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Musical Micas - RS Pro Mica Capacitors are ideal for audio applications

New RS Pro Mica Capacitors

The new RS Pro Mica introduction consists of three radial case sizes, CA1D 10mm, CA2D 20mm and CA3D 40mm. The capacitance range is 1pF to 200nF with the case changes occurring above 10nF and 50nF. The photo below shows the 510pF value.

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The benefits of Mica

Polystyrene capacitors were once the usual choice in precision audio applications, such as filters and RIAA amplifiers, but more recently these have become more difficult to source due to a shortage of film and a low melting point incompatible with modern soldering techniques.

Mica has the best stability of all dielectrics, with a temperature coefficient of +35 ppm/⁰C, as opposed to polystyrenes -150 ppm/⁰C. This is achieved by sintering of the silvered mica sheets at high temperature (560⁰C) with silver paste applied to reinforce the edges. Vacuum impregnation then takes place with Napelec C, a synthetic polybutene wax specially developed for high-voltage paper insulated cables. This makes a solid non-resonant structure shown in the below photo. Some inferior mica capacitors have a construction using steel clamps. A magnet and their bulbous ends usually give the game away. 

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Vacuum impregnation with Napelec C

Having been in production for over 100 years, mica capacitors have proven long-term stability. There is a long-term stress cracking problem that can affect polystyrene capacitors. The stacked extended-foil construction gives very low losses and excellent high-frequency characteristics. Polystyrene capacitors generally only have wound foils.

1% tolerance (±0.5pF below 10pF) is supplied as standard. They can be trimmed to very tight tolerances of up to 0.1%, matching the metal-film and tantalum-foil resistors used in high-quality audio equipment. Only mica has this capability. Mica is also the preferred dielectric for capacitor calibration standards and is used by the National Physics Laboratory.

Interestingly, mica is a naturally occurring silicate crystal that readily cleaves into very thin sheets. As with all natural products, variability can be high and careful selection is needed. RS Pro uses ruby mica plates sourced from India, as pictured below.

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Ruby mica plates sourced from India

Precision Manufacturing

The Mica capacitors are then assembled and individually trimmed by hand in the UK. This is still the most effective production method for precision specialist components. This also gives the flexibility to produce any value to special order. The testing is done using NPL traceable calibrated Agilent/HP test gear.

The new RS Pro mica capacitors use thick solid-silver wire attached by silver-loaded solder which means the whole conducting part of the assembly is effectively silver, eliminating any electro-potential barriers which could cause distortion. Standard copper-clad steel wires are not used since they can introduce third-harmonic distortion due to magnetic effects. The standard lead length supplied is over an inch long (25mm), or longer if needed, to facilitate the hand assembly employed in top-end audio equipment, such as hard-wired valve amplifiers. The 500v rating is also useful in this application.

Finally, the capacitors are sealed by dipping in a blue epoxy powder coat, cured at a 125⁰C and marked with the value and soon to be familiar CEL logo. They will perform and look the part in any high-end audio design.

WANT TO FIND OUT MORE? Click here to see the new range of RS Pro Mica Capacitors

Innovative product and technology marketeer, enthusiastic writer and translator, and author of the ultimately underwhelming blog "Oneday711": technology, travel, sport. Particularly interested in the impact innovation has on our lives - good and bad.
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