API 2500 Manual page 2

A BASIC OVERVIEW OF THE API 25OO COMPRESSOR
Thank you for choosing the API 25OO Stereo Bus 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 used three VCA's to minimize noise and distortion found in single and dual
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.
Just to the right of the VU meters, there is a set screw adjustment. This adjustment is to rock or tilt the
signal left or right when in the auto gain mode, allowing subtle corrections in the stereo image if it is off center
when compressing. This control only affects the image when the 25OO is above the threshold, and does not do
anything when the signal is not being compressed. Usually, this control should be in a vertical position.
One tip to get started: We try to design all of our processing modules with the understanding that there is never
enough time to read a manual and learn all of the features in the 10 seconds that is allowed during a setup. If
you simply place all of the knobs at the 12:OO position, the 25OO, and all of the 2OO series modules will have a
useful, but conservative effect on the signal.