Hum when connected straight to converters - Presonus Quantum

[redddog]

I have a few outboard pres that I'm using with the Quantum. When I hook them up through trs connections (in rear ) I get a buzz. Oddly, the out board pres  (Chameleon 7603 and MPA Gold) don't even need to be powered up to cause the hum - just connected. Even odder, I have cables plugged into all of the inputs back there and only the ones that are connected to preamps buzz. I have a couple channels that have TRS cables connected but not plugged into preamps that don't hum. Internal Xmax pres are silent.

Is there a setting I'm missing or is this something bigger.

Thanks all.

 

[Bo Deadly]

Is there anything in between like a patchbay? Try disconnecting everything except power and one TRS cable and terminate the end of the TRS somehow (meaning short the tip and ring to the sleeve). Then try adding one device. Then more and more until you discover the fault.

 

[redddog]

Is there anything in between like a patchbay? Try disconnecting everything except power and one TRS cable and terminate the end of the TRS somehow (meaning short the tip and ring to the sleeve). Then try adding one device. Then more and more until you discover the fault.

No sir. Pre straight into the interface via TRS. Its just starnge that the Pre doesn't even need to be on. Just connected.

 

[JohnRoberts]

No sir. Pre straight into the interface via TRS. Its just starnge that the Pre doesn't even need to be on. Just connected.

Plugged into the outlet?

Could be a "pin one problem" where 0V is corrupted by circulating chassid ground currents.

JR 

 

[redddog]

Plugged into the outlet?

Could be a "pin one problem" where 0V is corrupted by circulating chassid ground currents.

JR

So would making sure all pres and interfaces were plugged into the same strip make a difference?

Sorry, my level of understanding is lacking to say the least.

 

[JohnRoberts]

So would making sure all pres and interfaces were plugged into the same strip make a difference?

Sorry, my level of understanding is lacking to say the least.

Maybe...

perhaps search "pin one problem". Pretty well discussed around the internets. 

JR

 

[smilan]

Sounds to me like a ground loop, try to disconnect one end of the shielding cable that goes between the preamp to the convertor.

 

[Tubetec]

Ground loop is highly likely , both pre's are grounded through the plug , if you do happen to have them plugged into sockets far apart it just means a larger loop and most likely more noise . You may also have your monitors connected to the interface which might also have ground connections . 

Its not a good idea to lift ground wires in the plug , thats why we have moulded plugs nowadays , much safer like Smilan says to make a special patch cable with the ground unconnected at one end .

The classic situation you used find is a guitarist with two amps ,both grounded  and both signal and ground connected via the jack cable . In the old days guitarists would open the plug and disconnect the ground on one of the amps , but the problem is that now your relying on the jack lead for one of the amps to be grounded . If for what ever reason that amp gets used alone ungrounded ,it could present a shock hazzard.

Make up a cable with ground connected at only one end , mark the end with the floating ground and try that at the input side .

 

[JohnRoberts]

Be careful about defeating safety grounds, that could be unsafe.

JR

 

[Khron]

I vote for disconnecting the ground / sleeve connection on one end of the TRS cables as well. NEVER do it on the mains cable.

 

[redddog]

Here's some more....

So last night, I plugged everything (Chameleon 7603, MPA Gold, Quantum Interface) into the same channel strip. Still had the noise.

Since plugging a mic/xlr (SM 57)directly into the interface preamps on the Quantum caused no static, I wondered if going into the interface from the external pres via XLR rather than TRS would make a difference. It did.

No static going from external pre to interface using an XLR connection. I kept the right channel of the MPA Gold connected via TRS and the Left channel XLR and as predicted, no static from XLR, static from TRS. It's not a hum - more like a static noise - crunchy and hairy.

When doing this, I have to turn the interface pre levels to zero on the channels I'm plugging external preamps into or I would get super hot signals - because I'm essentially using the interface pres as well as the external pres. From what I understand, XLR connections do not bypass preamps on the Quantum.

So It's clearly the TRS connections on the interface and not ground issues, right? I have no clue why this would be. Maybe line input transformers on the interface? I'd be surprised if that unit had line in transformers.

I'd say it could be cabling but I hand made those cables and the liklihood that everyone of them was faulty would be a stretch - even for me.

 

[Bo Deadly]

Are the XLRs on the back also line inputs? If yes, there would have to be some kind of switch either on the panel (I know nothing of presonus anything) or in software. Or maybe XLR are mic only and TRS are line only. That would explain the difference in behavior. You were actually loading the external pres with low Z inputs. So maybe when connected to TRS you just need to fiddle with the software to get the right level. I have a motu that has software switches that switch the line inputs between -10 and +4. Meaning maybe you're sending low signal from the pres and then boosting that along with the noise floor and you're just listening to the little bit of hum that's not uncommon in a lot of gear. Play with the software to make sure it's set for receiving +4, turn the level way down, crank the output on the external pre and actually plug in a mic so that you have real signal and then bring the level up until it's reasonable. Then when you turn the external pre off do you hear hum? IOW, maybe you're just cranking gain in the chain to a level that you would never actually be used.

 

[redddog]

Are the XLRs on the back also line inputs? If yes, there would have to be some kind of switch either on the panel (I know nothing of presonus anything) or in software. Or maybe XLR are mic only and TRS are line only. That would explain the difference in behavior. You were actually loading the external pres with low Z inputs. So maybe when connected to TRS you just need to fiddle with the software to get the right level. I have a motu that has software switches that switch the line inputs between -10 and +4. Meaning maybe you're sending low signal from the pres and then boosting that along with the noise floor and you're just listening to the little bit of hum that's not uncommon in a lot of gear. Play with the software to make sure it's set for receiving +4, turn the level way down, crank the output on the external pre and actually plug in a mic so that you have real signal and then bring the level up until it's reasonable. Then when you turn the external pre off do you hear hum? IOW, maybe you're just cranking gain in the chain to a level that you would never actually be used.

Yeah, the software is incredibly basic (in a bad way) and doesn't really work.

At the moment, I'm not terribly impressed with this Quantum. It has no mixer so while there are 8 output channels, you can only run one set at a time. You have a heaphone amp, mains and a distibution amp, you're out of luck. It's just weird to me.

Then you have this issue with line-in noise. There might be a decent answer to that but it's still been a hassle.

I miss my FF800....a lot.

 

[Khron]

More often than not, the XLRs on interfaces are indeed mic only, while the TRS are line-level (and/or switchable to instrument). In most cases, the TRS are padded down and connected to the mic preamp inputs (as in most consumer mixers).

When you say it has "no mixer", have you downloaded / installed UC Surface?

 

[redddog]

More often than not, the XLRs on interfaces are indeed mic only, while the TRS are line-level (and/or switchable to instrument). In most cases, the TRS are padded down and connected to the mic preamp inputs (as in most consumer mixers).

When you say it has "no mixer", have you downloaded / installed UC Surface?

Yeah. That UC is actually just input levels. It also has an setting that allows you to select what the large knob controls. I can't get that to control anything. One would assume that it meant that if you selected "all," that with the knob turned up all the way, all outputs would be in use and sending oout audio. Not for me. I can only send audio out the L/R Mains. It makes no sense. The manual even shows all the outputs connected to varius sources  - like headphone amps and distribution amps - but the outputs only send audio one pair at a time. Unless you use Studio One. I've read that there is a matrix embedded in S1 that addresses this issue.

That's great but what if I don't want to spend weeks migrating to S1? Presonus should have outlined that full functionality of this interface is only possible when using with S1. Then there is this preamp/line-in issue. I still don't know how to work around that. being forced through the Xmax pres when I have outboard pres that are far better than the onboard interface pres is a major downer for me as well.

 

[Khron]

Well, that's a really dick move... A current day interface without (easy) internal mixing? Especially with a 4-figure MSRP? Jeeeeez...

And "Xmax preamps", eh? At least in the Audiobox 1818VSL they were discrete (NPN+PNP differential input pairs and an opamp), but since these are digitally controlled, I'm gonna guess they just use the good old TI PGA2500 (as in the Apogee Ensemble, and numerous MOTU units - I've done several teardowns on the blog). Just a hunch though...

 

[redddog]

Going to try Studio One (the Free Version) to see if for some reason, it's a Reaper thing.

I don't think it is but someone on another forum is certain it has something to do with it.

 

[redddog]

Going to try Studio One (the Free Version) to see if for some reason, it's a Reaper thing.

I don't think it is but someone on another forum is certain it has something to do with it.

As expected, the DAW made no difference.

I'm thinking that it's just that the line-in inputs on the Quantum suck. It's really the only explaination I can get to.

I have it set up with XLR connections that send signal through tha Xmax pres (though thay are turned down all the way at the interface level). It's a bummer that this is my only option but it works I guess.

Did I mention how much I missed my Fireface 800?

 

[Bo Deadly]

I'm thinking that it's just that the line-in inputs on the Quantum suck.

I very much doubt that. It's almost hard to make a crappy USB interface. Even a cheap USB interface has vastly superior noise performance than anything you would ever actually need.

Try terminating the input. Take an XLR cable and use a coin to short pins 2-3. But be careful not to touch the coin to the shell or pin 1. Use a shrouded alligator clip jumper if necessary. If that's quiet, then it's not the Quantum.