Line-level applications are typically low gain, less than 10-times (+20dB). Quite often buffering (1x, 0B) or a gain of up to 2x (+6dB) or 3x (+10dB) suffices. The circuit shown here can change between buffering and amplification with a simple switch or jumper-bridge.

Its core structure resembles of a diamond buffer as it is discussed in detail under the section buffers/bipolar buffers. To achieve ample current capablity additional booster transistors are connected in a complementary Sziklai configuration. The specialty are the resistors R8 and R11 that are connected in common via a switch or a jumper to either the output or signal-gnd. If connected to the output the resistors R6 and R7 as well as R10 and R11 are connected in parallel and the gain becomes 1. If R8 and R11 are connected to signal-gnd the gain raises to >1. The gain depends on the relationship of the resistors as well as the currents of the input- and output-transistors.

The calculation is further complicated by the fact that the input transistors Q1´s and Q2´s collectors are connected to the emitters of the opposite side output transistors Q3 and Q4, against the more common connection of the collectors to the opposite side supply rails. Their collector currents are routed through the output transistors too. The cross-coupling reduces the distortion level.

For Q1 through to Q4 Dual matched transistors would be good choice, which are available as same types as well as complementary types from NXP (BC847xx/BC857xx and BC807xx/BC817xx ) and other manufacturers.

THAT Corp. manufactures the 300 series which are available as same-gender- or complementary-pair-pairs.