Audio Interface line outputs, FOH, line isolators, DI Boxes and distortion :-)

[omega_deano]

Hey guys, I was hoping that the SOS hive mind might be able to help my tiny musician brain get to grips with an issue that has been bothering me.

So, we run a live rig which consists of tracks, live vox & live guitar. They all go into my MOTU Ultralite mk5, through Ablteton, and then out of the balanced line outputs to FOH.

In between the interface & FOH stagebox, I have some Orchid Electronics Line Isolators, which give me galvanic protection, and help with ground loops.

The problem I am having, is that if I turn my interface outputs up past a certain level (about -16db), the Iso boxes start distorting. The Iso boxes should be able to handle about +16dbu before distortion, so I think something fishy is happening. I have a limiter in Ableton set to -6db, so even when the outputs are at full on the interface, they aren't getting full level. I should also probably add that our music is VERY sub/bass heavy. It's basically a mix between electronic dub & dubstep, so you get the idea!

So, my one line of thinking was, maybe I need some Line Isolators with a higher quality transformer in them (such as the Radial J-Iso) as they should be able to handle the bass better, and generally have better performance.

But then I emailed John at Orchid about the issue, and he replied -

" The transformers used in the Orchid isolators are able to handle up to about +16dBu before distortion becomes evident.

Quite a lot depends on the source and load impedances of course, normally the input should come from a low impedance (almost always the case) - and the load should be medium/high (10K ohms or more).

Are you feeding your line level signals into a microphone input on a mixing desk?

It is possible that even with a PAD selected that the input impedance could be fairly low (to suit microphones of course).

Have you tried interfacing with DI Boxes? Although it is a good idea to use transformers to protect against Phantom power, which many devices (certainly iPads and computers) - are not protected against and can easily be damaged. "

This then got me thinking! Are my Iso boxes actually distorting because they are trying to feed essentially a mic input which has a too low input impedance? I had always assumed that as my outputs are balanced line outputs, I should keep it balanced, and send FOH line signal, but actually I am having to turn them down basically to mic level anyway because my isolators distort, and also, I assume that most stage boxes are going into mic inputs anyway!

So to get to the point, would I be better off getting some DI boxes with transformers in them (such as the Radial J-DI's), or would investing in a Line isolator with a higher quality transformer (such as Radial J-Iso) in it do the job?

A follow up question i have is, if DI boxes are the best option, what is the best way of connecting my balanced TRS line outputs to the unbalanced inputs of a DI box? My MOTU manual also states regarding the line outputs -

" Quarter-inch line outputs are not cross-coupled. Therefore, when connecting them to an unbalanced input, use a TRS plug with the ring disconnected. Not floating the negative terminal will short it to the sleeve ground and cause distortion. "

In this scenario would I take a TRS cable and disconnect the ring at one side, or both sides?

As you can see, I am rather confused!

Any help would be greatly appreciated!

Many thanks

Dean

[Hugh Robjohns]

I wrote a long expansive answer which my phone lost when I swapped to another app to check something!

But basically, I agree with John G.

I expect your iso transformer outputs are feeding the relatively low input impedance of a mic preamp at the FOH desk which, even with a pad selected, are likely to be only a few kilo Ohms instead of the 20-50k Ohms they need.

And all transformers will saturate and generate distortion with very high levels of LF. Cheap transformers are much worse at this than expensive ones, but they all have their limits! The Orchid transformers aren't top quality Jensens or Lundahls....

So, given your application needs, my recommendation would be to use a premium passive DI, such as the Radial stereo JDI. Yes, it's expensive, but it will last a lifetime and is as good as you'll get.

In my experience, every live sound system is set up to cope with mic-level DI outputs on stage, whereas few can cope with balanced line level sources. So going the DI route is likely to give much more consistent results without distortion issues (and using the JDI's input pad will reduce the risk of LF saturation further)

You need to be slightly careful in how you wire the balanced output from the interface, though. Most unbalanced cables short the cold side to ground and some balanced outputs are perfectly fine with this (and many even boost the hot output by 6dB to compensate).

However, lots of electronically balanced outputs really don't like it and will generate obvious distortion if connected that way — so this type has to be wired with the cold output left isolated (floating) rather than grounded. I think the MOTU is of this type.

In which case you'll need to find or make a cable with TRS at one end (for the interface) and TS at the other (for the DI box). Wire tip to tip, and sleeve to sleeve using screened single core cable, and leave the ring terminal isolated. As long as the cable is short (less than 2m) you are very unlikely to have any interference issues.

If you're handy at DIY, you could build your own passive DI box with balanced inputs. You'd just need a metal box, TRS jack sockets and XLRs, and a couple of high-end 12:1 (or similar) DI transformers from Jensen, Lundahl or Sowter (etc,). Pick something that offers the lowest distortion with the highest input level at 50Hz!

[omega_deano]

Hey Hugh. Thanks very much for the reply. I know the pain of losing entire messages well, so thanks for your perseverance

That all makes sense and I’m sure having a mic level output for FOH is going to make thing more consistent, and it will be seeing the correct load too.

I think I’ll just bite the bullet and get a few of those Radial JDI’s. Like you say, they’ll last a lifetime!

I’ll probably just buy some due to time restraints and my very basic electronics knowledge, although I am curious about building some with balanced TRS inputs. Do you know of any basic DI box building instructions that would suit? Might be handy as they need to fit in a particular space in my rig.

Thanks for the heads up on the TRS-TS leads. I can solder those, so if so go down the Radial route that will be fine.

Cheers again Hugh!

[Dave Rowles]

Hugh's detailed response is spot on of course. I'd highly recommend Radial for an easy stereo DI option. Personally I'd go for active, because that's what I see mostly on stages.

We use DI boxes to convert unbalanced to balanced for long distance signal transmission. Here's the thing, if you're coming out of the motu into a DI box, then you don't need to use balanced cables, you can use a standard instrument jack cable. The MotU will output fine and you don't need to mess around with soldering. As long as they distance from the MotU to the DI is short (5m say) then you're unlikely to have any noise problems.

The other reason to use a DI box is to protect your MotU from unexpected phantom power that will then blow the MotU outputs.

[Hugh Robjohns]

Here's the thing, if you're coming out of the motu into a DI box, then you don't need to use balanced cables, you can use a standard instrument jack cable. The MotU will output fine and you don't need to mess around with soldering.

As I mentioned above, you need to be REALLY careful when connecting balanced outputs to unbalanced inputs. Get it wrong and you will generally get peak or transient distortion... but in worst case scenarios you could get smoke!

It all depends on the specific design of the balanced output stage, and there are many different types with very different wiring requirements. There is no one-size-fits-all option here! Beware the dragons!

The clever balanced output stages act much like simple transformer outputs, in that they are quite happy if one side is connected to the ground terminal of an unbalanced destination. Most of this type will automatically adjust themselves to send the same signal level to the unbalanced destination as they would to a balanced one. Some can even sense ground-loop currents via the cold side connection and cancel them out to prevent audible ground-loop hums!

However, such 'cross-coupled' designs are more expensive to implement and so are typically only found on high cost products.

The majority of lower cost equipment with full symmetrical balanced outputs (as opposed to impedance-balanced outputs), simply use two independent op-amps, one to drive each side of the output, arranged to deliver signals in opposite polarities.

Connect this type to an unbalanced destination and the signal level will be 6dB lower than to a balanced destination simply because one half the signal voltage is being ignored. That's the least of the potential problems, though!

If the cold output is shorted to ground when connecting to an unbalanced input (the usual situation when using standard adapters and adapter cables), the cold-side op-amp will be forced to deliver higher than normal output currents through a low value resister into ground for extended periods.

This can potentially cause the chip to overheat (and eventually let the magic smoke escape). More likely, the op-amp will repeatedly enter a self-protecting current-limiting mode which will heavily distort its output signal.

Obviously, that signal isn't reaching the destination because it's shorted to ground... but it will be dumping all that heavily distorted output current straight into the local ground reference of the adjacent op-amp feeding the hot-side output to the unbalanced destination. Consequently, distortion in the grounded cold-side op-amp gets directly into the hot-side op-amp, and that's how it becomes audible.

This is the reason some manufacturers include statements in their manuals such as: ...when connecting a balanced output to an unbalanced input, use a TRS plug with the ring disconnected. Grounding the cold terminal will cause distortion. The manual for the OP's particular MOTU interface includes words to this effect, as the OP himself quoted above.

So, it's always VERY important to check the manufacturer's recommendations regarding balanced/unbalanced connections because different devices have completely different requirements.

[Dave Rowles]

That's good info Hugh, but I've never encountered that on stage using active DIs. Not a single interface I've ever plugged in with TS jacks has had any problems, though I've obviously not plugged in every interface that exists, I've had a wide range, both low and high cost, over my years, and they've all worked perfectly fine with TS.

[James Perrett]

Not a single interface I've ever plugged in with TS jacks has had any problems,

That's probably down to most interfaces using ground compensated outputs rather than having both legs actively driven.

It can be interesting to try plugging a pair of high impedance headphones into an output and see what it sounds like.

[Dave Rowles]

For clarity I thought I'd better add that this is always into 1/4" jack sockets usually labels "output 1/2" or "Main Outs L/R"

[Hugh Robjohns]

I am curious about building some with balanced TRS inputs. Do you know of any basic DI box building instructions that would suit? Might be handy as they need to fit in a particular space in my rig.

I don't know specifically of any, but for a passive DI you'd literally need a box, a DI transformer, a TRS connector for the input (with a second if you want a link out), and an XLR for the output... and maybe some wire!

Here's a schematic from Jensen for a standard (unbalanced input) passive DI box:

https://www.jensen-transformers.com/wp- ... /as007.pdf

The only modifications you'd need would be to replace the unbalanced TS input socket with balanced TRS input socket and, ideally, split the input resistance across both legs (although it might not even be needed...).

So, the primary yellow wire goes to the tip connection, while the primary orange wire goes to the ring connection (instead of the sleeve). The black and white transformer screening wires go to the sleeve as per the diagram. Here's a quick version I knocked up from a Jensen original (with better ground lift arrangements).

If you were to insert an unbalanced TS plug the orange wire would short to the ground and it would work just as a normal unbalanced DI. Insert a TRS plug from a balanced source, and you have a fully balanced connection.

The same idea will work with any step-down transformer intended for passive DI box applications from any of the well known transformer manufacturers — Jensen, Sowter, Lundahl, etc etc.

[Hugh Robjohns]

I've never encountered that on stage using active DIs. Not a single interface I've ever plugged in with TS jacks has had any problems...

I have with active balanced outputs. Not in every situation, but it has definitely happened to me... Audible distortion, usually only on loud peaks.

Obviously, it wouldn't occur with impedance-balanced outputs (as the cold-side isn't actively driven), as James says.

If I can't arrange a balanced connection in such circumstances, one solution which has always worked is a TRS-TS cable from my bodge box, wired with the ring isolated!

And manufacturers wouldn't cover their arses with such print disclaimers if it wasn't a thing!

[Dave Rowles]

Fair enough

[omega_deano]

I am curious about building some with balanced TRS inputs. Do you know of any basic DI box building instructions that would suit? Might be handy as they need to fit in a particular space in my rig.

I don't know specifically of any, but for a passive DI you'd literally need a box, a DI transformer, a TRS connector for the input (with a second if you want a link out), and an XLR for the output... and maybe some wire!

Here's a schematic from Jensen for a standard (unbalanced input) passive DI box:

https://www.jensen-transformers.com/wp- ... /as007.pdf

The only modifications you'd need would be to replace the unbalanced TS input socket with balanced TRS input socket and, ideally, split the input resistance across both legs (although it might not even be needed...).

So, the primary yellow wire goes to the tip connection, while the primary orange wire goes to the ring connection (instead of the sleeve). The black and white transformer screening wires go to the sleeve as per the diagram. Here's a quick version I knocked up from a Jensen original (with better ground lift arrangements).

balanced DI Box.jpg

If you were to insert an unbalanced TS plug the orange wire would short to the ground and it would work just as a normal unbalanced DI. Insert a TRS plug from a balanced source, and you have a fully balanced connection.

The same idea will work with any step-down transformer intended for passive DI box applications from any of the well known transformer manufacturers — Jensen, Sowter, Lundahl, etc etc.

Hey Hugh. Thanks very much for this! Seems like getting hold of Jensen transformers in the UK is pretty tricky, so I’m wondering how much dollar it would really save me in the end!
Does seem like a good DIY project though!