Phantom-powered compressor/limiter

.	The AMTEC PHANZEN is a unique compressor that has been designed to be used in conjunction with any standard microphone preamplifier. The purpose of the preamplifier is to restore the gain lost in the PHANZEN and, furthermore, to power the PHANZEN through phantom-power. The PHANZEN is a dual channel unit, where each channel is separate, but the control voltages of the both channels can be linked together by the LINK switch for stereo operation (the gain reduction in both channels will be the same, so stereo image will not shift to any side during compression) or for special effects like ducking. The input of the PHANZEN should be driven by a line-level signal, the output is at micprophone level that is amplified by the mic preamp back to the line-level. The preamp choice has great influence on the overall sound, so it allows the user to customize the combo's sonic character by selecting of the preamplifier paired with the PHANZEN. The gain of the preamplier should be set somewhere between +30dB and +40dB most of the time. The compression circuit uses a diode bridge to accomplish gain reduction. Although this topology is known for decades, it is relatively seldom used in modern devices. It delivers distinctive sonic character with subtle and pleasant distortion. The signal path is as simple as possible and consists of an input transformer driven by a constant-impedance switched pad network controlled by the INPUT control. Then the signal goes through the diode bridge attenuator, which controls the attenuation and shapes the frequency response and leaves the PHANZEN via output transformer which transformates impedances to the input of the following mic preamp and rejects the common-mode thump caused by variations in the control voltage. The signal path does not consist of any amplifier stages nor buffers. The current needed to control the attenuation of the diode bridge is derived from the sidechain circuit. The input of the side-chain circuit is taken from the output transformer, so it's post-compression (feedback type). The sidechain is powered by a mic preamp that follows the PHANZEN through phantom-power. When the phantom power is not present, the signal still goes through the PHANZEN, but there's no compression and the frequency response is altered a bit and some harmonics are added, thanks to the transformers and the diode bridge. The phantom-power current consumption is in the range of a typical condenser microphone and is below 5mA. The PHANZEN is capable of wide range of attack and release times, ranging from fast to very slow. It also offers the Overshoot mode (activated by the switch marked OVSH). This mode changes considerably the way the PHANZEN reacts to variations in amplitude of the input signal. The normal mode uses a well known configuration of two potentiometers (for attack and release times) and one timing capacitor. This mode is faster and more aggresive in its action, and sounds more traditional, bringing out the ring from drums, the decay from reverberant spaces and evening the levels. The OVSH mode adds another variable RC stage of control voltage filtering. It has a tendency to overshoot (hence the name), that is - not to follow the signal envelope precisely, but to overreact. The gain reduction is not as agressive, it's smoother and a bit lazy. The compression action is slower, the attack has a lag and has an oveshoot (overreaction), which gives an impression of slighty delayed action. Slower attack settings enhance transients and impact like no other compressor, without (due to its lazyness) the impression of being heavily compressed. This mode allows individual tracks to stand out in a mix. It's very good for program material as well. The INPUT control adjusts the amount of signal coming into the Phanzen. It's switched in ten 3dB steps ranging from -50dB to -20dB. It sets the overall gain loss, not including gain reduction from compression. The control is positioned in front of the input transformer, so it allows to perfectly match the whole PHANZEN's audio circuitry to the strenght of the incoming signal. The ATTACK is a continously variable control that allows to set the time of the compressor's reaction to the increasing signal above the threshold. It ranges from fast to very slow. The RECOVERY knob allows to control the release time of the compressor, that is the rate at which the gain is restored after compressing. It also ranges from fast to very slow. The setting of the attack and recovery pots is critical. Small movement makes a big change. A good starting point are middle settings. Please note! Setting both Attack and Release controls at minimum will introduce serious distortion and fluttering, particularly with bass-heavy signals. Gain reduction will try to follow the signal waveform, rather than the signal envelope. The THRESHOLD control sets the level at which the PHANZEN starts reducting the gain. Lower threshold means more compression. It should be balanced against the INPUT switch. Both controls have influence on the depth of compression. The COMP/LIM switch changes the slope of compression. In the COMP mode the compression action is gentler, whereas in the LIM mode the Phanzen clamps harder. Please note that both COMP and LIM slopes are soft knee and the actual ratio changes with the gain reduction, but typical numbers are 2.5:1 for COMP and 9:1 for LIM. The BIG switch filters out the lowest frequencies from feeding the sidechain and prevents these frequencies from causing the gain reduction. Gain (no compression): -20 to -50dB (in 3dB steps) Max. Gain Reduction: over 20dB Input Impedance: 2kohms Output Impedance: 200ohms Frequency Response: 20Hz-25kHz Phantom-power current consumption: 5mA Ratios: 2.5:1 (COMP setting) 9:1 (LIM setting)