To provide the extensive control and flexibility provided by the 552, its high-performance analog circuitry is infused with numerous control types. Digital control of analog signals provides the circuit designer with the power to offer features that analog alone can't offer. The mix of different control types for gain in the 552 include analog potentiometers, digital potentiometers, analog switches, micro-relays, and physical switches. Each location in the circuit where gain is controlled is described below.
Input Mic/Line Switching - Micro Relays
Each of the 552's five inputs is switch-selected for microphone or line-level inputs. In a departure from its predecessor which had physical switches adjacent to the input connectors, mic/line switching on the 552 is controlled from the front panel of the mixer. The benefit of using front-panel switch combinations to control mic/line level switching is most important in fast-changing setups; there is no need to reach into the production bag and find a switch to change input types.
Input Trim and Fader - Analog Potentiometers
Critical 552 gain adjustments are controlled with analog potentiometers. The combination of its wide gain-range trim control and its precision fader allows input to be controlled with high degree of accuracy and smoothness that is only possible with precision analog pots.
Pan - Digital Potentiometers
Input pan is a secondary control on the 552. In most applications, inputs are panned either hard-left, hard-right, or straight up the middle. To provide more control over the circuit the pan circuitry is controlled by a digital potentiometer. Audio signals flow through the digital potentiometer which is controlled by the panel-mounted potentiometer. As is the case with all digital potentiometers, gain levels are adjusted in discrete step. No audio is routed through the panel-mounted potentiometer.
Master Level Control - Digital Potentiometer
In most production applications, the Master Level Control is set-and-forgot at unity gain (o dB). To provide more control over the circuit both sides, left and right of the Master, are controlled by a digital potentiometer. Audio signals flow through the digital potentiometer which is controlled by the panel-mounted potentiometer. As is the case with all digital potentiometers, gain levels are adjusted in discrete step. No audio is routed through the panel-mounted potentiometer.
Master Output Mic/-10/Line - Physical Switch
While input assignments into the mixer may change during a production, rarely are its outputs reconnected. The output level is switch-selected using physical switches adjacent to the output connector.
Direct Output Mic/Line Switching - FET Switching
Using the Setup Menu, the level of the unit's direct outputs is switched between mic- and line-level. Micro-controller controlled analog switches accomplish this task.
Headphone Level Control - Digital Potentiometer
One of the most often touched controls, the Headphone Level Control is an essential component of the mixer. The Headphone Level Control also doubles as a selector for the Setup Menu. To operate in both modes, a digital potentiometer is used for headphone levels. A secondary benefit of the headphone level being controlled by a digital potentiometer is that the headphone volume level is shown on the output meter when the control is changed.
Return Levels - Digital Potentiometers
Like the overall headphone level, the return levels are controlled by digital potentiometers. The same rotary control used for headphone level is used to adjust the return level.
About Digital Potentiometers
It is possible to hear "zipper/stepping noise" in the 552 under the right circumstances when adjusting:
- master level,
- return levels, and
- headphone level.
This noise artifact is the result of those specific levels being controlled by digital potentiometers, which have finite and discrete gain increments. An analog or rotary switch controls the digital potentiometer. Of the controls above, the pan pot and the master output level controls are in the direct audio path to the master outputs.
Digital pots are used in many different devices by numerous manufacturers in our market, including wireless tx/rx, digital recorders, and analog and digital mixers. You can hear this zipper/stepping noise to some degree when they are used. In the 552, their benefit is exact repeatability of gain values at a specific pot rotation and no dropouts with excess or limited usage. The critical-path level controls (trim/fader) on the 552 use precision analog pots, such as the sealed, conductive-plastic fader pots.