Theory behind VERA Phono Preamplifiers Design
Moving Coil Preamplifier Theory
The design of VERA Phono Preamplifiers is rooted in sound electrical theory.
A moving coil cartridge is an electromagnetic generator and, thus, obeys the laws of electromagnetic induction. Electromagnetic induction is the production of voltage across a conductor situated in a changing magnetic field, or conversely a conductor moving through a stationary magnetic field.
A conventional MC preamplifier is a voltage amplifier with a predetermined (mostly adjustable) input impedance. A cartridge connected to a conventional MC preamplifier sees the input impedance of the preamp as load Rload. The current flowing through Rload generates a voltage drop across Rload which, in turn, is the input voltage fed to the preamplifier and to be amplified.
There are as many opinions about correct cartridge loading as there are cartridge users around. When using a conventional MC-Preamp, it is common practice to select a load in the region of a few tens of ohms up to 47k or 100k. Since the range recommended by cartridge manufacturers is so broad, the task of selecting the correct load is still not easy.
The VERA preamplifier takes a completely different approach. Instead of using a load resistor to create a voltage drop across it, which then is amplified, as is the case in conventional MC preamps, the input stage of the VERA produces a “pseudo short circuit” across the output terminals of the MC cartridge and makes use of the relatively high output current thus produced by the cartridge (see “Basics”). Simultaneously, it avoids the negative overdamping effect a real short circuit would have; in other words, the output terminals of the cartridge are left open.