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post #1 of 25 Old 08-22-2013, 05:47 AM - Thread Starter
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Hi,

I'm planning to run a 100' balanced line from a Mackie sound board (TRS connector) into another room to feed a consumer unbalanced Sony LCD monitor line in. My plan is to connect shielded Cat5e directly to the output of the mixer, shield connected. At the other end, I'll use an audio balun (matching transformer) with the shield left floating.

My question is, do any of you have recommendations for the balun? Can I use one that's intended for audio-Cat5 transmission or will the Mackie output be too hot for it and saturate it?

Thanks,
Bob
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post #2 of 25 Old 08-22-2013, 11:45 AM
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Quote:
Originally Posted by bjt08 View Post

Hi,

I'm planning to run a 100' balanced line from a Mackie sound board (TRS connector) into another room to feed a consumer unbalanced Sony LCD monitor line in. My plan is to connect shielded Cat5e directly to the output of the mixer, shield connected. At the other end, I'll use an audio balun (matching transformer) with the shield left floating.

My question is, do any of you have recommendations for the balun? Can I use one that's intended for audio-Cat5 transmission or will the Mackie output be too hot for it and saturate it?

Thanks,
Bob

That should work with a standard audio to CAT5 balun. In fact your wiring scenerio with the floating shield at the transformer is quite correct. If it is saturating the balun and causing distortion, just lower the output gain on whatever buss is feeding the monitor. IIRC. Mackie has a monitoring output with it's own gain pot.

Ideally the output from the Mackie should be balanced. But even if unbalanced, the transformer will still break any ground loop caused by the electrical system.

The only issue with a cheap transformer is good HiFi sound quality. But as you are feeding the internal audio system of a TV, that should not be an issue.

P.S. Glimmie grew up in Doylestown!
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post #3 of 25 Old 08-23-2013, 05:07 AM - Thread Starter
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Thanks for the reply, Glimmie. I took a tour through your home page. Wow! Very impressive.
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post #4 of 25 Old 08-23-2013, 05:38 AM
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Quote:
Originally Posted by bjt08 View Post

Hi,

I'm planning to run a 100' balanced line from a Mackie sound board (TRS connector) into another room to feed a consumer unbalanced Sony LCD monitor line in. My plan is to connect shielded Cat5e directly to the output of the mixer, shield connected. At the other end, I'll use an audio balun (matching transformer) with the shield left floating.

My question is, do any of you have recommendations for the balun? Can I use one that's intended for audio-Cat5 transmission or will the Mackie output be too hot for it and saturate it?

I've tried to overload the transformers in Radio Shack's ground isolator with professional levels, and failed.

I tested Radio Shack's "Ground Isolator" 270-054 using
test signals that maxed out around 2.5 v RMS.

ZSource = 150 ohms, ZLoad = 10K ohms.

The measured performance was truly amazing for a pair
of transformers case and cables selling for only $16.65.

All IM, THD, and noise artifacts were at least 80 dB
down with most in the -100 dB range or better. Frequency
response showed a 2 dB peak at 20 Hz and then 10 dB down
at 10 Hz. There was a 3 dB peak at about 51 KHz falling
to about 10 dB down around 100 KHz. +0.5 dB at 20 KHz
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post #5 of 25 Old 08-23-2013, 11:41 AM
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+1 to all the above. With a high-impedance input there is almost no chance of saturation because the current flow will be very low. The only times I have saturated audio cores was in low-Z systems (low-impedance loads) and high levels. Your scenario is nothing like that.

IIRC not all the Mackie TRS outputs are truly balanced (differential) drivers; some are pseudo-balanced if that. As Glimmie and Arny intimate, breaking any ground loop is usually key and the little RS balun will do fine.

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post #6 of 25 Old 08-25-2013, 04:31 AM - Thread Starter
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Yes, that was my thought so I tried simply breaking the ground loop with an isolation transformer but heard no improvement.

Don, you are correct. I checked out the sub out feed on the Mackie block diagram and it is not a differential driver. It never occurred to me that they would skimp like that.

I've order a couple of Intelix baluns and some CAT5e STP from Liberty AV Solutions' bargain bin. (Great prices.) I'll let you know how it works out.
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post #7 of 25 Old 08-25-2013, 01:02 PM
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Quote:
Originally Posted by bjt08 View Post

.....Don, you are correct. I checked out the sub out feed on the Mackie block diagram and it is not a differential driver. It never occurred to me that they would skimp like that .....

Unfortunately a lot of pro sound reinforcement gear (band stuff) pulls that stunt. I have a DBX bass processor that is the same way, unbalanced on TRS connectors. I have also seen this done with XLR connectors!

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post #8 of 25 Old 08-25-2013, 03:40 PM
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Quote:
Originally Posted by bjt08 View Post

Yes, that was my thought so I tried simply breaking the ground loop with an isolation transformer but heard no improvement.

Don, you are correct. I checked out the sub out feed on the Mackie block diagram and it is not a differential driver. It never occurred to me that they would skimp like that.

I've order a couple of Intelix baluns and some CAT5e STP from Liberty AV Solutions' bargain bin. (Great prices.) I'll let you know how it works out.
It is very typical to still have a balanced circuit-without a differential drive.

It is called an impedance balanced circuit-due to the fact that each leg has the same impedance to ground.

The big thing a differential drive offers is an extra 6dB of gain-due to the voltage being twice that of an impedance balanced circuit.
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post #9 of 25 Old 08-25-2013, 04:36 PM - Thread Starter
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Maybe that's the intent of the Mackie circuit, although I'm having trouble seeing how the impedance to ground can be the same for the two legs from their diagram (attached). Unless they took liberties with the block diagram and the schematic would show more detail.



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post #10 of 25 Old 08-25-2013, 06:17 PM - Thread Starter
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I think I see it now. It makes sense if the R on the non-driven output is the same as the source impedance of the driver. I'm assuming both are 600 ohms.
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post #11 of 25 Old 08-25-2013, 07:44 PM
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The problem with quasi- or pseudo-differential (balanced) drive is that common-mode rejection usually suffers. This is not often a problem for practical systems but long runs can be an issue, particularly if the feed bundles speaker and power with line level runs.

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post #12 of 25 Old 08-25-2013, 08:02 PM
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Bogen still makes that little mjracle called a WMT-1 which was designed specifically for the purpose the OP specified. 50 years old too. It was designed to be used and mounted to the old MX series mixer/amps. I build my own though. Very handy toy to keep in the tech kit.
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post #13 of 25 Old 08-26-2013, 09:51 AM
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Quote:
Originally Posted by DonH50 View Post

The problem with quasi- or pseudo-differential (balanced) drive is that common-mode rejection usually suffers.

The resistor to ground on the passive output pin is supposed to address that.

The thing that impedance balanced sources skimp on is total signal level. Higher signal levels tend to raise the signal out of the noise that is probably contributed over the long length of the cable.
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post #14 of 25 Old 08-26-2013, 11:46 AM
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Quote:
Originally Posted by bjt08 View Post

Maybe that's the intent of the Mackie circuit, although I'm having trouble seeing how the impedance to ground can be the same for the two legs from their diagram (attached). Unless they took liberties with the block diagram and the schematic would show more detail.



-- Bob
You're not looking at a full schematic. An opamp will have a Zout of a fraction of an ohm, but they usually have a build out resistor of 100R or so (often with a series cap) to protect the opamp from shorts. This build out will be replicated on the grounded polarity.
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post #15 of 25 Old 08-26-2013, 11:52 AM
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Quote:
Originally Posted by A9X-308 View Post

Quote:
Originally Posted by bjt08 View Post

Maybe that's the intent of the Mackie circuit, although I'm having trouble seeing how the impedance to ground can be the same for the two legs from their diagram (attached). Unless they took liberties with the block diagram and the schematic would show more detail.



-- Bob
You're not looking at a full schematic. An opamp will have a Zout of a fraction of an ohm, but they usually have a build out resistor of 100R or so (often with a series cap) to protect the opamp from shorts. This build out will be replicated on the grounded polarity.

Right, and the resistor to ground on the jack will have the same resistance. While these outputs are impedance balanced, they are rarely impedance matched. Impedance matching is rarely involved with balanced I/O,
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post #16 of 25 Old 08-26-2013, 12:32 PM
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Hmmm... The resistor to ground should match (maybe "equal" is a better word?) the impedance of the driven output (which will normally have a series resistor as was stated), but since it is not driven then there is essentially no rejection to ground on the receiver, obviating the high common-mode rejection a truly differential signal would provide. I call that circuit "pseudo-differential". It all depends upon how the ground is routed, natch, since to provide any sort of rejection the buffer opamp's input signal must reference the same ground (return path) as the other side (grounded through the resistor). The usual, or at least one, LF (audio) approach is to simply use another opamp to invert one side, creating a quasi-differential design. Fully-differential circuits are harder to find (not a normal opamp). My experience is based upon high-speed/wideband RF and data converter circuits that were fully-differential in and out so maybe I am missing something in the audio world's implementation of "balanced" vs. the differential circuits I am used to seeing.

Aside: If nothing else changes, a balanced output using a second amp to replicate and invert the signal should provide 6 dB voltage gain (twice the signal), the higher signal level Arny is referring to (I assume?) I could not say how much that matters for a given system (depends on the noise floor and headroom).

FWIWFM, I have used a number of Mackies and other similar systems and IME they do not do well at isolating ground or CM noise the way their XLR or truly balanced connections do. But, experience (including mine) varies widely, or maybe wildly...

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post #17 of 25 Old 08-26-2013, 04:00 PM
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Quote:
Originally Posted by DonH50 View Post

Hmmm... The resistor to ground should match (maybe "equal" is a better word?) the impedance of the driven output (which will normally have a series resistor as was stated), but since it is not driven then there is essentially no rejection to ground on the receiver, obviating the high common-mode rejection a truly differential signal would provide.

Not exactly. The interference rejection is entirely provided by the differential receiver. The balanced output improves the SNR by delivering a 2x signal to the differential receiver. The differential receiver provides cancellation even if the resistor is shorted out.
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I call that circuit "pseudo-differential". It all depends upon how the ground is routed, natch, since to provide any sort of rejection the buffer opamp's input signal must reference the same ground (return path) as the other side (grounded through the resistor).

I don't know about the pseudo part. The differential input has to be real to provide any benefit.
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The usual, or at least one, LF (audio) approach is to simply use another opamp to invert one side, creating a quasi-differential design. Fully-differential circuits are harder to find (not a normal opamp).

A true differential input can be made from general purpose op amp(s) - from 1 to 3 of them depending on what sort of performance you desire. A purpose built differential input op amp chip includes all or most of the active and passive components required, usually including some resistors that need to be carefully matched.
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Aside: If nothing else changes, a balanced output using a second amp to replicate and invert the signal should provide 6 dB voltage gain (twice the signal), the higher signal level Arny is referring to (I assume?) I could not say how much that matters for a given system (depends on the noise floor and headroom).

Agreed. The differential input costs you 3 dB of noise performance (2 uncorrelated noise sources instead of just one with an ordinary unbalanced input) but the balanced source gives you 6 dB more signal or if you will puts the noise floor 6 dB further below the signal, for a net gain of 3 dB less noise.
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FWIWFM, I have used a number of Mackies and other similar systems and IME they do not do well at isolating ground or CM noise the way their XLR or truly balanced connections do.

Remember the different ways of building a balanced input with 1, 2, or 3 op amps or using a dedicated chip? The more op amps the less dependence on matching resistor values. The dedicated chip usually has factory-trimmed resistors and keeps them tightly thermally coupled. Improving the matching of these resistors improves common mode rejection.
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But, experience (including mine) varies widely, or maybe wildly...

What you saw is what one gets with the lower cost implementations of active balanced inputs. The really nifty chips usually end up in mic preamps, but one sometimes sees them in premium equipment. For example among audio interfaces the M-Audio Delta 44 and 66 use impedance balanced outputs, the Delta 24/192 uses active balanced outputs, and the LynxTWO uses dedicated chips for both inputs and outputs. The output chip senses when one output line is grounded and doubles the voltage on the other output, which simulates what a transformer output does. The audio interfaces cost $100, $200, and $800 respectively and have dynamic range to match.
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post #18 of 25 Old 08-27-2013, 05:03 AM - Thread Starter
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Not exactly. The interference rejection is entirely provided by the differential receiver. The balanced output improves the SNR by delivering a 2x signal to the differential receiver. The differential receiver provides cancellation even if the resistor is shorted out.

I would think that if the resistor is shorted out, the impedance of each leg of the line would differ, causing the voltages induced by the (correlated) common-mode interference to differ and would prevent the differential receiver from cancelling the CM signal.
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Agreed. The differential input costs you 3 dB of noise performance (2 uncorrelated noise sources instead of just one with an ordinary unbalanced input) but the balanced source gives you 6 dB more signal or if you will puts the noise floor 6 dB further below the signal, for a net gain of 3 dB less noise.

I'm much more concerned with cancelling correlated interference (hum) and would accept the 3dB (uncorrelated) noise hit to do it.
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post #19 of 25 Old 08-27-2013, 07:26 AM
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Not exactly. The interference rejection is entirely provided by the differential receiver. The balanced output improves the SNR by delivering a 2x signal to the differential receiver. The differential receiver provides cancellation even if the resistor is shorted out.

I would think that if the resistor is shorted out, the impedance of each leg of the line would differ, causing the voltages induced by the (correlated) common-mode interference to differ and would prevent the differential receiver from cancelling the CM signal.

That is all true, but it only reinforces my contention that the cancellation itself takes place only in the receiver.

The cancellation is not an all-or-nothing effect.

For example, if the source impedance of the op amp were 75 ohms and the input impedance of each side of the receiver were 10K, the difference the resistor would make would be 0.75% and still allow > 40 dB CMRR.

For comparison, a cheap balanced input made with 1% resistors might have CMRR in the same range - about 40 dB.

40 dB CMRR can turn a relatively loudly humming system (unbalanced input) into something that might be subjectively acceptable.
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post #20 of 25 Old 08-27-2013, 07:37 AM - Thread Starter
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Yes, I agree that the differential receiver is where the payoff is with a balanced line. However I had in mind a balun with an input impedance of 600 ohms. In that case, shorting the resistor would make much more of a difference.
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post #21 of 25 Old 08-27-2013, 10:31 AM
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I use transformers all the time. No problems at all. Even in noisy environments and when running cables hundreds of feet from a mix desk inside a convention hall to a satellite uplink and a recording truck outside running on a combination of generator power, portable distro systems and house power.
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post #22 of 25 Old 08-28-2013, 11:47 AM
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^ Ditto. At least in the past when I was setting up such systems, and my experience is vastly less than Gzimo's.

@Arny: I think we are crossing wires, or I don't fully understand how differential drivers and receivers are used/defined in audio systems. I agree with most of what you said, but even a differential receiver does not help if you ground one input -- it becomes effectively a SE receiver at that point. It will still reject noise common to both sides (ground and signal), natch, depending upon the input configuration and CMRR/CM loop design. I do understand the various op-amp configurations. I probably need to draw it out, been too busy with other things and my comprehension is suffering. Chances are if we were in a room with a chalkboard/whiteboard or piece of paper this would be a one-minute discussion.

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post #23 of 25 Old 09-01-2013, 04:08 AM - Thread Starter
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It turns out this line was only one of several contributors to the hum problem. I ran the cat5e with the 1/4" TRS plug at the Mackie sub out and the balun at the Sony receiver and the hum was significantly reduced but not gone.

Next I found that there was a ground loop between the Kramer VP-728 video switcher and the Mackie. Both devices were plugged into the same receptacle. When I lifted the ground pin of the Kramer and disconnect all but one audio input to the switcher, all was quiet. Of course I didn't leave it like that but rather added a ground loop isolator in the between the Kramer and the Mackie which brought the hum way down to an acceptable level.

Now I notice that when the PC input is selected on the switcher, I'm getting a video dependent buzz in the audio which disappears when the PC's VGA cable is disconnected from the Kramer. I'll try another isolator between the PC audio and the Kramer this morning and report back.

Thanks for the informative discussion and balun above.

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post #24 of 25 Old 09-01-2013, 08:05 AM - Thread Starter
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The isolator in the PC audio line eliminated the video crosstalk into the audio as near as I can tell with just a quick test.

Two isolators seems like a hack of a solution but I'm not sure there's anything else I can do given the nature of the internal wiring of the equipment we're using.

Thanks again for all of your input,
Bob
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post #25 of 25 Old 09-01-2013, 02:46 PM
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You can also use a video 'hum bucker.' There are some DB15 HD equipped units but the more common uses 5 BNCs on both I/O connections.
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