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Help! - do I need these unbalanced patch leads in my rack set-up?

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Help! - do I need these unbalanced patch leads in my rack set-up?

Postby Li-rocchi » Fri Aug 19, 2011 8:13 am

Hi all

I have a real mental block when it comes to working out how to connect a variety of different units together in a rack system, especially considering that some are balanced and others unbalanced.

I now have all the leads I need to connect them all to the back of my (BALANCED jacks) patchbays, so just need the patch leads to make the connections at the front.

My questions are:

1. Can I use balanced jack to jack leads exclusively for patching at the front? I have loads of them. But I also have a fair few unbalanced jack to jack patch leads. Someone has offered to buy them (the unbalanced ones) from me, but before I sell I wanted to check that I won't be needing them for patching duties later down the line.

2. The leads to connect the units to the back of the patchbay...... Have I made these correctly? For unblanced units I've used unbalanced cable, for balanced units I've used balanced cable. Now that I'm reading more I'm uncertain as to whether I shold have connected some units with "pseudo-balanced" leads.


If anyone could help me through this quagmire I'd appreciate it hugely.

Many thanks

Max
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Re: Help! - do I need these unbalanced patch leads in my rack set-up?

Postby James Perrett » Fri Aug 19, 2011 9:22 am

It sounds like now is the time for testing...

Try patching things together in every possible way you can imagine. Do things work as expected and do you ever get any unexpected noise?

Pseudo balanced leads are sometimes useful if you have ground loop problems but most of the time unbalanced leads will be fine. The one thing to watch out for is plugging TRS plugs into TS sockets as some sockets make the ground connection right where the plastic separator between R and S happens to be.

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Re: Help! - do I need these unbalanced patch leads in my rack set-up?

Postby Hugh Robjohns » Fri Aug 19, 2011 9:38 am

If it's a balanced patchbay then I'd stick with balanced patch cords and sell or junk the unbalanced ones. If you don't it's inevitable that an unbalanced one will creep into the rig at some time and leave you wondering why something doesn't work properly!

As for the source and destination wiring, as James says it often doesn't matter if some of the sources are unbalanced and some balanced, and on the output side the worst that's likely to happen if you send balanced outputs to unbalanced inputs is a 6dB level drop...

However, wiring unbalanced gear together via a patch bay is a good recipe for ground loop hums and the proof will be when you start testing everything.

Personally, if you're making your own cables up, I'd wire unbalanced sources using the pseudo-balanced cable technique, as that is the best way of avoiding potential ground loop issues. It's not strictly necessary, but it makes sense to work that way if you're making your own cables and have a balanced patch bay.

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Re: Help! - do I need these unbalanced patch leads in my rack set-up?

Postby Li-rocchi » Fri Aug 19, 2011 5:59 pm

Huge thanks to both of you for the replies.

I certainly see that now would be a good time to test, but unfortunately that's not possible because the rack unit has not been built yet, and many of the leads have not been soldered up yet either (I have the cable and the plugs ready but they still need joining).

James Perrett wrote:The one thing to watch out for is plugging TRS plugs into TS sockets as some sockets make the ground connection right where the plastic separator between R and S happens to be.

That should not be a problem I don't think, as I have TS plugs for TS sockets. The only thing close to this scenario will be having a TS in the BACK of the patchbay (from an unbalanced unit) with a TRS in the corresponding socket at the FRONT to patch it elsewhere. Is that scenario ok?

James Perrett wrote:...but most of the time unbalanced leads will be fine.

Hugh Robjohns wrote:If it's a balanced patchbay then I'd stick with balanced patch cords and sell or junk the unbalanced ones.

It is indeed a balanced patchbay. Hmmm....So which is it? Are they needed/fine or ok to sell? To be honest, at the moment this is the most essential question because I have someone lined up to buy around 30 unbalanced patchcords (Neutrik Van Damme ones), but only want to sell them if I definitely won't need them in the set up.

Hugh Robjohns wrote:Personally, if you're making your own cables up, I'd wire unbalanced sources using the pseudo-balanced cable technique, as that is the best way of avoiding potential ground loop issues. It's not strictly necessary, but it makes sense to work that way if you're making your own cables and have a balanced patch bay.

Thanks for the advice Hugh, and I am indeed wiring up my own cables. But would you mind clarifying please.....? Are you talking here about the cables connecting the units to the back of the patchbay for ALL unbalanced units? And then use balanced patch cables for all connections at the front of the bay?

Thanks once again for all the help and sorry for being so slow with it all!

Max
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Re: Help! - do I need these unbalanced patch leads in my rack set-up?

Postby Hugh Robjohns » Sat Aug 20, 2011 10:45 am

Let's deal with the money question first: If it's a balanced patchbay then it makes sense to use balanced patch cables exclusively. Any thing else is likely to cause confusion and trouble at some stage now or in the future.

So flog the unbalanced patch cables without a second moment's thought or concern.

Okay, so now you have a balanced patchbay and balanced patch cables. Lovely. The next question is how to connect the unbalanced gear.

If you plug an unbalanced TS plug into a balanced TRS socket, the hot side mates with the TS tip, and the cold side mates with the TS sleeve. (The TRS sleeve also mates with the TS sleeve, of course). So the signal is passed perfectly well, but it is ubalancing the balanced connection through the patchbay.

Is that a problem? Probably not. If you're patching an unbalanced synth to an unbalanced interface or console line input it will work fine -- as it will if you patch it to a balanced line input. All that happens in the case of the latter is that the cold side is grounded instead (potentially) of seeing an inverted signal. The input gain might need to be cranked 6dB higher as a result, but it's no big deal.

However, the potential problem with this scenario is that the patchbay acts like a giant earthing point for all your unbalanced gear, and that could lead to a very difficult to cure mess of ground loops. It might not... but few of us are that lucky!

On the other hand, if you adopt the psuedo-balanced wiring approach for your unbalanced gear, you avoid the problem of tying equipment grounds together at the patchbay completely, and as a result you should have a hum-free system.

The patchbay is still a grounding hub, of course -- all those TRS socket sleeves must be grounded, after all. But the DIY cables don't extend that ground directly to the unbalanced equipment.

The patch bay TRS plug is wired in the usual way with normal balanced cable.

The unbalanced plug is wired with the hot wire to the tip, the cold wire to the sleeve and the cable screen can either be cut off and isolated completely (if you're lazy) or, and better, linked to the TS sleeve via a 100 ohm resistor.

The value isn't even slightly critical -- anything from about 100 to 300 ohms would be fine -- and if you're feeling posh, you could solder a little 0.1uF bypass capacitor across the resistor too to improve the RF screening properties.

In this way, if you patch the unbalanced signal to a balanced input, the hot sie sees the signal and the cold side sees the synth's ground reference -- so the signal is transferred in it's entirety quite happily. But there is no direct conection between the unbalanced source's ground and the balanced destination's ground (there's 100 ohms in the way). Hence no ground loop problem.

Okay, so it's a bit of a faff making up a load of leads and having to squeeze a resistor into all those TS plugs, but the advantage is a ground-loop free system, no need to worry about whether you're patching balanced or unbalanced things about, and no need to rewire anything as your equipment improves and becomes balanced everywhere.

Hope that helps

Hugh
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Re: Help! - do I need these unbalanced patch leads in my rack set-up?

Postby ef37a » Sun Aug 21, 2011 7:02 am

It is indeed a headache when you have balanced and unbalanced sources and sinks in a patchbay system.
My prefered solution was to impedance balance all my unbalanced outs and because I have a "thing" about special leads (and a son who will happily use a speaker lead twixt amp FX send and mixer! Musician you know.) I actually modded the ubal kit, fitting TRS sockets in some cases (my Philips DCC)and making up wee boxes for others, MDisc,Dolby S cassette. In any event this kit is RCA connectors anyway.

There is no such quick fix to make a balanced input but I did not seem to have many problems with that.

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