Interface outputs

ecc83

Well-known member
Read something the other day that I "knew" but had not thought the circumstances through?

Problem: a chap having trouble sending an AI's line outputs, via transformers to a venue desk, cannot get much above -6dBu without distortion.
That problem seems to be due to the 'line' signals feeding mic inputs on the desk which have an input impedance of only 1 to 1.5k. Now, most line outputs would drive such a low Z but in this case, and with the transformers, apparently not. The suggested fix? Use DI step down transformers not 1:1 line jobs. It seems standard practice for stage feeds to always go into mic inputs?

However, feeding your common or garden DI from a line output is also fraught. This is because they use a TS jack and so a TS plug. If that goes into a TRS balanced AI line out then it will short the ring signal to sleeve and also cause distortion. New cables needed TS one end, TRS the other but ring not connected. Needless to say, TRS goes into AI!

Dave.
 
Read something the other day that I "knew" but had not thought the circumstances through?

Problem: a chap having trouble sending an AI's line outputs, via transformers to a venue desk, cannot get much above -6dBu without distortion.
That problem seems to be due to the 'line' signals feeding mic inputs on the desk which have an input impedance of only 1 to 1.5k. Now, most line outputs would drive such a low Z but in this case, and with the transformers, apparently not. The suggested fix? Use DI step down transformers not 1:1 line jobs. It seems standard practice for stage feeds to always go into mic inputs?

However, feeding your common or garden DI from a line output is also fraught. This is because they use a TS jack and so a TS plug. If that goes into a TRS balanced AI line out then it will short the ring signal to sleeve and also cause distortion. New cables needed TS one end, TRS the other but ring not connected. Needless to say, TRS goes into AI!

Dave.
The “venue desk” doesn’t have line level inputs? Sometimes, like some AIs, they have combo jacks that operate as mic level for an XLR and line if a 1/4” is inserted. And the AI’s line out is not balanced?? Get a new AI if not, or, yes, just step the 1/4” down with any DI that has a -10-20 pad, I suppose.
 
The “venue desk” doesn’t have line level inputs? Sometimes, like some AIs, they have combo jacks that operate as mic level for an XLR and line if a 1/4” is inserted. And the AI’s line out is not balanced?? Get a new AI if not, or, yes, just step the 1/4” down with any DI that has a -10-20 pad, I suppose.
Yes Keith, desks will always have line inputs, hopefully 10k or more but it seems sound guys habitually just stuff feeds into mic inputs.

"A new AI"? Could be a worse situation. Many earlier interfaces (and most budget ones) use an impedance balanced output therefore there is no danger of shorting the 'cold' ring signal to ground. Newer and better interfaces use two op amps sections to give a true differential output. My MOTU M4 is one such.

It was never my intention to cause any disharmony here! I merely pass on the information and it might save someone some time chasing a low, possibly distorted signal problem?

Dave.
 
Why not just use TRS to XLR cables? If the AI output is too hot, can't it be turned down by the AI itself?
'Sigh' i wonder if anyone here knows who Bernard Bresslaw was and his catch phrase?

The problem arose in an existing system and it is not the level of the line out that is the issue but the loading caused by the low input Z of the mic channels. And before anyone else has a pop! Yes! The line outs SHOULD have been able to drive about 1k but in this particular case they couldn't.

This is in fact one of several shortcomings of the TL072 op amp that must have been used in many, many thousands of audio circuits.

(Gonna feed me H hogs now)

Dave.
 
Okay, it wasn't clear that the AI outputs were tried without the transformers. I can't say I've had that problem.

I have had cases where a line output was too hot for a mixer's mic input, so it seemed like something to consider. Some of these newer digital mixers don't have pads on their mic preamps. Setting the gain control doesn't help because the signal has already clipped upstream.

I'm not sure I've had the problem of distortion from connecting a balanced output via unbalanced cables. Maybe it's because I try to use balanced whenever it's appropriate. Years ago when I was poorer and less informed, I did use TS cables to connect some balanced gear, but that was often impedance balanced stuff. That just led to typical grounding issues like hum.
 
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