Typical Audio Interface Balanced Output > TS (unbalanced) 1/4" Jack (OUT). Safe???

[777funk]

This would of course short circuit the Negative side of the balanced signal to ground.

I'd think most interfaces would be designed to tolerate this. No?

I ask because I have an M-Audio Profire 610 that I've connected this way and after a few months of this, that output is having some issues. It's both channels of the stereo out that are having issues as well as the headphones out for channels 1+2 out. I'd think the actual outputs would have different opamps than the headphones, which makes me think it's further upstream and not the output opamps that have been damaged. Of course it's hard to know. But back to the question, wouldn't most manufacturers design the opamp out circuits to handle shorting half of the balanced signal out when someone (perhaps technically incorrectly) uses a regular 1/4" jack?

 

[ccaudle]

There isn't really "a" typical balanced output circuit, there are three.

wouldn't most manufacturers design the opamp out circuits to handle shorting half of the balanced signal out

For TRS outputs typically yes. For XLR practice varies quite a bit.
For best results you can modify your connections based on type of output circuit, but unfortunately the ProFire 610 user manual does not seem to give any clues as to what type of output circuit it uses.

 

[scott2000]

I'd think most interfaces would be designed to tolerate this

after a few months of this, that output is having some issues

 

[NoisyIndividual]

The line outs have a series electrolytic capacitor and a mute transistor on both hot and colds.
The headphone outs don't have inline capacitors, just the mute transistors.

Normally these sort of problems are down to the capacitors exceeding their sell by date, but as the headphone also seem to be affected which doesn't have them, you would have to suspect the mute transistors, or the mute circuit that is driving them.

As all these are affected I would be looking at the mute circuit, which is not generically uncommon.


Op-amps don't usually get damaged, they either work or they don't, so this is not likely.

 

[NoisyIndividual]

Mute circuit here, I would suspect C121.

 

[ccaudle]

I'd think the actual outputs would have different opamps than the headphones

Is there any possibility that the second topology I drew here is used? In that case changes to the line output could affect the headphone out.
I don't know how likely that is, my guess would be not terribly high.

 

[NoisyIndividual]

Is there any possibility that the second topology I drew here is used? In that case changes to the line output could affect the headphone out.
I don't know how likely that is, my guess would be not terribly high.

View attachment 146281

Is there any possibility that the second topology I drew here is used? In that case changes to the line output could affect the headphone out.
I don't know how likely that is, my guess would be not terribly high.

View attachment 146281

Line outputs and headphone outputs have dedicated op amps.
This is normal, that is not.

I don't see any value in guessing here.

 

[MagnetoSound]

Your thread title implies that you are connecting two outputs together. What are you actually doing?

 

[NoisyIndividual]

No they are not.

They are simply saying they have been connecting a balanced output via an unbalanced jack, thus shorting one leg and questioning whether this might have damaged anything.

They then have postulated on here the fault they are now having is due to this fact.

Now I am saying the fault they have is nothing whatsoever to do with using a mono jack in a balanced output, but is in fact due to a fault in the Mute circuit.

 

[Voyager10]

@777funk - can you describe what 'the problem' here is? Do you get any audio? Audio plus noise or distortion? Nothing at all? Is it intermittent? Is it the same for left and right channels? Tell us everything you know.

 

[777funk]

@777funk - can you describe what 'the problem' here is? Do you get any audio? Audio plus noise or distortion? Nothing at all? Is it intermittent? Is it the same for left and right channels? Tell us everything you know.

The audio volume is in and out like a dirty pot. And it's distorted and clear also. On both L + R channels. Interestingly, channels 3 and 4 are fine.



Thanks NoisyIndividual. I will look into the mute ckt.

 

[NoisyIndividual]

You can check if the mute transistors are in mute by measuring the voltage on their base. It should be around zero. If it is around 0.6v they are muting, or partially muting. I have found in the past that a faulty mute circuit doesn't always fully mute, the transistors can start to conduct, and you get different levels from different outputs, and/or noise/distortion.
The problem may just be all the series caps need changing which is very common, and maybe the headphones not working is a separate issue.

 

[777funk]

You can check if the mute transistors are in mute by measuring the voltage on their base. It should be around zero. If it is around 0.6v they are muting, or partially muting. I have found in the past that a faulty mute circuit doesn't always fully mute, the transistors can start to conduct, and you get different levels from different outputs, and/or noise/distortion.
The problem may just be all the series caps need changing which is very common, and maybe the headphones not working is a separate issue.

Well the great thing here is that there are 2 headphone outs. One is for Outputs 1+2 and the other is for Outputs 3+4 (this gives the user two mixes). So in order to make this thing work well now, I just send the mix to Out 3+4. So it's easy enough to verify that the headphones are good. Also the mix going to speaker Outs 1+2 sounds similar (in and out slightly and slightly distorted). At first I thought it was my amp (TPA3118 class D) but when I found that it's also in the headphones, I figured that something is going bad in the M Audio.

 

[777funk]

Apparently this is an intermittent issue as it works great again now. It's always so confusing with interfaces because there's also a software / digital element involved. I didn't change anything in software, but sometimes things boot up differently etc. It's odd that it'd be both channels (unless a single opamp is driving both channels plus headphones).

 

[Voyager10]

Can you post photos of the board, in as much detail as you can?

It would be worth trying to identify the DAC chip for these outputs - typically an audio interface will have one stereo DAC chip per pair of output channels - and take a look at the components around it.

( @Khron has an excellent website with teardowns of similar units - e.g. https://khronscave.blogspot.com/2020/03/59-motu-828-mk2-teardown.html - which illustrates the sort of thing you're looking for).