Good article explaining phono stages. I’m writing the next one about the audio business!!
Imagine the tiny signal coming out of a moving coil phono cartridge: 30,000 to 50,000 times smaller than what comes out of your preamplifier. It has come a very long way and through much amplification to become that much bigger. Like a weary long-distance traveler who struggles to stay intact along the way, its journey is a difficult one with detours, traps, pitfalls and dangers aplenty. But sometimes a designer is so adept at navigating the perilous journey that what comes out the other end shines.
The trick to making a successful phono preamplifier that honors and preserves this tiny signal is two-fold: make sure the vanishingly low output from the cartridge is perfectly preserved, then keep the progressively louder signals away from any limiting agents.
The first challenge is often the most difficult. Interfacing with and amplifying without adding noise can often be more art than textbook solvable. For example, most phono preamplifiers use bipolar transistors to interface with the cartridge because they are low noise and easy to work with. Unfortunately, bipolars and their non-linear diode inputs are not the best at working with tiny signals like those coming from moving coil cartridges. A pure voltage device, like a FET or a tube, is a better choice, yet getting these devices to be low noise can be a challenge. In the new Stellar Phono, engineer Darren Myers solved this problem by lining up multiple FETs in parallel. Each added device reduces noise. The greater the number, the lower the noise.
If one manages to get the tiny signal through the input it’s off to the races, though not without challenge. As the signal grows exponentially larger, it begins to approach the power supply limitations of the circuit. When this happens, linearity suffers—which is why Darren uses high voltage throughout Stellar.
It’s often tempting to just meet spec and call it a day, but then you probably won’t get Stereophile’s Michael Fremer to write: “The midrange on this phono preamp is as open, uncongested, transparent, and revealing as that of any phono preamp I’ve heard at any price.”
It’s a rare treat when a designer maps with care the perilous journey of a signal, then clears a free path for it to arrive at the other end unscathed.