Proper wiring to avoid ground loops

[Brian Roth]

600-10K load defines any current studio input impedance....unless you mult a bunch of 600 Ohm destinations.

Bri

 

[Brian Roth]

Of course, IIRC....ABC's New York "master" facility once used 150 Ohms for in/out in the entire facility.

Bri

 

[JohnRoberts]

Well they misapplied the circuit and used it in the wrong application.
There is several line driver style op amps that can go as low as 50 ohms that would work better.
But as going to a higher input impedance, it defeats the intended balance operation by definition. That is why no one in their right mind use it for a main mix out and not use a balanced line driver circuit. Because they would want their mixer to be compatible with what is connected.
The worst case was a studio monitor that was 47K input impedance that was fixed by putting a 150 ohm resistor across 2&3. Of course others in the process freaked out on a forum, but the guy did it and it worked and PM me later, but didn't post anything about it because of the argumentative people.

I really dislike defending Mackie, but Greg, and Rick Chinn, pretty much knew what they were doing in this respect.

There is no need in modern devices to drive lower than 600 ohms for audio interfaces. Some digital and video interfaces require lower impedances (50,75, ?). The 600 ohm drive capability is only required to support the occasional legacy (old) gear left over from the days when 600 ohm terminations for all I/O were common. Professional gear is typically designed to drive as low as 600 ohms without problems. Semi pro gear is typically only capable of driving 2k loads, and expecting 10k or higher input impedances.

Modern gear is designed to work with bridging terminations. Putting 150 ohms across a modern line level input is IMO bad advice, do not do that.

JR

 

[Khron]

Some will even plot the expected THD at different operating impedances that is applied in the circuit.

Yes, they will plot the opamp's THD for a range of LOAD impedances of whatever the output of the opamp is feeding (ie. INPUT impedance of the device / circuit that the output of the opamp is connected to).

https://www.mouser.com/datasheet/2/294/NJM4565_E-42952.pdf
Top left on page 3, a graph of max output voltage swing. See the subscript "L" next to the R, above each of the two plot line? That usually stands for "Load". Not output impedance. If anything, as i've mentioned before, output impedance of most opamps nowadays is so low that it's borderline-irrelevant, not even being mentioned in datasheets anymore.

To my surprise, OPA134 seems to be an exception, they actually DO specify the closed-loop output impedance (10 milliohms..!!!) as well as the open-loop output impedance (10 ohms, but how often will you be running one without a feedback network?).

https://www.ti.com/lit/ds/symlink/opa2134.pdf

Please note: The non driven side is the monitor in our example.

How and why??? Was the input circuitry of that monitor you mention, somehow single-ended, for some reason?

That is why a popular line driver is DRV134/135 with 50 ohm output Z

Yes, that happens to have a built-in fixed 50 ohm resistor on-die, but you'll note it's only specified for 600 ohm loads. Lower than that may well start bumping against the current-limiting circuitry, which will cut down the headroom for higher amplitudes. 15V peak into 150 ohms "already" would require 100mA, which the DRV134 can't supply even in the best-of-the-best-case scenarios (fig.14 on page 8 of the datasheet).

https://www.ti.com/lit/ds/symlink/drv135.pdf?ts=1703606495774
If i didn't know any better, i'd say there's some serious semantic(?) misunderstanding going on somewhere...

 

[Brian Roth]

@JohnRoberts

Just an aside....it's Rick CHINN.... whom I've known from the Tapco (Greg Mackie early) days in the 1970's.

Rick is a Cool Guy and we still stay in contact. I haven't seen his annual "family holiday letter" this year.....hope Rick is doing well.....

Bri

 

[JohnRoberts]

@JohnRoberts

Just an aside....it's Rick CHINN.... whom I've known from the Tapco (Greg Mackie early) days in the 1970's.

Rick is a Cool Guy and we still stay in contact. I haven't seen his annual "family holiday letter" this year.....hope Rick is doing well.....

Bri

thanks for the correction. I only met him once back in the 80s before he was working for Mackie (while he may have worked for a different earlier Mackie incarnation). I only knew him as a competent mixer design engineer working for a competitor.

edit,,, fixed it thz /edit

JR

 

[Brian Roth]

https://www.aes.org/aes/rickchinn
Rick's website is often linked on the internet when folks ask about attenuators:

http://uneeda-audio.com/
More specifically:

http://uneeda-audio.com/pads/
Bri

 

[Newmarket]

Ok, my bad, its a 150 ohms, but they used the wrong op amp that is designed for 400 ohm operation.
View attachment 119456

The resistance value quoted isn't the problem. Others here have tried explaining why there's no problem but you seem intent on resisting all expertise so I'll leave it there.
btw if you wanted to point to a potential shortcoming of that circuit you could point out that the passive leg omits the capacitor. But it's a moot point.

 

[Brian Roth]

Yep....needs the same 150 uF in series for the corresponding (undriven) leg.....

Bri

 

[abbey road d enfer]

But maybe something was impedance balanced in a mismatched circuit?
I've always considered impedance balanced connection not a true balanced connection because rarely things are the same impedance,
The application impedance balance circuit was rarely used in instances where equipment outside certain devices was interconnected because the impedance is standardized. Like Soundcraft making a sudo-balanced tape send, because tape decks standardized their inputs a decade or two before they fell out of commercial retail. But if you use that board on a standard interface, it will not go in as a balanced signal.

Equipment manufacturers in the past and present use balanced line drivers to achieve balanced connection across a wide range of impedance.
So for a monitor controller, to be truly a universally applied across different monitors, and amps, it should have this driver.
However, the budget ones use "impedance balanced" connections and people get hum city and wonder why. Mackie Big knob is 400 ohms impedance balanced, so every monitor device has to be 400 ohms input impedance or adapted to it or suffer from noise and hum people misunderstand as a ground loop or unbalanced power.

So much misconceptions in this post, and some others of yours.
Balancing is not related to nominal impedance. A balanced line driver designed for 600ohms can and should work perfectly with a 100k balanced receiver.
The impedances at play in the performance of a balanced connection are the two common-mode output impedances and the two common-mode input impedances; the differential output and input impedances are anecdotical.
You seem to ignore that a connection can be perfectly balanced with largely different output impedances, because a balance connection is a Wheatstone bridge.
I don't need to remind you of the equation for equilibrium of the Wheatstone bridge, since you are a "doctor".
Souncraft EBOS were not pseudo-balanced, they were just improperly set-up, because the guy who created the set-up procedure did not understand that balancing is about impedances, not about "equal level on each leg".
Actually, impedance-balanced outputs would have provided better performance at a lower cost, except for the max output level.

 

[Brian Roth]

Amen...the "Tascam Problem" design did supply equal signal levels between pins 2 and 3 referenced to "audio ground", but with inverted phase. Sadly, Avid made the same mistake.

Bri

 

[abbey road d enfer]

View attachment 119462

The effect of the added 150r resistor is just to drop teh level by 10dB. It does not help at all with CMR and noise rejection.
It may attenuate externally induced interference by a tiny amount, but considering the 10dB reduction of wanted signal, the balance is negative.

 

[abbey road d enfer]

Amen...the "Tascam Problem" design did supply equal signal levels between pins 2 and 3 referenced to "audio ground", but with inverted phase. Sadly, Avid made the same mistake.

Bri

The Tascam problem was different. The two signals were of equal impedance, but had no "floatability". The problem was they could not drive unbalanced receivers correctly.
Soundcraft EBOS had all it takes to make a good floating output, plus one thing that pertained to a misguided procedure and ruined everything.
The procedure consisted in balancing the hot and cold output voltages with brute force, a trimmer that loaded the legs, introducing distortion, and actually degrding the impedance balance.

 

[Brian Roth]

OK...I was on a different "tangent"......"Tascam" designs would cause one poor little opamp to SCREAM when it's destination went to "ground" if the destination was unbalanced.

Bri

 

[abbey road d enfer]

PROVE IT

You don't understand the basics of voltage divider?

 

[Khron]

Obviously you don't know the 150 resistor keeps the cable in balanced connection.

The 150 ohm resistor you added in parallel with the 47k of the monitor, right?

What's your definition of "balanced connection", then? And perhaps doodle some examples for us simpletons to comprehend how and why?

 

[Newmarket]

The 150 ohm resistor you added in parallel with the 47k of the monitor, right?

What's your definition of "balanced connection", then? And perhaps doodle some examples for us simpletons to comprehend how and why

I'll get my e-popcorn for this

 

[Khron]

It was the termination resistor at the monitor's xlr across 2&3. On the input circuit of the monitor when I had him take the amp plate off the back, was two 47K resistors to the input on signal + and - of the first op amp and no coupling caps.

Ok, that answers my first question; what about the second (and third) one?

 

[Newmarket]

Go take a hike

 

[Khron]

ask them one by one

What's your definition of "balanced connection", then?

And perhaps doodle some examples for us simpletons to comprehend how and why?

(Really sorry, couldn't ask them separately since the latter is directly related to the former, and would have made no sense, taken out of context...)