ABSTRACT
It is a truth universally acknowledged, that if you are designing an active crossover, you will need active devices. In this day and age that almost always means opamps. Active crossovers can be built with discrete transistor circuitry if this is desirable for performance benefits or marketing reasons. This is relatively straightforward for Sallen & Key filters that require only a unity-gain buffer or voltage-follower, which can be implemented as some form of emitterfollower. It is a bit more complex for MFB filters and some equaliser circuits, which require a high-open-loop-gain inverting amplifier, and more complex again for configurations such as state-variable filters, which require multiple differential amplifiers. (The first active crossover I designed for production was in fact a discrete-transistor system, solely on the grounds of performance, for the affordable opamps of the time were really not very good.) However, even if the crossover architecture is confined to Sallen & Key filters, matching the distortion performance of the newer opamps is going to be no easy matter. The noise performance of discrete circuitry should be slightly better, but in a typical application the differences will be marginal. Discrete transistor circuitry undoubtedly scores on the issue of headroom, because you can use supply rails that are pretty much as high as you like; the downside is that the rail voltages have to be increased considerably to get a meaningful increase in headroom when it is expressed in dB, and equipment capable of producing very high output voltages can be dangerous to other parts of the system if the output levels are mismanaged. While the study of suitable discrete circuitry for active crossovers would be fascinating, its doubtful utility means that space cannot be given to it here, and this chapter concentrates solely on opamps.
