Using VHD
VHD � Variable Harmonic Drive is a unique feature of the Alpha Channel pre-amp. It allows a signal (mic or line) to be over driven from subtle harmonic colouration through to full on distortion characteristics. The VHD circuit is inherent to the pre-amp, but usually only comes into noticeable effect when the knob is in the last quarter of it's travel. This allows the pre-amp to be used in the normal manner (without distortion!) for most of it's range and the distortion characteristics to be brought into play at higher gain settings. The level of the incoming signal will have an affect on the VHD circuit, as will the PAD button which offers 20db of gain reduction for loud signals. The second knob in the input section, labelled 0-11 changes the character of the distortion from 3rd harmonic to 2rd harmonic. This will only have an affect at high gain settings. 2nd harmonic distortion is associated with the harder, brighter sound achieved from solid state devices, where as 3rd harmonic is more akin to the warmer sound produced by overdriven valves. You are informed by the Alpha Channel of the distortion state where VHD will become noticeable. The PAD LED will glow red when this is happening, and this is generally best avoided for a clean signal. The VHD is highly dependant on the input signal. It becomes more obvious when used with a signal that contains mainly low frequencies, i.e. Bass guitar. If the input signal is predominantly high frequency, then the change in effect will be less noticeable as the harmonics generated by the overdrive may be above the bandwidth of normal hearing. Be aware of the situation that occurs if the pre-amp is over driven. This will have an affect on the gain of all parts of the unit which follow in the signal chain. The Alpha Channel has of course been designed to allow these high gain settings without overloading other parts of the circuit, but you may find that the output gain will need to be taken down to interface correctly with other equipment. You could engage the Lite Limit here to stop the A/D converters overloading if using the digital output.
Parallel Processing
This is where a processed signal and it's unprocessed counterpart are summed together to provide a single source. It often allows you to apply extreme processing but still retain the dynamic and character of the original. The Alpha Channel provides an easy method of performing this function: If the SUM is depressed whilst using a compressor in the send/return loop then the unprocessed signal will be added to the compressed. The output gain of the compressor can then be used to balance the two.
