Radial Engineering Twin-Servo 500 Series Preamplifier

Update of a classic. Originally developed in 1988, the Twin-Servo 500 is an adaptation that fits the 500 series format. The design begins with two 100% discrete 990 op-amps cascaded in tandem to deliver 60dB of gain with less than 1/10th the distortion and twice the bandwidth of a single stage design.

This maximizes the gain-margin so no matter what the input, the lowest possible distortion will be realized. This also reduces noise at mid and low levels, a critical advantage for those that record with a wide dynamic range. In order to minimize phase distortion and retain the delicate balance of fundamentals with harmonics, all capacitors have been removed from the audio signal path and replaced by two DC servo feedback circuits. This not only reduces low-frequency noise, but also eliminates the ugly distortions associated with electrolytic capacitors that are especially degrading to low-level microphone signals when used to block phantom power in transformerless designs.

To deliver the widest possible bandwidth, Jensen transformers are placed at both the input and output producing a frequency response that extends from 0.2Hz to 150kHz. The transformers further benefit with unmatched common-mode noise rejection from offending devices such as power supplies, light dimmers and motors that may be connected to the same power service.

Features include a convenient front-panel Neutrik combo connector with choice of XLR and ¼” inputs, a dual-gang potentiometer for level adjustment along with a 10 segment LED meter for signal management. This is supplemented with a 180º polarity reverse switch that toggles the absolute phase, a gentle high-pass filter to eliminate excessive resonance, a -15dB pad to prevent overload, and a 48V phantom power activation switch.