Technical Articles and Editorials
- Written by Dr. David A. Rich
- Published on 23 September 2013
- AVR - Audio Video Receiver - Build Quality: Part I
- Page 2: Understanding DAC Specifications
- Page 3: Digital Reconstruction Filter
- Page 4: Number of DACs per Chip
- Page 5: Improved Distortion and Noise Performance with Balanced DAC Output
- Page 6: Enhanced Distortion Performance with Current Mode DACs
- Page 7: Multiple DACs Combined to Produce a Single Channel
- Page 8: Chart Presenting Build DACs used in AVRs Across Manufacturers and Price
- Page 9: The Right Side of the Chart: More Details about the AVRs and Pre/Pros
- Page 10: The concept of Effective Bits
- Page 11: Single Chip Analog AVR LSI
- Page 12: Enhanced Performance with SSI Parts
- Page 13: Limitations of Operational Amplifier Performance with the Single Chip Analog AVR LSI
- Page 14: Limitations on the Performance of Semiconductor Switches with the Single-Chip Analog AVR LSI
- Page 15: Use of Relays to Achieve Better Performance
- Page 16: A Very Brief Look at Changes in Power Amps in AVRs
- Page 17: Conclusions
- All Pages
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
- 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:
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.