How to choose the cable for the loudspeakers Opera

 

In principle, all cables should be shielded.  The quality of a cable is determined by:

 

 the velocity of propagation of the signal in it

must be as high as possible (greater than 0.65c) and is in turn determined by the quality of the dielectric, geometry and stranding.

the quality of the conductor

silver

OFC copper (silver or tinned)

aluminum

shielding

 Single, double or triple should be 100% (normal is 95% or 97%)

the dielectric

Polypropylene (PP), polyethylene (PE), Teflon, kevral.

The cables with dielectric PVC and PVDF should not be used.

 

So that the propagation velocity is high, capacity and inductance of the cable must be both very low. It follows that a cable with high speed has a low capacitance and inductance, and then is better. The low impedance cables are "slow" and mainly capacitive. Cords with “privileged direction” or requireing a period of "running" are not suitable for connecting Hi-Fi (and should not be used with products from Unison and Opera).

Wire size is often specified with reference to the American standard AWG. According to this convention the smaller numbers correspond to the larger sections and then an AWG 6 is "better" than an AWG 10. In passing from one AWG number to the next (or previous) section decreases (or increases) of about 26%. Decreasing numbers of 3 units (for example from 13 to AWG 10) doubles the section and the resistance is halved.

A power cable is constituted by two conductors (by convention one “red”  or positive and  one “black” or negative) and then the resistance of the whole cable should be calculated on a double length: 2.5 meters of cable are in fact five meters of conductor. In Italy it is decòared the number and diameter of the conductors: for example 2x2.5 means two wires with 2.5 mm  diameter (equivalent to AWG 10). For the avoidance of doubt always ask if the “size” of the cables refers to the diameter or to the section of the conductor.

Another tip: If a cable costs tens of euros per meter is already expensive enough and then you should expect to know, as well as size and length, the capacity and inductance per meter, the quality of copper (if silver, tinned , etc.), the speed of propagation, the materials used for insulating and dielectric, the percentage of shielding, and any other data. It is intolerable to spend $ 1000 only to find to have bought a simple  13 AWG  with PVC jacket (which may cost 3 euro per meter) which probably only have a beautiful transparent color......

This table shows the resistivity section and according to the American classification AWG cables with the correspondingsection in diameter and metric units.

 

Some cables allow propagation velocity close to 92% of the speed of light (coaxial with special dielectrics). A large section of cable (other parameters being equal) is always preferable to a thinner cable even if more expensive. Avoid wire coated with PVC and PVDF: time release chlorine, which produces CuCl with copper and AgCl with silver wire with 

 

Cables and amplifier damping factor

 

Amplifier

 

Internal amplifier impedance

Camping factor refered to 8 ohm

Valve Amplifier without output transormer

OTL

Da 0.7 a 7 ohm

Less than  2

Amplifier with output transormer

Without feedback

Da 1 a 4 ohm

Less than  8

With feedback

Da 0.2 a 1 ohm

Less than  40

Solid State Amplifier

With feedback

< 0.2 ohm

More than  40

 

In principle, the cable resistance should be negligible respect to the internal impedance of the amplifier. Since, normally, the damping factor is much greater than 1, this ensures that the resistance of the cable is also much higher than the impedance of the speaker. In fact today many solid state amplifiers have a damping factor  greater than 200 refered to 8 ohms. Then simply choose a cable with resistance much less than the minimum impedance of the speakers. For lengths of 2.5 meters and with minimum speaker impedance greater than 3 ohm, an AWG 10 (two-wire, 2.5 mm diameter) is good (R = 0,017 ohms).

However, the do not exceed the following resistivity values:

- 120 milli ohm for 8 ohm nominal impedance speakers
- 60 milli ohm for  4 ohms nominal impedance speakers

With these values ​​the overall response of amp + cable + speaker (whose impedance complies with DIN) will produce a maximum change in the order of 0.1-0.2 dB in the whole frequency respnce of the system while the damping factor will remain at values ​​ranging between 20 and 40 (depending also on the internal impedance of the amplifier).

 

Example of calculation:

suppose you need a 5 meters long cable for a 4 ohm speaker:
60 / (5 x 2) = 60/10 = 6 milli Ohm x meter
the  calculated resistivity corresponds to a value between 10 and 12 AWG: choose an AWG 10 cable.


Suppose you need a 20 meters long cable for a 8 ohm speaker:
120 / (2 x 20) = 120/40 = 3 milli Ohm x meter
the  calculated resistivity corresponds to a value between 8 and 9: choose an  AWG 8 cable.

All Opera loudspeakers are in accordance with DIN 45500 and have minimum impedance grater or equal to  3.2 ohms in module and at least 3 ohms in the real part of the impedance. In general, however, you can find speakers with a minimum impedance well below 1 ohm (Apogee Scintilla, Martin Logan, and others). Minimum values ​​around 2.5 ohms are fairly common. In this case the section of the cable must be increased to achieve a resistivity of at least 50-100 times lower than the minimum impedance value of the speaker. An AWG 6 cable, 2.5 meters long, is suitable, 99.99% of the speakers on the market. A good 6 AWG can cost more than the speakers themselves. For the multi-amplification or bi-wiring we recommend to use cables of the same type, cross section and of equal length.
The rules for calculating the cable size for bi-wiring are the same as for the mono wiring.

 

More ...
 

The cables connecting the speakers should not be "twisted" or too close to power cables and other interconnects. The best solution is to adopt shielded cables for all connections (signal, diffusers and power  supply).

The weak points of the stereo system (in terms of connections)are two:

- The power supplies (which must all be brought to a single power strip with star connection)

- The rear panel of the amplifier to which converge a number of cables that are all very close to each other (so it is best if they are all shilded).

Then there is the quality of the voltage supplied by ENEL (in Italy)... but that's pretty much a matter of luck.