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Low impedance, matched speaker cables represent the ideal path from the amplifier to a loudspeaker but in rare cases may cause small percentage of solid state amplifiers to become unstable when combined with certain speakers. The symptom is audible oscillation or overload shutdown. Fortunately the problem is easily solved, by installing an Alpha-Core RC link across the terminals of each speaker.
The advantage of the low impedance, matched path is that the amplifier is rendered able to exert control and damping of voice coil resonance, and all frequency bands are transferred with the same interrelated magnitude irrespective of cable length. This also means that the cables to the left and right channels of a stereo system can be of unequal length if desired, largely without ill effects.
Read on for full technical explanation:
The impedance of voice coils in dynamic loudspeakers and their crossover networks rises with frequency, causing a corresponding decrease in audio output at high frequencies.
At low audio frequencies a voice coil behaves almost like a purely resistive load but at for example 20 kHz its impedance may rise to more than 10 times the DC resistance. This is mainly due to self-inductance, which causes the almost purely inductive impedance to rise even further above 20 kHz.
Unfortunately the feedback loop in certain solid state amplifiers does not satisfy the Nyquist or Bode criterion for stability. The amplifier may at times exhibit gain band widths in the internal feedback loop in the megaHertz order, meaning that things may start happening above the audible range. The problem arises when this type of amplifier is connected with loudspeakers that exhibit high impedance at high frequencies, via an otherwise ideal low impedance matched cable. Under certain circumstances, instead of stabilizing the gain via the internal feedback loop as intended for a low impedance output, the amplifier is turned into a HF power oscillator operating at a frequency well above the audible range. This may load the amplifier excessively, resulting in higher than normal operating temperature or overload shutdown.
Our remedy is to place a resistive load across the speaker terminals, which becomes effective at frequencies well above the audible range. Tests have shown that a 0.1 microfarad capacitor in series with a 10 ohm resistor will do the trick if applied across the speaker terminals. The combination RC Link also called a Zobel network, is supplied free of charge by Alpha-Core to be applied when needed.
A simple calculation shows the impedance of the RC link to be 14 ohms at 159kHz, amply illustrating that they in no way affect performance at audio frequencies.
Incidentally, the situation described in the above does not occur with electrostatic speakers or with tube amplifiers containing output transformers. |
