Cross-talk on Focusrite Saffire Pro 24 Outputs

[BigRedX]

MacBook Pro 13" Mid 2012, 16GB RAM, ElCapitan, Logic 9, Saffire Mix Control 3.7

My current band wants to ditch their ancient Minidisc player that they have been using for their backing tracks and I suggested using my MacBook and Focusrite interface as it would give us more flexibility live.

However at our first rehearsal last night I noticed that we were getting bleed/cross talk of the click track on the main backing track.

In Logic I have 2 tracks on the arrange page one with the backing in stereo, and the second with the click track in mono. The stereo backing is routed to Saffire line outputs 1&2, and the click to Saffire line output 3. When I play the track I can hear the click faintly in the stereo outputs. Not a problem at rehearsal, but live it will be easily notable through the PA. The strange thing is that the click cross-talk only happens when I plug in the lead from output 3 to the drummer's headphone amp.

Things I have already checked:
1. It doesn't appear to be a routing problem since the click is only audible in Outputs 1&2 when the lead is plugged into output 3.
2. The click is definitely bleeding through into the main outputs and not some psycho-acoustic effect caused by it being massively loud in the drummer's headphones
3. I have tried routing the click to the other line outs, with the same result. As soon as the lead to the headphone amp is plugged in the click also appears faintly in outputs 1&2.

Any ideas?

[Hugh Robjohns]

Sounds like a problem either with the grounding or perhaps taking an unbalanced connection to the headphone amp.

For example, if you are using an unbalanced cable to the headphone amp, the shorted cold side of the interface's balanced output will be dumping a lot of signal into the local ground path, and if the PCB layout isn't scrupulous that could be finding its way back into the adjacent output stage for channels 1/2, hence the unwanted crosstalk.

The quick and easy potential solution I'd try first would be to connect the headphone amp via a line isolator box (ART DTI or similar).

Use a balanced TRS-TRS (or TRS-XLR) into the DTI box, and whatever cable is appropriate from the box output on to the headphone amp.

H

[ef37a]

I would put my money on mixed up ground paths/lack of balanced working as well.

If you look on Amazon you can buy an 'Earth Isolator' for about a fiver. It is a black metal tube about 50mm diameter and has two sets of RCA plugs and line sockets.

The quality is easily good enough for a gigging headphone link and if it doesn't sort the problem, no great loss.

Dave.

[BigRedX]

Are the outputs from the Saffire balanced? They are just jack sockets with "line out" printed beneath them. Nothing to indicate they might be balanced. If they are TRS sockets can I get away with making up a cable with only the tip and sleeve connected for the headphone amp

I looked up earth isolator but they all appear to be stereo with phone connectors. I need a jack to jack lead. I'm reluctant to spend money on an item when the first thing I've got to do is irreversibly modify it.

[Hugh Robjohns]

Are the outputs from the Saffire balanced?

Yes.

They are just jack sockets with "line out" printed beneath them. Nothing to indicate they might be balanced.

Well... you could always try reading the manual that came with it!

Manual Page 7: Back Panel
The Back Panel provides the majority of input and output connections on the Saffire PRO 24.
1. 2 x TRS jack sockets Line Inputs 3 and 4 *.
2. 6 x TRS jack sockets for Line Outputs 1-6 *.
3. 1 x Optical input socket **.
4. 1 x IEEE1394 6 pin FireWire socket
5. 2 x Din5 MIDI Input and Output sockets.
6. 1 x Power switch.
7. 1 x 2.0mm DC Power inlet socket - Use supplied power supply only.
8. 2 x RCA SPDIF input and output sockets.
* Either 1/4 inch TRS (balanced) or TS (unbalanced) Jack connectors can be used.
** Optical input can be used as either ADAT or S/PDIF optical input.

...can I get away with making up a cable with only the tip and sleeve connected for the headphone amp

Possibly... it depends on the headphone amp and it's input connection format.... neither of which you've told us!

I looked up earth isolator but they all appear to be stereo with phone connectors.

Why am I so strongly reminded of the phrase, 'you can lead a horse to water but you can't make it drink'?

The ART DTI that I recommended above is two channel, but has pretty much every connector format you could possibly need, which is partly why it's such a useful box to have in your toolbox!

H

[ef37a]

"The ART DTI that I recommended above is two channel, but has pretty much every connector you could possibly need, which is why it's such a useful box to have in your toolbox!"

Indeed Hugh and in that case, and because the OP does not seem DIY inclined, it seems good value at about a nifty.

Dave.

[Hugh Robjohns]

... it seems good value at about a nifty

It is! I think the weakened pound has put the price up a bit, but the bax-shop have it at £46 which is pretty good.

https://www.bax-shop.co.uk/miscellaneou ... solatorBax

It's such a useful box I don't understand why everybody doesn't have at least one -- I have three!

It can be used for ground isolation, balanced-to-unbalanced conversion, unbalanced-to-balanced conversion, connector conversion, and signal duplication/distribution.

H

[BigRedX]

Hugh,

Thanks for the replies, and sorry I didn't read the manual, but TBH I have no idea where it is - hopefully it's in the box for the interface somewhere in my attic should I ever need to sell it.

All that extra info is going to be very useful. Since all the outputs on the Saffire are balanced, I take it that means I don't need a DI box to connect it to the PA - a TRS jack plug to XLR male plug with Sleeve > 1; Tip > 2; and Ring > 3; should do it?

Then on the headphone amp. Sorry I don't know what it is as I'm the FNG in this band. It belongs to one of the guitarists and comes out of his box of bits and disappears behind the drum kit at rehearsals, but I am trying to find out. However I do know that when they were using the Minidisc player it was connected to the right-hand channel with a phono > mono jack lead. So I'm thinking a lead with a TRS jack at one end with only the Tip and Sleeve connected going to a standard TS jack at the other.

I am perfectly happy to DIY and modify stuff. What I don't want to be doing is buying things and immediately making permanent modifications to them unless I know 100% that it will solve the problem. The other thing is, that this set up is for live use where ideally I want as little complexity as possible. IME each extra connection and extra piece of equipment in the signal chain is another source of potential failure once the gear arrives at the venue. My ideal solution will be three leads from the interface 2 to the PA and one to the drummers headphone amp and nothing else. If I do need to make up special leads I'll be making 2 of each so I have spares.

BTW if I do need to completely isolate the headphone amp will any transformer-based DI box do? I have 4 EMO ones in my spares box at the moment.

[ef37a]

Just like to add...You don't need to alter the RCA connectors in any way.
Just use gash leads and cobble an adaptor together, at these lowish impedances you can use finger twisted joints and bit of Gaffer or Elastoplast in extremis!

But then the box I mentioned WAS less than a fiver and an ISO box is ALWAYS handy.

Dave.

[Hugh Robjohns]

Since all the outputs on the Saffire are balanced, I take it that means I don't need a DI box to connect it to the PA

You don't need one, no, a TRS-XLR cable will get the job done if the PA can accept line level inputs.

However, there are some important safety benefits in using a DI box since its transformer will isolate the Saffire from unwanted phantom power down the cable or any mains-related faults in the PA system. Personally, I always connect my interface/keyboards to an unknown PA system via passive DI boxes.

I do know that when they were using the Minidisc player it was connected to the right-hand channel with a phono > mono jack lead.

Sounds like it has an unbalanced input then.

So I'm thinking a lead with a TRS jack at one end with only the Tip and Sleeve connected going to a standard TS jack at the other.

Probably, but it depends on the nature of the output driver in the Saffire. If it's an active symmetrical type then it should work as you require, but the level may be 6dB lower than might be expected as you're only taking half the signal on that configuration.

The other issue when working with unbalanced gear is the risk of creating a ground loop by linking the interface and headphone amp grounds directly, which is why the isolation box I mentioned is so handy for situations like this.

BTW if I do need to completely isolate the headphone amp will any transformer-based DI box do? I have 4 EMO ones in my spares box at the moment.

No. A DI box and ground isolator box are different things.

A ground isolator box works at line level in and out, accepts balanced or unbalanced connections on either side, and has a moderate input impedance.

A DI box takes line or instrument level in but gives out mic level. It is also unbalanced in but balanced out. And it has a very high input impedance. So not at all what you need in this situation!

The EMO boxes will be great for protecting the Saffire output from the PA system, though, and deliver a mic level into the stage multicore along with all the stage mics -- just what most PA guys like!

H

[BigRedX]

Thanks Hugh!

The EMO DI box inputs are all unbalanced. Will I be OK with standard mono jack to jack leads for connecting this to the Saffire? Doesn't having a part unbalanced cable run even if it's only a few inches long defeat the object of using balanced cables and the interface having balanced outputs?

For the headphone amp I'll make up a TRS > TS jack lead and see how we get on with it, and if it doesn't cure the cross-talk problem or is too quiet I'll invest in one of those ART boxes. Is it suitably low-profile to fit into a 1U rack space?

EDIT: I've just found out that the Headphone amp is a Yamaha MM10 being run from it's external PSU rather than batteries. AFAICS from the manual the inputs are unbalanced.

[Hugh Robjohns]

The EMO DI box inputs are all unbalanced.

Ah yes... good point! Brain disengaged.

Will I be OK with standard mono jack to jack leads for connecting this to the Saffire?

Normally I'd say yes, but given that we don't yet know the cause of the crosstalk issue you had before I'd be cautious about it at the moment. It won't cause any damage, but crosstalk remains a possibility.

Doesn't having a part unbalanced cable run even if it's only a few inches long defeat the object of using balanced cables and the interface having balanced outputs?

With line level signals interference isn't so much of a problem with unbalanced connections, but ideally you would keep everything balanced.

Normally, you use DI boxes with unbalanced sources anyway (keyboards, guitars etc). With balanced outputs you can use the isolating transformer for protection (that's why I have three DTI boxes), and some passive DIs are actually wired with TRS sockets and work as balanced or unbalanced inputs depending on the type of plug used ( I modified my own EMO boxes to work that way).

For the headphone amp I'll make up a TRS > TS jack lead and see how we get on with it...

Good plan.

...and if it doesn't cure the cross-talk problem or is too quiet I'll invest in one of those ART boxes. Is it suitably low-profile to fit into a 1U rack space?

It is 1U high:

http://artproaudio.com/isolators/product/dti/

EDIT: I've just found out that the Headphone amp is a Yamaha MM10 being run from it's external PSU rather than batteries. AFAICS ... the inputs are unbalanced.

Yes, I figured as much. It is quite likely, then, that the external PSU is a double insulated type and so the headphone amp will rely on getting its earth reference from the Saffire...

If that's the case using an isolating transformer may well cure the crosstalk, but may also make the headphone amp buzz badly as it would then be lacking an earth reference... so you'd have to add a ground reference to the output side of the DTI box... which is easy, but an added complication.

H

[BigRedX]

Hugh,

OK... so if I'm reading this correctly:

1. I should be OK with a short unbalanced run between the Saffire and the EMO DI box but I would probably be better off with an ART DTI box to connect the interface to the PA.

2. Disconnecting the R connection of the Saffire to the Headphone amp might sort everything out, but if it doesn't I will definitely need an ART DTI box here.

3. I might end up with an earth buzz which I assume can be fixed by connecting a lead from the earthing terminal on the output side of the ART DTI to the earth of the lead going to the Headphone amp.

Which brings me to another potential problem. All this equipment is going into a rack box with detachable top, back and front lids mounted on metal rack shelves. I'm now a little worried that I'm going to be introducing potential earth issues caused by the various item's cases touching each other in the rack, which I definitely wouldn't have had with just 3 leads coming out of the Saffire going to the PA and headphone amp...

[Hugh Robjohns]

1. I should be OK with a short unbalanced run between the Saffire and the EMO DI box but I would probably be better off with an ART DTI box to connect the interface to the PA.

Correct.

2. Disconnecting the R connection of the Saffire to the Headphone amp might sort everything out, but if it doesn't I will definitely need an ART DTI box here.

Not definitely... but probably (with the caveat I mentioned earlier about ensuring the headphone amp has a reference earth)

An alternative might be simply to use the headphone amp in the Saffire to drive the drummer's phones directly!

3. I might end up with an earth buzz which I assume can be fixed by connecting a lead from the earthing terminal on the output side of the ART DTI to the earth of the lead going to the Headphone amp.

The earthing terminal on the DTI box is on the input side -- as clearly shown in the circuit schematic printed on the box and in the manual. You would need to provide an earth by using one of the other (spare) output connector's ground terminal.

All this equipment is going into a rack box with detachable top, back and front lids mounted on metal rack shelves. I'm now a little worried that I'm going to be introducing potential earth issues caused by the various item's cases touching each other in the rack, which I definitely wouldn't have had with just 3 leads coming out of the Saffire going to the PA and headphone amp...

Life is never simple is it?

Given that you haven't yet proved beyond doubt the cause of the problem it might be a little presumptuous to say this... but it seems to me that the cause of all your problems is (almost certainly) the unbalanced nature of the headphone amp.

So perhaps investing in a better headphone amp with a balanced input would be justified given the extra expense, inconvenience, and potential reliability issues you are now facing to solve that problem.

You know what they say: Buy cheap, buy twice!

H

[BigRedX]

Thanks Hugh!

The guitarist in question who owns the headphone amp makes his living buying a selling musical equipment second hand - a lot of it through car boot sales - so I would imagine that this model is something he has picked up on his travels, and up to now with the Minidisc player it has done pretty well.

I could try running the headphones directly from the interface, but that would require a long extension lead and would remove control of the click volume from the drummer. And no I'm not letting him anywhere near my MacBook!

So I would agree with you that it might be prudent to upgrade to a dedicated headphone amp which is a bit more robust and with proper balanced line inputs. Is there a model (or two) that you would suggest I look at?

[BigRedX]

For anyone who has been following this and is interested, I have a fix that works for me.

I'm running the main stereo backing out of outputs 1&2 on the Saffire using unbalanced cables into an EMO dual channel DI box with ground lift switched on (with the ground connected I was getting a nasty buzz through the rehearsal room PA).

I'm then running the click track from output 3 on the Saffire to the Yamaha Headphone amp using a special lead I made up that has a TRS jack plug at the end which plugs into the Saffire with nothing attached to the R terminal and a standard TS jack plug at the end which plugs into the headphone amp.

Everything worked fine at last night's rehearsal, and there was no bleed through of the click track into the main output. The gig is on Saturday (Leeds Goth City Festival) so hopefully it will all still be working fine then.

I suppose the long-term solution is to invest in the ART DTI box for the connection from the main outputs to the PA and a headphone amp that accepts a mono balanced line input, so I can used balanced connections through the system and not have to worry about custom cables.

Can anyone recommend a suitable headphone amp? - must be nice and loud for the drummer's headphones.