INGRAMENGINEERING WHITE PAPER

Signal Attenuators

Single Value vs. Variable Attenuators

 

It is desirable to use an attenuator before the first active stage of a pre-amp so that high level signals may be attenuated as much as needed to keep from overloading the active stage. Many pre-amps use a 20dB attenuator pad and include an indicator to let the user know if the active stage is being overloaded. When an overload indication is given, the pad may be activated and the signal reduced to a level that results in linear active stage performance.

 

A major drawback of a high value, single mode attenuator pad like this is that the pad provides much more attenuation than is needed in many cases, and dynamic range of the signal is sacrificed.

 

For example, consider a music source. In order to maximize the dynamic range of the recording, the music signal level at the input to the active stage should be as high as possible but still in a suitably linear region of the active stage. The music signal level may then be as high as this maximum and as low as the active stage input referred noise. This defines the dynamic range capability of the pre-amp.

 

Assume that the music signal has infrequent peaks that are 4dB above the maximum peak level that was previously set at the maximum suitably linear region of the active stage. When using a pre-amp configured with a 20dB pad, the pad will need to be activated, and the signal level reduced so that the infrequent peaks do not overload the active stage. However, only 4dB plus a comfortable headroom margin was actually needed. If 2dB headroom is desired, then a total of 6dB attenuation was needed, yet 20dB attenuation was the only choice. As a result, 14dB extra attenuation were used. Since input referred active stage noise is constant for this scenario, the dynamic range was reduced by 14dB more than necessary.

 

To maximize dynamic range for all recording scenarios, the Ingram Engineering pre-amps utilize variable stepped attenuator pads at the input. These highly flexible variable attenuator pads provide maximum control of the signal.

 

Furthermore, if the system is properly designed, there is usually no real need to know if attenuation is used or not. The only real useful information is whether the active stage(s) is in an overload condition. The Ingram Engineering pre-amps with variable attenuators address this by having a single stepped input control that is used to set the input level as high as can be set before an "overload" LED lights. The stepped switch controls both the stepped attenuator and the active stage gain steps. The pre-amp design is set so that attenuation is automatically used when needed, and automatically switched off when not needed. When not needed, only active stage gain is changed when the stepped switch is adjusted. And, as long as the input switch is set as high as can be set with appropriate headroom, maximum dynamic range results. For stepped switch settings one click below overload indication, a minimum of 2dB headroom is realized.

 

The variable gain portion of the stepped switch settings utilize both 2dB and 3dB steps. Two dB steps are used for small to moderate level adjustments and 3dB gain steps provide more coarse control.

 

For example, a graph of the MPA683 attenuation/gain scheme is shown below. Fifty dB adjustment range is provided, while allowing dynamic range to be maximized with one simple procedure for any signal level.

 

Figure 1: Attenuation or Gain vs. Stepped Switch Position