Triband / PWM FM Processor

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Inovonics' Model 255 is an ideal companion for standalone stereo generators or for exciters with built-in stereocoders. It offers slow, "gain-riding" AGC and three bands of compression and limiting. A complement of hidden front-panel controls allows the user to tailor the processor’s sound "signature" over a considerable range.

The 255 is a "spectral loading" processor that helps maintain a consistent balance between program midrange and the frequency extremes. Includes power cable and bound paper manual.


  • Aggressive 3-band processing for contemporary FM music formats

  • Feed-forward pulse-width-modulation program gain control

  • Use it with Inovonics’ Model 708 or other stereo generator



Frequency Response

±0.5dB, 20Hz–20kHz.


Better than 80dB below 100% modulation, 10Hz–20kHz.


(With full AGC correction and “subjective” adjustments centered)

<0.25% THD, 1kHz–20kHz; <0.5% THD, 250Hz–20kHz; <1% THD, 20Hz–20kHz.


Better than 65dB, 10Hz–20kHz.


Active-balanced, bridging; accept nominal line levels between –20dBu and +10dBu.


Active-balanced, 600-ohm resistive source; deliver 0dBm to +15dBm into 600 ohms.

Gated AGC

  • Gating inhibits AGC “hunting” and compression release during brief pauses in the program.

  • The gating threshold is frequency-weighted and fixed at –25dB, relative to the corrected program level.

  • AGC corrects input level variations over a ±12dB range.

  • AGC response to program content is quasi-peak, conforming to the 10ms UK/EBU “PPM” characteristic.

  • Conversely, the slow correction rate is variable between 0.25 and 1.0 dB-per-second.

3-Band Compressor/Limiter

  • Compression and Limiting share common gain control circuits in each of the three frequency bands. The two functions are separated by a time-domain, “floating platform” attack/release characteristic.

  • The spectrum is divided with a LOW/MID crossover at 200Hz (795μs), and a MID/HIGH crossover at 2120Hz (75μs) or 3180Hz (50μs).

  • A high frequency “activity detector” monitors dynamics at the upper program frequencies to determine an optimum limiting/clipping ratio.

Power Requirement

105–130VAC or 210–255VAC, 50/60Hz; 12W.

Size and Weight

31/2”H x 19”W x 7”D (2U); 10 lbs.