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Author Topic: Pseudo balanced outputs?  (Read 27022 times)

kristianjohnsen

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Re: Pseudo balanced outputs?
« Reply #20 on: March 02, 2012, 07:51:13 am »

..He may be running 24 tracks of playback from the stage?? AFAIK, a pseudo-balanced output uses a non-differential driver with common tied to Cold/Ring. Easy enough to check with a multimeter.

Hello, just starting to get into multitracking so please forgive me if this is a dumb question:  In what situations would one run the outputs of the multitrack down a long snake?  The best I can think of is virtual soundcheck where both monitor desk and FOH desk needs the audio at the same time.
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brian maddox

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Re: Pseudo balanced outputs?
« Reply #21 on: March 02, 2012, 08:09:46 am »

Hello, just starting to get into multitracking so please forgive me if this is a dumb question:  In what situations would one run the outputs of the multitrack down a long snake?  The best I can think of is virtual soundcheck where both monitor desk and FOH desk needs the audio at the same time.

actually, i suspect the OP is running tracks as additional enhancement for a live show and they are being run from on stage so that the performers have control over when and where the cues happen.  this has become increasingly commonplace as bands try to recreate the '85 guitar tracks' studio sound live.  running it from the stage does also make everything show up in whatever consoles are being used, which goes to your question.

the hd24 is a tool that has found some popularity in this application as it is a hardware recording system with 24 track capability and it is a fairly stable platform.  and yes, the outputs are balanced and can easily be run down a standard 100 meter or less analog snake without any issues.  or course, DI's are still a good idea for ground loop isolation, etc. but that's another topic....
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kristianjohnsen

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Re: Pseudo balanced outputs?
« Reply #22 on: March 02, 2012, 08:27:18 am »

actually, i suspect the OP is running tracks as additional enhancement for a live show and they are being run from on stage so that the performers have control over when and where the cues happen.  this has become increasingly commonplace as bands try to recreate the '85 guitar tracks' studio sound live.  running it from the stage does also make everything show up in whatever consoles are being used, which goes to your question.

the hd24 is a tool that has found some popularity in this application as it is a hardware recording system with 24 track capability and it is a fairly stable platform.  and yes, the outputs are balanced and can easily be run down a standard 100 meter or less analog snake without any issues.  or course, DI's are still a good idea for ground loop isolation, etc. but that's another topic....

That makes sense.  Most of the act I work with that need "enhancement tracks" have already mixed their tracks to stereo and just run off a mac with an external soundcard, but I definitively see the advantages and why people would do in the manner you describe.  Thanks.
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Mac Kerr

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Re: Pseudo balanced outputs?
« Reply #23 on: March 02, 2012, 10:47:23 am »

Maybe I'm misunderstanding, but your two paragraphs seem to be saying opposite things.  I disagree with your second paragraph: a balanced audio circuit requires that all parts of the circuit (output, cable, and input) have balanced common-mode impedance (as you describe in your first paragraph), but imposes no requirement for voltage symmetry.  Voltage symmetry might help in some cases, but that's a minor subtlety in comparison.

It is a question of unclear terminology. We are saying the same thing. When I said "Audio" I meant the electrical signal, and when I said "Circuit" I meant the electrical circuit.

Mac
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Marty McCann

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Re: Pseudo balanced outputs?
« Reply #24 on: March 02, 2012, 01:02:18 pm »

Quasi or Pseudo-balanced outputs are found on many entry level mixers.  And although they function as line drivers, they are not good current sources (adequate or worse) depending on the source impedance and the load impedance.

If you have more than a few power amps that have the inputs paralleled, there can be issues as 20 K Ohms won't require as much current from the source as say a 2 K Ohm load.

And yes quasi or pseudo-balanced outputs are NOT balanced voltage wise.   Often on these entry level mixers, they simply have two 100 Ohm resistors (or higher on the real cheapies) in series with ground with one pin picking off at the 1st resistor and the one pin picking off or tapped at the junction of the two resistors.  This results in one of the two lines having twice the voltage of the other or a difference of -6 dB on the low side.

Now many of you do understand that this should not be a problem.  Should not as again in all things audio the answer may be it depends.  It depends on what it is driving.   

Sending the signal to a single power amp with a balance differential input of 10 K Ohms on each leg (20 K total Z) should not be a problem.

However if there should be a DSP in between, you may have a potential problem with unsymmetrical clipping of the analog front-end or even problems in the A/D conversion.  How is that?

Some DSP designers recognize that is is best to not permit symmetrically clipping of the input of an A/D converter.  So in the gain stages before the A/D they may actually choose a scheme where they have unsymmetrical DC rails on the IC.   Yes this is done.  They may use +5 VDC on one rail and -8 VDC on another. 

Now think about this possibility . . . .  You happen to interconnect your system in a manner where the signal line that is +6 dB hotter is  on the lesser voltage rail.  You would then have -10 dB less headroom on one side of voltage window.

Now I think we all would agree that this is far from a professional audio signal chain.

Your welcome!
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John Livings

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Re: Pseudo balanced outputs?
« Reply #25 on: March 02, 2012, 01:24:13 pm »

On a Alesis Adat Hd24 it states it has pseudo balanced outputs what exactly does that mean? Would I be better of running the outputs through a di for better shielding and longer runs? Is it just the ground and shield tied together?

This may help,

http://tech.groups.yahoo.com/group/hd24/

Regards,  John

P.S. I should clarify, The link was to answer the OPs post,

"I'm talking about the outputs. For instance running signals down a snake to FOH a couple hundred feet away."

I have great respect for the many people that contribute to this forum and meant no disrespect, I was only offering a link to a forum I use when I need help with my HD-24 XR.

Regards,  John
« Last Edit: March 02, 2012, 07:04:50 pm by John Livings »
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John Roberts {JR}

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Re: Pseudo balanced outputs?
« Reply #26 on: March 02, 2012, 03:43:26 pm »

Hi Marty I hope you are well... For any new guys, Marty has decades of experience

[edit- oops confused two threads /edit]
« Last Edit: March 02, 2012, 03:49:42 pm by John Roberts {JR} »
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Dave Dermont

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Re: Pseudo balanced outputs?
« Reply #27 on: March 03, 2012, 04:03:59 am »

What about lecterns...er, podiums...or...


 ::)

You stand ON a podium.

You stand AT a lectern.

 8)
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Nick Hickman

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Re: Pseudo balanced outputs?
« Reply #28 on: March 03, 2012, 07:26:56 am »

Hi Marty,

Quasi or Pseudo-balanced outputs are found on many entry level mixers.

True, but they're not the exclusive preserve of budget gear.  I already mentioned their use on Neumann and AKG microphones.  (I haven't confirmed that to be the case, BTW, but I'm sure I remember hearing it from Ray Rayburn.)

Quote
And although they function as line drivers, they are not good current sources (adequate or worse) depending on the source impedance and the load impedance.

I'm not sure why the single leg of an asymmetric output is inherently any different with regard to current delivery capacity from the legs of a symmetric (push-pull) output.  On inexpensive kit, I imagine either could be done badly and/or cheaply.

Quote
Often on these entry level mixers, they simply have two 100 Ohm resistors (or higher on the real cheapies) in series with ground with one pin picking off at the 1st resistor and the one pin picking off or tapped at the junction of the two resistors.  This results in one of the two lines having twice the voltage of the other or a difference of -6 dB on the low side.

That isn't an arrangement I'm familiar with and seems to yield unequal common-mode impedances on the two legs, rendering it unbalanced.  When referring to an asymmetric (or "hot-driven") impedance-balanced output, I'm thinking of the hot leg driven by an opamp (just as each side of a push-pull output would be), and the cold leg tied to 0V through the same components as the driven leg to give it the same common-mode impedance (at least within the parameters of normal use of the driven output).

Quote
However if there should be a DSP in between, you may have a potential problem with unsymmetrical clipping of the analog front-end or even problems in the A/D conversion.  How is that?

Some DSP designers recognize that is is best to not permit symmetrically clipping of the input of an A/D converter.  So in the gain stages before the A/D they may actually choose a scheme where they have unsymmetrical DC rails on the IC.   Yes this is done.  They may use +5 VDC on one rail and -8 VDC on another. 

Now think about this possibility . . . .  You happen to interconnect your system in a manner where the signal line that is +6 dB hotter is  on the lesser voltage rail.  You would then have -10 dB less headroom on one side of voltage window.

I'm not sure of the reason for using asymmetric supply rails on a converter but I'd hope that, as the user of a piece of audio equipment rather than of an ADC chip, such details would be hidden from me.  I'd hope the piece of equipment would have an input spec in line with normal interfacing conventions, i.e. balanced common-mode impedance on its input legs, and a reasonable maximum input voltage on both.

Nick
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John Roberts {JR}

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Re: Pseudo balanced outputs?
« Reply #29 on: March 03, 2012, 09:44:51 am »

Hi Marty,

True, but they're not the exclusive preserve of budget gear.  I already mentioned their use on Neumann and AKG microphones.  (I haven't confirmed that to be the case, BTW, but I'm sure I remember hearing it from Ray Rayburn.)
+1
Quote
I'm not sure why the single leg of an asymmetric output is inherently any different with regard to current delivery capacity from the legs of a symmetric (push-pull) output.  On inexpensive kit, I imagine either could be done badly and/or cheaply.
Yup, while all things equal the dual differential outputs drive can result in 4x the current draw of single legged drive so products will rarely have 2 legged drive "and" low drive capability.

For value gear it's a fielders choice, so YMMV.
Quote
That isn't an arrangement I'm familiar with and seems to yield unequal common-mode impedances on the two legs, rendering it unbalanced.  When referring to an asymmetric (or "hot-driven") impedance-balanced output, I'm thinking of the hot leg driven by an opamp (just as each side of a push-pull output would be), and the cold leg tied to 0V through the same components as the driven leg to give it the same common-mode impedance (at least within the parameters of normal use of the driven output).
+1 I don't recognize that eother.
Quote
I'm not sure of the reason for using asymmetric supply rails on a converter but I'd hope that, as the user of a piece of audio equipment rather than of an ADC chip, such details would be hidden from me.  I'd hope the piece of equipment would have an input spec in line with normal interfacing conventions, i.e. balanced common-mode impedance on its input legs, and a reasonable maximum input voltage on both.

Nick

That may be an isolated design quirk related to a specific opamp having different drive capability to each rail.  IIRC 5V and 8V are standard regulator voltages. So an opamp requiring slightly more power supply headroom on the negative rail, could end up with +5v /-8V . This should all be invisible to the end user, and there shouldn't be any difference between treatment of + and - inputs, other than polarity.

JR

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Re: Pseudo balanced outputs?
« Reply #29 on: March 03, 2012, 09:44:51 am »


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