Introduction; Features - API 2500+ Operator's Manual

1.0 Introduction

Thank you for choosing the API 2500+ Stereo Bus Compressor. The 2500+ Bus Compressor is API's
flagship compressor that provides the unmistakable sound of API's compression technology combined
with unmatched parametric control. The result is one of the world's best sounding and versatile stereo
bus compressors that's capable of handling an infinite number of production applications. Carefully
designed to deliver a wide range of compression options, the 2500+ delivers warmth, clarity and
punch, whether used for subtle adjustments or heavy compression effects.

1.1 Features

• "Over Threshold" LED illuminates when input audio crosses the set threshold
• Patented
• 3 selectable compressor curve KNEE settings
• Selectable NEW or OLD type switch for feed-forward or feed-back operation
• Preset or variable release time control
• Variable Left/Right link with peak and RMS control voltage filters
• Selectable automatic or manually variable make-up gain
• Variable input-output blending for onboard crossfaded or parallel compression
• Audio circuit uses 2510 and 2520 discrete op amps with transformer output
• Stereo VU meters show selectable input, output, or gain reduction levels
• Full hard-wired relay bypass
The 2500+ has several unique features that will allow you to tailor the sound of the compression in
many different directions.
THRUST
The patented
consoles, as well as the Pro-6 Input Strip. This circuit places a filter in front of the RMS detector, with
a slope of 10dB per decade, which is the inverse of the pink noise energy curve. In acoustics, the
pink noise curve is used to equalize energy vs. frequency over the audio spectrum, as sound requires
more low frequency energy than high frequency energy to sound correct to your ear.
equipment, a "LOUDNESS" contour is used to equalize the music at lower levels so it sounds correct.
Even with this curve, there is still a substantial amount of low frequency information compared to high
frequency information in the audio signal path. When that signal is fed into an RMS detector, the
detector will process the signal into a DC control voltage based on those louder low frequencies,
resulting in a control voltage that favors the low frequencies of the signal, causing pumping and a loss
of punch. Sometimes, this is not desirable. By switching the
LOUD positions, this inverse filter is placed in front of the RMS detector, evening out the energy by
lowering the energy in the low frequencies and increasing the energy in the high frequencies, so each
octave has the same energy instead of each octave having half the energy as the one lower. This
creates a unique compression effect that still reduces the overall gain, but the sound is much more
punchy and the signal actually sounds much less compressed.
The L/R LINK feature is also unique to the API 2500+. First, there is a variable link control, ranging
from IND (independent) to 50% through 100% in six steps. Variable linking allows combining of the
Left/Right control voltages over a range, minimizing the interaction between channels, while still
linking them to retain the stereo image. While engaged, the link control has selectable detection
speeds, FAST (peak) and SLOW (RMS). The 2500+ has two complete, identical compressor circuits,
from the input through the RMS detector, the VCA's and to the output section.
2500+ Bus Compressor
THRUST
® filter with 3 settings for frequency dependent side chain control
® feature has been used for many years in the famed ATI Paragon and Paragon II
THRUST
®
6
In Hi-Fi
button in either the MED or
API