Source Impedence & Matching
Understanding what is meant by source impedence is essential when deciding how to interface various equipment. From an electrical standpoint, optimal interfacing is referred to as 'matching'. Good matching is obtained when the output of a signal source is connected to an input having a suitably high input impedence, such that no degradation to signal level or frequency response occurs. The term impedence may be thought of as AC resistance under signal conditions. In audio equipment, and signal cables in particular, impedence will vary with frequency. With poorly matched equipment, this varying impedence can cause frequency response problems.
Audio Signals
Low-level balanced microphone: This covers signals from all good quality studio and stage microphones. Although the signal level can be very small, from microvolts to millivolts, microphones also have a low source impedence of typically 600ohms or less. In practice, these signals may be sent down very long cables, much longer than would be found in any small studio layout, without degradation, so long as good quality shielded cable is used.
Unlike most bynamic stage microphones, many studio types require a source of power, typically 48V, to be sent to them down their cable. This is known as 'phantom power' and is normally provided by the preamps on your mixing desk or input stage. Care should therefore be taken not to connect a microphone input to any item which does not require phantom power ie. an effects unit. It may be damaged if you supply it with phantom power. From a matching standpoint, microphones should only be used with dedicated microphone inputs.
Unbalanced Instrument Level: Signals from various string instruments such as guitar and bass will fall into this catagory, whereas those from electronic instruments such as keyboards and drum machines fall under the catagory of 'unbalanced line-level'.
Instrument -level signals are particulary fragile, as they are not only very small, but also have a high source impedence. They therefore requre special treatment if they are not to be seriously compromised. If these signals are sent down any appreciable cable length, noise will be introduced and the higher frequencies will be attenuated. This type of signal is best connected to a mixing desk via a DI box, which should balance the signal and reduce it's impedence, allowing it to be used without degradation and with longer cables if necessary.
Balanced Line Level: These signals are found on most, if not all professional audio equipment, including outboard effects units, and tape machines. Due to the level being quite high, typically greater than one volt, and the source impedence being low, often in the order of 10ohms. In modern equipment balanced line-level signals are very robust. They may be sent over long cables without loss of quality. Most well designed balanced outputs will happily tolerate being unbalanced (one pin being connected to ground) to enable connection to unbalanced inputs. The unused pin must always be grounded to obtain the correct output level.
