balanced / unbalanced
- Thread starter Anders
- Start date
SouthSIDE Glen
independentrecording.net
A balanced cable has a third conductor in it. The hot signal is split into two conductors (wthe the third being the ground). This is why balaced hables have three contacts in their plugs (a 1/4" Tip-Ring-Sleeve instead of the unbalanced 1/4" Tip-Sleeve connector, for example). But the phase inverted on the second hot line. Then on the other end the phase is un-inverted on the second hot cable and the signals re-combined.
The trick behind this is that most RF or other noise picked up in the cable length itself is picked up equally on the two hot conductors. When the phase on the second conductor is "un-inverted" at the receiving end, the signal itself is un-inverted, but the noise is inverted. Therefore when the two lines recombine, the un-inverted hot signal is combined with the regular hot signal and the result is the original hot signal. However, the now-inverted noise on the 2nd line is combined with the un-inverted noise on the first, and the two cancel each other out. The end result is a "balanced" cable is much less noisier over long distance or dangerous runs than an "unbalanced" cable is.
G.
The trick behind this is that most RF or other noise picked up in the cable length itself is picked up equally on the two hot conductors. When the phase on the second conductor is "un-inverted" at the receiving end, the signal itself is un-inverted, but the noise is inverted. Therefore when the two lines recombine, the un-inverted hot signal is combined with the regular hot signal and the result is the original hot signal. However, the now-inverted noise on the 2nd line is combined with the un-inverted noise on the first, and the two cancel each other out. The end result is a "balanced" cable is much less noisier over long distance or dangerous runs than an "unbalanced" cable is.
G.
Farview
Well-known member
The search function would help a lot.
Unbalanced cables have 1 conductor and a shield, like a guitar cable.
Balanced cables have 2 conductors and a shield, like a mic cable.
Balancing is just a way of keeping noise from being picked up along the cable run. It does this by sending two signals along the cable one of them with the polarity reversed. As the signal travels the cable it picks up noise on both conductors equally. At the other end the reversed signal is put back right. The two signals add together while the noise gets cancelled. That's it.
Unbalanced cables have 1 conductor and a shield, like a guitar cable.
Balanced cables have 2 conductors and a shield, like a mic cable.
Balancing is just a way of keeping noise from being picked up along the cable run. It does this by sending two signals along the cable one of them with the polarity reversed. As the signal travels the cable it picks up noise on both conductors equally. At the other end the reversed signal is put back right. The two signals add together while the noise gets cancelled. That's it.
SouthSIDE Glen
independentrecording.net
Show me, Sergeant!boingoman said:Unless, of course, the gear uses one of the balancing schemes that doesn't work like that.
G.
Hi..
don't mean to steal this thread..but I'm wondering something..
I have an audio snake, which has 4 XLR (f) return connectors. (the other end is paralleled XLR (m) and TRS)
I would like to send headphone mixes in stereo from my digi 002R to the snake. I would have to go dual mono 1/4" cable to male XLR..is this possible? is there such a Y cable adaptor?
Here's the item I own..
http://cgi.ebay.ca/ws/eBayISAPI.dll...0037641122&rd=1&sspagename=STRK:MEWN:IT&rd=1
thanks for your help,
-LIMiT
don't mean to steal this thread..but I'm wondering something..
I have an audio snake, which has 4 XLR (f) return connectors. (the other end is paralleled XLR (m) and TRS)
I would like to send headphone mixes in stereo from my digi 002R to the snake. I would have to go dual mono 1/4" cable to male XLR..is this possible? is there such a Y cable adaptor?
Here's the item I own..
http://cgi.ebay.ca/ws/eBayISAPI.dll...0037641122&rd=1&sspagename=STRK:MEWN:IT&rd=1
thanks for your help,
-LIMiT
SouthSIDE Glen
independentrecording.net
You're right, by definition, a "balanced" signal works only if the impedance of the two conducters is equivalent. And this equivalence is the value needed for the inversion trick to work at maximum efficiency. But to describe how balanced cables operate by describing the matching impedance value of the two conductors is like describing how a train gets you to work by measuring and describing the constant width between the track rails.boingoman said:Later, gotta go to work!
For now, suffice it to say it's the impedance that's important, not the signal symmetry, for good CMR. The inverted signal is common, but not necessary, for balanced operation.
But that does nothing to explain the difference between balanced and unbalanced, or to actually explain the balance mechanism, which was the question posed in this thread. The real difference is in the number of conducters; it's by definition impossible to have a "balanced" line unless you have two conductors refereced to the ground, it's that second conductor that allows it to become balanced. And the purposes of that second conductor is to provide the inverse polarity and to remove the idea of a shield acting in that capacity as well.
Anders, if you want the full story it in good detail, check out
http://www.audioholics.com/techtips/audioprinciples/interconnects/Balancedvsunbalanced.php
G.
SouthSIDE Glen said:
I should have made the disitinction between signal and noise. The second conductor need not carry signal, it only needs to carry noise to both sides of the balanced input. In one balancing scheme, on the output, the second conductor carries no audio at all, but is hooked to ground by a resistor, for instance.
SouthSIDE Glen
independentrecording.net
True. The key is to cancel the noise, and the signal split is not necessarily required for that aspect of it.boingoman said:
G.
BigRay
New member
http://www.vandenhul.com/artpap/wiring.htm
Human nature is highly non-linear, and definitely
reluctant to let previously bad decisions die
– Examples: the RCA phono plug, XLR pin 1
• In the Bible it is written that the sins of the fathers
are visited unto the 3rd and 4th generations…
– In other words, we must live with design practices and
standards based upon bad technical decisions that are
sometimes many decades old
• NO amount of shielding seems to be effective!
Analog Signal Connections
• There are only two basic types of analog signal
connection:
• Unbalanced
– usually associated with consumer audio or very low
impedance applications (power amp outputs)
• Balanced
– usually associated with Pro-Audio and Broadcast
The Unbalanced Connection
• The signal is carried on one line and referenced to the
ground line or chassis connection
• Only the signal line has a specified source impedance
– The reason it is called “unbalanced”
Typical Unbalanced Circuit
• Signal is referenced to “ground” as it is perceived
by the local output and input circuits
– However there is usually ground potential differences
between the output and input circuits (Vcrap)
Ground Potential Differences Are
Often >1 mV
• With a 1 V audio signal and 1 mV of Vcrap the
typical S/N ratio of an unbalanced connection is
only 60 dB
– 300-1000 V signal levels would be needed to achieve a
S/N equal to the dynamic range of today’s converters!
• There are two significant sources of ground
potential differences
Leakage Currents into Ground
• Leakage currents from AC Mains or other sources
can cause residual voltage potentials between the
chassis grounds of difference pieces of equipment
• Usually results from the capacitance between the
AC mains circuit and ground
– inter-winding capacitance of power transformers
– stray capacitance
– intentional capacitance (EMI filters)
The Ground Loop Problem
• If a ground loop exists, stray magnetic fields will
induce unwanted potentials in series with the ground
connection between any two pieces of equipment
– loop usually consists of the shields of interconnecting
cables and/or the earth safety grounding
The Simple Stereo Connection of
Two Unbalanced Cables Inherently
Creates a Ground Loop
• How the stereo cables are dressed significantly
affects the loop area
• Twisting stereo cables greatly reduces hum and
interference effects from stray magnetic fields
– Besides minimizing loop area, it also causes alternating loop
polarities
A Typical Setup has Many Loops
Basic Ways to Minimize Problems
with Unbalanced Connections
• Minimize the ground potential differences
in the first place
• Minimize the sensitivity of the circuit to
ground potential differences
“Brute Force” Grounding
• Idea is to create a much lower impedance connection
so any potential differences caused by leakage
currents will be carried by a large, multi-strand cable
– think of the circuit as a current divider
– may increase magnetic induction problems due to creation of a
new loop (watch location of the strap versus the signal cables!)
Floating Unbalanced Source
• Requires a transformer or complex output stage design
– uncommon in consumer gear
• Removes the ground loop possibility, but signal is still
susceptible to other common mode influences
Quasi-Floating Input Design
• If the input unbalanced connector can be isolated
from ground, differential input design can give big
improvements
– Generally need HF bypass capacitor to prevent EMI
susceptibility problems (typ 10-100 nF)
– Also need some DC shunting resistance if source is
truly floating (typ 10-500 Ohms)
– Used in Audio Precision analyzers
• Quasi-floating outputs are also possible
The Balanced Connection
• The signal is transmitted using two signal lines having
equal source impedance (Zs).
– Voltage symmetry is NOT a requirement!
• The signal path does NOT
include ground
• Ground loops involving
cable shield do not affect
the differential signal
(in theory)
Typical Balanced Circuit
• Gives best possible immunity to ground potential
differences provided both source impedance and
input impedance are well balanced
– Cable construction is also a factor
Balancing Offers Vastly Improved
Rejection of Ground Potentials
• Typical component tolerances give 40-50 dB CMRR
without circuit trimming or matching
– Capacitive balancing is usually more difficult to achieveresulting in some loss of S/N performance above 2-10 kHz
• 60 dB S/N with an Unbalanced connection can easily
improve to 100-110 dB S/N with a Balanced circuit
having the same signal amplitude
The Grounded Balanced Source
• Examples
– Active balanced driver
– Transformer with grounded center-tap
• Symmetrical or equal
output voltages
• Ground loop in signal
path is not possible
– But the cable shield can be
part of other loops
The Floating Balanced Circuit
• No explicit ground reference--but it’s still there!
• Common mode capacitance is usually present
• Output voltage symmetry not assured
– measuring one side to ground gives spurious readings
• Ground loop in the signal path is not possible
– but the cable shield can be part of another ground loop
Interfacing Balanced Outputs to
Unbalanced Inputs
– No problem with
truly floating
sources
– Try using only 1/2
of the output with
grounded or ground
referenced sources
Interfacing Unbalanced Outputs to
Balanced Inputs
• Not a serious problem as long as the cable is balanced
and the input impedance greatly exceeds the source
impedance
– Cable capacitance imbalance will cause problems
Tip for Connecting an Unbalanced
Output to a Balanced Input
• Add a resistor equal to the source impedance in the
ground segment of the unbalanced output to create a
balanced output
• Voltage symmetry is not a requirement of balancing!
Testing for connection problems:
• Analyze noise spectrum
– requires a high dynamic-range FFT analyzer
• Try a “Brute Force Ground”
– use an additional, separate, better ground connection
• Use the 400 Hz High-Pass filter as a quick check
of improvement
– compare noise readings with and without filter
Noise Spectrum Analysis
• Will almost always show some AC mains frequency
• 2nd harmonic (100/120) can indicate full-wave or bridge
rectifier filter inadequacy
• 3rd harmonic (150/180)
caused by magnetic fields
of power transformers
• Higher frequency noise
and spurious spikes can
be created by motors and
lamp dimmers
Brute Force Ground
• Provide a very low resistance ground connection
between the devices and see if the problem is reduced.
– use #12 or #10 with many strands
THD+N or Noise vs. Hi-Pass Filter
• Compare Noise or THD+N readings in absolute units (V,
dBu, dBV, etc.) with and without 400 Hz high-pass filter
– An improvement of more than about 0.4 dB is usually an
indication of significant hum (AC mains related products)
The XLR Pin 1 Problem
• NEVER connect Pin 1 to the signal circuit ground or
circuit board ground plane
• ALWAYS connect Pin 1 directly to chassis ground at
both ends of cable
– use the lowest possible resistance and inductance
– leaving Pin 1 floating at either end of the cable invites
serious EMI problems
Summary
• High quality analog signal connection requires:
– (1) understanding of basic electronic principals
– (2) careful attention to detail
• High quality analog signal connection does not
happen by chance!
Human nature is highly non-linear, and definitely
reluctant to let previously bad decisions die
– Examples: the RCA phono plug, XLR pin 1
• In the Bible it is written that the sins of the fathers
are visited unto the 3rd and 4th generations…
– In other words, we must live with design practices and
standards based upon bad technical decisions that are
sometimes many decades old
• NO amount of shielding seems to be effective!
Analog Signal Connections
• There are only two basic types of analog signal
connection:
• Unbalanced
– usually associated with consumer audio or very low
impedance applications (power amp outputs)
• Balanced
– usually associated with Pro-Audio and Broadcast
The Unbalanced Connection
• The signal is carried on one line and referenced to the
ground line or chassis connection
• Only the signal line has a specified source impedance
– The reason it is called “unbalanced”
Typical Unbalanced Circuit
• Signal is referenced to “ground” as it is perceived
by the local output and input circuits
– However there is usually ground potential differences
between the output and input circuits (Vcrap)
Ground Potential Differences Are
Often >1 mV
• With a 1 V audio signal and 1 mV of Vcrap the
typical S/N ratio of an unbalanced connection is
only 60 dB
– 300-1000 V signal levels would be needed to achieve a
S/N equal to the dynamic range of today’s converters!
• There are two significant sources of ground
potential differences
Leakage Currents into Ground
• Leakage currents from AC Mains or other sources
can cause residual voltage potentials between the
chassis grounds of difference pieces of equipment
• Usually results from the capacitance between the
AC mains circuit and ground
– inter-winding capacitance of power transformers
– stray capacitance
– intentional capacitance (EMI filters)
The Ground Loop Problem
• If a ground loop exists, stray magnetic fields will
induce unwanted potentials in series with the ground
connection between any two pieces of equipment
– loop usually consists of the shields of interconnecting
cables and/or the earth safety grounding
The Simple Stereo Connection of
Two Unbalanced Cables Inherently
Creates a Ground Loop
• How the stereo cables are dressed significantly
affects the loop area
• Twisting stereo cables greatly reduces hum and
interference effects from stray magnetic fields
– Besides minimizing loop area, it also causes alternating loop
polarities
A Typical Setup has Many Loops
Basic Ways to Minimize Problems
with Unbalanced Connections
• Minimize the ground potential differences
in the first place
• Minimize the sensitivity of the circuit to
ground potential differences
“Brute Force” Grounding
• Idea is to create a much lower impedance connection
so any potential differences caused by leakage
currents will be carried by a large, multi-strand cable
– think of the circuit as a current divider
– may increase magnetic induction problems due to creation of a
new loop (watch location of the strap versus the signal cables!)
Floating Unbalanced Source
• Requires a transformer or complex output stage design
– uncommon in consumer gear
• Removes the ground loop possibility, but signal is still
susceptible to other common mode influences
Quasi-Floating Input Design
• If the input unbalanced connector can be isolated
from ground, differential input design can give big
improvements
– Generally need HF bypass capacitor to prevent EMI
susceptibility problems (typ 10-100 nF)
– Also need some DC shunting resistance if source is
truly floating (typ 10-500 Ohms)
– Used in Audio Precision analyzers
• Quasi-floating outputs are also possible
The Balanced Connection
• The signal is transmitted using two signal lines having
equal source impedance (Zs).
– Voltage symmetry is NOT a requirement!
• The signal path does NOT
include ground
• Ground loops involving
cable shield do not affect
the differential signal
(in theory)
Typical Balanced Circuit
• Gives best possible immunity to ground potential
differences provided both source impedance and
input impedance are well balanced
– Cable construction is also a factor
Balancing Offers Vastly Improved
Rejection of Ground Potentials
• Typical component tolerances give 40-50 dB CMRR
without circuit trimming or matching
– Capacitive balancing is usually more difficult to achieveresulting in some loss of S/N performance above 2-10 kHz
• 60 dB S/N with an Unbalanced connection can easily
improve to 100-110 dB S/N with a Balanced circuit
having the same signal amplitude
The Grounded Balanced Source
• Examples
– Active balanced driver
– Transformer with grounded center-tap
• Symmetrical or equal
output voltages
• Ground loop in signal
path is not possible
– But the cable shield can be
part of other loops
The Floating Balanced Circuit
• No explicit ground reference--but it’s still there!
• Common mode capacitance is usually present
• Output voltage symmetry not assured
– measuring one side to ground gives spurious readings
• Ground loop in the signal path is not possible
– but the cable shield can be part of another ground loop
Interfacing Balanced Outputs to
Unbalanced Inputs
– No problem with
truly floating
sources
– Try using only 1/2
of the output with
grounded or ground
referenced sources
Interfacing Unbalanced Outputs to
Balanced Inputs
• Not a serious problem as long as the cable is balanced
and the input impedance greatly exceeds the source
impedance
– Cable capacitance imbalance will cause problems
Tip for Connecting an Unbalanced
Output to a Balanced Input
• Add a resistor equal to the source impedance in the
ground segment of the unbalanced output to create a
balanced output
• Voltage symmetry is not a requirement of balancing!
Testing for connection problems:
• Analyze noise spectrum
– requires a high dynamic-range FFT analyzer
• Try a “Brute Force Ground”
– use an additional, separate, better ground connection
• Use the 400 Hz High-Pass filter as a quick check
of improvement
– compare noise readings with and without filter
Noise Spectrum Analysis
• Will almost always show some AC mains frequency
• 2nd harmonic (100/120) can indicate full-wave or bridge
rectifier filter inadequacy
• 3rd harmonic (150/180)
caused by magnetic fields
of power transformers
• Higher frequency noise
and spurious spikes can
be created by motors and
lamp dimmers
Brute Force Ground
• Provide a very low resistance ground connection
between the devices and see if the problem is reduced.
– use #12 or #10 with many strands
THD+N or Noise vs. Hi-Pass Filter
• Compare Noise or THD+N readings in absolute units (V,
dBu, dBV, etc.) with and without 400 Hz high-pass filter
– An improvement of more than about 0.4 dB is usually an
indication of significant hum (AC mains related products)
The XLR Pin 1 Problem
• NEVER connect Pin 1 to the signal circuit ground or
circuit board ground plane
• ALWAYS connect Pin 1 directly to chassis ground at
both ends of cable
– use the lowest possible resistance and inductance
– leaving Pin 1 floating at either end of the cable invites
serious EMI problems
Summary
• High quality analog signal connection requires:
– (1) understanding of basic electronic principals
– (2) careful attention to detail
• High quality analog signal connection does not
happen by chance!
