Technical & Editorial

AVR – Audio Video Receiver – Build Quality: Part III – Component Choices for a High Performance Design

ARTICLE INDEX

A More Detailed View of the Internal Signal Flow in the Large Scale Integrated Circuit (LSI) Analog AVR chip

Below is a block diagram of the chip when connected to the DAC which, in turn, is connected to the preamp outputs. The DAC and reconstruction filter are external to the LSI chip at the left of the drawing. The switch in the LSI chip is positioned to allow the DAC output to appear at the volume control input.

As noted on the diagram, the volume control represents a low impedance load established by the resistor string. Current must flow through the switch to drive the volume control input.

Current flowing through a transistor switch causes distortion. The complex phenomenon is well explained by Douglas Self in his "Small Signal Audio Design" text. Chapter 16 covers the complete topic of switch design for audio circuits. I also mention this text as a solid reference in the op-amp section above.

For this simplified diagram, the transistor switches and the tapped resistor for the electronic volume control are not shown. These components are assumed to be inside the round block.

Transistors switches used to select the appropriate tap on the resistor string of the electronic volume control have no current flowing in them. The other end of the electronic volume control switches are connected to the IC buffer following the volume control.

The input impedance of the buffer is very high, and no current flows from the switches inside the volume control to the input of buffer; consequently, there is no voltage drop across the switches inside the volume control.

The output impedance of the buffer is low, and it has no problem driving the preamp outputs or power amp inputs.

Pre/Pros implemented with Small Scale Integrated (SSI) chips have an additional op-amp buffer placed between the switch (on its own chip) and the volume control (on its own chip). With the added buffer in place, there is no current flowing in the switch connected to the volume control; consequently, the switch has no voltage drop across it. This is described in more detail below.

Some Rohm versions of the analog LSI AVR chip have this additional buffer, adding eight more op-amps to the chip. The Rohm chip's data sheet suggests the chip has half the distortion of the top of the line New Japan Radio (JRC) LSI AVR chip, which has no added buffer

Like all active electronic devices, a buffer adds noise to the signal that passes through it. In an SSI implementation, very low noise standalone op-amps would be used for both buffers. The small opamps in the LSI AVR chips produce more noise. This can be seen in the data sheets with the New Japan Radio (JRC) chip emitting less output noise than the Rohm chip. Herein is the engineering tradeoff between noise and distortion performance resulting from the performance limitations of the op-amps inside the LSI chip.

The Pioneer SC-79 uses the Rohm BD3473KS2 LSI AVR, which has been reviewed in Secrets.

For this sample, measured SNR was 17.8 bits, equivalent flat 20Hz – 20KHz referenced to 2VRMS at unity gain. This is an admirable result. Distortion was less impressive. No product with a JRC chip has been bench tested with the Audio Precision at Secrets.

The block diagram of the LSI chip when the 7.1 input is selected is shown below.

This diagram illustrates the signal flow when the switch position allows voltage at the 7.1 input to transfer to the switch. The signal path for the DAC and 7.1 direct mode are the same; only the switch position is changed. The amounts of noise and distortion introduced by the switch, volume control, and output buffer are the same for the 7.1 channel analog input signal, in direct mode, as when the DAC is applied to the LSI chip (previous block diagram).

The next figure is a block diagram of the LSI chip when a pair of stereo inputs is selected.

Analog stereo signals are further degraded since they must pass through another small switch that is part of the input selector. When switches are connected in series, the voltage drop across the pair is larger and the distortion more pronounced.

Some JRC chips place a pair of buffers after the two-channel selector switch to prevent this problem, but the buffers introduce a high DC offset that must be removed by external blocking capacitors. Added external components increase cost as well as and take PC board space, and as a result, some designers may use a different part in response to this. The JRC NJW1299 AVR LSI used in some Denon products does not have the added buffers.