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Old 31st March 2011, 08:40 PM   #101
RCruz is offline RCruz  Switzerland
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Quote:
Originally Posted by john curl View Post
Today, we try to SERVO, in order to remove all the coupling caps, either: input, output, or feedback, if possible (more later)
Looking forward for the (more later) info to come.

I had enourmous trouble "voicing" my Riaa preamp because it depends so strongly upon caps behaviour... I Experimented with many and now I am satisfyed with my present "cooking" but it is far from perfect. I could not use expensive caps so I had to try severall bypass combinations until I got pleasant results.
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Old 31st March 2011, 09:16 PM   #102
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How do you make your measurements ?
See Linear Audio Vol 1 for lots of data.
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Old 31st March 2011, 09:54 PM   #103
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Originally Posted by simon7000 View Post
Or in complex terms, does the statistical model hold at the extremes?

ES
Without a major research facility at your disposal you could never get into the realm of the extreme.

Even two #40 wires just touching with 1uA in them involves millions of atoms and ~10^12 electrons per second. Gold wire stretched until there is a single atom left will demonstrate the quantized Hall effect as it breaks, this would be a good place to start.
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Old 1st April 2011, 07:23 AM   #104
MiiB is offline MiiB  Denmark
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What were the silent resistors...??
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Old 1st April 2011, 09:12 AM   #105
RCruz is offline RCruz  Switzerland
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Takman
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Old 1st April 2011, 12:56 PM   #106
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Default Use of DC servos

As John said, the use of DC servos is a preferred technique for managing DC offset and avoiding the use of DC blocking capacitors that may degrade sound quality. In fact, I have an entire chapter on the use of DC servos in my Designing Audio Power Amplifiers book. Although treated there in the context of power amplifiers, most of the material applies equally well to the use of DC servos in phono preamps. There are some design subtleties that are important in applying DC servos that need to be understood.

Servo is a feedback loop and all of its components are in the signal path. This is especially true of the op amp(s) and the integrating capacitor(s). The op amp should be of audio grade, as it can inject noise and distortion into the signal path. Similarly, the integrating capacitor should be of high quality, polypropylene at minimum. Fortunately, the integrating capacitors used in DC servos do not usually have to be very large, so cost consideration of using high-quality capacitors are mitigated a bit.

Never use a DC servo that employs a non-inverting integrator. Unfortunately, this approach is popular with some designers. This circuit configures the integrator as a differential amplifier, using two integrating capacitors. In most circuits the use of the non-inverting integrator saves an op amp at the expense of an additional integrating capacitor. This is a mis-guided and poor trade-off in terms of both board area and cost. Worse yet, the common mode rejection of the non-inverting integrator depends on precise matching of the two capacitors, which is usually not available. Poor common mode rejection in the differential integrator can alter the servo frequency characteristics and allow unintended amounts of output back to the input stage.

Cheers,
Bob
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Old 1st April 2011, 02:24 PM   #107
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I've been toying with optical servos. True integration (I into C) and low value caps 1000pF will do.
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Old 1st April 2011, 02:25 PM   #108
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The use of servos and +/- supplies allows coupling cap free design. This has an extra benefit of reducing the number of IN SERIES connections in the audio path. The 'problems' are put in a parallel path and can be minimized or avoided, instead.
It must be said that a capacitor is not just the ideal device represented by a symbol on a schematic. It has leads (sometimes magnetic) dissimilar metal junctions (sometimes lousy) and it has an imperfect dielectric material, in virtually every case, except vacuum.
Care must be taken with the servos, however, and they should be DECOUPLED as much as possible from the audio itself.
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Old 1st April 2011, 02:43 PM   #109
SY is offline SY  United States
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Quote:
Originally Posted by john curl View Post
Care must be taken with the servos, however, and they should be DECOUPLED as much as possible from the audio itself.
Put that in big letters. With some attention, the requirements that Bob rightly brought up can be minimized- a properly implemented servo can get by with moderately good opamps and caps, nothing fancy needed.
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Old 1st April 2011, 03:09 PM   #110
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I've been toying with optical servos. True integration (I into C) and low value caps 1000pF will do.
In power amplifier input pairs I place power resistors next to the devices. These allows me thermal control of the offset from the bipolar input transistors! I also use lm34s to set the reference level!
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