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Author Topic: Active balanced output topologies that can safely drive an unbalanced input  (Read 926 times)

drew gandy

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I only ran into one case where impedance balanced stumbled and that was back during the pin 2/pin 3 hot transition where an unbalanced input was looking for signal on the wrong one hot leg (embarrassing because it was two Peavey SKUs).


I ran into a problem many years ago with what I believe was a cheapskate impedance balanced output. On a corporate talking head gig (some music playback for stingers and maybe a video or two) I was looking for a post fader aux output to feed some subwoofers at the other end of the snake. The Midas Venice had 2 aux outputs on XLRs that are supposed to be balanced and 2 "FX" aux sends on 1/4" TRS that are also supposed to be balanced. Since my XLR aux sends were already in use I used one of the "FX" sends with a TRS to XLR adaptor feeding through a 150'+ snake and into a DSP in the amp rack. It buzzed - badly enough that I needed a different solution. Switching the subwoofer send to one of the XLR "real" aux sends worked cleanly.

I believe (and I think professor Whitlock has basically said this) that it's the receiving end that does most of the heavy lifting. I'm not remembering now what model the DSP was but I suspect it was taking some shortcuts. In this situation, at least, the sending end made a difference.
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Mac Kerr

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I believe (and I think professor Whitlock has basically said this) that it's the receiving end that does most of the heavy lifting. I'm not remembering now what model the DSP was but I suspect it was taking some shortcuts. In this situation, at least, the sending end made a difference.

I think the issue was as likely the TRS>XLR adaptor as anything else you mentioned.

Mac
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Frank Koenig

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Impedance balanced is "balanced" and is perfectly fine for 99.99% of applications. Don't hate it because it is so cost effective.

I don't hate impedance-balanced. It even works quite well for balanced digital signals such as AES. I've made passive (all-resistor) SPDIF to AES convertors. If you don't impedance balance the output and try to send the resulting (pseudo) AES over a cable, it fails. Present equal source resistances to both legs and it rocks. (Not for critical-gig use, laboratory only, etc.  ;) )

--Frank
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John Roberts {JR}

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I ran into a problem many years ago with what I believe was a cheapskate impedance balanced output. On a corporate talking head gig (some music playback for stingers and maybe a video or two) I was looking for a post fader aux output to feed some subwoofers at the other end of the snake. The Midas Venice had 2 aux outputs on XLRs that are supposed to be balanced and 2 "FX" aux sends on 1/4" TRS that are also supposed to be balanced. Since my XLR aux sends were already in use I used one of the "FX" sends with a TRS to XLR adaptor feeding through a 150'+ snake and into a DSP in the amp rack. It buzzed - badly enough that I needed a different solution. Switching the subwoofer send to one of the XLR "real" aux sends worked cleanly.

I believe (and I think professor Whitlock has basically said this) that it's the receiving end that does most of the heavy lifting. I'm not remembering now what model the DSP was but I suspect it was taking some shortcuts. In this situation, at least, the sending end made a difference.
Bill is now active over at GroupDIY where I'm a mod. I could ask him for you, but I already know the answer.  8)

 In a balanced interface everything matters wrt the impedance balance.

JR
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David Sturzenbecher

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Bill is now active over at GroupDIY where I'm a mod. I could ask him for you, but I already know the answer.  8)

 In a balanced interface everything matters wrt the impedance balance.

JR

Bill is also very active over on the Syn-Aud-Con forums as well.
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drew gandy

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I think the issue was as likely the TRS>XLR adaptor as anything else you mentioned.

Mac

Given what little I've said so far, I understand your comment. But what do you suspect was wrong with the adaptor?
It's been a long time but I'm sure I checked it for correct contact to pin connections.
i.e.:
tip -> pin 2
ring -> pin 3
sleeve -> pin 4
(Ha! gotcha! I meant pin 1)  ;D
Maybe there is something else that could have been wrong with the adaptor?  [btw, in my personal drag around kit I carry TRS plug to XLR plug short cables as adaptors rather than those cheap one piece units.]

That said, I more than likely tried another adaptor as well. I know I tried different sends in the snake. I remember being surprised by the situation and spent some time trying to troubleshoot/understand it.

This is the only case that I can remember where I think a resistor tied impedance balanced connection didn't work as well as a "full" solution. I only call it cheapskate because it is indeed a lot cheaper to implement. All that said, there are so few pieces of "Pro kit" that use this type of impedance balanced output topology that my sample size is probably too small to mean anything.

Bill is now active over at GroupDIY where I'm a mod. I could ask him for you, but I already know the answer.  8)

 In a balanced interface everything matters wrt the impedance balance.

JR

If the receiver has high common mode impedance then impedance imbalance issues at the source (or the wiring) become less significant. Bill's work showed that the reason transformers work so well compared to the typical active balanced input is because of this 'HCMI'. Then he worked up a brilliant way to mimic it with active circuitry. THAT signed a deal and released the in-genius chip. I think I have this story right. But if Bill is interested in commenting on this discussion I would love to learn a thing or two.
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John Roberts {JR}

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This is the only case that I can remember where I think a resistor tied impedance balanced connection didn't work as well as a "full" solution. I only call it cheapskate because it is indeed a lot cheaper to implement. All that said, there are so few pieces of "Pro kit" that use this type of impedance balanced output topology that my sample size is probably too small to mean anything.

If the receiver has high common mode impedance then impedance imbalance issues at the source (or the wiring) become less significant. Bill's work showed that the reason transformers work so well compared to the typical active balanced input is because of this 'HCMI'. Then he worked up a brilliant way to mimic it with active circuitry. THAT signed a deal and released the in-genius chip. I think I have this story right. But if Bill is interested in commenting on this discussion I would love to learn a thing or two.
It is not accurate to think of balance (impedance or otherwise) in terms of just the transmitter or receiver. Balance is the result of both the sending end and receiving end combined together. I am repeating myself but impedance balanced when properly implemented is fully functional.

Transformers are widely embraced because they are so forgiving of operator error.... like accidentally grounding one leg or the other. Transformers just keep on working until the operator accidentally grounds both legs.  :o

For decades (even back when I was still designing gear) we used cross-coupled negative feedback between two active output stages to better mimic transformer outputs for a fraction of the cost. These circuits were not trivial to implement and required trim pots or precision resistors for best performance. This is a natural for integrated circuits with superior resistor matching.

The major IC companies have been trying to win design-ins for all the I/O sockets in audio SKUs. They still didn't implement my wish list*** that I gave them last century but the off the shelf canned solutions behave as well or better than we could do using conventional circuitry. No engineering manager will get fired for instructing their junior engineers to use these canned input and output solutions in new designs. 

JR

*** Features I asked TI to include in their IC output drivers, when they visited Peavey last century for one of their "dog and pony" shows. They pretend to want advice on features we need/want, but in reality are there to do a hard sell on products they already have finished and ready to ship. My list included features like 1) built-in muting for silent turn-on/turn-off, 2)Voltage gain of a few dB and ability to operate from unregulated rails for more usable real world output, 3)  robust protection against static and rogue external over voltage hits. 
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