Four Pole Electrolytic Capacitor

No matter how fast the amplifiers signal processing circuits are, you cannot utilize that speed if your power supply is too slow to follow rapid signal changes.
The power supply's reservoir capacitor constitutes a vital element in the amplifier chain effecting the signals on the main signal paths as well, because the most power amplifiers reservoir is conceptually placed in series with the loudspeaker line.

The main issue is not only that the capacitor can give you enough charge and quickly enough, but the attenuation of the power supply toward the amplifier. In spite of the fact that the audio band is nominally 20-20 kHz, the stability of the amplifier and the overall sound quality is strongly influenced by reservoir capacitor behaviour at very high frequencies.

The function of the reservoir capacitor is not only to store energy, but also filtering, providing decoupling between the power supply and the amplifiers signal processing circuits. For energy storage a conventionally constructed aluminum electrolytic capacitor with sufficient Capacitance (F) x Voltage (V)/ Volume (cm3) ratio and satisfactory low ESR and inductivity at higher frequencies would be suitable. However the capacitors filtering characteristic plays a very important role in decoupling and suppressing unwanted transients and other i.e. digital high frequency noises.

Extremely low inductance makes excellent high frequency capabilities


The high inductance value of a capacitor is particularly harmful when they are used for filtering at higher frequencies, as the impedance of a high capacitance capacitor over the resonance frequency - which is typically a few tens of kHz - is strongly dominated of the inductance of the capacitor winding.
In consequence of the construction, the conventionally designed, even multitabbed or extended foil electrolytic capacitors provide a significant undesired and increasing impedance vs. frequency response in the critical frequency field. The most acceptable compromise and so far the most utilized solution for this problem was the usage of several high capacitive electrolytics in parallel connection, in order to reach the desired low inductance value.

The best and the ultimate solution is no doubt a four pole electrolytic capacitor having the same Capacitance x Voltage/Volume ratio as the best high capacitive electrolytics with an inductance, which is only the fraction of theirs. Let us take a typical reservoir capacitor size of 15.000 µF 40 V and compare the Z impedance versus frequency characteristic of a traditional and a four pole device. The curve B shows the traditional two pole, and curve A the four pole capacitors frequency versus impedance characteristic. (Figure 1 .)

The two curves run together for a while, but already before the resonance frequency the curve A is falling steeper than B and reaching a minimum value of 0.5-0.8 m in the frequency interval of 10-50 kHz. This value is almost two orders of magnitude lower than the impedance of a traditional two pole electrolytic capacitors. At the resonance point the inductive and the capacitive part of the impedance is zero and only the pure ohmic component (ESR) define the impedance. From this point the capacitor behaves as an inductor and the impedance consisting mainly of the inductive component increases again. It is very easy to realize that not only the equivalent serial resistance (ESR) but also the inductance of a four pole reservoir capacitor is only the fraction of a conventional capacitor in a wide high frequency range. The very low transfer impedance value at higher frequencies make the four pole capacitor applicable also in other high frequency and digital signal handling equipments and circuits as preamplifiers, A/D and D/A converters, switched mode power supplies etc.

The capacitor also works as a filtering/attenuation circuit

The four pole electrolytic capacitors have other additional advantages, which are especially useful for reservoir capacitors in high end amplifiers. If you use a two terminal, two pole capacitor the entire AC and all noises coming from the power supply appear on the capacitor and are sent toward the high frequency signal path, disturbing the load because of the common resistive and inductive parts which force the input and the output together electrically.
( Figure 2 .).

All these disturbances can be avoided by using a four terminal capacitor construction, where the AC on the input terminal is decoupled from the load. The inductances and resistances coming from the lead to foil connections (tab foils) form an advantageous filter circuit which attenuation increases with frequency. ( Figure 3 .)

As a consequence of the damping/suppressive effect of this inherent LR filtering, often a single four pole capacitor can replace a complicated filter arrangement.

Complex construction inside

The filter capability (the measure and the frequency dependence of the attenuation) and the transfer impedance of a capacitor is determined by the inside construction and different geometrical factors of the capacitor winding itself. As these factors depends on the dimension of the can, the length and width of the used capacitor and tab foil, and are different from type to type, the optimization process must be accomplished for every single capacitance/voltage/dimension combination.

The manufacturing of the four pole capacitor is also more complex then the traditional one.

As the yield capacitance value of the electrolytic capacitor foils (anode and cathode) can vary up to 20%, the length of the foil should also be adjusted accordingly to achieve the needed nominal capacitance. On the other hand this adjustment will destroy the hair-fine geometrical balance optimized for best filtering and impedance so you either need to re-optimize the capacitor with the new foil length or use raw materials, which have always the same features. It's a really difficult choice/dilemma both technically and economically.

Application remarks/advices

Due to the special construction of the capacitor the DC load current pass through the capacitor.

Any heat generated by this DC current must be taken into account when the capacitor's operating temperature is calculated.

The mixed DC/AC load can be calculated with the help of the Figure 4 .

The 100% of Idc (See the table in figure 4 ) is coming from the DC resistance and the construction of the capacitor, and mainly depends on the heat dissipation capability of the capacitor. For Idc values of different types and constructions see the table above. For calculation of the full load the Iac values are given as max. admissible ripple current values on the capacitor data chart.

When it is necessary the DC load can be increased by means of a by-pass ferrite core winding according to Figure 5 . (Not recommended for high end audio applications). As a rule of thumb the DC resistance of the by-pass winding must be about the 1/20 of the capacitors DC resistance (measured between the input and output terminals on the same polarity poles) and its AC resistance is about 20 times higher than the capacitors AC resistance (impedance) on the working frequency.

The input and output terminals are marked on the capacitor. It is recommended to use this connection thus the inherent inside filter is optimized for this signal direction.  However reversing the inputs with the outputs will not damage the capacitor and the consequence is only shown as decreased damping/suppression effect and slightly higher transfer impedance.

Frequently asked questions

I would like to replace the traditional two pole capacitors in my power supply with four pole ones. Is it possible and how?

Yes it is possible and neither so difficult. You should make some minor changes in the circuit where the traditional two pole capacitors should be replaced by the four pole ones.

First of all remove the two pole capacitor and mark the polarity on the panel or circuit board.

When you have a two pole capacitor in the circuit, you have also wires leading to positive and the negative pole of the capacitor. These two wires (or in printed circuit copper band leads) should be cut off near to the original capacitor connection. By this way you get two wires for each pole. One, which leads to the circuit and another which leads to the old two pole capacitors connection point. Simply connect the short wire, leading to the positive terminal of the two pole capacitor to the positive output and the other part of this positive wire leading toward the circuit to the positive input of the four pole capacitor. Make the same on the negative side and your four pole capacitor is connected.



What kind of improvement of the sound quality can I expect when I replace the traditional two pole electrolytics with fire pole ones in the power supply of my CD player/amplifier?

The degree and the type of the improvement you will get replacing the two pole electrolytic capacitors (even if they are one of the highly regarded types with graphite particles in the capacitor paper or with ceramic particles in the electrolyte) with the four pole ones of course depends on your equipment, but we guarantee a significant and audible improvement in transparency, high frequency capabilities, detail, dynamics and micro dynamics, immediacy and clarity.

I need large capacitances in my circuit; can I parallel connect the fire pole capacitors to increase the capacitance and at the same time maintain the advantageous filter/suppression effect?

Yes you can, and what is more you can also parallel connect two pole capacitors to the four pole ones and reuse your old two pole capacitor and in the meantime take advantage of the four pole capacitors filter and suppression effect.

When you connect the four pole capacitors in parallel, in the classical way by connecting one capacitor is positive input to the others positive inputs, the positive output to the others positive output and so on, you are not only getting the capacitance multiplied, but at the same time the direct current resistance reduced, as the resistance of the foils also is parallel connected.

It is very advantageous when you need high DC current flow on the capacitor bank.


By parallel connection of the four pole capacitors, their filter/suppression effect is fully maintained. It means that the transfer impedance corresponds to the capacitance value you have achieved by parallel connecting of two or more four pole capacitors.
For example 10.000 µF 40 V four pole capacitors transfer impedance is about 1.3 mohm (check in the table on the datasheet). When two of them are parallel connected the capacitance is the double, about 20.000 µF and the transfer impedance is about 0,9 mohm, almost the same as the transfer impedance of a 22.000 µF 40 V capacitor seen in the table. In case of parallel connection of three pieces of the same type, the transfer impedance decreases to about 0,7 mohm and so on.

The filter/suppression effect is fully maintained but the DC resistance is multiplied when you parallel connect four pole electrolytics by connecting the first capacitors positive output to the seconds positive input and the first's negative output to the seconds negative input and so on if you want to connect more capacitors.


This method has both advantages and disadvantages. The disadvantage is coming from the fact that the DC current must flow trough both the anode and cathode foil of all capacitors you connect in parallel. The DC resistance could be up to a few ohms, when connecting three or more capacitors in parallel in this way. The loss coming from this source together with losses caused by the ripple will be so high that this connecting method could be considered as unacceptable in most applications.

However if your current requirement is low or you definitely need to implement a few ohms of inductance free resistance in the line, this coupling method is your choice. The wiring of this variation is also much more simple.

Parallel connecting of two pole and four pole electrolytics is just straightforward.
The only rule is that the two pole capacitors must be connected to the input terminals of the four pole. The transfer impedance of the system depends only the resulting capacitance and can be calculated as on described above.

No, we do not recommend series connecting of our four pole electrolytics.
The voltage on the series connected electrolytic capacitors is divided in proportion of the capacitance of the capacitors. As the capacitance tolerance of the electrolytic capacitors is generally rather wide (-10/100% to -10/+30%), it could happen that one of the series connected capacitors get a higher voltage, than it was originally calculated to.

The voltage overload reduces the lifetime of the capacitor, make unbalances in the circuit and in worst case might make the capacitor to explode.

There are of course work arounds for the problem as i.e. using capacitors with very similar characteristics or the application of bleeder resistors in parallel connection with the capacitors.

Even if this solutions might work satisfactorily in different power applications as welding machines and laser equipments, they are unacceptable for high end audio applications.