Gilbert's cell

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In yesterday's postwe covered how a potentiometer, or pot, worked - the standard volume control found on most preamplifiers and a few DACS with analog inputs and volume control. Yet another way to control the volume on a DAC is by changing the actual gain of the DAC's analog output stage. Typically the gain of the analog output stage of a DAC or preamplifier is fixed with nothing more than a couple of resistors. However, there is a means to have the stage gain variable and this can be accomplished with a pretty snazzy circuit called a Gilbert Cell or, more commonly referred to as, a voltage controlled amplifier (VCA). The VCA (Gilbert Cell) was invented many years ago by Barrie Gilbert of Analog Devices and is in wide use today throughout the recording and consumer industries. Properly designed a VCA is a cool device that can change its gain over a very wide range without changing its sonic characteristics in the process. We used a special form of VCA in several of our older designs and called them Gain Cells. Gain Cells take the signal in from the internal DAC output (or an input on a preamplifier) into the Cell. There the signal is combined with a DC voltage to form a multiplier and the higher the voltage you put in, the more the input signal is multiplied and the music gets louder. Unique to this amplifier is the means by which it changes gain which is through the addition of a voltage. Imagine how much freedom the designer has with a Gain Cell to control the level. Because the only requirement is to change a voltage, the front panel control for the volume turns a voltage up or down but the actual signal itself never goes through the volume control. Designers are then free to use a simple pot for the voltage or a remote control, switch, etc. No longer does the quality of the controlling device matter to the sound since the sound is never passing through the control. Future PS designs that include both analog and digital inputs will use this form of Gain Cell to keep the path simple, short and clean. Tomorrow let's cover the last type of volume control, the stepped attenuator.
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Paul McGowan

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