Hunting Down System Noise, Looking For Clues

[zamproject]

Do firewire  isolators exist?

seems like that would be ideal

Be aware that despite motu PCIe system use standard FW cable, it's not at all FW data type also maybe different pining for supply/0v ref
So maybe won't work ?

Best
Zam

Best

 

[Matt C]

How did you go from the balanced TRS converter output to the unbalanced console?

TS to TS? This would short pin3 and ground at the converter which I'm guessing is ok with the Motu....

I'm sure you already know but......

Some balanced outputs want pin 3 floating some want it shorted to ground when interfacing to unbalanced.  Depends on the circuit ......

I'm leaving pin 3 floating. But I did try both ways and it made no discernible difference

 

[scott2000]

I'm leaving pin 3 floating. But I did try both ways and it made no discernible difference

Floating at the output of the converter?

You can't just decide what sounds better to you in regards to noise. You're Motu may be servo/cross coupled balanced or electronically balanced and The outputs of the converter HAVE to either have the pin 3 not connected or it has to be shorted to ground........ if you're feeding unbalanced inputs down the line depending on which output you have.. I'm just making sure as I was unclear ......

http://www.rane.com/note110.html

Like either 9 or 11 on the Rane page...........

Using TS to TS should automatically give you 11

 

[Matt C]

Floating at the output of the converter?

You can't just decide what sounds better to you in regards to noise. You're Motu may be servo/cross coupled balanced or electronically balanced and The outputs of the converter HAVE to either have the pin 3 not connected or it has to be shorted to ground........ if you're feeding unbalanced inputs down the line depending on which output you have.. I'm just making sure as I was unclear ......

I plugged a TRS cable into the converter output, and another TRS cable into the tape return input.  I then used alligator clips to test different wiring schemes. What I've been using and what gave the best (but still unacceptable) noise performance was connecting T-T, S-S, and leaving the ring (pin 3) floating at both ends.

I believe the MOTU outputs are electronically balanced but I haven't been able to confirm that. If they were cross-coupled, leaving the pin 3 floating would interrupt the signal, correct?

 

[scott2000]

I plugged a TRS cable into the converter output, and another TRS cable into the tape return input.  I then used alligator clips to test different wiring schemes. What I've been using and what gave the best (but still unacceptable) noise performance was connecting T-T, S-S, and leaving the ring (pin 3) floating at both ends.

I believe the MOTU outputs are electronically balanced but I haven't been able to confirm that. If they were cross-coupled, leaving the pin 3 floating would interrupt the signal, correct?

Not exactly.....

Here's a good reference....

https://www.presonus.com/learn/technical-articles/balanced-unbalanced


Also....I wouldn't count on the Motu not being cross coupled.

If it is, TS to TS is what you want......(which means tying pin 3 to ground at the converter output which a TS will do automatically usually))

 

[Matt C]

Here's a good reference....

https://www.presonus.com/learn/technical-articles/balanced-unbalanced

Thanks for the link.  I tested that out and it seems that the MOTU is electronically balanced like I thought, not cross-coupled.

A little bit of progress - I was able to get rid of the buzzing from the computer screen just by lifting the ground prong on its power cable. 

The noise from the computer itself remains though.  I realized that some of it is linked to activity on the computer - as I click and scroll through windows, drag faders around in my DAW, etc, those actions create little bursts of noise, like high pitched screeching. This is what makes me concerned that it's an issue with a poorly designed motherboard or power supply in the computer.

 

[scott2000]

Thanks for the link.  I tested that out and it seems that the MOTU is electronically balanced like I thought, not cross-coupled.

A little bit of progress - I was able to get rid of the buzzing from the computer screen just by lifting the ground prong on its power cable. 

The noise from the computer itself remains though.  I realized that some of it is linked to activity on the computer - as I click and scroll through windows, drag faders around in my DAW, etc, those actions create little bursts of noise, like high pitched screeching. This is what makes me concerned that it's an issue with a poorly designed motherboard or power supply in the computer.

Awesome... Yeah those monitors can be a headache....KVM switches too.....

Weird about the Motu.....

My experience has been that when they mention that you can use balanced or unbalanced with a unit, and don't mention any special instructions, they are assuming the customer will just be using TS cables when interfacing unbalanced gear. Which would mean they are wanting pin 3 tied to shield (- shorted to ground). Must be something different with these...... Maybe they're just counting on the TRS outputs interfacing the unbalanced gear with TRS which would work  unless the unbalanced inputs are wired differently.... Which is possible .....
Because the scenario of getting this interfacing part wrong can definitely cause weird noise issues.... Probably more so when shorting when you aren't supposed to though....

Getting there!!! You'll get it!

 

[Tubetec]

I had similar things happen before ,Spdif optical link from the pc to the convertors solved it for me .
Laptops worked perfect on battery ,the moment you used the power supply with any kind of ground connected to the audio path the fuzzy buzzy thing came back . I know it doesnt offer you any possible solution with your set up ,but I said it might be worth adding it into the discussion anyway .

Galvanic isolation between the pc and the convertor module is what you need ,there are small digital transformers that might help , switchmode psu's can often interact in weird ways . Have you tried lifting the ground on the audio interface ,and just allowing it to be grounded through the console ?

 

[Matt C]

Have you tried lifting the ground on the audio interface ,and just allowing it to be grounded through the console ?

I didn't hear any change when lifting the ground on the converters, but after lifting the ground on both the computer and the screen, the noise was noticeably reduced. Definitely still there, but quiet enough that I can get out of panic mode.  My instinct tells me it's a sloppy/unwise fix but it'll do for the moment at least.