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Preamps

PreAmps

Schoeps Circuit
Schoeps is a professional microphone manufacturer with a long history of innovation. In 1964, their CMT-20 was the first mic to use "phantom power". It remotely supply 8.5VDC to the mic preamp and a local oscillator circuit; which, in turn, provided the necessary high-voltage bias for the capsule. This was revolutionary to say the least. Since then, many have used the basic concepts when building remote, phantom-powered condenser mics; including the Neumann U67-FET in 1967 and recent LCMs like the Studio Projects C1. The basic design concept has been widely published, so there are no secrets here, though the microphone manufacturers all have their own unique variations. So, what is this "Schoeps"-type circuit and how does it work?

Below is a basic block diagram showing the functional concepts but without the circuit details. Essentially:
  • The 48VDC phantom power is separated to supply power to the preamp circuits
  • An oscillator, running at a frequency far above the audio range, generates an AC signal (which is multiplied by about 4x using a diode-capacitor chain) which provides the zero-current high voltage bias for the capsule
  • The FET provides impedance transformation, and a little gain, to the capsule signal
  • A bipolar transistor amplifier then drives the mic signal at much lower impedance down the XLR cable



Phantom Power
The XLR connector has 3 wires; [1] ground and [2]&[3] balanced signal + phantom power. Today, +48V is supplied to both [2] and [3] and the circuit taps this voltage to power the preamp. The signal from the preamp is coupled back on the same two [2]&[3] wires in "push-pull" fashion. This means they are out of phase with each other by 180 deg in a balanced 600 ohm configuration which gives good noise immunity when using long cables. One very clever trick is that the preamp voltage is actually extracted directly from the "common collectors" of the preamp output stage transistors as shown below; so it has a  nice low impedance. The phantom bias point also include zener diode and capacitor stabilization.

Push-Pull Stage
As mentioned above, what is interesting about the push-pull output amp, which drives the XLR cable, is its dual function. The balanced FET output drives the push-pull transistor bases (center biased in balance) which amplify that current to drive the XLR line at low impedance. Since the transistor collectors are tied together, a common, low-impedance voltage point is created: a) which is centered relative to the audio signal and b) which provides a low-impedance supply node for phantom power. The audio signal thus drives the balanced line while the phantom DC voltage remains stable and filtered at the collector common node. One zener diode keeps the common node biased and stable above ground and the pair of emitter-to-collector zeners maintain balanced and symmetrical emitter DC voltages. This elegant solution as shown below.




FET Preamp
The capsule is constructed as two isolated "plates" (typically a gold-plated 3-6 micron thick membrane held very close to a plate) which form a capacitor with about 80pF of capacitance. This is where the LCM gets its name; "condenser" is the old name for a capacitor. Since a capacitor is just some space between two conductors, there isn't direct contact -- so zero DC resistance. When the membrane moves, rapidly from sound waves, the change can be detected but the signal is very weak. In fact, in electrical terminology, there is a voltage change but essentially no current (very, very high impedance). This is where the FET (an N-channel FET usually) is so useful. Like the grid of a tube, the gate of a FET places almost zero "loading" on this ultra weak signal from the capsule and the FET circuit amplifies this signal. This FET design performs several tasks:
  • It lowers the capsule impedance by about 1000x
  • It provides gain (amplification) without adding much noise
  • It requires almost no supply current (unlike a tube)
  • For these small signals, it provides a balanced output to drive the output stage which drives the XLR cable.
Capsule Bias
A condenser mic capsule's bias is typically 50-100 VDC but since there is no DC current (no DC conduction) the power source just supplies a voltage at essentially zero current. These days, small modules can do this, but historically mics just used discrete parts. A transistor oscillator, running at a frequency much above the audio range, feeds a diode multiplier, along with some DC filtering, to generate the ~50 VDC required.

In the circuit below, the capsule is shown as a capacitor which is grounded with a very high resistance (like 1G ohm). Acoustic pressure on the capsule diaphragm causes a capacitance change, at audio frequencies, which thus provides a changing impedance relative to the 1G ohm bias resistor; essentially an AC voltage divider. This very weak signal is capacitively coupled to the gate of the N-Ch FET.