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AES NEW YORK 2011

AES NEW YORK 2011. Product Design Session PD1. The Quietest Link Douglas Self. Noise and CMRR in balanced interconnections. The advantages of a balanced link. Noise due to current flowing through the link ground is cancelled out.

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AES NEW YORK 2011

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  1. AES NEW YORK 2011 Product Design Session PD1 The Quietest Link Douglas Self Noise and CMRR in balanced interconnections

  2. The advantages of a balanced link • Noise due to current flowing through the link ground is cancelled out. • Noise coupled to the link conductors (eg magnetically) is cancelled out.

  3. The basic balanced link

  4. The basic balanced input In all cases the input impedance at the +ve input is 20 KΩ With balanced input drive input impedance at the –ve input is only 6.66 KΩ The noise output (using 5532) is -104.8 dBu

  5. The basic unbalanced input Much quieter than 10K balanced input: by 14 dB Noise output = -119 dBu

  6. A practical balanced input EMC filters added (R5,C3 and R6,C4) DC blocking added (C5,C6 non-polar) Bandwidth limitation added (C1,C2)

  7. A high-end balanced input Common-mode choke added for greater EMC immunity R4 will usually be made trimmable for optimum CMRR

  8. N O I S E

  9. Noise advantage of a balanced output Noise out of inverter only = -113.5 dBu Bal input amp only noise out = -104.8 dBu Noise out of complete link = -104.5 dBu For a 1 Vrms input (+2.2 dBu) The link S/N ratio is 2.2 + 6 +104.5 = 112.7 dB. Balanced advantage = 5.4 dB If R2, R4 of input amp reduced to 5KΩ, for 1 Vrms internal: advantage = 5.9 dB

  10. The signal/noise ratio of a balanced link is determined by the noise performance of the balanced input amplifier

  11. High impedance balanced input amplifier Noise out with R = 10K is -105.1 dBu (22 – 22 kHz) For minimum input impedance of 47 KΩ via IN-, R = 68 KΩ (68 KΩ x 2/3 = 47 KΩ) Noise out = -98.6 dBu: 6.2 dB worse than with R = 10 KΩ

  12. High impedance balanced input amplifiers Using 5532 * R = 15 K gives a minimum input impedance of 10 K

  13. Low impedance balanced input amplifiers Too low for direct connection to outside world Using 5532 * Gives the minimum input impedance for loading a preceding 5532 stage

  14. 5532 opamp noise consistency Test circuit is the R = 10K balanced input amplifier. Noise output in dBu. (22 Hz - 22 kHz) All but two within a 0.5 dB range

  15. Improving the noise performance of the balanced input amplifier • Add input buffers so lower resistor values • can be used in the balanced amplifier stage 2) Use multiple amplifiers

  16. Improving the noise performance: 1 Added input buffers and lower balanced amp resistors Noise output = -110.2 dBu 5.1 dB better than the standard 10K balanced amp (-105.1 dBu)

  17. Improving the noise performance: 2 Adding a second balanced amplifier and averaging the outputs Noise output = -112.5 dBu 7.4 dB better than the standard 10K balanced amp (-105.1 dBu) 2.3 dB better than the previous amplifier There is also an analogous improvement in CMRR

  18. Improving the noise performance: 3 With four balanced amps, noise output = -114.0 dBu 8.9 dB better than the standard 10K balanced amp (-105.1 dBu) 1.5 dB better than the previous amplifier

  19. Improving the noise performance: 4 Doubling the input buffers Noise output = -116.2 dBu 11.1 dB better than standard 10K amp 2.2 dB better than previous version Looks unwieldy but can be very compact with SM technology. Only 4 opamp packages.

  20. Improving noise performance: summary • Standard 10K balanced amp -105.1 dBu 0 dB • Buffers + 820Ω balanced amp -110.2 dBu -5.1 dB • Buffers + dual 820Ω balanced amps -112.5 dBu -7.4 dB • Buffers + quad 820Ω balanced amps -114.0 dBu -8.9 dB • Dual buffers + quad 820Ω bal amps -116.2 dBu -11.1 dB

  21. Noise improvement: Balanced input stage noise out = -106.7 dBu 5.8 dB better Bandwidth defn filter noise out = -105.7 dBu 4.7 dB better

  22. Noise improvement in real life Cambridge Audio 840W Balanced input 0.9 dB quieter than unbalanced input CES Innovation Award in January 2008

  23. C M R R

  24. Optimising balanced amp CMRR • Source impedance imbalances • Input amplifier resistance imbalances • Finite op-amp gain • Finite op-amp bandwidth • Finite CMRR of op-amp itself What affects it?

  25. The basic balanced link again

  26. The effects on balanced amp CMRR Source impedance imbalance Balanced input amplifier R = 10 KΩ

  27. The effects on balanced amp CMRR Source impedance imbalance Balanced input amplifier R = 100 KΩ

  28. The effects on balanced amp CMRR Source impedance imbalance Balanced input amplifier R = 1 MΩ

  29. The impedance-balanced output The XLR connector is likely to be much more expensive than an extra 5532 section that would make it a true balanced output with an almost 6 dB noise advantage.

  30. The effects on balanced amp CMRR Finite opamp gain alone: opamp CMRR infinite

  31. The effect on balanced amp CMRR Finite opamp gain alone: opamp CMRR infinite

  32. The effects on balanced amp CMRR Finite opamp bandwidth LF open-loop gain of 100,000x gives an LF CMRR of -94 dB

  33. The effects on balanced amp CMRR Finite CMRR of the opamp itself If all resistors are exactly correct and opamp gain infinite the CMRR of the link is that of the balanced input amplifier opamp CMRR.

  34. The effects on balanced amp CMRR Input amplifier resistance imbalances Worst-case CMRR with all resistors at limit of their tolerance in the most unfavourable direction T is tolerance in %

  35. The effects on balanced amp CMRR Input amplifier resistance imbalances Worst-case CMRR with all resistors at limit of their tolerance in the most unfavourable direction This will never happen. In real life 1% resistors reliably give CMRR better than -40 dB

  36. The effects on balanced amp CMRR Using 5532 • Source impedance imbalance -80 dB • Finite opamp open-loop gain -94 dB • Op-amp CMRR -100 dB • Resistor imbalances -40 dB Resistor imbalances have the greatest effect on LF CMRR

  37. Optimising balanced amp CMRR As used in top-grade mixing consoles by the thousand. EMC & DC-blocking components not shown.

  38. Optimising balanced amp CMRR Trim improves CMRR from -50 dB to better than -80 dB up to 500 Hz 5532 opamp

  39. Other balanced input amplifier configurations

  40. Other balanced input amplifier configurations • High-impedance 2-opamp balanced amplifier • The “Superbal” balanced input amplifier • Gain-switched balanced input amplifier • Variable-gain balanced input amplifier • The instrumentation amplifier (see later)

  41. A high impedance balanced input amplifier

  42. A high impedance balanced input amplifier Built with AD8397 for operation from a +5V supply rail With Rg = 62Ω, R1A etc = 1 KΩ, Gain = +30.6 dB Noise out = -86.0 dBu Equiv Input Noise = -116.6 dBu Noise Figure = +13.8 dBu (ref 150 Ω source res; Johnson = -130.4 dBu)

  43. A high impedance balanced input amplifier CMRR plot Effect of capacitor to ground at end of R1A for single-rail operation

  44. A high impedance balanced input amplifier Snag: if gain is set to less than 2 times, headroom is impaired because the first opamp stage now has a gain of greater than one, while second stage has a gain of less than one.

  45. The “Superbal” balanced input amplifier Equal input impedances on the two inputs Attributed to Ted Fletcher of Alice. (Studio Sound Dec 1981) Reported by David Birt of the BBC 1990

  46. Gain-switched balanced input amplifier Gain is intermediate with switch in mid-position

  47. Variable-gain balanced input amplifier Gain range is -20 to +10 dB

  48. The instrumentation amplifier Noise and CMRR

  49. Application for balanced input with gain of 4 times Active crossover unit with elevated internal levels to increase signal/noise ratio by reducing effect of filter noise Assumption that there is only a limited-range level trim between crossover and power amplifier, so that power amp will always clip first

  50. Application for balanced input with gain of 4 times The active crossover is followed by a ±10 dB gain trim network that is normally expected to work at the 0 dB setting, ie giving 10 dB of attenuation and so reducing the noise from the input stage.

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