What is gain reduction on a compressor

Summing compressor

Stereo compressor

In addition to the standard controls for Threshold, Attack, Release and Ratio, the stereo compressor is equipped with a number of additional options for compensating for side effects and interference from compression, which considerably expand its range of application.


The crest switching adapts the AC-DC converter and enables the choice of a very fast peak value rectification or an RMS rectification with a total of 4 intermediate stages. This selection determines the basic characteristics of the compression. Depending on the crest setting, the summing compressor works like an aggressive FET compressor or a soft opto compressor.


The envelope function influences the behavior of the compressor when processing bass tones and enables less distortion with short release times without pumping. The potentiometer determines the starting frequency in the range up to 120 Hz.


Fill enables parallel compression directly in the device. The addition of the uncompressed signal effectively hides disturbances in the signal structure, which occur as an unpleasant side effect with hard compression.


LT-Int is a steplessly adjustable auto-release over a wide range. Depending on the position of the controller, the release time is controlled by the density of the signal and changed to a greater or lesser extent. If the setting is adjusted, this increases the naturalness without sacrificing loudness.

Threshold, attack, release and ratio
The control ranges for Threshold, Attack, Release and Ratio are more than sufficient for setting different modes of operation of the summing compressor. From hard, aggressively audible compression to soft compression, anything is possible. A switchable side chain insert and a switchable autogain function for constant output level without readjustment are also available, as well as an additional manual gain control and an LED chain to display the gain reduction with 10 light-emitting diodes. The stereo compressor can be switched on and off separately from the limiter.

Stereo limiter

The Stereo Limiter is designed as an extremely fast peak limiter for the inaudible control of signal peaks in the range of a few dB and optimized for use in the Mixbus.

The attack time is sufficiently short to turn down within the first oscillation of a 20 kHz signal. The attack and release times can be adjusted over a wide range. The threshold can be adjusted between - 10 dB and + 20 dB. The 0 dB mark on the scale is calibrated internally. The Vernier control enables fine adjustment of the threshold with a range of ± 2 dB. The gain control is used to adjust the limited output signal with a range of ± 10 dB. An LED chain with 5 light-emitting diodes and a 10 dB display area is used to display the gain reduction.

LF attack

With the short attack times of less than 0.1 ms required for effective limitation in front of an AD converter, there is a risk of pick-up distortion in the bass range. Such distortions are caused by the hard cut in the signal curve. These effects disappear with an attack time in the range of 1 ms or more. Then, however, the attack time is long enough for peaks in the mids and highs to overdrive the converter. The LF-Attack control acts as a 'softener' and reduces attack distortion in the bass range.

Dynamic release
The dynamic release time modulation works in a similar way to the LT-Int function in the stereo compressor. With dense signals, the release time is extended depending on the control position. With an adapted setting, the problems with physically caused bass distortion with short, pump-free release times are reduced.

Stereo peak meter

The stereo peak meter in the summation compressor is a completely separate assembly that can either display the level of the output signal or an external stereo signal. The selection is made using the INP2 switch on the front panel.

The version in the illustration has a display range of -40 to +15 dB. Other scale plates with the following display ranges are available:

The level for the 0 dB display is adjusted according to the customer's specifications. The factory default setting is +6 dBu. The setting range is sufficient for all common converter levels as well as for 'old' analog devices.

In relation to the -40 to +15 dB scale, the resolution is 1 dB from +15 to -10 dB, 2 dB to -30 dB and 2.5 dB to -40 dB. can be displayed. The resolution of the peak meter with 40 LEDs is 1 dB in the range of - 10 dB to + 15 dB.

The versions FAST = 60 µsec, PCM = 1 ms and IRT = 10 ms can be selected for the settling time. The return time can be set between 0.5 and 2 sec / 10 dB. The return is linearized in the lower area.

The inputs for external stereo sources are electronically balanced and have an input resistance of more than 10 kΩ.


All devices in the Tool series have a headroom of + 30 dBu in the frequency range from 10 Hz to above 50 kHz. The frequency responses in the rasterized 0 dB positions are adjusted to a deviation of less than 0.2 dB. The harmonic distortion in control amplifiers depends on the gain reduction, the settings of attack and release and the frequency. For physical reasons, the distortion factor increases at lower frequencies, higher gain reduction and shorter attack and release times. With inactive control of the compressor and limiter, the distortion factor remains up to a level of 1 dB below the overload limit of 0.1%. The dynamic range is greater than 120 dB based on the 0 dB setting. The overall slew rate is greater than 10 V / µs.

further 1U devices:
Master equalizerMultiband EQ type 1Multiband EQ type 2