Floating/grounded source and balanced wire question

[Coytee]

I'm looking at a publication by Crown. It's in paper form in my hands or else I'd post it here. I'm trying to find it again and have posted a question to Crown to attempt to do so.

It shows various ways to wire XLR wires, depending on how things are wired.

Example: Balanced, grounded source... "for use with components equipped with three-wire grounded AC line cord or other ground connection" This diagram then shows the output from source XLR being wired "1,2,3" but, the input to the amplifier end of the cord is wired "2,3" and pin 1 wire is "shield not connected at this end". I don't know if that means this wire is totally disconnected or perhaps, attached to the sheathing since the diagram shows it to be circular as though it's wrapped around the other two wires, as contrasted with snipped and simply truncated prior to reaching the terminal end point.

Example 2: Balanced, floating source..."for use with components equipped with two-wire AC line cord or battery power" This diagram is more straight forth than the prior one in that the three wires from the "output from source" are wired 1,2,3 and the input to amplifier side is also wired '1,2,3' so it's simply a straight through wire.

Ok, that's the setup without pictures, sorry...I'll post them if I can get them.

Here's my question.... is this the "Pin #1 problem" that I've heard about before? Suggesting that sometimes you might need to alter how these are connected depending on having a grounded verses a floating source?

What constitutes the source?

If EVER SINGLE of my components has a 3 prong wire then I presume I can conclude that I have a grounded source.

What if however, my preamp has 3-prong (Peach) but my CD player is only two prong? Do I then have a grounded or floating source?

What if my preamp and 2-prong cd player are plugged into a 3-prong power strip so they use a ground prong on the plug side of the strip but, they are not grounded to the strip?

What if I change preamps and insert my Lexicon preamp which is a 2-prong?

Or.... is this only concerned with the immediate item PRIOR to the power amp, in my case, the Dx38 which has 3 prongs? How far up the food chain does one need to worry about?

[TNRabbit]

Perhaps I'm mistaken about what you're talking about, but when you brought up XLRs, realize you're talking apples & oranges. AC power & what is being transmitted via XLR interconnects are two separate things.

The XLR connections have:

Hot -a positive version of the audio signal

Cold - a Negative version of the same signal (out of phase 180 degrees)

Ground - normal ground

When the signal arrives at the next component, the negative is inverted (cancelling any noise picked up on the way) & summed into one signal.

Connecting any other way is erroneous. Balanced connection are also at a different dB level than unbalanced connections.

This is completely different than the three wires on the A/C signal.

[Coytee]

But alas...I'm only reporting that which I'm reading. This is why I'm bummed that I can't find the article to post

[TNRabbit]

Balanced Interconnections

Balanced inputs on power amplifiers are used to prevent noise and
crosstalk from affecting the input signal, especially in applications
where long interconnections are used. They are standard on professional
amplification equipment, and are steadily becoming more common in the
world of hi-fi. A balanced input amplifier is sometimes called a line
receiver. The basic principle of interconnection is to get the signal
you want by subtraction, using a three-wire connection. In some cases a
balanced input is driven by a balanced output, with two anti-phase
output signals, one signal wire (the hot or in-phase) sensing the
in-phase output of the sending unit, while the other senses the
anti-phase output.



In other cases, when a balanced input is driven by an unbalanced output,
as shown in Figure 20.3, one signal wire (the hot or in-phase) senses
the single output of the sending unit, while the other the cold or
phase-inverted) senses the unit's output-socket ground, and once again
the difference between them gives the wanted signal. In either of these
two cases, any noise voltages that appear identically on both lines
(i.e. common-mode signals) are in theory completely canceled by the
subtraction. In real life the subtraction falls short of perfection, as
the gains via the hot and cold inputs will not be precisely the same,
and the degree of discrimination actually achieved is called the
common-mode rejection ratio (CMRR), of which more later.

It is tedious to keep referring to non-inverting and inverting
inputs, and so these are usually abbreviated to 'hot' and 'cold'
respectively, though this does not necessarily mean that the hot
terminal carries more signal voltage than the cold one. For a true
balanced connection, the voltages will be equal. The 'hot' and 'cold'
terminals are also often referred to as In+ and In—, and this latter
convention has been followed in the diagrams here.

The subject of balanced interconnections is a large and subtle one, and a
big fat book could be written on this topic alone. A classic paper on
the subject is by Muncy. To keep it to a reasonable length, this section
has to concentrate on the areas most relevant to power amplifier
interconnection.

Advantages

• Balanced interconnections discriminate against noise and
  crosstalk, whether they result from ground currents, or electrostatic or
  magnetic coupling to signal conductors.
• Balanced connections make ground loops much less intrusive, and
  usually inaudible, so people are less tempted to start 'lifting grounds'
  to break the loop. This tactic is only acceptable if the equipment has a
  dedicated ground-lift switch that leaves the external metalwork firmly
  connected to mains safety earth. in the absence of this facility, the
  optimistic will remove the mains earth (not quite so easy now that
  molded mains plugs are standard) and this practice is of course
  dangerous, as a short-circuit from mains to the equipment chassis will
  result in live metalwork. A balanced interconnection incorporating a
  true balanced output gives 6 dB more signal level on the line, which
  should give 6 dB more dynamic range. However, this is true only with
  respect to external noise — as the section below describes, the electronics of a standard balanced input is more than 6dB noisier than the electronics of an unbalanced input.
• Balanced connections are usually made with XLR connectors. These
  are a professional three-pin format, and are a much superior connector
  to the phono (RCA) type normally used for unbalanced connections (more
  on this below).

Disadvantages

• Balanced inputs are inherently noisier than unbalanced inputs by a large margin,
  in terms of the noise generated by the input circuitry itself rather
  than external noise. This may appear paradoxical but it is all too true,
  and the reasons will be fully explained in this chapter.
• More hardware means more cost. Small-signal electronics is
  relatively cheap; unless you are using a sophisticated low-noise input
  stage (of which more later), most of the extra cost is likely to be in
  the balanced input connectors.
• Balanced connections may not provide much protection against RF
  intrusion — both legs of the balanced input would have to demodulate the
  RF in equal measure for common-mode cancelation to occur. This is not
  very likely, and it is important to provide the usual input RF filtering
  to avoid EMC difficulties.
• There are more possibilities for error when wiring up. For
  example, it is easy to introduce an unwanted phase inversion by
  confusing hot and cold in a connector, and this can go undiscovered for
  some time. The same mistake on an unbalanced system interrupts the audio
  completely.

[Don Richard]

Some folks float the receiver end of a balanced line for installed equipment, and hook the shield up for portable systems but that is not a requirement. I have the shields connected at both ends on all of my interconnects on my home system.

[Coytee]

Yes, that's a much nicer presentation of one half of what I'm looking at. The image you post reflects what I'm seeing in this Crown publication when they talk about Balanced, Floating Source. They have the three wires hooked up as (pardon my wording) straight pass through. Pin 1 to Pin 1, Pin 2 to Pin 2 and Pin 3 to Pin 3.

With the grounded source display, they have Pin 2 (female end) to Pin 2 (male end), Pin 3 (female end) to Pin 3 (male end) and Pin 1 (female end) wrapped around the male end saying "shield not connected at this end".

[TNRabbit]

More from "Audio Power amplifier design handbook" Douglas Self

Common-Mode Rejection Ratio

Figure 20.4 shows a balanced interconnection reduced to its bare
essentials: two source resistances, and a standard differential
amplifier. The balanced output in the source equipment is assumed to
have two exactly equal output resistances Rout + and Rout -, and the
balanced input in the receiving equipment has two exactly equal input
resistances R1, R2. The balanced input amplifier senses voltage
difference between the points marked In-+ (hot) and In— (cold) and
ideally completely ignores common-mode voltages that are present on
both.

The amount by which discriminates is called the common-mode
rejection ratio or CMRR, and is usually measured in dB. Supposes
differential voltage input between In + and In— gives an output voltage
of 0dB; then reconnect the input so that In+ and In— are joined together
and the same voltage is applied between the two of them and ground.
Ideally the result would be zero output, but in this imperfect world it
won't be, and in real life the output could be anywhere between —20 dB
(for a had balanced interconnect: and —140 dB (for a very good one). The
CMRR when plotted may have a flat section at low frequencies, but it
commonly degrades at high audio frequencies (more on this later).
In
one respect balanced audio connections have it easy. The common-mode
signal is normally well below the level of the unwanted signal, and so
the common-mode range of the input is not an issue.

The extremely simplified circuit of Figure 20.4, with a little
SPICE simulation, demonstrates the need to get these resistor values
right for good CMRR, before you even consider the rest of the circuitry.
The differential voltage sources Vout+,Vout- that represent the actual
balanced output are set to zero, and Vcm, which represents the
common-mode voltage drop down the cable ground, is set to 1V to give a
convenient result in dBv. The output resulting from the presence of this
voltage source is measured by a mathematical subtraction of the
voltages at In+ and In— so there is no actual input amplifier to confuse
the results with its non-ideal performance.

Let us start out
with Rout+, Rout_ = 100 ohm and R1, R2 = 10k, which are plausible values
and nice round figures. When all four resistances are exactly at their
nominal value, the CMRR is infinite, which on my simulator rather
worryingly appears to be exactly —400 dB (presumably that is the
mathematical 'noise floor'). If one of the output resistors or one of
the input resistors is then altered in value by 1%, then the CMRR drops
like a stone to —80dB. If the deviation is 10%, things are predictably
worse and the CMRR degrades to —60dB, as shown in Table 20.2. That would
be quite a good figure in real use, but since we have not begun to
consider op-amp imperfections or other circuit imbalances, and have only
altered one resistance out of the four that will in real circuitry all
have their own tolerances, it's a bit unsettling. Clearly we need to
understand how to improve things at this theoretical level before we
start to complicate the circuitry.

The essence of the problem is that we have two resistive
dividers, and we want them to have exactly the same attenuation. If we
increase the ratio between the output and input resistors, by reducing
the former or increasing the latter, the attenuation gets closer to
unity and variations in either resistor have less effect on it. If we
increase the input impedance to 100k, putting aside the technical
implications of doing this for the moment, things get 10 times better,
as the Rin/Rout ratio has improved from 100 to 1000 times. We now get
—100 dB for a l % resistance deviation and —80dB for a 10% deviation. An
even higher input impedance of 1 Mohm, if it can be managed, raises
Rin/Rout to 10,000, and gives —120 dB for a 1% resistance deviation and
—100 dB for a 10% deviation.

As another angle of attack, we can reduce the output impedances
to 1052, ignoring 'for the moment the need to secure against HF
instability caused by line capacitance, and return to an input impedance
of 100k. This again yields, as you have probably guessed, —120 dB for a
1% deviation and —100 dB for a 10% deviation.

In practical circuits, the combination of 68 ohm output resistors
and a 20 kohm input impedance is often encountered; the 68 ohm resistors
are about as low as you want to go with conventional circuitry, to
avoid HF instability. The 20kohm input impedance is what you get if you
make a basic balanced input amplifier with four 10 kohm resistors. I
strongly suspect that this value is chosen because it looks as if it
gives standard 10kohm input impedances — in fact it does nothing of the
sort, and the common-mode input impedance, which is what matters here,
is 20 kohm on each leg (more on this later). It turns out that 68ohm
output resistors and a 20 kohm input impedance give a CMRR of —89.5 dB
for a 1% deviation which is not at all bad. All these results are
summarized in Table 20.2.

The conclusion is simple: we want to have the lowest possible
output impedances and the highest possible input impedances to get the
maximum common-mode rejection. This is highly convenient because low
output impedances are already needed to drive multiple amplifier inputs
and cable capacitance, and high input impedances are needed to minimize
loading and maximize the number of amplifiers that can be driven.

[TNRabbit]

Combined Unbalanced and Balanced Inputs



Very often both unbalanced and balanced inputs are required, and it
is extremely convenient if it can be arranged so that no switching
between them is required — switches cost money. A handy way to do this
is shown in Figure 20.9, which for clarity omits most of the extra
components required for practical use that are referred to above. For
balanced use, simply connect to the balanced input and leave the
unbalanced input unterminated. For an unbalanced input, simply connect
to the unbalanced input and leave the balanced input unterminated. No
mode switch is required. These unterminated inputs sound as though they
would cause a lot of extra noise, but in fact the circuit works very
well and I have used it with success in high-end equipment.

As described above, in the world of hi-fi, balanced signals are at
twice the level of the equivalent unbalanced signals, and so the
balanced input must have a gain of 0.5 or —6 dB relative to the
unbalanced input to keep the same system gain by either path. This is
done here by increasing the value of R1 and R3 to 20 kQ. The balanced
gain of this circuit can be made either greater or less than unity, but
the gain via the unbalanced input is always unity. The differential gain
of the amplifier and the constraints on the component values for
balanced operation are shown in Figure 20.5, and are not repeated in the
text to save space. This applies to the rest of the balanced inputs in
this chapter.

There are a few compromises in
this scheme. The noise performance in the unbalanced input mode is worse
than for the sort of dedicated unbalanced input circuitry described
earlier in this chapter, because R2 remains effectively in the
signal path in unbalanced mode. Also, the input impedance of the
unbalanced input cannot be very high because it is determined by the
value of R4, and if this is raised all the resistances around the op-amp
must be increased proportionally and the noise performance is markedly
worsened. A vital point is that only one input cable should be connected
at a time. If an unterminated cable is left connected to an unused
input, then the extra cable capacitance to ground will cause
frequency-response anomalies and can in bad cases cause HF oscillation. A
warning on the back panel is a very good idea.

There, how's that for info overload?? []

[Coytee]

Whoo hoo....finally heard back from Crown

Page 7 is attached, otherwise, go to location & scroll to document page 7

http://www.crownaudio.com/pdf/133472.pdf

XLR.pdf

XLR.pdf

[Coytee]

There, how's that for info overload??

Mission accomplished [Y]

I will be (im)patiently waiting for the Cliff note version in english []

[TNRabbit]

Basically, Balanced is better at rejecting exterior noise sources but in and of itself is a noisier medium (due to it's internal summing devices) than dedicated uunbalanced where there is no external RF noise.