Bipolar electrolytic capacitors (audio frequency capacitors)

Bipolar electrolytic capacitors are electrolytic capacitors that are suitable for AC voltage and can therefore also be used directly for the music signal (e.g. in loudspeaker crossovers or as a coupling capacitor). Their decisive feature: They have two anode films or an internal structure that behaves as if two polar capacitors were connected head-to-head to each other. As a result, there is no fixed plus/minus polarity — the capacitor tolerates changing polarity without the oxide layer acting as a dielectric being operated “backwards” during one half-period.

How is a bipolar Elko constructed — and why is it resistant to AC voltage?

In the case of classic (polar) Elko, the capacity is mainly generated by:

• an anode film with an oxide layer, the actual dielectric
• an electrolyte as a conductive countermedium or counterelectrode
• a cathode film as a “counter contact” for contacting the electrolyte

In the case of bipolar Elko, this principle is extended so that there is a “safe” electrode arrangement for both polarities of the applied voltage. In practice, this means that in each half-period, an electrode film takes on the role of anode with a stable oxide layer. This makes the capacitor resistant to AC voltage and can carry music signals.

Important in practice: “Bipolar” does not mean “arbitrary”. Bipolar capacitors also have limits in terms of AC voltage, current load, temperature and frequency — and (due to their design), they show higher losses than film capacitors.

Typical applications in the audio sector

• Speaker crossovers (often with larger capacities, when film capacitors would be very large/expensive)
• Coupling capacitors in simple circuits (when a DC voltage component can reliably be excluded)
• Auxiliary and correction elements (equalization, level adjustment, impedance shaping — depending on the concept)

Sound and measurement relevance: losses and non-idealities

Electrolytic capacitors are not “ideal capacities”, but have additional properties that can be relevant in audio applications:

• Series resistance: leads to level losses and heating, affects filter quality and attenuation
• Frequency-dependent losses: the behavior is not equally “clean” across all frequencies
• Tolerances and aging: Capacity and losses can vary over time/temperature
• Mechanical sensitivity: The coil, electrolyte and connections can react to vibrations (a real effect in crossovers)

This is the reason why bipolar capacitors are often deliberately used in demanding signal paths only where large capacities are required and the circuit forgives its drawbacks — or where the alternative (film technology) becomes disproportionately large.

“Rough” and “smooth” — what does that mean when it comes to audio frequency capacitors?

In the case of bipolar electrolytes (and electrolytes in general), a distinction is often made according to the surface structure of the films:

Roughly produced electrolytic

Rough electrolytes use films whose surfaces have been roughened through an etching process. This significantly increases the effective surface area — and thus you get:

• more capacity per frame size
• large capacity values can often be achieved at low cost

The price is typically:

• higher losses (and therefore greater influence on filter quality/heating)
• tends to be less linearity than with smooth films (depending on design/quality)

Smooth elcos

Smooth electrolytes are made from unetched (smooth) films. They are usually larger for the same capacity value, but often offer:

• lower losses
• a “quieter” electrical behavior over frequency and temperature

As a rule of thumb:

• If the focus is on size/price, the rough version is more likely to be used.
• When losses and signal purity are more important, the smooth design is often the better choice — as long as the size is acceptable.

Selection and interpretation rules (practical)

• Don't choose dielectric strength too tight: Depending on amplifier power and loudspeaker impedance, AC voltages of relevant levels can occur in switches.
• Take temperature and installation situation into account: Heat accelerates aging and increases losses.
• Note the current load (heating): High currents can flow, especially in low-frequency branches.
• Calculate long-term drift: Capacity and losses change over years — this is a real issue when it comes to critical filters.
• When maximum neutrality is required: Test film condenser as an alternative (or mixing concept depending on development goal)