Sytek MPX-4Aii 4-channel Microphone Preamplifier - Used  From Sytek

With modern techniques, the MPX-4Aii's transformerless input design rivals and even surpasses the best transformer-input designs that were the standard in the industry earlier. This is especially true in low-level input applications to achieve extremely low noise floor, minimum distortion, wide bandwidth and linear phase response. This approach can be obtained a warm sound, crystal clear and fast response, getting a step closer to the human ear characteristics.

Each MPX-4Aii unit uses a Class A, auto-bias bridge configuration, hybrid input stage preamplifier based on hand-selected transistors for the lowest of noise, and the lowest possible base-spreading resistance and collaterally matched for perfect balance, and biased at the precise levels that make them maximally insensitive to thermal drift. As a result, no offset crowding occurs, and the dynamic range remains wide open. Because of the wide gain-bandwidth parameters of these specially-selected transistors, and the super-low impedance characteristics, and the vanishing circuit reactance, the impulse response is optimum, and phase-delay distortion undetectable. Gain adjustments are made with balanced differential-mode cancellation, preserving the excellent DC offset characteristics, without altering the loop feedback characteristics from their optimal configuration.

Controls

A premium quality gain control is followed by three rugged but nimble pushbutton controls for each of the MUTE, PHASE, and 48V (Phantom Power) functions.
The MUTE control allows the individual preamplifier section to be silently switched out of service.
The PHASE control allows the inversion of signal polarity.
The 48V control switches on phantom power, necessary when using condenser microphones.
Additionally, each preamplifier section has a Peak indicator, set at -8dB before clipping the signal output.