Legacy AXS API
Appendix
API 2500C Stereo Bus Compressor
A BASIC OVERVIEW OF THE API 25OO COMPRESSOR
The 25OO has several unique features that will allow you to tailor the sound of the compression
in many different directions. There are two features that can only be found on this unit, as they are
either patented or patent-pending.
The patented "THRUST" feature has been used for many years in the famed ATI Paragon and
Paragon II 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. In
Hi-Fi 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 THRUST button in either the MED or 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 LINK feature is also unique to the 25OO. First, there is a variable link control, ranging from
IND (independent) to 50% through 100%. The variable linking allows combining of the left and 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 a selectable HI-Pass, LOW-Pass and BAND
Pass filter that can be inserted, but only into the LINK circuit. This feature can reducing peaks from
cross- linking, reduce low frequencies from cross-linking or both. The value of this feature is shown
when the signal contains a large amount of percussive instruments, spread around the stereo field.
When all other compressors are linked, any peak in the left side will result in a gain reduction in the
right side that will shift the stereo image, resulting in a less than desirable effect. By inserting the high
or low pass filters, you can eliminate this undesired image shift while still linking the preferred frequency
range.
The 25OO has two complete, identical compressor circuits, from the input through the RMS
detector, the VCA's and to the Output section. When many compressors are linked to the "STEREO
LINK" mode, only one channel is controlling the compression. When the 25OO is linked, the control
voltages are summed together, but both channels are still detecting their own control voltages. This
also eliminates changing of the tone of the compressor when linking is used. Many popular units actually
change when linked!
Additionally, each channel uses multiple VCA's to minimize noise and distortion found in single
VCA compressors. The signal path from the input to the output is ALL DISCRETE, using the API 2510
and the API 2520.
The GAIN pot can be either switched IN for manual gain control, or it can be left OUT and the
25OO will bring the output level up and down automatically, keeping the signal at the same level
regardless of where the THRESH or RATIO controls are set. This is the same as the API 525 Ceiling
control.
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