LTCC, (Low Tension Capacitor Coupling)

One of the most interesting phenomena that has been discovered and researched by Q-tron is the distortion caused by the negative effect of high voltage gradients over coupling capacitors.

It has long been known that capacitors used as coupling element can have a large effect on the sound quality in tube amplifiers. This phenomena has so far not been fully understood but it is well known that different type of capacitors have different sound characteristics.

We have concluded that capacitors influence on sound is mostly due to the negative effect of high voltage gradients. We have also been able to eliminate this distortion by our amplifier design.

Technical explanation

The electrical properties of the capacitor depend on the structure of the material used as dielectric. If the molecule is not symmetrical it will have a dipole moment giving increased dielectric constant. On the other hand, the dielectric constant and tan(d) are then dependent on frequency. For instance ceramic materials used in ceramic capacitors have very high dipole moment giving them high capacity with small dimensions but other characteristics are not very good.

If a capacitor is built using a material with high dipole moment it will create noticeable distortion if a DC voltage is applied. This is because of the unsymmetry caused by the dipole moment. The amount of distortion is not very high but is most severe at low AC voltages, (like in preamplifier circuits). Note that this phenomena exist in all capacitors to a certain extent as all dielectric's have a non zero polar moment, there are however large variations between different type of capacitors.

It is well known that paper in oil capacitors have very good characteristics at high voltage gradients this is because the oil used for impregnating the paper normally have low polar moment, (note! not all paper capacitors use this kind of oil, some are also built using polar type oil in order to shrink the dimensions as much as possible).

Paper in oil capacitors have good characteristics in some respect but not in others, for instance so are high frequency characteristics really bad compared to some plastic capacitors.

In a normal anode coupled amplifier stage the coupling capacitor can be subjected to a voltage gradient of several 100 volts. This high voltage will build up a static field inside the capacitor and will severely load the capacitor and cause excessive distortion.

At Q-tron we have made thorough research regarding this phenomena. We have investigated most type of capacitors that exist on the market using several test circuits.

We discovered that all type of capacitors, (although to different extent) give higher distortion with DC voltage applied. The level of distortion is also dependant on the applied DC voltage.

We have therefore concluded that it is important to reduce the voltage gradient as much as possible in order to reduce distortion. It is also therefore important to choose capacitors depending on the application, (low or high signal levels, high or low voltage gradient).

As a summary here are some important factors to consider when designing circuits and choosing capacitors:

Polar moment, high polar moment give higher distortion with applied DC
Low DC gradient, If the applied DC voltage is low the distortion caused by the polar moment is eliminated
Low loss factor, tan(d), low loss factor is important in order to give good performance in the audio frequency band.

We have found that the best capacitors overall are made using polypropylene as dielectric, but important to note is that there are different grades of polypropylene, some are purer than others.

At Q-tron we don't use paper in oil capacitors, although these also have low polar moment they have high frequency dependant losses that will affect the sound of the circuit they are used in. We don't believe in using passive circuit elements to influence the sound of an amplifier. If sound with an amplifier is improved by for instance changing capacitors to paper in oil type it shows that there is a serious fault in the active circuit itself, and we believe this is better cured by correcting the faulty circuit design than to compensate the fault by using non ideal components.