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AUDIO OUT AUDIO OUT L R By Jake Rothman Potentiometer scratching – Part 2 L ast month, we introduced the problem of ‘pot scratching’. An annoying fault that causes audio output to go ‘scratch, crackle and bang’. This month, we’ll examine how to tackle the problem and offer some pot recommendations. Measurements and testing The Radio Designer’s Handbook states that a hot-moulded carbon track pot will typically give 1-2mV/V applied across the track when rotated at a speed of one rotation per second. I made some measurements and found that a current of just 1.5µA through the wiper could just make audible noise on a 25kΩ linear Plessey MCH 0.5W moulded-carbon pot listened to at line-level. This is a problem, because most electrolytic capacitors have a leakage of around 3 to10µA. On the other side of the pot feeding the clockwise end of the track, 5µA could be tolerated. This experimental set-up is shown in Fig.11. Fig.12. Most moulded-track pots have carbon brush wipers, such as this Plessey MH1. This can reduce rotational noise compared to a metal wiper. assembly oxidation in British humidity. (The wiper itself is immune, being made from carbon as shown in Fig.12 – it’s the rest of the device that oxidises). The otherwise excellent Wayne Kerr AMS1 audio measurement system (Fig.13) suffers from scratchy devices made by ModPots. This is not helped by the lack of DC blocking capacitors in the circuit. This was further exacerbated by the use of NE5534 op amps, which have high input bias currents and output offsets. Furthermore, the system’s thermistor-stabilised phase-shift oscillator configuration (Fig.14) is one of the most sensitive circuits to pot noise, which causes the amplitude to bounce all over the place as the frequency is altered. Another problem is that the anti-log dual-gang 10kΩ pot used has an unusual 20% taper. Changing it to another Oxidation Oxidation of the metal components of potentiometers, such as the wiper and its central pick up contact (slip ring) is a particular problem in damp (eg, British) climates. The modular Allen-Bradley ModPot 70 series is very successful in the US market, but fails from wiper slip-ring Apply R1 across C1 to set track current 100kΩ: noise quite noticeable = 40µA 470kΩ: noise just noticeable = 10µA 1MΩ: noise gone = 5µA 100nF polyester R1 5µA max C1 + +5V 1.5µA max 22µF tantalum low leakage Pot under test manufacturer makes the scale markings wrong. Last, but not least, the other pot configurations of tracks and switches are also unique and the only solution is total disassembly and servicing. Luckily ModPots are held together with screws as shown in Fig.15, so access to the innards is possible. When I was responsible for maintaining Solid State Logic 4000G series mixing desks I often stripped down and fixed their unique multi-section Clarostat 388 series pots. At £30 a device in the 1980s, it was worth it. Old Omeg pots which used Radiohm parts had a peculiar slip-ring problem because the wiper is brass and the centre pin contact is steel (see Fig.16). The dissimilar metals reacted badly in damp and sometimes even acted like a rectifier. Around 1993 the centre ring was replaced To 1MΩ input impedance amplifier 30dB gain Connect R2 to set wiper current 4.7MΩ: no noise = 1.6µA 2.7MΩ just audible = 2.7µA R2 + This part of the circuit injects CW 25kΩ lin Fig.15. Luckily for service engineers, the AMS1’s custom ModPots are screwed together, facilitating servicing. 7.2V – current into the wiper. Results show that wiper current causes more noise than track current. 0V Fig.11. The maximum currents allowed through pots before rotational noise becomes apparent are very small, as shown in this experiment. 62 Fig.13. This old Wayne Kerr AMS1 audio analyser was cursed with scratchy pots and clicky switches. Practical Electronics | July | 2023 All op amps NE5534A (power: pin 4 is –15V, pin 7 is +15V) C6 4.7pF VR1a 100kΩ REV log 20% taper VR1a 100kΩ REV log 20% taper CW Frequency control R2 120kΩ 2 3 R4 4.3kΩ 6 IC1 5 R8 120kΩ R6 33kΩ C8 22nF 2 3 8 C5 22pF R7 6.2kΩ R10 4.3kΩ R19* RA25 NTC C15 4.7pF R13 10kΩ R9 10kΩ CW – + C12 4.7pF Positive feedback R5 10kΩ R3 10kΩ *Evacuated glass bead thermistor: 200kΩ at 20°C, Rmin = 890Ω R18 10kΩ – + R14 33kΩ 5 2 6 IC2 3 8 C11 22pF C2 22nF R15 10kΩ R20 33kΩ – 6 IC3 + 5 8 C14 22pF R23 10kΩ Output (load 10kΩ) 0V Fig.14. The AMS1’s low-distortion oscillator circuit suffered from pot noise. It can be fixed by using 10µF polyester DC-blocking capacitors in series with the pots. with a printed carbon polymer track (Fig.17) and the problem was solved. (This caused me constant problems with Colorsound guitar pedals and I dislike sourcing parts from overseas for a product with such a unique British history. I’ll be going back to UK sourcing from Omeg.) Servicing pots A common cause of pot noise is dirt on the track causing intermittent contact. There is also the build-up of carbon from track wear. Pots tend to be open or sealed. Open types are more likely to become noisy but they can easily be cleaned by squirting cleaner through the gap above the tags. Sealed ones can be quite a challenge to open non-destructively. A switch contact cleaner lubricant aerosol, such as Super Servisol 10 (now marketed as Kontakt Fig.16. Old Radiohm/Omeg P20 (pre1993) pots had particularly crackle-prone centre terminal slip-ring assemblies due to dissimilar metals being used. Fig.17. Later Omeg P20 pots had a carbon polymer track and centre slipring, which eliminated the above failure mode and were much quieter. Practical Electronics | July | 2023 super 10) is the standard cleaner. This is available from most suppliers such as Cricklewood Electronics. The very expensive Contralube 770 connector protector from Newgate Simms Ltd (see Fig.18) is an excellent synthetic gel pot track lubricant. I sourced my 75g tube from Rapid, but sadly it now seems to be discontinued. It’s still possible to get 8g packs for £8.00 from specialist automotive suppliers, and there is an alternative compound called Tribogel. Don’t use the ubiquitous WD40 spray, since its solvents soon evaporate, leaving an oil film not dissimilar to 3-in-1 mineral oil. This then turns to a sticky resin over time. It is meant for freeing seized bolts and locks, not cleaning pots and switches, although it’s useful for loosening stiff pot shaft bushings. Newgate Simms also offer mechanical damping grease for pot bushes. If you want that ‘velvety feel’ to pot rotation then a low-viscosity type is recommended. Kilopoise grease can be too stiff, but it’s what Omeg used to use. There are reactive cleaners that claim to dissolve metallic oxides and sulphides. DeoxIT F5 by Caig Chemicals in the US is possibly the best known compound. They also do a special Fader Lube for conductive plastic pots. These products are distributed by Hot Rox UK in Nottingham. I use the tube versions of the liquid shown in Fig.19 and their wipes (Fig.20) which require the pot to be disassembled, unlike the aerosol options. Do remember that most aerosols are around 80% propellant and are therefore an expensive way of dispensing chemicals. They also pollute the workshop air. One oxide-removing aerosol I do recommend however is Kontakt 60 (Rapid 01-0744) since it is relatively cheap. Remember to re-lubricate afterwards to prevent rapid re-oxidation. These chemicals often need a bit of extra help when dealing with oxidised wiper assemblies and a little abrasion is needed. Fig.18. Super Servisol and Contralube, old mainstays of many a studio maintenance engineer’s tool kit. Fig.19. Specialist products such as this DeoxIT fader lubricant can be very useful if you do a lot of pot maintenance. Fig.20. To get rid of mild oxidation from contacts and wipers these wipes can save the day. 63 Solder slaughter Soldering can kill pots – not immediately – but worse than that, a few months later. The riveted joints from the tags to the silvered track portion are vulnerable to going open circuit due to the corrosive effect of solder flux. I learnt this the hard way when I had this ‘great idea’ of soldering wires in the rivet holes to adapt wired tag pots into PCB-mount ones for my Colorsound Fig.21. With a bit of cutting and prizing out of its plastic frame the moulded-track element of the ModPot can be extracted for cleaning and polishing. It will then be good for another ten years. Fig.22. Roberts mains-powered table radios are very popular, but often have bad on/off volume controls that blow your ears off. The earlier RM20, AMonly model is on the top – it does not have this problem since it has a discrete amplifier rather than the high-gain chip used in the two FM models below. 64 Input from tuner output Replace parts: VR1 with 10kΩ log R2 with 2.2kΩ C3 with 1µF 23V TDA2611A 2.2kΩ 7 100nF 1 9 R2 22kΩ 4.7nF C3 100nF 2.2nF Volume VR1 100kΩ log 330pF CW 150µF 10V 2 + A fine file or emery paper may need to be used. I have very rarely resorted to applying a current-limited high voltage from an insulation tester to clear oxide from sealed wipers, but it can be destructive. It’s essential to apply a bit of fader lube to seal the metal surface once oxides have been cleared; otherwise, oxidation will reoccur. Occasionally, the surface of moulded carbon tracks can become a bit crazed over time and these can be polished down on a glass surface with T-Cut until they are shiny and smooth again. This works wonders with the ModPot carbon tracks once they have been cut out from their plastic frame (Fig.21). The final polish is achieved by rubbing on a flat sheet of paper or cloth. + R1 5.6kΩ C1 6.8µF Increase negative feedback by adding R1 and C1 C2 2.2nF Cut track, insert C2 to reduce gain except at high frequncies 12Ω 2W 68Ω 0V Fig.23. Modifications required to fix the Robert’s radio problem. It is quite tricky to reduce the gain of the TDA2611A without it oscillating. The pot value can also be reduced to 10kΩ without distortion. guitar pedals. I had to change a lot of pots under guarantee. Be careful – never allow flux to migrate up the pins to the track. Switches As an aside from pots, all the above servicing and DC leakage details also apply to signal switches, another major source of crackles. Wonky pots A lot of old radios used combined on/off switch volume controls. With some, the switch was on the mains input, which caused a mild hum. I don’t like the idea of mains a few millimetres from the low voltage side of the circuit. (Although, if the metal body of the switched pot unit is connected to mains earth and it passes a 500V insulation test it will be safe.) Another problem with switched pots is that the volume is turned up before the circuit has time for its DC conditions to settle down and the coupling capacitors to charge up, causing noise as currents are flowing through the pot. One possible solution is to use a push/pull switch, assuming the volume level is at the previous level. Another fix is a time-delay output mute circuit. Best of all would be to use a separate switch and volume control. The Robert’s mains table radios shown in Fig.22 have integral mains switches and over 40dB of gain after the volume control. There’s practically an audio ‘explosion’ when you turn them on. This can be fixed by reducing the pot value from 100kΩ to around 5-22kΩ, increasing the input coupling capacitor and reducing the amplifier gain with additional negative feedback components around the chip. This is shown in Fig.23. These radios have exposed metalwork screws and only two core mains leads. To make them safe and compliant they should be earthed and upgraded to a three-core lead. Going over the edge One of my least favourite types of pot is the ‘edgewise’ design – intended for thumb operation. When they fail it’s often impossible to find a replacement, so I usually give up and try to fit a rotary device. I recently spent six months struggling to source an edgewise switched audio-taper pot when designing the latest Dubreq synthesiser because the ‘style concept’ demanded a thumbwheel. I tried eight mostly useless samples, and when the first units came off the line, the knobs were rubbing against the slot, needing more rework (see Fig.24). Avoid these nuisances in your designs! Sliders I also dislike slider pots because they’re a pig to fit and dirt falls into the open slot. Also, they come in 20 different sizes with multiple non-standard knobs that easily get lost. On my ‘can’t be bothered to repair’ rack a lot of the equipment Fig.24. This little edgewise switched pot on my latest synth design caused a few months of headaches. In your designs, use a sensible rotary pot and a latching power switch. Fig.25. A nice synth ruined by stuck sliders used only for ‘style’ purposes. Fixing these will be a £200 job. In this case it’s worth doing because the machine is a highly collectable ARP. Practical Electronics | July | 2023 Fig.27. Typical fader rubbish to be cleaned out. Fig.28. A nicely cleaned track ready for wiper de-oxidation and application of lubricants. Fig.29. A small sable-hair painter’s brush was used to apply the track lubricant. Input impedance is 7.15kΩ at 1kHz 8µF gives a 2.8Hz cut-off, which is much bigger than needed. 82kΩ Input from detector stage + Volume 5kΩ log Output to driver stage OC71 –1.5V CW Leakage current 15kΩ 680Ω 1kΩ *Replace with 1µF poyester 120µF + 0V Positive earth Typical DC voltage at this point = +0.55V 560Ω 11µA To amplifier CW To previous IF stages 10nF 10nF Final IF transformer stage OA90 10nF Practical Electronics | July | 2023 1.5kΩ Fig.32. The volume control and post-fader amplifier circuit on the Bush TR130. Note that the wiper Potting bushes In some cases, just replacing coupling capacitor should be replaced with a lower the input electrolytic capacitor capacitance plastic-film unit to reduce leakage and on the wiper with a polyester settling time. if the designer is daft enough to route the of lower value solves scratching. I do negative feedback DC path through the this in the popular Bush TR130 radio pot. I had endless trouble with an Omeg circuit shown in Fig.31. I find a Mullard volume pot in the Douglas Self Wireless 1µF C280 ‘tropical fish’ capacitor looks World 1979 pre-amp even though it was right in a 1967 radio and fits in the PCB properly coupled. This was transformed nicely. The resulting higher bass cut-off when I replaced it with a Bourns 91 frequency is of no consequence given conductive plastic unit coupled with the cut-off frequency of the speaker and metal-cased ITT TAA tantalum capacioutput transformer. The circuit is shown tors. These had 10x less leakage current in Fig.32. than the Mullard 017 blue electrolytic A common problem with old AM radios capacitors I used originally. is the use of the volume control track element as the diode detector load. This can put over a volt of DC across the track. Settling time The solution is to provide a separate Equipment that has not been switched on resistor for this. This should be coupled for a long time may suffer leakage currents via a capacitor to a high-impedance and terrible pot noise while wet electrolytic load at least 4x higher. This is needed capacitors reform. This is why I have always because distortion can arise if the load advocated tantalum coupling capacitors in seen by the diode is different for AC and my designs (along with the necessary distorDC – see Fig.33. tion-mitigation measures). There is also the In some circuits, pots are placed in normal time all capacitors take to charge up negative feedback loops such as in Fig.34. when turned on. In some designs – using These active volume controls are extremehigh value pots (around 100kΩ) and large ly sensitive to pot noise because if the value (around 100µF) capacitors – settling wiper loses contact it goes to maximum time can take a few minutes. This is usually volume rather than minimum. In some the result of a misguided attempt to extend circuits the output can even go hard offset bass and obtain ultra-low distortion down OA90 Fig.31. Bush TR130 with a Plessey switched volume-control pot. –2.7V *8µF Power rail Fig.30. Re-crimping the tabs by twisting with a screwdriver. –7.25V 5.6kΩ 10µF + Fig.26. Opening up an Alps mixer fader for cleaning. Squeeze the tabs to get the side plate off. needs new unobtainable sliders like the synthesiser shown in Fig.25. As an engineer, I want to avoid odd-shaped pots, industrial designers please take note. I only make an exception for mixer faders because they are a cultural norm. The Alps fader was popular on older semi-pro mixing desks. Like all slider pots they suffer from debris entering the slot. Fig.26 to Fig.30 shows a sequence of photos detailing how to recondition them. 560Ω DC blocking capacitor 10nF Volume 5kΩ log 0V Buffer amplifier with at least 20kΩ input impedance 470nF 5.1kΩ load resistor 1.5µF To amplifier CW Volume 5kΩ log 0V Fig.33. A lesser known cause of pot scratching on many old radios, including the TR130, is the practice of using the volume pot track as the detector load. 65 Input This stage oscillating at high frequency Volume 100kΩ lin + 10µF Low-leakage CW 3.3kΩ Bursts of radio frequency due to track irregularities demodulated into audible noise by output amplifiers +24-26V Noise 100kΩ 100kΩ 1kΩ CW 330nF Volume BC143 220kΩ BC547 8.2kΩ + 47µF 12kΩ 47µF + BC549C +24-26V 22Ω Output 4.7kΩ 2N1711 Red LED 56Ω 0V to subsonics. Recording engineers are well aware of these settling phenomena and usually allow their equipment to stabilise before use. Use it or lose it Pots must be rotated over their full rotation every six months or so, otherwise they may seize up or oxidise – just like a car or any other machine that relies on lubrication, they have to be used. High-frequency oscillations If a pot is fed with an ultrasonic high frequency signal, for example from an oscillating op amp, it can be demodulated later in a following amplifier (Fig.35). The result sounds just like DC leakage crackling and it’s caught me out a few times. This is a relatively rare pehenomenon, but is worth knowing about if apparent pot crackle is resisting all attempts at being cured. Fig.35. (above) Occasionally, radio-frequency oscillation or breakthrough from inadequate filtering may give pot crackling similar to DC on the track. Fig.34. (left) When a pot is used as a negative feedback volume control element, such as in this Baxandall topology circuit, a jump to high resistance caused by dirt on the track can cause the gain to go open loop with horrific noise. Where necessary, arrange for the feedback to be AC-coupled through the pot with a separate path for DC. (RL) specification before they wear out. Those with a life of over 20,000 turns are expensive (£4-12) compared to the £0.70 to £2.00 for ‘regular’ carbon track units. This means ‘life cost’ should be considered. There’s no point using a high-spec pot for a rarely used treble control on a Hi-Fi amplifier, but it’s a good idea for the volume control, since a £50.00 service fee could be avoided later. A lot of these pots are no longer made and there have been many mergers and acquisitions in the components industry. This means old designs often turn up again under a different name. They are included because they are regularly found in old equipment and as NOS ‘new old stock’ on auction websites. Plus, I’ve taken all of these apart for analysis so you don’t have to! In this section we’ll list a selection of pots with their expected rotational life Allen-Bradley series 70 ModPots (100,000 RL) Any configuration possible, easy to disassemble. Poor humidity resistance. conductive plastic, cermet and moulded track versions. Fig.36. The Allen-Bradley J-series; one of the toughest pots made and with low rotational noise due to a highly polished moulded-track element. I’ve never had to replace them, only scavenged them from old equipment for reuse. Fig.37. The Allen-Bradley W-series, a quiet sealed pot that’s very small. Pot pourri 66 Allen-Bradley J series moulded track (100,000 RL – Extra life (EJ) version 1,000,000 RL) Very strong, sealed, large 2W mil spec. Used in ‘Cry Baby wah-wahs’ effects pedals. Been in production for over 60 years. Now made by Honeywell. Stainless-steel case. Similar types made by other manufacturers as the military RV4 form. Clarostat make a conductive plastic version called the 380 series. See Fig.36. Allen-Bradley W series 10mm sealed miniature moulded-track (50,000 RL) Very solid design. See Fig.37. Alpha (16,000 RL) The best cheap pots, a Taiwanese version of the Japanese Alps pots. I use them in my consumer products. They use four silvered wipers on track, three on the slip ring. Alps (16,000 RL) Good for most Hi-Fi – the 27mm Blue Velvet is a definitive Hi-Fi volume control. Fig.38. A pot loved by Hi-Fi amp designers, the ALPs Blue Velvet – the smoothest carbon-track pot made. Available from Mouser and eBay. Practical Electronics | July | 2023 Fig.39. AB (not to be confused with AllenBradley) CTS pot. Tough old-fashioned carbon track product made in Wales by Blore Edwards Ltd. Easily identified by the plastic spindle pip at the back. Fig.40. BI P260 – the most costeffective sealed conductive-plastic pot. This device is very small, good for PCBs, uses a four-fingered wiper and is distributed by Profusion. Fig.41. The Bourns 91 is my favourite volume control. Its plastic construction is a bit weak, but its noise performance is the best – available from Farnell and Audio Out. Excellent track matching, especially at low levels. See Fig.38. stereo 10kΩ log 91A2A-B28-D15/D15L they are expensive, but worth it. Unique blue appearance, as shown in Fig.41. Erie/ITT (100,000 RL) Very old moulded-track design, very quiet for the era – see Fig.44. Clarostat RV6 conductive plastic (now Honeywell) (100,000 RL) Good, 10mm miniature, but expensive – see Fig.42. Has a dynamic noise resistance spec on its datasheet; in this case, ±1% of the track resistance. For the 50kΩ linear version I stock the expected variation would be ±500Ω. Omeg (20,000 RL) Early versions suffered from dissimilar slip ring metal, causing scratching after a year or so. These will have to be stripped out if found in equipment. Later versions with carbon polymer slip ring are much better. AB CTS series (now Blore Edwards) (25,000 RL) Physically strong, used in EMS 4000 series synths and the Leak Delta 70 amplifier. See Fig.39. BI P260 series (1,000,000 RL – this seems optimistic!) High initial jump-on step resistance, good price for a sealed conductive plastic pot. See Fig.40. Bourns 51, 81, 82, 91, 95, 96 series (100,000 RL) My favourite, even the cermet versions (50,000 rotations) are quiet. Used in professional broadcast audio. At around £12 from Farnell (order code 9357858) for a Fig.42. Clarostat RV6, a tiny mil-spec conductive plastic pot with a sealed stainless-steel case. I used these as the tuning control in my Pocket Theremin for many years. Practical Electronics | July | 2023 Clarostat 388 modular series conductive-plastic (100,000 RL) Top notch, used in SSL mixing consoles. (Citec/TE connectivity MC1/U 25,000 RL) Small size 0.5-inch, cermet high 2W rating. Well sealed, very solid. Used in Tornado jets. Similar in style to Plessey/ Citec MH1 25,000 RL moulded-track version 0.25W rated. A bit inconsistent. The newer CLQ version has a rear epoxy end-seal to protect the pins from soldering damage. See Fig.43. Fig.43. Citec/Plessey CLQ moulded track pot. Used in aviation equipment, very expensive, very tough. There is also a cermet MCU/1 version, but it’s too noisy for volume controls. Penny and Giles (>1,000,000 RL) The very best, but stopped production recently. Audio Maintenance Ltd in Manchester still have a few stereo 104mm faders for £90. Piher (10,000 RL) Pretty rough, cheap with some awful plastic models. Plessey E series (25,000 RL, but in reality more like 100,000 RL) Very big, 2W-rated, solid moulded-track Fig.44. Erie moulded track pot, one of the few early ones designed for low rotational noise. I’ve used them in my guitar pedals with no problems. 67 some are log, marked ‘+exp’. I used to use their faders which were physically strong. They were marked ‘RUF’, a rather unfortunate name since they were quite smooth. See Fig.46. Sfernice (100,000 RL) Mainly cermet, but some are conductive plastic. Posh French units, very expensive, intended for instrumentation. Spectrol 149 conductive plastic, small black cube shape (25,000 RL) UK made, solid instrumentation grade. Fig.46. Very small Ruwido pot. Looks like a cermet due to its ceramic track substrate, but it actually has a log carbon film track with a carbon wiper. CE Distribution in Arizona USA have a good selection of pots, mainly guitar related. Just say ‘no’ to pots Fig.45. An old classic industrial mouldedtrack pot, the Plessey E series – they are almost indestructible. and never seem to die. High torque, so they need a big knob, a bit of rotational play in the spindle. RS used to sell these under their own brand as 30mm, advertising them as having low rotational noise. However, given the roughness of the moulding it wasn’t exceptional and had poor resistance tolerance. See Fig.45. Ruwido (20,000 RL) Good quality, small, sealed, sprayed-carbon instrumentation pots, popular with German audio engineers. Unusually, BACK ISSUES Practical Electronics Practical Electronics radios and synthesisers) transconductance amplifiers, such as the LM13600/700 or the better quality CA3280 can be used with a log control pot. Where stepped level control and a bit of ‘zipper noise’ is acceptable, there are digitally controlled switched resistive ladder-network chips available, such as the Dallas DS1802. These can use a rotary encoder, which can be optical, for a very long life. Of course, there’s no scratching. All these solutions are relatively expensive, and much more complex than the humble pot. So, it seems the two main options today are ‘software’ volume controls or the simple, ancient potentiometer approach. One solution to all of the above problems is to simply avoid putting audio through pots and use a VCA (voltage-controlled amplifier) instead. I find the THAT VCA range of chips, such as the 2180xx series have sufficiently low distortion and noise for domestic Hi-Fi use. They also have an inherent logarithmic control characteristic. A stereo or six-channel surround volume control can be fed by a control voltage from a single linear pot smoothed by a capacitor. This increases the life of the pot several times, but for almost infinite life, a hall-effect sensor with a magnet on a shaft could be used. 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