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AUDIO OUT AUDIO OUT L R By Jake Rothman Using Capacitors for HiFi, Part 2 T his series started last month by explaining how suitable different types of capacitor are for audio use. In this second and final instalment, we'll continue that investigation, concentrating on some of the more unusual types of capacitors for HiFi, and give a ranking of all the major types. Non-polarised solid tantalum capacitors exist, but don’t be fooled, they are just two standard tantalum capacitors in one case (see Fig.24). I sometimes solder a bias wire to the case. I have also seen capacitors wired in inverse parallel. Of course, this does not work well with voltages above 1V either way, but it can be useful for blocking very low offset voltages while maintaining a high capacitance. The distortion performance is shown in Fig.26; it's about half that of a single capacitor of the same type. When using back-to-back polarised capacitors, you might expect the capacitance of the pair to be half that of an individual unit, but that is not necessarily so. Depending on the biasing and signal level, one capacitor may be acting like a diode at any time if it is sufficiently reverse-biased. Switching in and out of that mode can introduce extra distortion. This is mainly a problem with series back-to-back polarised capacitors in long time-constant integrators, such as LFOs in sweep generators. One way to avoid this is to provide a centre-bias to the junction of the capacitors. With audio coupling, where there is little voltage drop across the pair, the capacitors will act more like you would expect. The general consensus appears to be if the frequency is above 1Hz and the voltage across the composite bipolar capacitor is kept below about 10% of the rated voltage, there is no capacitance doubling. Bypassing It’s often said by HiFi buffs that bypassing an electrolytic capacitor with Fig.24: a bipolar tantalum capacitor. I was shocked to find it was just two standard polarised units in one case! INPUT 22mF 22mF 10V 10V TANT. TANT. OUTPUT 150W LOAD 470nF 63V POLYESTER Fig.25: bypassing two back-to-back tantalums with a 470nF polyester cap. Total Harmonic Distortion (%) 0.5 0.2 0.1 0.05 0.02 0.01 .005 .002 .001 .0005 .0002 .0001 20 50 100 200 500 1k Frequency (Hz) 5k 2k R1 47kW R2 470W POWER AMP C3 2200mF + 47kW RF CURRENT FEEDBACK C2 100mF 8W R3 0.22W Fig.27: this power amplifier circuit applies feedback around the output electrolytic, linearising it. 48 INPUT 1.5mF V+ – 20k Fig.26: the distortion curve of a reverse parallel network (two 47µF 10V caps). It isn’t as bad as expected, seeing as they were being thrashed at 6V RMS. V+ (74V) INPUT C1 10k 33kW POWER AMP 8W 4700mF + 80V ½ V+ +50V 8W 1000mF + 40V INPUT TRANSISTOR POWER Q1 AMP C INPUT 1mF B 22kW 33kW 10mF 50V 47kW 3.3mF FILM – 3.9kW 330W (SEE AUDIO OUT, MARCH 2018) Fig.28: low distortion can be obtained using a standard 4700µF 80V output capacitor in the amp’s feedback loop. 47kW FILT. E V+ C Q2 1MW 22kW ACFB B E – DCFB CURRENT BOOSTER 1mF FILM 2 20 W Fig.29: G. Nalty’s solution for linearising the output capacitor (Q2 allows the DC feedback resistor value to be high). Practical Electronics | April | 2025 INPUT 100nF 220kW NE5532 47mF TANT. OUTPUT C2 C1 INPUT 10kW 220kW 330nF 1kW LOAD CAUSES HUMP IN RESPONSE <at> 4Hz 100nF PLASTIC FILM 1k W NE5532 OUTPUT 10mF TANT. INPUT OUTPUT C1* * CRITICAL CAPACITORS 220kW HIGH-PASS SALLEN-KEY FILTER 10kW INPUT C1* Fig.30: an op amp follower with its electrolytic output coupling capacitor in the feedback loop. Fig.31: an inverting amplifier trick to bootstrap C1 on a virtual earth to increase its effective value by 10 times, a plastic film type can reduce distortion, as shown in Fig.25. Since capacitor distortion is a low-frequency effect, it doesn’t really work. It does work for high-frequency decoupling, however. When I made some measurements, it did reduce some of the high-order distortion harmonics when gross LF distortion was occurring by about 30%. But if that’s happening in your circuit, it probably isn’t what you’d call HiFi, anyway. well with normal power amps, as shown in Fig.28. Fig.29 shows a technique developed by engineer/PE writer Graham Nalty for single-rail amplifiers with a single input transistor. He used this in his Class One Sound amplifier in Everyday Electronics, January 1989. Op-amp followers (Fig.30) can also have the output capacitor included in the loop. Another trick is to apply NFB to input capacitors on inverting amplifiers, as shown in Fig.31. This allows a smaller capacitor C1 for a given LF roll-off to be used. Calrec used this in their virtualearth mixer amplifiers. The output capacitor, C2, can also be included in the feedback loop to linearise it. The input capacitor is not linearised, but it can be reduced in capacitance enough to be a film capacitor. Care has to be taken to prevent gain rise at subsonic frequencies when applying NFB around capacitors. The circuit should be tested for LF instability with tone bursts and the required load. If the capacitor values are not carefully selected so the frequencies are spaced apart, LF bouncing on tone burst and pulses may occur. Sometimes LF oscillation or ‘motorboating’ can result from the phase shift. Negative feedback LOW-PASS SALLEN-KEY FILTER C2 C2 OUTPUT Fig.32: minimising the voltage drop across C2 allows a cheap polyester capacitor to be used in an active filter. second bootstrapped capacitor (C2) in Sallen-Key active filters was much less sensitive to non-linearity than the first, C1 (Fig.32). This reduces the part cost for high-performance active crossovers. Mechanical construction The classic axial cylinder construction tends to give the lowest distortion, and most high-quality audio capacitors are of this shape. The late Cyril Bateman, a capacitor engineer at Erie (later ITT) found that flattened oval shapes and stacked-foil forms increased distortion. I suspect this is because the mechanical stresses are less evenly distributed, and there may be regions where movement could occur, leading to self-modulation. Foil capacitors are large and expensive. They can be inductive if each lead is attached at only one point to the foil. This was later eliminated with the extended-foil technique, where the whole of the foil on the end was soldered together (see Fig.33). Most plastic film capacitors today are metallised film types, which have higher losses but are smaller and cheaper. They also have the benefit of possessing selfhealing properties (Fig.34). Foil capacitors are designated F in the European (DIN) system, with M for metallised. Some Wima foil capacitors are shown in Fig.35. Capacitor distortions produced within a negative feedback (NFB) loop, such as driver-stage bootstrap capacitors, emitter bypass, and the VBE multiplier bypass in power amps are all reduced by the feedback. Feedback compensation capacitors, such as CDOM (the dominant pole ‘compensation’ capacitor, usually across the VAS transistor), must be linear, though. I once used an X7R ceramic capacitor instead of an NP0 type for CDOM and the high-frequency distortion went up by a factor of four. (By the way, NP0 and C0G are different terms for the same specification). The distortion-reducing powers of NFB can be applied to output capacitors on power amplifiers. The distortion reduction is usually in the order of 10 times (20dB). I discovered this effect when making constant-current power amplifiers, as shown in Fig.27. Including the output electrolytic in the NFB loop reduced the distortion. NFB around the output capacitor works While researching his book The Design of Active Crossovers (2nd ed, Routledge, 2020), Douglas Self observed that small polyester capacitors improve with use. However, the effect was not permanent. He also showed that the Most leaded components now have copper-clad steel leads. This can Fig.33: the clear epoxy end-fill on Wima polyester foil capacitors reveals that the contact is on the extended foil. Fig.34: self-healing of a metallised film. Metal burns away from the defect, isolating it (note how thin it is!). Fig.35: a Wima metal-foil capacitor compared to its smaller metallised version. Practical Electronics | April | 2025 Subtler stuff Capacitor leads 49 Fig.36: a Wima MKB3 metal-cased polycarbonate capacitor. Unfortunately, mechanical resonance ruined its sound. Fig.37: dipping silvered-mica capacitors at Charcroft in epoxy powder. This protects them from moisture etc. Fig.38: the final encapsulated mica capacitors. Other types of capacitor, like ceramics, are protected similarly. Fig.42: a sheet of capacitor-grade Indian ruby mica (Charcroft). Fig.43: a polyester foil capacitor being unwound. Fig.44: an unusual cellulose acetate (MKL) cap, the first plastic film type. cause hysteresis-induced distortion in some high-current situations. It may also increase distortion from the contact inside the capacitor. There is now a market for the older versions of capacitors that used copper leads. Generally, high-quality audio capacitors have pure copper leads; you can check this using a magnet (it will stick to steel but not pure copper). capacitors in their tweeter crossovers so one would dampen the resonance of the other. It's always better in audio to have multiple low-level resonances than a high-amplitude single one. These effects are only audible to skilled listeners with good high-frequency hearing, though. has to be done in a vacuum chamber to remove bubbles (see Fig.40). Acoustic resonance Total Harmonic Distortion (%) A subtle effect in film capacitors is where the capacitor has its own acoustic resonances that are driven by and contaminate the original signal. The effect is usually in the 10-20kHz region and can occur in large plastic-film capacitors used in passive crossover tweeter sections. I had it once with some Wima MKB3 polycarbonate capacitors (Fig.36). You could hear an acoustic metallic “ting” around 7kHz when you tapped the metal case. Apparently, there were two versions of this capacitor; one was epoxy resin filled and one that had free air inside. The epoxy ones were nonresonant and sounded better. Mordaunt Short used to parallel two Encapsulation Most capacitors are dipped in epoxy powder and then heated (see Figs.37 & 38). Higher-quality units are epoxy potted. Fig.39 shows a special potted 80nF mica capacitor made by Charcroft. All capacitors are susceptible to moisture ingress, especially paper and solid tantalum types. It is possible to obtain glass/metal hermetically sealed tantalum types at high cost. However, there are plenty on the military surplus market. Because they are hermetic, they are usually perfect, even if they have been stored since the Cold War. Paper and mica capacitors were traditionally impregnated with silicone wax to improve stability and moisture resistance before encapsulation. This Here's a list of capacitors from best to worst for audio work. • Teflon (PTFE), K=2.1. These types are very rare and expensive. Teflon is still used for trimmer capacitors. • Polystyrene foil (FKS). K=2.56. Easily melted by soldering and difficult to source, but still the best for audio filters such as RIAA equalisers and active crossovers. Typically they have a 1-2.5% tolerance. • NP0/C0G class 1 ceramic, K=30. These are good for HF compensation capacitors for amplifiers. They typically have 2–5% tolerance with a small size, excellent for SMT up to 10nF. Sometimes marked with a black paint band across the top. • Polypropylene foil (FKP), K=2.2. They are good for values up to 47nF. • Metallised polypropylene (MKP; see Fig.41), the standard quality plastic film capacitor type for audio. Note that there are fakes around. I've unwound a 0.05 0.02 0.01 .005 .002 .001 .0005 .0002 .0001 20 50 100 200 500 1k Frequency (Hz) 2k 5k 10k 20k Fig.48: the very low distortion a 100µF 35V bipolar Suntan electrolytic <at> 8V RMS. 50 Audio ranking Fig.49: Nichicon make excellent bipolar electrolytics. This one is suitable for speaker coupling. Practical Electronics | April | 2025 Fig.40: top-quality capacitors are wax impregnated in a heated vacuum chamber to exclude air bubbles. Fig.41: standard audio grade polypropylene capacitors, as used in speaker passive crossovers. Fig.45: a wax-covered paper foil capacitor popular in 1950s valve gear. Fig.46: old metallised paper capacitors. These black Hunts types are unreliable. Fig.47: Philips ceramic plate capacitors are colour-coded by dielectric type. few “audio polypropylene” capacitors and found a standard polyester type inside. You can tell as polypropylene is soft and polyester is hard. • Mica (Fig.42), K=6. A proven natural material for capacitors over 100 years. Very stable, with 0.1% tolerance available, but expensive. • Polycarbonate (MKC). Used in original BBC LS3/5A crossovers. A more stable version of polyester with a higher dielectric strength. K=3, and lower losses above 20kHz. They are no longer made. • Polyester (FKT/Mylar/PET), K=3.2, used for soft drink bottles and the first practical plastic film dielectric. It was developed by German chemists during the Second World War to deal with the capacitor paper shortage. They are smaller than paper foil capacitors by 15%. Fig.43 shows a polyester foil capacitor being unwound. • Metallised polyester (MKT). The general low-cost audio capacitor. 50% smaller than foil types. • Glass, K=5.6. The original dielectric used for Leyden jars. They are radiation-resistant and have a very wide operating temperature range. • Lacquer film/cellulose acetate (MKL), K=4. Like paper, but very small and radiation resistant. Were only made by Siemens (see Fig.45). • Paper in oil. Used by valve enthusiasts. The viscously damped soft paper gives a very low mechanical resonance. • Impregnated paper-foil K=4. Often only dipped in wax, as shown in Fig.45. They absorb moisture, so they can be leaky. • Metallised paper. Avoid the Hunts (Fig.46) and Rifa types like the plague. • Mixed dielectric, made by Dubilier. They look like a blue-and-white chewy sweet. They have the low leakage of polyester with the sound quality of paper. Polyester is the main dielectric between the foils. They were popular in old TVs. • Medium-K ceramic. Unstable, with a large negative temperature coefficient. Philips used to make a whole range of square ceramic plate types with differently coloured tops (see Fig.47). Orange was -150ppm/°C and violet was -750ppm/°C. I used them for temperature compensation in tuned circuits with ferrite inductors. Esoteric electrolytics Fig.50: a pair of Cerafine low-distortion electrolytic capacitors. Practical Electronics | April | 2025 Total Harmonic Distortion (%) Fig.39: or this big ‘special’ 80nF silvered-mica capacitor was potted in epoxy resin. • Wet aluminium bipolar types are the cheapest option for a low-distortion electrolytic (Fig.48). Nichicon's UEP series (Fig.49) capacitors are excellent. • Cerafine by Elna, with a paper separator impregnated with fine ceramic particles. The distortion specification is ﹣120dB at 10kHz & 0.1A. Distortion does increase at low frequencies in the normal way (see Figs.50 & 51). • Slit-foil electrolytics (Fig.72). These capacitors are made by Kemet and designed by Denis Morecroft of DNM. They are the best speaker coupling and main reservoir caps. The foils are slit to reduce circulating eddy currents, lowering distortion (in theory). They were used in Mission Cyrus II amplifiers. There is even a four-terminal version, the T-network capacitor, with internal connections at optimum points on the foil for minimum rectifier spike induction, giving 10dB less HF noise. • Silmic branded audio electrolytics with special silk paper made by Elna. They have lower distortion than Cerafine types. 0.05 0.02 0.01 .005 .002 .001 .0005 Suntan .0002 Cerafine .0001 20 50 100 200 500 1k Frequency (Hz) 2k 5k 10k 20k Fig.51: the distortion of Cerafine versus normal (Suntan) electrolytic caps. 51 Total Harmonic Distortion (%) 0.05 0.02 0.01 .005 .002 .001 .0005 F95 tantalum STC tantalum .0002 F95 back-to-back F95 back-to-back, biased .0001 20 50 100 200 500 1k Frequency (Hz) Fig.52: a Black Gate electrolytic cut open (from Jimmy’s Junkyard). Note the black carbonised winding. Fig.53: distortion plots for F95 (single & back-to-back) vs normal STC tantalums. Fig.57: solid tantalum bead types. They can suffer from moisture ingress. The type on the right seems prone to failure. Fig.58: Oscon solid-aluminium polymer capacitors from the SG audio series. These have an epoxy-sealed base. Fig.59: Panasonic SEPF, Nichicon FP and Würth Elektronik solid polymer electros. These all have rubber bungs. • Black Gate by Rubycon, an audio electrolytic with graphite-impregnated paper separator. The paper winding looks black if you open them up (Fig.52). They had the best distortion, down to -150dB at 10kHz as per the data sheet. They were discontinued in 2006. Audio Note Kaiseis are the new replacement, but their cost is beyond engineering comprehension. • Audio F95 tantalum by AVX (Fig.54). Sadly they are only available in SMD packages, with a quarter the distortion of conventional solid tantalums, as shown in Fig.53. Apparently, the lower distortion is possible due to a thicker dielectric and frameless design. When biased, distortion drops further to 0.001%. These are the electrolytic caps to use for top-quality, high-reliability audio. At £1.92 a pop, they are expensive, but very small. • Metal-cased solid tantalum (Fig.55). These seem to last forever. They are made mainly by Kemet today, eg, the T110, CSR13, Vishay CTS13 and STC TAA series. They are very expensive at over £5 new, but cheap on the surplus market. These were used in Nagra tape recorders and avionics. • Wet tantalums are very rare, and the most expensive, an eye-watering £40 plus. Good for high voltages. Distortion should have been very low, but when I measured the few military surplus samples I had, such as the bipolar 15µF 100V Stantalum in Fig.56, it was worse than a normal solid tantalum. I ran it at 8V RMS into a 600Ω load, whch gave 0.15% distortion at 20Hz. • Tantalum beads (Fig.57). Kemet's and Union Carbide's are excellent. The old ITT/STC red and blue types have poor reliability; their later yellow ones were better. • Polymer tantalums have lower ESR and don't ignite if abused, like manganese oxide tantalums. Their distortion may be lower, but I haven't checked yet. • Oscons, the first solid polymer electrolytics, developed by Sanyo but now made by Panasonic. Distortion is still high, but the ESR and hence decoupling ability was in a new league. They did an audio version shown in Fig.58, the SG, which had copper leads, but the distortion was still slightly higher than solid tantalum. However, their CV product went up to a higher level than tantalum types, allowing more wet electrolytics to be eliminated from high-reliability designs. • Solid aluminium manganese dioxide electrolytic, a Philips invention to get around the high cost of tantalum. They have too much leakage for audio coupling use, and high ESR, although this is beneficial for regulator output damping. They were the most reliable electrolytics, with high reverse voltage ratings, but are now obsolete. • Generic solid polymer electrolytic (Fig.59). Their very low ESR makes them fantastic for decoupling, but can cause regulators to oscillate. To stop this, I've had to insert a resistor in series, but this does allow the ESR to be optimised and defined. Their leakage is four times higher than wet aluminium, so it says “not suitable for coupling” on the datasheet. Also, the distortion is twice as high as tantalums; back-to-back connection and biasing does not seem to help. They easily go short circuit if reverse biased. I've found them to be excellent emitter bypass and bootstrap capacitors in audio amplifiers, though. • Hybrid polymers contain liquid electrolyte but it's impregnated with polymer particles, giving the low ESR of polymer but with the self-healing characteristic of normal electrolytics. They give the best of both types, but can still dry up. I bought a couple of samples of a Kemet hybrid polymer (Fig.60). These are built into the almost hermetically sealed Rifa-style axial case design with a metal end disc. They show great promise as a speaker coupling capacitor, being similar to Black Gate types but at an industrial price, rather than an “audiofool” price. • Niobium oxide; although advertised as “Oxicap audio caps”, these have higher distortion than tantalums. I think it is because the oxide's linear bandgap region is smaller. • X7R multilayer strontium titanate ceramic, K=250. Use for decoupling only. Their distortion is ten times that of tantalum types. • Z5U/Y5V barium titanate ceramic, K=600-1200. Use for decoupling only. Even worse distortion than X7R! • Barrier-layer ceramic class 3, K up to 50,000. Introduced by Erie (Fig.61) in the 1960s for transistor decoupling. They often have very low voltage ratings, eg, 12V. They use a semiconductor effect to give high capacitance with a single layer but can't be made with multiple layers. The distortion is so bad that they are useful for audible demonstrations of capacitor distortion! 52 2k 5k 10k 20k Practical Electronics | April | 2025 Fig.54: the F95 audio tantalum from AVX. The spike indicates the positive terminal. Fig.55: metal-cased hermetic tantalums from the ITT TAA series and Sprague 150D series; these last for decades. Fig.56: a 15µF 100V Stantalum bipolar wet tantalum by STC. I’m not sure what is so special about this type. Fig.60: a Kemet hybrid polymer electrolytic, which has a very long life for wet technology. Fig.61: surface-barrier ultra-high-K ceramics with distortion in the order of 2%. Maybe useful for fuzz boxes! Fig.62: Mullard Mustard capacitors, officially called type C296. A valve amp maker’s dream for the last 65 years! Classic audio capacitors These axial polyester-foil capacitors never go wrong, apart from the leads falling off. They were used in the Philips Electronic Engineer kits and they were the first UK plastic film types, introduced in 1958. They are no longer made, highly collectable and I've seen a fight at a radio rally over them. guitar people. Retro Vibe have reissued the Bumble Bee for £33 (see Fig.63)! The Fender Stratocaster forum showed a picture of a fake Bumble Bee cap (Fig.64), they are meant to be paper foil types. With a $5000 guitar, you want to maintain authenticity. Maybe I should start dipping my Chinese polyesters in paint to make “tropical fish” caps. They would be better than the real thing! Wima The plastic film capacitor was a German invention pioneered by the Wima company, which is still going strong today. Cambridge Audio uses them and they advertise that fact. The MKS2B051001N00JS 10µF 50V type is only 11mm wide and 16mm high with a 5mm lead pitch. However, it costs £3.50. Mullard/Philips This company had a huge following in capacitors. They later reduced production, handing it over to BC Components and Vishay. Their transparent shrink-wrapped blue electrolytics have always looked good, but go leaky if not polarised. “Mustard caps” These are Mullard's most famous capacitors (Fig.62), more like Dijon mustard rather than English. If you see these in an old amplifier or radio, do not replace them. Fig.63: a Sprague “Bumble Bee” cap. This is a Thomann TAD Jupiter reproduction model for guitar restorers. Practical Electronics | April | 2025 Tropical Fish, Mullard/Philips C280 capacitors These are loved by guitar pedal builders and are so pretty they have been used as ear-rings. They are quite poor capacitors with fragile lead attachment, 10-20% tolerance and high distortion. I had an old Solina organ full of these and I got a thrill just looking at the PCBs. They were introduced in 1962 and discontinued in 1984. They became the Vishay MKT368, which have a nice orange epoxy coating, a European version of the US “Orange Drop” made by Sprague. “Black Beauties” and “Bumble Bees” These are more nostalgia fest capacitors from US company Sprague, loved by Mullard/Philips polystyrene One of the most stable capacitors ever made, usually with a 1% tolerance. They are now banned by environmental laws because they used lead foil. This was, of course, their great advantage because the soft lead supporting the hard brittle polystyrene avoided the cracking that occurs when aluminium foil is used. The lead also ensured an excellent soldered connection on the end of the winding in comparison to the difficulties in soldering the wire contact to an aluminium electrode. The original yellow “Mini Poco” version (Fig.65) used copper leads and is highly sought after, along with its Fig.64: a fake “Bumble Bee” capacitor. Once opened, it reveals there was a cheap polyester capacitor inside it. Fig.65: “Mini Poco” polystyrene capacitors. They are top-notch audio filter capacitors if rather large. 53 Fig.66: Philips 444-series polystyrene capacitors. Similar to the “Mini Poco” but with tougher epoxy encapsulation. Fig.67: the final version of Philips’ polystyrene capacitor. The blue cap is their polypropylene replacement. Fig.68: the biggest polystyrene I have seen, Suflex 432nF. The double leads give lower parasitic inductance. Fig.69: current polystyrene capacitors by LCR. Some versions now use tin foil. Fig.70: a TCC paper-in-oil capacitor from the Cold War. Fig.71: a “Vitamin Q” military paper-inoil capacitor (from an old eBay photo). replacement, the 444 series, shown in Fig.66. The newer green version (Fig.67) was smaller and, because it had iron leads was okay with flow soldering. There is now a blue polypropylene replacement, which is almost as good. oil type by TCC Telegraph Condenser Company is shown in Fig.70. The most desirable type is the Sprague Vitamin Q (Fig.71) which, being hermetically sealed, doesn't suffer from electrical leakage. mechanical resonance, around 15dB down compared to normal similar capacitors. The end spray is zinc with a copper sponge matrix soldered on with thick copper leads. KEF use 5.7µF Clarity capacitors in their LX50 reference speakers. One trick top HiFi companies use with specialised audio capacitors is to get their own branding printed on. Fig.73 shows a pair of branded slit-foil reservoir capacitors in a Cyrus amplifier. So there you have it, just part of the mountain of tacit knowledge on the audio aspects of capacitors. I think I PE should write a book! Suflex HS polystyrene These are very stable, low in cost and popular in RIAA networks in HiFi amplifiers. Their weakness is melting while soldering. Suflex were the go-to choice if you wanted an odd custom value. I had an old lady neighbour who used to work at their Welsh plant. A sideline to her main assembly job was to inform MI5 where weird custom capacitors were going! She gave me a few boxes before she died. There are some very odd values in there; 432nF ±1%, anyone (Fig.68)? I'll have to design a circuit especially for them. The world's last polystyrene capacitors (Fig.69) are still made by LCR in Wales, who bought up Suflex's stock of film. They still make the HS series. Charcroft (ICW) Clarity Caps I have to confess a vested interest here since I did some work for the owning company, Charcroft Electronics, who make these, and Salford University, who performed the research. Clarity Caps are axial series-wound metallised polypropylene types with a mixture of polyurethane resin for damping and epoxy resin end-fill (Fig.72). This gives them the lowest Sprague Vitamin Q Paper-in-oil capacitors are highly favoured by valve amplifier makers. Like all wet capacitors, they have very low resonance. An old British paper-in- Fig.72: Clarity Caps ­– highly developed audio polypropylene (Charcroft). 54 Fig.73: special slit-foil electrolytics in a Cyrus amplifier. Note the company’s branding in gold. If you are paying for it, you should show it off! Practical Electronics | April | 2025