for such winding – it can be a tricky doing it with two rolls. Driver transformers with centre-tapped or split secondaries are wound this way to avoid switch-off spikes from the output transistors operating in class B. The disadvantage of the bifilar approach is that the inter-winding capacitance is high and the maximum isolation voltage is low. Fig.32. This is the amount of fine wire in a Greenweld X7920 interstage transformer. 1770 turns on the primary and 330 turns on the secondary, giving a 5:1 ratio (the measured ratio was somewhat less due to losses). Note the nylon moulded bobbin and laminations. 56 Fig.31. An old Belclere potted VTX-A-style transformer which I have ‘de-potted’ (so you don’t have to!). Note the soldered joint on the copper screen and the lighter-coloured 49% Nickel (Ni) laminations. Impregnation In the old days (from the valve era up to the early 1960s), the varnish insulation on enamelled winding wire was inferior to the more uniform polyurethane coating of today. It was essential to insulate between each layer of the winding with paper, then impregnate the whole lot in wax to render the unit impervious to damp. However, the wax used to melt in service (when running hot) and drip out of the unit. Later, the wax was replaced with polyurethane which was much better… unless you needed to disassemble the transformer for repair. Sometimes transformers are potted in epoxy resin which minimises microphony, but it’s rigidity can cause lead-out wire breakages and its irreversible solidity makes repair impossible. Impregnants are useful to reduce emitted noise, but they also increase capacitance. I have occasionally heard output transformers ‘singing’ due to loose windings and/or laminations. Fig.33. There are many Chinese ‘clones’ of classic British audio products, such as this Neve-copy microphone preamplifier. It suffers from bad hum due to a PCB PSU earth loop and the input transformers having just two steel (not Mumetal) cans which give poor shielding from extraneous magnetic fields. Practical Electronics | October | 2022 Fig.34. Screening can for Vigortronix VTX-A transformers. Fitting the transfomers can be a challenge, but it’s easier with a little silicone grease. Fig.35. The base plate of a transformer shielding can must be soldered to the transformer’s ground pin before it is mounted on the PCB. Interwinding screens Copper foil screens are often placed between windings on input balancing transformers to prevent interference being capacitively coupled; for example, high-frequency switchmode power supplies buzzing digital audio sources. Care has to be taken to ensure a shorted turn isn’t created, the ends of the screen must be insulated from each other by wrapping insulation tape around one end. The screen must be earthed or the interwinding capacitance between the windings is made worse. These screens are often referred to as ‘Faraday shields’. Although a screen is not essential on output transformers it is still worthwhile connecting it to earth or chassis if the transformer provides one, especially if it is being driven by something using an un-earthed power supply. Fig.31 shows a Beclere/VTX line-output transformer cut open to show the screen under the primary. This is also connected to the core. (As an aside, since I was in a dismantling mood, Fig.32 shows a Greenweld interstage transformer revealing a mass of fine wire.) I sometimes connect the shield via a 39Ω resistor to signal ground (if that is the only ground available) to reduce instability in the amplifier driving the transformer caused by the capacitance from the primary to the shield. Normally, the shield is connected to chassis/mains earth and pin 1 on the input or output XLR connector. A better approach to isolating the shield capacitance from the input driving amplifier is to use an LR network. Jensen sell these, (load isolator, part number JT-OLI-3) – it is simply 40 turns of fine 30 AWG wire wound around the body of a 39Ω 1W carbon composition resistor (5.5mm x 15mm non-magnetic). This gives an inductance of around 3.7µH. This sometimes isn’t enough, so I may use a separate inductor of 10 to 82µH. Fig.36. The upper part of a transformer shielding can must also be earthed. This can be soldered along the base plate or use a bit of wire from the PCB to the can. say ‘gently’, because one must never bend or bash annealed Mumetal because the permeability is reduced. Restoring lost permeability is possible, but difficult for the home constructor – you have to re-anneal it by heating it to 1150ºC for four hours, and then cool it down at 250ºC per hour. Note that these cans must be earthed. This is done by soldering the can base to the transformer’s screen/core earthing pin (see Fig.35). The upper can must also be soldered to the base plate, as shown in Fig.36 before mounting. Vigortronix supply versions with the cans ready fitted and I usually get these if I need screening, often the VTX-101-003, which is numbered VTX-102-003 when supplied with the can. Next month That’s all for Part 3. Next month, we will conclude with comprehensive information on sourcing audio transformers, plus a very handy pair of PCBs for mounting Vigortronix audio transformers. Your best bet since MAPLIN Chock-a-Block with Stock Visit: www.cricklewoodelectronics.com O r p h o n e o u r f r i e n d l y kn o w l e d g e a b l e st a f f o n 0 2 0 8 4 5 2 0 1 6 1 Components • Audio • Video • Connectors • Cables Arduino • Test Equipment etc, etc Screening cans Input transformers can easily act like pick-up coils causing hum. Totally enclosing the transformer in a metal can is the solution. Mumetal is the best material (ordinary steel doesn’t work very well). There has been a spate of Chinese Neve microphone preamplifier clones, such as the one shown in Fig.33. These suffer badly from hum when mounted in a rack near other equipment because they don’t use the correct (pricier) materials needed for effective transformer screening cans – just ordinary steel. Some transformers add a copper ‘belly-band’ which also reduces hum pick up. Very expensive input transformers will have many layers of screening material. Optional screening cans (VTX-102-000) are available for the Vigortronix/OEP range, as shown in Fig.34. They are sized 30mm long x 25.5mm wide x 23.75mm high and it’s quite a squeeze to insert the transformer – and much harder to remove. A bit of silicon grease is useful, and they should be pushed on gently with a vice along each edge in turn. I Practical Electronics | October | 2022 V is it ou r S h op , C a ll or B u y on lin e a t : w w w . c r ic klew ood elec t r on ic s . c om 0 2 0 8 4 5 2 0 1 6 1 V is it ou r s h op a t : 4 0 - 4 2 C r ic klew ood B r oa d w a y L on d on N W 2 3 E T 57