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Pseudo Balanced query

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Pseudo Balanced query

PostPosted: Mon Nov 12, 2018 1:19 pm
by BigDeano
Hey guys, was wondering you might be able to help.

I have an Orban Spring reverb hooked up to my Focusrite Pro40, connected via balanced TRS - XLR. The only thing is, the input on the spring is actually unbalanced. Would this be a good time to use a Pseudo balanced cable? There is some ground hum present so wondering if this would help the situation.

The other slight issue, is that the Orban has XLR plugs wired to it, as it has the old terminal type connections, so what i think i need is a pseudo balanced TRS - unbalanced XLR. I already have a couple of the SOS pseudo cables, but none have an unbalanced XLR side. Would someone be able to remind what i need to do to turn the current xlr cable into a pseudo one? I.e, which wire i need to disconnect, and i think people add a resistor?

Any advice on the best way to hook her up would be great!

Thank you
Dean
:-)

Re: Pseudo Balanced query

PostPosted: Mon Nov 12, 2018 2:02 pm
by Sam Spoons
You might find this useful if you don't already know of it.https://www.rane.com/note110.html

Re: Pseudo Balanced query

PostPosted: Mon Nov 12, 2018 2:15 pm
by BigDeano
Yeh i saw this article, is pretty helpful, although doesn't quite have my specific situation of have an XLR unbalanced input, or what the cable might be wired like.
Cheers tho!

Re: Pseudo Balanced query

PostPosted: Mon Nov 12, 2018 2:45 pm
by Sam Spoons
Good point. :oops: If the XLRs on the reverb are 'custom' wired we'd need to know how they have been connected to the I/O and chassis/screen of the Orban unit.

Re: Pseudo Balanced query

PostPosted: Mon Nov 12, 2018 3:13 pm
by Sam Spoons
edit :- info not relevant

Re: Pseudo Balanced query

PostPosted: Mon Nov 12, 2018 4:03 pm
by Hugh Robjohns
BigDeano wrote:I have an Orban Spring reverb hooked up to my Focusrite Pro40, connected via balanced TRS - XLR. The only thing is, the input on the spring is actually unbalanced. Would this be a good time to use a Pseudo balanced cable?

No! Pseudo-balanced cables are useful for connecting the unbalanced output of a mains safety-grounded device to a balanced input purely because they avoid a direct ground connection and thus minimise the chances of a ground loop between the source and destination. They are of no use if the unbalanced source is not a mains-grounded device (and could actually make the situation worse), and are of no use feeding a balanced output to an unbalanced input, either!

The correct approach in this case would be to connect the 'hot' side (tip/XLR2) of the Focusrite's balanced output to the unbalanced input (could be XLR2 or 3 depending on vintage!), with the FF's 'cold' terminal (ring/XLR3) disconnected and isolated, and leave the cable screen/ground connected to pin 1/sleeve at both ends.

There is some ground hum present so wondering if this would help the situation.

A pseudo-balanced cable might help if the device has an unbalanced output and you are connecting it back to a balanced input on the focusrite, by avoiding creating a second ground loop. However, in this case any hum may not be due to ground loops at all. The springs and pickups in this kind of reverb device are quite susceptible to stray magnetic radiation from the mains transformers of other nearby equipment, so it would be worthwhile experimenting with where the unit is placed relative to other equipment, especially if in a rack!

Would someone be able to remind what i need to do to turn the current xlr cable into a pseudo one? I.e, which wire i need to disconnect, and i think people add a resistor?

pseudo-balanced cable image.jpg


The balanced destination end is the output that plugs into your Focusrite's input, and is fitted either with an XLR or TRS. The 'hot' wire goes to the TRS tip/XLR2, the 'cold' wire goes to the ring/XLR3, and the cable screen goes to the sleeve/XLR1.

For the unbalanced source's output, you would normally have a TS jack plug, wired such that the 'hot' wire foes to the tip, the 'cold' to the sleeve, and the cable screen is linked to the sleeve via the resistor (and optional parallel capacitor).

In your case, you'd need to use an XLR connector, with the hot wire going to whichever terminal carries the unbalanced output signal (could be pin 2 or 3 depending on vintage), the 'cold' wire to pin 1, and the cable screen to pin 1 via the resistor/capacitor combo.

H

Re: Pseudo Balanced query

PostPosted: Mon Nov 12, 2018 5:02 pm
by BigDeano
Thanks for that supper detailed reply Hugh! I’ll double check how the terminals are wired on the back of the spring tonight and maybe post a picture just so I know what I’m doing is right! Wiring is not my strong suit!

Re: Pseudo Balanced query

PostPosted: Tue Nov 13, 2018 1:42 am
by BigDeano
So here are the connections on the back of the orban spring, which are being sent on balanced jack-XLR, and returned on balanced XLR-XLR currently..

https://drive.google.com/file/d/1Itgf-d ... sp=sharing

https://drive.google.com/file/d/1As9Txy ... sp=sharing

https://drive.google.com/file/d/1mCilnK ... sp=sharing

https://drive.google.com/file/d/1KwVZN5 ... sp=sharing

Not sure if this changes anything?

Re: Pseudo Balanced query

PostPosted: Tue Nov 13, 2018 1:49 am
by James Perrett
Hugh Robjohns wrote:The correct approach in this case would be to connect the 'hot' side (tip/XLR2) of the Focusrite's balanced output to the unbalanced input (could be XLR2 or 3 depending on vintage!), with the FF's 'cold' terminal (ring/XLR3) disconnected and isolated, and leave the cable screen/ground connected to pin 1/sleeve at both ends.

While this may work for the Focusrite output design it won't necessarily work with all balanced outputs. Some (like those on my A&H desk and anything using transformers) require both hot and cold to be connected.

Re: Pseudo Balanced query

PostPosted: Tue Nov 13, 2018 2:00 am
by Hugh Robjohns
Yes, there are several different forms of balanced output, and if wiring to an unbalanced destination it is necessary to know which type you're dealing with to ensure it is wired correctly. The apposite phrase in the quoted text is 'The correct approach In this case'... :-)

The arrangement I described is appropriate for most active balanced and impedance balanced outputs, but not for floating transformer balanced outputs or cross-coupled active outputs.

H

Re: Pseudo Balanced query

PostPosted: Tue Nov 13, 2018 10:28 am
by Sam Spoons
BigDeano wrote:So here are the connections on the back of the orban spring, which are being sent on balanced jack-XLR, and returned on balanced XLR-XLR currently..

https://drive.google.com/file/d/1Itgf-d ... sp=sharing

https://drive.google.com/file/d/1As9Txy ... sp=sharing

https://drive.google.com/file/d/1mCilnK ... sp=sharing

https://drive.google.com/file/d/1KwVZN5 ... sp=sharing

Not sure if this changes anything?

It would not be difficult to remove the screen connection from the ground terminal to see is that breaks any earth loop, then try again but by removing the -ve/cold terminal instead (or, maybe even remove both on the unbalanced input side of the Orcam Reverb). Try this with only one channel connected to eliminate the possibility of a loop in the other channel.

Re: Pseudo Balanced query

PostPosted: Tue Nov 13, 2018 10:38 am
by Wonks
You could try disconnecting the shield wire tag on the input to the Orban as it will take just seconds to do. The shield will still be grounded at the Focusrite end, so will still help protect the cable from noise, but will break a prospective ground loop path.

Re: Pseudo Balanced query

PostPosted: Tue Nov 13, 2018 12:47 pm
by Sam Spoons
Why didn't I think of that ;)

Re: Pseudo Balanced query

PostPosted: Tue Nov 13, 2018 12:56 pm
by Wonks
You definitely hadn't posted that when I wrote it.

Re: Pseudo Balanced query

PostPosted: Tue Nov 13, 2018 1:01 pm
by Hugh Robjohns
BigDeano wrote:So here are the connections on the back of the orban spring, which are being sent on balanced jack-XLR, and returned on balanced XLR-XLR currently.
Not sure if this changes anything?

Possibly...

Currently, the unbalanced input is wired such that the 'cold' side of the input XLR is shorted to ground (the blue wire going to the ground tag). Most active outputs won't mind that, but some do. With the Focusrite, I'd probably disconnect and insulate that tag, just to minimise the stress on the output driver... but it's not essential.

The output is clearly balanced, so no need for any pseudo-balanced malarky there. A straight XLR-XLR or XLR-TRS cable will be fine.

I see someone has strapped a 600 resistor across the output... I assume that's to load an output transformer properly -- very old school! ;-) But removing that will probably change the tonality a bit (make it brighter, I expect) and provide slightly more output level, so it might be worth experimenting with it attached and removed to see which you prefer.

If the hum you mentioned earlier doesn't improve when you move the unit away from other mains-powered devices in your rack (or wherever you have placed this thing) then you could experiment removing the ground terminal connections on those input and output cables -- you could try removing them individually, but I suspect you'll need to remove both together to see if there is a ground-loop issue.

H