A preamplifier (or preamp) is an electronic amplifier that prepares a small signal for further amplification or processing. A preamplifier includes active devices such as transistors, vacuum tubes, or op-amps, which perform the initial amplification of a weak input signal.[1]
The first stage of a typical audio amplifier is a differential input stage, which amplifies the difference between two input voltages but suppresses any voltage common to both inputs. This stage is usually followed by one or more single-ended stages, each using only one active device per channel to provide additional voltage gain. The final stage in the audio amplifier chain is the output stage, whose purpose is to drive the load impedance with as much current as required while ensuring that the average voltage swing remains within the power supply rails.
In many cases, particularly in professional audio applications, it is desirable to use a balanced line level signal throughout the system from microphone through to loudspeakers. To achieve this, it is necessary to insert an extra stage into the amplifier chain between the differential input stage and the single-ended gain stages. This extra stage is known as a balun (balance->unbalance) transformer and its function is to convert the balanced input signal into an unbalanced output signal suitable for driving single-ended gain stages.
However, there are some circumstances where it may not be possible or practical to use a balun transformer; for example, when interfacing with equipment that only has unbalanced inputs and outputs. In these cases it may be necessary to connect the balanced output of one piece of equipment directly to the unbalanced input of another piece of equipment; this type of connection is known as a high-impedance bypass (HT bypass) connection.
The most important consideration when making an HT bypass connection is that both pieces of equipment must have their ground connections connected together; if they are not then there will be a risk of ground loops occurring which can introduce hum and other noise into the system. Once both ground connections have been made then it simply becomes a case of connecting the positive terminals of each piece of equipment together and connecting the negative terminals together; no special wiring configuration is required.