Damping Control

It is a common design technique to incorporate "Global Negative Feedback" (GNF) into an amplifier. This is where a small sample of the output signal is fed back into the pre-amp or driver/phase inverter stage of the amplifier. Feed out-of-phase to help minimize output distortion (the bad kind that is due to imbalances, transformer or tube artifacts, etc.) and lower the apparent drive impedance of the amplifier. Lowering the drive impedance is not the same as the matching impedance (4, 8 16 ohms etc.) but is actually related to the ability of the amplifier to drive a reactive load (such as the typical loudspeaker). In mathematical terms, (simplified for this explanation) the product of 1/(net output impedance) is known as the Damping Factor.

Because loudspeakers typically have wide variances in their impedance as the frequencies of the signal change, it is difficult to drive them stably with good efficiency. By using a small amount of feedback, the drive impedance, and hence the amplifiers damping factor can be improved and helps stabilize the output stage over a wider range of signal.

Occasionally a designer will incorporate a switch selectable feedback circuit (often called something like "tight/loose" or "modern/vintage" etc.). This changes the amount of GNF to slightly tailor the sound to the player's taste.

DST takes this idea one step further by utilizing continuously variable control of the GNF. We incorporate a user variable control called "Damping" that the player can tweak to best match his sound, style and loudspeaker choice. We carefully select the control range to maintain a stable, broad band output while allowing adjustment of the gain and damping factor of the output stage.

Have a question or want more details? Contact Bob Dettorre, VP Engineering, DST Engineering
bob@dst-engineering.com

DST Engineering Damping control - Copyright (c) 2002