Most modern audio equipment has “electronically balanced” outputs, while older units might use transformers to feed their outputs.
The use of electronic alternatives to transformers results in a worthwhile reduction of cost and weight, as well as usually improving the audio performance. A possible downside to their use is that the unit’s output(s) are directly connected to the electronics, making it possible to damage the circuitry if unsuitable voltages are applied to the outputs.
In most cases, the electronic output stage uses a number of integrated circuits powered with a dc voltage of typically +15V and -15V relative to the unit’s internal ground connection. The output socket may be connected directly to the output driver circuit or via a low value resistor. Eventide uses special output driver circuits designed for this application which can sustain a short circuit to ground indefinitely without damage.
Under normal operation, the only voltage present at the outputs should be that produced by the output electronics, which will normally feed the input circuit of the following equipment. If for any reason some other voltage is introduced into the circuit, as well as corrupting the audio signals, it may damage the electronics.
In particular a voltage outside the power supply range (+15V to -15V in the example above) will cause the circuitry to operate outside its design range and may cause damage, either by overheating or by breakdown of the internal insulation. Some equipment will have protection diodes connected to the outputs to reduce the risk from applied voltages, but these will be limited in the amount of abuse they can take, and may increase distortion levels.
The following notes apply to most electronic equipment, not just that supplied by Eventide.
There are a number of possible causes for extraneous voltages to appear at the outputs:
This is a voltage (usually 48V DC) that can be supplied by a microphone input circuit to power the circuitry of some microphones. While this is usually of too low a power level to damage output circuitry, it should always be turned off or be otherwise disabled when the input is being fed by anything other than a microphone. The use of an isolating transformer will protect against possible phantom power damage (see below).
In a perfect world, all ground points would have exactly the same electrical potential and any ground point would be the same as any other. In practice, this is of course not the case. Different buildings will have local electrical ground connections, and mains leakage from connected equipment will further change ground potentials.
We can protect ourselves against the mains hum that this introduces by using balanced input and output connections (usually meaning XLR rather than ¼” jack), and disconnecting grounds (shields) from one end of cables where appropriate. Such voltages will then become common mode voltages, since they will be present on both input connections, and will be largely cancelled out by the input circuitry.
Should the ground potential between two pieces of equipment differ by more than about 5V rms (usually as a result of faulty wiring or electrical problems), output damage may result. The use of an isolating transformer will provide some protection against differing earth potentials (see below).
Note that any difference between grounds of more than a fraction of a volt will probably introduce noticeable hum and almost certainly represents an electrically dangerous situation that should be immediately rectified.
A lightning strike can produce very high voltage and current levels in its vicinity, which can be transmitted to any equipment with connections to the area of the strike. Protection against lightning is beyond the perspective of this document and requires specialized professional expertise.
Suffice it to say that lightning is very likely to damage equipment or systems exposed to it – our experience is that this is not as rare as might be expected.
External Transformer Isolation In most cases, audio equipment is located in a recording studio or similar facility, with all connected equipment powered from the same source and connected to the same ground. As long as the phantom power notes above are observed, output circuitry damage is unlikely.
More risky situations arise when equipment is located in different places, as might be the case in a broadcasting environment. In this case, the use of external transformer isolation may be worth considering. The transformer should be located close to the output circuit, and the shield of the cable driving the following input should usually be disconnected at the transformer end. Such a transformer will also isolate any phantom power signals.
A transformer such as the Jensen JT-11-BMCF will provide protection against applied common mode voltages of up to 250V rms without damage to itself or degrading audio performances even at peak levels. Note that Eventide equipment can produce output levels of up to 26dBu – this can overload a small or unsuitable transformer, causing noticeable distortion.
Unfortunately, a high quality transformer capable of handling high signal levels will be neither small, light or cheap – this is why they are not installed as standard.
Created 20 September 2007