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Balanced connections

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Ash View Drop Down
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    Posted: 17 Mar 2021 at 9:42pm
I guess it depends on the susceptibility to interference for the particular scenario. Short cable runs may not benefit from balanced connections. With balanced connections, you have three signal options to compare. Standard phase (single-ended/unbalanced), inverted phase (so-called R3) or balanced (both phases). Between DAC and amps, I use inverted phase Lautus 1.5m instead of balanced Libran 1.5m as the benefit of sheath ferrites probably outweighs the benefit of having both phases. Perhaps an ultra-short Libran might beat the 1.5m Lautus though, just because shorter is better?
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 17 Mar 2021 at 10:32pm
Originally posted by layercake1 layercake1 wrote:

It probably costs more to design a fully balanced circuit so that’s a reason to exclude it on budget gear. 



What are you insinuating?
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Post Options Post Options   Thanks (0) Thanks(0)   Quote lfc jon Quote  Post ReplyReply Direct Link To This Post Posted: 17 Mar 2021 at 10:37pm
I was told shorter is better if using RCAs, but XLR was better over long distances, that was said to me as a general rule 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 17 Mar 2021 at 10:39pm

To be honest, my patience at my time of life is wearing rather thin. Especially after serving as senior engineer designing all-balanced professional broadcast mixing desks (the above being a stereo desk based on my BBC journalist workstation desk). Do I have to constantly fend off the bullsh*t brigade?





Edited by Graham Slee - 17 Mar 2021 at 10:49pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 18 Mar 2021 at 12:08am
Balanced is the feeding of phase and anti-phase signals down a twisted pair, from a 600-ohm source to a 600-ohm receiver (150 ohms using a transformer).

Show me any modern hi-fi gear capable of sending at 600 ohms? OK then, I'll name one - the Majestic.

600 ohms is suitable for the telephone system and theatre snakes, whilst high-quality transmitter feeds will be 150 ohms via transformers. That is how program leaves a studio, connected to underground "BT lines," and emerging at a transmitter site some one to two miles away. Feeds to repeaters (AM and FM) might then be by microwave.

Interesting fact: The Viking AM "Euro transmitter" is on the other side of the Humber estuary, so either the BT line goes the long way round or goes by an underwater cable. The transmitter site remains; although the station closed, it could be picked up anywhere in western Europe. The feed was line level.

To obtain the required balance for noise cancellation, required a gain tweak in the non-inverting opamp input attenuator. The minimum signal gain is 2, although "noise gain" is unity. The reason for using opamps is their gain accuracy. The balance tweak was set on-test using an audio analyser measuring precision mixing resistors, such that the signal was reduced to an absolute minimum, below the noise floor.

The arrangement used was designed by BBC engineer David Birt. Virtually all high quality feeds used NE5534 opamps.

An improvement on Mr Birt's design is the all-inverting version because of it having no common-mode distortion. The conventional feed uses inverting and non-inverting stages for the phase and anti-phase signals. The non-inverting stage suffers common-mode distortion, which is compounded by the same occurrence in the receiving circuit.

In most studios, balanced was mandatory only because it was used everywhere else in the station, and it is cost-effective to stock XLR's and TRS jacks rather than a mix and match of connectors.

Some might think the XLR is used exclusively, but they'd be wrong. TRS features extensively in rack rooms for signal routing via jackfields. These tend to use the old-style PO jacks and sockets (the Majestic accepts both). My colleague, John C, has many years of experience wiring them into racks.

The reason for using balanced is NOT to avoid grounding problems (although the bullsh*t hi-fi mantra goes like that).

To say that balanced sounds better than a single-sided coax over a few metres is, in my experience, highly biased. It is marketing hype, and that is why I have so little patience when, after years of talking about balanced audio, another starts yet another topic about it.

As I've said, please show me some balanced hi-fi unit which complies with impedance matching. Show me some hi-fi unit where the balance has been tweaked on a precision test to be actually balanced, and not just for boast!
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Post Options Post Options   Thanks (0) Thanks(0)   Quote BAK Quote  Post ReplyReply Direct Link To This Post Posted: 18 Mar 2021 at 12:46am
Graham quote: "To obtain the required balance for noise cancellation, required a gain tweak in the non-inverting opamp input attenuator. The minimum signal gain is 2, although "noise gain" is unity. The reason for using opamps is their gain accuracy. The balance tweak was set on-test using an audio analyser measuring precision mixing resistors, such that the signal was reduced to an absolute minimum, below the noise floor." 
 Balanced signal will not be truly balanced unless the + and - signal components (XLR pins 2 and 3) are exactly equal.
 When the designer makes the effort to make the + and - signal components exactly equal and 
makes them able to drive a 600 ohm load,
 this is a sign of a true professional.

 Not all balanced outputs are equal.
Many cut corners for cost only to say they have balanced outputs.
Bruce
AT-14SA, Pickering XV-15, Hana EL, Technics SL-1600MK2, Lautus, Majestic DAC, Technics SH-8055 spectrum analyzer, Eminence Beta8A custom cabs; Proprius & Reflex M or C, Enjoy Life your way!
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Ash Quote  Post ReplyReply Direct Link To This Post Posted: 18 Mar 2021 at 7:08am
I think the rationale behind having two phases of equal/matched amplitude, one inverted and one non-inverted, is that when both signals are overlaid, the interference common to both can be completely eliminated.
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