High-End-Preamp （2）

Published:2011/7/22 2:47:00 Author:Phyllis From:SeekIC

By Benjamin Hinrichs

The PGA23I1 stereo audio volume control IC

Admittedly, the idea of using a ’digital’ IC for volume adjustment, and further more controlling it using a microcontroller, may evoke a sceptical frown from many an audiophile. Ten years or more ago, this scepticism would certainly have been justified, but the semiconductor industry has made enormous progress in this area. All of the major functions can now be integrated into a single chip, with results that can easily hold their own against the best mechanical solutions.

The Texas Instruments PGA2311 volume control IC used in this project is moderately priced and provides out-standing tracking without any degradation of tracking accuracy when balance adjustment is used, and it also has very good technical specifications. Another major advantage is that it can be digitally configured. This makes user-friendly operation possible (including remote control). An example of a high-end builder who uses this technology is Jeff Rowland.

The PGA2311, whose internal structure is shown in Figure 1, is a digitally controlled analogue stereo volume control with certain refinements. The two channels can be independently adjusted over a range of -95.5 dB to + 31.5 dB in steps of 0.5 dB, which yields an adjustment range of 127 dB. The tracking error between the two channels, as well as the absolute setting accuracy of each of the channels, is ±0.05 dB. This naturally means that a balance adjustment can also be implemented without any problems, since the high absolute setting accuracy prevents any offset from occurring. Another noteworthy feature is that the IC can directly drive 600-Q loads.




The multiplexer (MUX) switches individual resistors to set the attenuation. After the attenuator, the signal passes through an output buffer, which can also provide gain via an adjustable feedback resistance.

The IC is controlled via a serial SPI interface. Clock signal SCLK transfers a single 16-bit word to the IC via the SDI line. The first eight bits set the volume level for the right channel, while the second eight bits set the level for the left channel. The minimum value (0) represents Mute, and the maximum value (255) represents a gain of +31.5 dB.

For fully noise-free switching, zero-crossing detection can be enabled via the ZCEN lead. If it is enabled, the IC analyses the music signal and attempts to perform the switching during a zero crossing. If no zero crossing is detected within 16 milliseconds, the switching is performed without waiting any longer. Due to system design constraints, the ramp function of the software does not work properly if zero crossing detection is enabled.

In principle, four different types of ICs can be used in this circuit. Table 2 lists the differences among these ICs. The original design was developed for the Crystal (Cirrus Logic) type CS3310. The equivalent competitive product from Texas Instruments is the PGA2310, which is not only pin-compatible, but also has significantly better internal specifications. A particularly attractive feature of the latter IC is that it can handle signals up to 27 Vpp if the analogue supply voltage is increased to 30 V. The improved type PGA2311 has even better channel separation, and the selected A’ version has a better specification for total harmonic distortion plus noise (THD-i-N). For this reason, we selected the PGA2311A for his project.




We can also mention the PGA4311 in passing. This is a four-channel version of the PGA2311 and is only available in the SOIC package. It can be used with only minor modifications to the circuit board layout and software.

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