Technical Articles

AVR - Audio Video Receiver - Build Quality: Part I

ARTICLE INDEX

Introduction to AVR Build Quality – Part I: Range of Options by Supplier and Price

What does your money purchase inside the chassis of an AVR or Pre-Pro in an audio / video system?  In this article, one in a three part series, I attempt to answer this question: I will open the top and look at the parts inside and show how each of the individual parts can affect the audio quality. We will see that dramatic differences exist at a given price point. While this article is focused on electronic components that perform the complex tasks in the audio channel of the AVR, I have always kept in mind the need to make the material understandable to a motivated reader.

Herein, we examine the components embedded in thirty AVRs and Pre/Pros from twelve manufacturers. I draw my observations from service manuals and the technical information found on the manufacturers' websites. Some of my manuals are dated by a couple years, but little has likely changed in the preamp sections of the units on which this article concentrates, with the exception of a handful of products that recently had a DAC upgrade.

It is not possible to focus only on current products because there is often a significant lag between a product's announcement and when I can get access to the schematic or service manual. Access to manuals from some companies is restricted to authorized service centers, or companies may sell them to consumers at very high prices.

For this article, there are no schematics to read, and I have kept the electronics jargon to the level of the features and specifications sections on a manufacturer's website. More technical material is deferred to Part II of this article. Part I covers all the concepts.

My attention centers on the data converters, e.g., DAC, and analog electronics that follow the preamp. Not shown in Figure 1 above are the multiple sets of stereo inputs. In direct mode, the stereo inputs can be connected to the volume control to bypass the digital signal processor (DSP). Alternatively, the inputs can be converted to a digital signal by an analog-to-digital converter (ADC) to drive the DSP. This path is also not shown in the diagram. A direct path for a set of 7.1 analog inputs to electronic volume controls (volume controls that have gain circuits embedded in the volume controls themselves) is also not shown in this diagram.

Many older AVRs had eight ADCs instead of the current standard of two. With eight ADCs, the DSP could process 7.1-multichannel analog signals. The additional six ADCs were costly. HDMI obviated them by enabling high-resolution multichannel digital signal transfer.

To make the discussion more manageable, I do not highlight quality differences in the stereo ADC path. This may be the subject of a later article.


Understanding DAC Specifications

The most important aspect of DAC performance is SNR, followed by level linearity error, pattern noise and THD. Other factors to note are whether the DAC is balanced or unbalanced, whether the DAC is a current-source device or a voltage-source device and the number of DAC channels inside the package"

The price of a DAC IC increases with performance. Improved performance is typically related to the size of the silicon utilized by the chip. Over time, performance improves for a constant price, especially for the entry level DACs. This is a consequence of innovations in circuit design and process technology.

A DAC manufacturer's worst-case specification on a datasheet establishes a performance bound (the lowest performance acceptable). If the DAC chip does not exceed the tests specification with the bound, then it is thrown out. Most chips that pass have better than worst-case performance. AVR designers must work with the worst-case numbers for all integrated circuits (ICs) to determine the unit's overall worst-case performance. The final worst case terminal specifications of the AVR provided to the consumer are set by the summation of the worst case specifications of the individual IC in cascade inside the unit. Unfortunately, most datasheets for analog ICs present worst-case values for only a few specifications. Most specifications provide only "typical" values. The percentage of parts that achieve this typical value is not specified.

Signal to Noise Ratio (SNR) dominates the DAC's performance across the majority of the signal levels. Only as the signal level approaches 1/20 of the full-scale signal level does Total Harmonic Distortion (THD) assert itself.

Most DAC suppliers only provide SNR measurements with an A-weighted filter in the signal path. The A-weighted filter rolls off the low end of the spectrum, which is said to correspond to hearing test curves that show the ear is less sensitive to low frequency noise. The sensitivity of the ears to low frequency tones changes with SPL, so A-weighting often underestimates the perceived noise level.

http://en.wikipedia.org/wiki/A-weighting

Since the noise of a transistor, especially a MOSFET, tends to rise at low frequencies, adding the filter improves SNR when compared with an unfiltered measurement more than if the noise source were flat for all frequencies.

The deviation in amplitude of a sine wave from the theoretical value, as the digital input signal level is reduced, is also important. This is often called level linearity error. Level linearity error should also be checked at different frequencies and sampling rates, but one is lucky to find a single specification on most data sheets.

The noise floor of some DACs may have audible tones that cannot be observed using standard tests for DACs. These quasi-periodic oscillations are called pattern noise, idle-channel tones, or modulation noise. Quasi-periodic signals are difficult to measure; rarely, will it show itself as a distinct line in a spectral plot. Nonetheless, our ears can detect the tonality. These quasi-periodic tones can be observed by changing the digital equivalent of a DC voltage as it is applied to the DAC and monitoring the SNR. Other test methodologies have been proposed to produce a measurable result when the converter is exhibiting this behavior. The following reference offers additional information and includes extensive references:

www.stereophile.com/content/super-audio-cd-rich-report-postscript-december-2000

The paper below by ESS engineers looks like an IEEE paper, but in fact it was never reviewed or published. Section IV of the paper addresses how ESS attempts to remove the quasi-periodic coloration of the noise floor.

http://www.esstech.com/PDF/sabrewp.pdf

Full scale THD at 1 kHz does not fully characterize the performance of the DAC. Distortion performance varies with the full frequency range from 20 Hz to 20 kHz. THD tends to increase with increasing frequency.


Digital Reconstruction Filter

Data sampling systems require reconstruction filters to eliminate out-of-band spectra that are artifacts (e.g., aliasing) of the sampling process. Modern audio DAC ICs use a digital filter to remove most of the off- band components. By doing so, the analog filter at the output of the DAC can be a simple circuit. The more complex the digital filter, the better its ability to reject the out-of-band components from the sampling process.

A more complex filter also has less frequency response ripple in its pass band. With high-resolution material sampled at 96k samples / second and above, the DAC filter may provide different, digitally selectable, filter roll-off rates. A slow mode improves the impulse response in the time domain by reducing the slope of the transition band. If the slow mode is used for a CD with 44.1 samples / second the response will start to rolloff at 10kHz and will be down -3dB at 20kHz which is obviously audible. For sampling rates above 96k samples / second slow mode is flat past 20kHz. In most cases, these options are documented in the datasheets for the DACs. However, ESS does not disclose this publically.

Information on digital reconstruction filtering process is found in Part II of his article

Some designers perceive an aural effect with different digital reconstruction filters. The paper below does extensive testing of different digital reconstruction filters and concludes different approaches do not change the sound.

http://www.stereophile.com/reference/106ringing/index.html

The next link, below, is from Wolfson Semiconductor. It was presented at an AES conference, but never in the AES journal which has a more demanding review process. Wolfson draws different conclusions than the paper above.

http://www.wolfsonmicro.com/documents/uploads/misc/en/
Ultra_High_Performance_DAC_whitepaper.pdf

The potential changes in sound quality of digital filters and the quasi- periodic tonal behavior of some DACs may explain an AVR designer's justification of selecting DAC ICs that do not maximize SNR and THD for a given chip price.


Number of DACs per Chip

The number of DACs per chip is typically two or eight. The eight-channel chips are often called octal DACs. The ability to increase the number of channels on a single chip results from transistor scaling as process technology advances, at least for the digital transistors. Analog components often do not scale, since this can affect noise, distortion, and level matching between channels. It thus should come as no surprise that the parts with a lower performing analog section were the first to be made available as octal DACs. These parts also minimize the amount of silicon taken by the digital filter. The datasheets for lower priced octal chips show the performance degradation of the digital filters in which the most significant is the amount of attenuation in the stop band. Digital filter specifications will be discussed in Part II.

The performance of DACs with eight channels improved when Cirrus offered parts with worst case SNR and THD improved by 10dB over previous octal parts in 2001. In the last few years ESS has shown that the analog performance need not be compromised offering almost all its parts with eight independent channels.

The advantage of octal chips is that only one chip is required instead of four on the PC board. An added benefit is the matching between channels is improved since all 8 are on a single chip.

In highly integrated chips, the two ADCs for conversion of analog stereo inputs to digital for DSP processing may also be integrated on the same chip. When both ADCs and DACs are on the chip, it is called a CODEC.

The most highly integrated mixed signal ICs for AVRs may also include the SPDIF input selector, as well as SPDIF timing and data recovery system. This is sometimes called the DIR for Digital Input Receiver. Combined with the latest multicore DSP chip and a highly integrated analog chip, to be discussed below, most of the audio portion of an AVR are found on three chips.

Recovery of digital audio from the HDMI inputs is performed by ICs on the video switching board of the AVR. The PCM streams from USB and Apple digital audio dock cables are produced by other chips in the AVR.


Improved Distortion and Noise Performance with Balanced DAC Output

All but the lowest cost DACs provide two outputs per channel. Each output actually comes from two separate DACs. One DAC output is out of phase with the other. This is called a balanced signal. A circuit called the balanced to single ended converter is connected to the balanced outputs of the DAC IC.

The balanced signals are subtracted from each other to form a single-ended output. In the process of converting the balanced analog signal to the single-ended output, some distortion produced by each DAC, in the balanced pair, is partially cancelled. Comparing the THD levels for single ended and balanced output DACs shows significant improvement for the balanced DAC. This will be seen on the chart below. In addition to improving distortion, balanced DACs may also show an improvement in SNR level.

The balanced-to-single-ended converter is also designed to provide the final reconstruction filtering in the analog domain. This low order filter removes folded tones that remain after the digital reconstruction filter. They are typically above 100 kHz and low in level, so only low-order filtering is required to remove them. Additional details on this analog filter and its typical frequency response can be found at the following link:

http://www.hometheaterhifi.com/integrated-amplifiers/integrated-amplifiers-reviews/harman-kardon-hk-990-stereo-integrated-amplifier-with-digital-room-correction-and-dual-subwoofer-bass-management-part-ii/page-8-hk-990-backend-analog-circuitry.html

The analog reconstruction filter must be in the signal path for single ended and balanced DACs. Since the reconstruction filter uses an opamp in all but the lowest cost (below $350) AVRs, only a few passive parts are needed to add the balanced-to-single-ended conversion function.


Enhanced Distortion Performance with Current Mode DACs

DACs come in two types. DACs with a current output (current-mode DAC) require high-quality external opamps to convert the DAC's current output to a voltage. The current-to-voltage converter stage is often abbreviated as I/V.

A current-mode DAC allows the output pins of the DAC to stay at analog ground instead of moving with the music signal. This improves the distortion performance of the DAC. The I/V converter, as the name implies, produces a voltage output in proportion to the current flowing from the DAC. For reasons that will be outlined below, producing a very high performance opamp is difficult when the chip must perform other function. Since the I/V converter is a stand-alone opamp, it does the job more accurately (lower noise and distortion).

All current mode DACs have balanced outputs thus two I/V converters are required

Voltage output DACs put the opamps inside the chip, which prevents inherent THD from exceeding -100 dB worst case. The advantage of a DAC with a voltage output is that the two external opamps and the associated passive parts need not be placed on the PC board, reducing cost. All lower performance DACs have transitioned to voltage outputs to save the cost of the extra opamps.


Multiple DACs Combined to Produce a Single Channel

Data sheets for a company's top of the line two channel DACs often provide specifications for the chip running as a mono device. In this mode, the output of the two DACs are summed together so that a single output is produced. When this topology is used, the noise floor is reduced. If the DAC supplier intends a part to be used in mono mode, the SNR improvement is provided on the datasheet for mono DAC performance.

An internal digital control bit in the DAC sets the DAC to accept a single mono PCM input stream.

The simplest DACs to place in mono mode are the current mode topologies. The outputs of the two channels are connected together. This connection is specifically shown in the Analog Devices in the Analog Devices application manual below and is also described in the ESS datasheets. TI appears to require a more complex setup discussed in Part II of this article.

http://www.analog.com/static/imported-files/user_guides/UG-048.pdf

Mono modes are found only in top-of-the line DACs in a vendors lineup because it is typically cheaper to move up the product line for stereo DACs rather than stay at a lower rung and use two DAC chips in mono to produce a stereo signal. The only time this does not apply is if the lower rung parts are being used in very large quantities across a large number of AVRs at a specific company. The company would get a significant discount given the large quantities they purchase. The next rung up the ladder part would be targeted only for a top of the line AVR, and the price of the DAC could be much higher since high quantity discounts would not apply to it.

Octal ESS DACs provide to option to be wired as stereo DACs (four current output lines shorted together) or mono DACs (eight current output lines shorted together). Thus three sets of specifications are provided on the data sheet (mono, stereo and eight-channel). With the choice increased to three, it is less clear which ESS DAC will yield the lowest cost solution for an AVR that requires eight or more channels of conversion. Some products use a given ESS chip for mono or stereo operation for the main channels (L and R) and another single chip with the same part number for the remaining six outputs in a 7.1 system.

Cirrus Logic does not have a DAC which is specified to operate in a mono mode.

It is possible to add circuitry so an AVR with 6 stereo DACs (to provide 12 outputs for 11.1 applications) can be switched to use the DACs in mono for SNR improvement, but only 6 channels are available. Some 9.1 channel AVRs switch only the front channels to mono allowing 7.1 operation to be preserved.


Chart Presenting Build DACs used in AVRs Across Manufacturers and Price

Performance of the digital-to-analog (DAC) converter chip is wide-ranging across manufacturers and models. DAC performance with decreasing SNR is listed on the following chart. The ranking includes all DAC manufacturers actively producing parts found in AVRs. The main column for the SNR is for all channels of the DAC producing independent signal (two or eight channels).

The rankings are done with the worst-case values of SNR shown in red. For the ESS parts, this article in SECRETS is the first public disclosure of the worst case SNR figures. Only typical numbers are found on the ESS datasheets on the website.

The SNR values in the chart are the A-weighted values.

A separate column of SNR values supplied when the DAC is operated in mono mode if the DACs data sheet provided a value for this. For the special case of an ESS Octal parts operated in stereo mode (four current outputs connected together), the SNR value is shown at the rightmost position on the chart.

The THD of the DACs is also shown. ESS and Wolfson do not supply worst case distortion numbers. I have not provided information about the performance of the digital filter incorporated in the DAC chip to prevent the charts from becoming too cluttered with data.

Performance of some DACs changes with the sampling rate. Datasheets often provide specifications for sampling rates of 96K samples/sec and 192K samples/sec in addition to 48k samples/sec. Again, to make the chart readable I have only shown the data for 48k samples / second.

At the right of the chart are products with the DAC. As can be seen, several different companies may deploy the same DAC. The price of the AVR need not correlate with DAC performance. One product, for example, the $500 Harman Kardon AV1700 has DACs that have 108dB worst-case SNR spec and -94dB worst case THD spec. On the other hand, a $1600 Marantz AV1700 has a DAC with a 102dB worst case and -86dB worst case, respectively.

Towards the negative, I identified a couple companies who relatively recently downgraded a DAC for a given price point.

Towards the positive, improved DAC performance is evident in the just announced top of the line products from Pioneer and Yamaha. I have listed these units alongside older offerings to illustrate the improvements.

Most changes in DACs, for a given company, at a given price point, are lateral with a change less than +/- 3dB change in SNR or THD. Another semiconductor vendor may have won the socket or a part may have been discontinued and replaced by the incumbent vendor.

Some of the best DACs from the top manufacturers are not currently being used. Under these circumstances, I listed an older AVR that used the part. In some cases, I could only find two-channel products currently using these parts. I still listed the DACs since I wanted to show differences among the best parts from all DAC producers.

A small subset of AVRs may use lower performing DACs for extra surround or subwoofer channels. I do not note this in the chart. The chart (Table 1) is seen on the next page.


The Right Side of the Chart: More Details about the AVRs and Pre/Pros

Different AVR and Pre/Pros listed on the right side of the chart (Table 1 below) may have interesting aspects of the design not associated with the DAC IC. A small number shown after the product indicates that an aspect of the design deserves more discussion. A page of notes following the DAC list deciphers the numbers.

The chart and the note page offer a relatively complete picture of the build quality of the analog audio channel of the products to the preamp output at a level that obviates a deep dive to the schematic. The only missing item is the ADC and the circuits that drive it.

Service manuals are generally not circulated for the more expensive North American products because of their small production runs. Service calls only occur at corporate headquarters. I was able to include the Anthem AVM 50 and D2 because their designs were discussed on the website when the products debuted. The discussion was at such detail that almost every chip in the audio signal path was described. Subsequently, this information was removed from Anthem's website.

For the Anthem MRX 700, I referred to the review by Kevin Nakano in the LA Audiofile website. Kevin removes some boards in the unit to identify the key chips mounted on them. This requires great care to avoid damaging the unit. This is an especially difficult task in the absence of dis-assembly instructions found in the service manuals. www.laaudiofile.com/anthem_mrx700.html.

To see a larger version of the table, left mouse click on the small image and the larger image will appear. If you want to see it full size, right click on the larger image and select "View Image". Then you will see a mouse cursor with a + symbol. Left click on the larger image, and it will become full size. Use the back arrow to return to the text.

All DAC data are shown in dB. Typical values are black. Guaranteed minimum limits are shown in RED. SNR values are with an A-weighed filter in the signal path.

Part marked SI have a single ended output. Parts marked Bal have balanced outputs (see above)

Parts marked CM have current mode output (see above)
DACs with current output require two additional opamps and passive components per channel on the board for current-to-voltage conversion.

Specifications are for a sampling rate of 48k samples per second. Some parts degrade at higher sampling rates.

Parts marked ADC have 2 channels of conversion. If a part marked DIR the part has an SPDIF input selector, clock and data recovery.

* CS4226 6ch CS4382, CS4385 and CS4228 8ch The CS4226 and CS4228 include the ADC and DIR

Notes Section

  • AVRs and Pre/Pros listed in black have a single Large Scale Integrated (LSI) chip that subsumes the majority of the unit's analog electronics.
  • AVRs and Pre/Pros shown in green use multiple Small Scale Integrated (SSI) parts for analog signal switching and digital electronic volume control function.

Unless otherwise noted, external operational amplifiers are LM833, RC4558, or equivalent Far East second sources. The parts are the lowest-priced opamps with performance specifications just sufficient for audio applications.

1) Old design for reference with respect to the DAC usage only.

2) Asynchronous Sampling Rate Converter (ASRC) for jitter reduction. This can be internal to the DAC chip (ESS DACs) or precede the DAC as a dedicated IC. Other products may subsume the jitter reduction functions within the DSP chip. If this is the case, I cannot observe the circuitry.

3) Quasi-current mode balanced to single ended converter circuit that uses only one opamp instead of three. Use of this circuit increases distortion from value shown. Additional details in Part II of this article.

4) The operational amplifier quality improved to NE5332 (NJM2114 equivalent for Arcam). Roughly a 30% - 50% price increase. These have better performance and are found in many high-end two channel products.

4a) NE5532 opamp used in I/V converter stage that follows the current mode DAC. Other opamps are typically lower quality parts.

5) Significantly better opamp quality. There may be a two- to five-fold jump in price relative to the lowest cost opamps useful for audio applications.

6) Attention has been paid in reducing the nonlinear effects of DC block capacitors. More information on these effects can be found in a paper written in Audio Magazine that was placed on the web:
www.reliablecapacitors.com/pickcap.htm

Service manuals often lack sufficient transparency to determine the grade of electrolytic capacitors in the signal path. The service manual does indicate if the number of DC blocking capacitors in the signal path has been minimized (higher quality opamps with lower DC offset voltages enable this). The presence of a film bypass capacitor in parallel with the electrolytic is another clear sign that the designer is worried about signal distortion from the electrolytic. If I observed this, a notation is placed next to the product.

7) Balanced electronic volume for DAC.

Details on ASRC, opamp quality and the balanced volume control are found in Part II of this article.


The Concept of Effective Bits

Signal-to-noise ratio (SNR) and Total Harmonic Distortion (THD) of the DAC are expressed in dB. This is the convention adopted by DAC manufacturers for presenting the metrics on datasheets. It is a challenge to decipher the relationship of these numbers relative to the requirements to reproduce music recorded in high resolution. It is nonetheless possible to convert the SNR to an equivalent specification called Effective Bits of Performance with minimal effort. As the name implies, effective bits indicates the limit of the performance of the DAC.

http://en.wikipedia.org/wiki/Effective_number_of_bits

We can calculate the effective bits for both SNR or THD.

While high resolution music is recorded with a 24 bit word per sample, the ADCs and analog electronics used for the highest quality recordings are limited to the 20 – 21 bit range.

Effective bitsSNR in dB
14 86
15 92
16 98
17 104
18 110
19 116
20 122
21 128
22 134

When the number of bits of the word coming into the DAC is subtracted from the effective bits the remainder is often called the marketing bits. For example if a DAC that has 20 effective bits of signal to noise and has an input word length of 32 bits, it has 12 marketing bits.

Effective bits are also a useful measure for analog signal blocks. The SNR and THD specifications are converted with the same equation to clarify when the performance of the analog block will swamp the performance of the DAC. More specifically, this occurs when the effective bit specification of the analog part is lower than that of the DAC.

For a few data sheets on data converters, notably the ESS products, the number of bits in the data path, or multiply-accumulate section of the digital signal processor, are presented. The large number of extra bits, relative to the analog resolution of the DAC in effective bits, are required to prevent round-off error in the digital computations occurring in the on-chip digital reconstruction filter and Asynchronous Sampling Rate Converter.


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.

 


Enhanced Performance with SSI Parts

Previously, a single chip analog AVR LSI was associated with lower priced AVRs. In contrast, higher-priced units would use Small Scale Integrated circuits (SSI) to provided performance advantages.

An opamp is an SSI chip. Opamp packages separately contain as many as four units.

To form the electronic volume control, an SSI part containing only resistor strings and switches is used. External opamps are added to form the complete circuit.

The switches are grouped in SSI packages with 4 – 50 switches pairs per package. Multiple switch packages are dispersed throughout the AVR to perform functions such as stereo analog input selection and signal routing to the electronic volume controls.

One downside of some AVRs built with SSI chips is the latitude afforded to the designer to incorporate extra functions to the analog path. This is most common in top-of-the-line Japanese designs using SSI chips. Additions, such as analog bass management high-pass filters, analog tone controls, and analog down mix add solid state switches and extra opamps to the main signal path even when the functions are bypassed. The additional components reduce channel separation and can increase distortion.


Limitations of Operational Amplifier Performance with the Single Chip Analog AVR LSI

Shoehorning these parts into one chip can limit performance because of size constraints. Power consumption of the opamp must be reduced to avoid overheating the LSI chip. The opamps in the LSI chips consume about half the current of a standalone audio opamp. The area of silicon taken by each opamp on the LSI chip must be reduced to fit them into the chip. Decreasing the size of an opamp and the current it consumes increases the noise of the opamp and causes more distortion, especially when driving resistor loads below 10 Ohm.

The power supply of the LSI chip decreases to ± 7 V, half of the standard value for analog audio chips. Doing so decreases the power consumption of the total chip by half, thereby permitting more parts to be placed in a single LSI chip. A smaller power supply provides less margin between the maximum signal swing of the signal at the output of the LSI chip and the power rails. The reduced headroom is another aspect of the LSI chip's design that increases distortion.

Process technology to produce an LSI analog chip differ from a process for SSI opamps. For example, MOS switches are not required. Some performance specifications for the individual bipolar transistor will be improved in the optimized opamp process. In turn, better measured performance from the opamp can be achieved than were it manufactured with LSI process technology. The typical process for opamps supports ± 15V.

The selection of specific opamps for optimal performance in an AVR is discussed in Part II of this article.


Limitations on the Performance of Semiconductor Switches with the Single-Chip Analog AVR LSI

Akin to the opamps, there is tradeoff between the level of integration and the size of switches. Smaller transistors pairs that make up the switches have higher resistance, which can result in more distortion.

Transistor switch resistance is also proportional to power supply voltage. The power supply voltage of the LSI chip has been reduced to half of what an SSI set of switches can support (±15 V). Lower voltage on the switch pair reduces the pair's effective resistance. In the Funk preamp reviewed on this site, the voltage on the switches was increased to ±20 V to reduce distortion further at the risk of exceeding the recommended maximum voltage for the SSI switch parts and reducing the lifetime of the product.

www.hometheaterhifi.com/preamplifiers/preamplifiers-reviews/funk-lap-2-v3-preamplifier.html

Combining the issues of the opamp and semiconductor switch performance degradation, the distortion of the LSI chip is 4 – 10 times larger than what can be achieved with an SSI solution. The LSI chip typical distortion is in the 15.5– 16 bit equivalent range. This drops by 3 bits worst case. SNR performance is, however, better than expected at 20 – 20.5 bit equivalent typical with a 2 bit drop worst case. The best implementation I found in a Pre/Pro using SSI parts produced an SNR about a bit equivalent better than this for both typical and worst case values.


Use of Relays to Achieve Better Performance

In stereo equipment, relays often substitute for solid-state switches. This is costly and can create reliability issues since the relay has mechanical parts; to the positive, the substitution imposes no noise or distortion. In an AVR or pre/pro built with SSI parts, a relay can replace transistor switches for routing analog signals inside the AVR. None of the products for which I have a service manual deployed relays for stereo analog input selection. I suspect higher-priced pre/pros produced in North America may use relays for this function, but cannot confirm this. I did find a relay routing the signal to the electronic volume control in one product.


A Very Brief Look at Changes in Power Amps in AVRs

The design quality of a power amplifier can vary across units at a given price point. Quality variations do not correlate with the design quality of the preamplifier section. Dissecting the power amplifier requires an understanding of schematics and analog circuitry at the transistor level. I will not attempt to rank the power amplifiers from different manufacturers.


Conclusions

When the hood is lifted on AVRs that are currently in production or have recently been discontinued, we find changes have occurred in the analog section (excluding the power amp) in medium to higher-priced products.

Intuition suggests the quality of the DAC in an AVR moves inversely with the added features list over time, i.e., to keep prices constant as features are added, DAC quality suffers. I was surprised to discover that the intuition turns out to be incorrect. Only after completing the exercise of compiling the products list did I fully appreciate the tradeoffs between functionality and cost control. In many cases, DAC quality is not the point of vulnerability; instead, the pinch point is the substitution of the SSI chips for the single-chip analog AVR LSI.
 
A key takeaway: circuit quality in the direct mode (stereo or 7.1) is almost always invariant to AVR prices in the range of $400 to $2,000. As examples, the $250 Yamaha RX-V367 and Marantz AV8801 ($3000) use the same Renesas LSI chip (R2A15220FP). With the LSI analog chip in these products, the sound of the direct mode is relatively constant, although a more robust power supplies, addition a quality output buffer and enhanced DC blocking capacitor quality can make small differences.

Unfortunately it is not possible to actually do this listening test just proposed in practice because the Yamaha RX-V367, like almost all low cost AVRS produced today, does not have preamp outputs. The experiment would be possible using the older RX-V665 ($550)

Even a low-cost stereo integrated amplifier will offer better performance in its analog input to its preamp output than an AVR in direct mode, since stereo products are made from SSI chips and, occasionally, relays. Value-priced stereo preamps with exceptional internal parts quality, including the electronic volume control chip, such as the Emotiva XSP-1 recently reviewed in Secrets (link shown below), will reveal any coloration of the LSI chip in the AVRs in direct mode.

www.hometheaterhifi.com/preamplifiers/preamplifiers-reviews/emotiva-xsp-1-balanced-stereo-preamplifier.html

The LSI analog chip also degrades the performance of the AVR when listening to the DAC for reasons outlined above.

Using multiple SSI analog chips was, a technique found in all top of the line AVRs for Japan but now only one is left. The use of multiple SSI analog chips remains in Pre/Pros designed and built in North America. Unfortunately, these units have higher prices. I cannot make definitive statements about top of the line units designed in the UK since I have no current service manuals

Most, not all, manufactures now fail to provide specific chip numbers on their website for components like the DAC, ADC, electronic volume controls, and opamps, leaving the consumer limited visibility with respect to parts quality. They have never specified the IC used for the multichannel electronic volume control if it is produced by Renesas, Rohm, or JRC.

A significant delay can occur between the time a product is released and when I gain access to the service manuals. Thus most of the products examined for this article are discontinued

Part II of this article will  provide more technical information about:

  • Construction options for improving performance of an AVR, Pre/Pro and stereo units with better performing analog SSI circuits.
  • Signal flow in the LSI analog AVR chips.

I would like to thank Peter Aczel, Jim Clements, Howard Ferstler, Jay Haider, Stephen Hornbrook, Chris Heinonen and Robert Kozel for reviewing this article.