Technical Articles

AVR - Audio Video Receiver - Build Quality: Part I

ARTICLE INDEX

Single Chip Analog AVR LSI

The AVRs in the table, regardless of price, tend to have a single chip that subsumes almost all of the unit's analog electronics. This single chip serves a variety of functions:

  1. Eight channels of electronic volume controls to control the level at the preamp or power amp output.
  2. Eight channels of analog buffering for direct connection to the preamp output jacks or internal power amp inputs.
  3. A switch at each electronic volume control input to select the DAC output or the 7.1 analog inputs.
  4. A selector switch for the two-channel analog inputs (8 – 14) to be sent to the ADC, or in direct mode, sent to the electronic volume controls at the preamps output.
  5. The FL and FR volume control inputs have an additional position on the switch to the volume controls. This is for the direct (DSP bypass mode) mode of operation for two channel inputs. All other volume controls are grounded when stereo direct is selected.
  6. Two channels of electronic volume controls for the ADC input to adjust levels to prevent overload of the ADC, followed by a pair of opamp buffers.
  7. An independent selector switch for the two-channel analog inputs to be sent to the record output. An opamp buffer is placed between the selector switch and the chips output to isolate the selector switch from the load. In addition, switches are in series with the output that open to prevent a tape recorder self-loop fault condition. Some AVRs have no tape output and in that case this selector is for zone 2.
  8. Another independent selector switch for the two-channel analog inputs to be sent to an alternate record output. This output can also be used for zone 2 or 3 outputs. The selector switch is again buffered by a pair of opamps

Block diagrams will be supplied in the Part II of this article.

The number of transistor switches ranges from 100 to 150, and the opamp count typically lies between 16 and 30. Parts with high opamp counts support other functions such as analog bass management or add unity gain buffers into the main signal path.

Each electronic switch in a CMOS process requires two transistors in parallel: a PMOS that provides the lowest resistance path as the signal moves to its maximum voltage, and an NMOS switch that provides the lowest resistance as the signal moves to its minimum voltage.

Each electronic volume control consists of a resistor string to attenuate the incoming signal, and a block of transistor switches. Each switch selects a different volume level at each tap in the resistor string. A simple electronic volume control is shown in Figure 2 below. It has four resistors to provide four distinct output levels.

Consider the 4-position example in Figure 2 above, with each tap having the same value resistor between taps.

Level Reduction

The top point is always 0 dB
Tap 1 is down to 0.75 full level
Tap 2 is down to 0.5 full level
Tap 3 is down to 0.25 full level

Volume controls for audio have steps that decrease in constant decibel increments. The resistor values in an audio volume control are different between each tap to obtain this results. Each volume control in an AVR must have a large gain adjustment range, for example -96 dB to +32dB. When half dB steps are desired, 256 resistor segments and 512 transistors (2 transistors per switch) are required. The LSI chip has ten volume controls, eight for adjusting the level to the power amp and two, with a smaller range, for adjusting the level to the ADC to prevent overload. In total over 2000 resistors and 4000 MOS transistors are needed. More transistors are on the chip as digital gates to allow an external microcontroller to close the correct switch.

The number of pins for these parts runs between 80 and 100. Integrated circuits incorporating this quantity of electrical components are called Large Scale Integrated (LSI) circuit.

Missing from the LSI chips are the ADC, the DAC, and a pair of standalone opamps at the input of the ADC for anti-aliasing filtering. Eight standalone opamps are resident at the outputs of the DAC for reconstruction filtering and if the DAC is balanced the balanced to single ended conversion function.

By far, the Renesas R2A152XXFP is the most common single chip analog LSI family. The XX are the last two numbers of the chip and designate the size of the input switch network. Other similar chips are from Rohm (e.g., BD3473KS2) and New Japan Radio Co (JRC) (e.g., NJW1299). The datasheet for one of these chips can be found at

http://semicon.njr.co.jp/eng/PDF/NJU72340A_E.pdf

This chip (NJU72340) has a simpler input switching than what is found in current AVRs.
Datasheets for more complex chips listed above are not on the IC company's website. The Rohm and JRC chips are most readily accessible by entering the part number in Google and using a third-party datasheet storage website to download it. Renesas datasheets are not currently available on the web.

The block diagrams in the datasheets show every opamp and the switching blocks. The Rohm datasheets shows every switch pair. Some datasheets are more comprehensive than others especially with respect to the worst-case performance numbers. Different chip suppliers may not use the same test procedure. Taking this into account, it appears the typical THD and SNR of different LSI chips found in the AVRs I examined varied about 3 dB typical, but 6 dB worst case. No single chip had the best specs in all categories. A chip with the lowest noise might not have the lowest distortion.

The latest generation of AVR LSI chips has less noise and distortion than the ones offered in the prior decade, which explains their deployment in more expensive products. This is mostly a result of innovations, at the circuit level, in the design of the volume control section. The distortion performance, however, has not improved to the point where the AVR LSI volume chips are better than all but the least expensive DACs. Thus the final measured distortion at the preamp output is often dominated by the analog AVR LSI, and not the DAC. For higher performance DACs the noise will also be dominated by the AVR LSI chip.

An LSI chip can handle a maximum of eight channels. On products with more channels, such as 11.2 receivers, there will be two LSI chips to provide volume controls to cover the additional channels and the second and third zones.