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ELECTRIC GUITARS AND BASSES
Pickups in electric Guitars and Basses are mostly designed to drive a very high impedance input. (≥250Khz). If the impedance the pickups sees is too low, upper frequency response and dynamics will suffer... The preamp section of tube guitar amps feature very high input impedances allowing a guitar pickup to drive them easily... (which is one of the many reasons tube guitar amps sound so good!) Solid-state guitar preamps don’t always have such a high impedance and often sound a little dull and lifeless. (at least compared to a tube preamp) In addition the inputs to most consumer or pro audio equipment (ie... consoles, eq’s, comp/limiters etc..) have even lower input impedances. Usually on the order of 10kΩ to 20kΩ. In reality, this is not much greater than the actual impedance of the pickup itself, which average around 8kΩ... (give or take) So directly plugging a high impedance instrument (guitar or bass) into the line or mic input of a vintage Neve module will basically sound like crap...
Passive DI’s or direct inject boxes are commonly used to overcome this issue of interfacing a guitar or amp directly to a piece of pro audio gear... Initially they were just step down transformer with a high (but not always high enough) input impedance and a low(er) output impedance. By virtue of being a step-down transformer the level of the instrument was also attenuated some amount, making it compatible with the high-gain, low-impedance inputs of microphone preamps... (usually between 150Ω to 1,200Ω)…
JFET’s, when properly used, can be wonderful devices and they feature incredibly high input impedance. Active DI’s using a JFET for their input can rival or exceed tubes in this regard... they also (usually) feature very low output Z and can easily drive loads of 600Ω and above... These DI’s also lower the output level to be compatible with microphone preamp inputs
TROUBLE IN PARADISE?
In both cases above, (using DI’s) the attenuated signal is boosted back to a usable level by the mic pre. While acceptable in many instances this gain make-up is less than optimum. By adding gain, distortion and noise is also added to the signal... granted sometimes a necessary evil, but evil none the less...
discrete class-a jfet buffer