Drip LA2A Loud Hum ONLY when used as an External FX

[adamfiasco]

Hi guys,

Disclaimer: this is my first post on here and I am by no means an electronics DIY expert, I am in fact completely useless but I desperately need some help before I go insane!

I have today taken delivery of my LA2A (Opto 6 ??) from Gregory at Drip (which he built for me) and first off I have to say up to now his service has been stellar.

I have hooked the unit up and recorded some vocals through it and it sounds great!! Perfect in fact, just as I had hoped. However when I use it as an external FX in Cubase there is a loud hum that will not go away! So far this is what I've tried:

Different cables
Different In/Out on interface for external FX
Different IEC power cable
XLR ground lifts in all different positions
Running the la2a straight into a wall plug socket rather than through my samson powerstrip

So far none of these things have worked! The hum seems to be somewhere between 60-100Hz, and does not get any louder or quieter when I turn the gain knob on the unit.

My setup is a Steinberg MR816x interface and all my pre's and comps run through a Samson patch bay...whether recording directly or using external FX.

Gregory mentioned an impedance mismatch as the LA2A runs at 600 ohms but then the output of my MR816x is 600 ohms so should be matched perfectly. If I unplug the input to the LA2a the hum remains, whereas if I unplug the output then it goes!

I just can't understand why when I record through it, it works perfectly but when it is hooked up as an external FX I have this humming problem!

Anyone have any ideas as I am tearing my hair out here!!

Thanks
Adam

 

[saxmonster]

Does the LA2A Pass audio when it is off?

When the unit is off do you still hear the hum? 

Are you using balanced cables from the la2a to the batch bay?

Can you bypass the patch bay and connect the La2a right to the unit and try using it as an effect insert that way?

Also when using as an effect insert don't forget about the delay once your sound goes the complete trip from D/A and back from A/D, this doesn't create hum but is anything thing to watch out for.

It has to be something stupid we are missing. 

-Scott

 

[adamfiasco]

Hi Scott, thanks for your help, to answer your questions,

No the la2a doesn't pass audio when off
No hum when unit is off
I am using balanced cables to the patchbay and mr816x interface
I can try bypassing the patchbay but will require disconnecting a lot of things behind my desk tomorrow!
I have the delay compensation sorted ...so easy in cubase

I have come to a workable solution...for now....if I pad the input of the interface and use the gain on the LA2A the hum is inaudible! Probably not the best solution as the hum is still there just very quiet but at least I can use the LA2A as an external plugin!

Actually thinking about it...originally I said when recording directly through it it worked fine, I had the input padded!

Adam

 

[saxmonster]

Have you talked to Drip yet about it?

 

[adamfiasco]

I have spoke to Gregory at Drip! He insisted the power supply is perfect and said that its quite common for LA2As to exhibit noise and hum with modern sound cards!

Since using the unit a fair bit the hum has definitely got quieter and perhaps a little lower...now around 50Hz maybe?

But I can work around this problem by making sure the inputs on my audio interface (Steinberg MR816x) are padded, and then use the gain from the LA2A.....this makes the hum inaudible!

 

[Biasrocks]

What is your calibration for 0VU on your converters? -18 and -20 is common.

My apogees are set for -18dbfs = +4db/0VU in and out.

I have mine set up with the volume at about 30% and gain reduction at around 30% with no input pad. This provides a clean, hum free signal when using it as an external processor or as a insert in Pro Tools.

Gain staging is everything on the LA2A, you don't want to hit it too hard.

To verify your converter is unity gain, run a patch between input and output and record the result in your DAW. If everything is calibrated correctly it should come back at the same level it went out.

You can also verify your calibrated level by sending out a -18dbfs 1khz tone to the LA2A and switch the meter to signal and look to see if the LA2A is reading 0VU. If not, you've got a calibration issue.

The LA2A should also be terminated with a 620ohm resistor when interfacing with modern gear to ensure proper frequency response and meter readings.

Mark

 

[Bowie]

I have spoke to Gregory at Drip! He insisted the power supply is perfect and said that its quite common for LA2As to exhibit noise and hum with modern sound cards!

Since using the unit a fair bit the hum has definitely got quieter and perhaps a little lower...now around 50Hz maybe?

But I can work around this problem by making sure the inputs on my audio interface (Steinberg MR816x) are padded, and then use the gain from the LA2A.....this makes the hum inaudible!

I've built a few P2P units and they exhibited next to no hum.  Even less than the UA version, which is already extremely quiet.  I'm not sure if this is just a DLA2A problem or if you have an issue, but I wouldn't accept any noticeable hum out of an LA-2A.
FWIW, the 12AX7 in the gain stage (not the sidechain 12AX7) will influence the noise floor heavily and you can easily pick up hum there.
Also, if you aren't familiar w/ LA-2As, the gain knob is usually only around 20~35, depending on source material.  They are gain monsters and don't get turned up like most modern gear.

I have today taken delivery of my LA2A (Opto 6 ??) from Gregory at Drip (which he built for me) and first off I have to say up to now his service has been stellar.

Glad to hear, though I'm extremely frustrated that he won't reply to my emails.  I have a noisy Drip preamp that I'm trying to get one of their DC heater boards for.

 

[bernbrue]

I have always padded the inputs of my UR824 and pots set to 10 o'clock for line operation, otherwise the inputs of the interface a way too hot. The LA2a has got a lot of gain as well, so it's very likely that your problem is caused by a wrong gain staging.
Bernd

 

[dustbro]

Sounds like a loop. What grounding scheme are you using.
Try dropping the ground at the output of the compressor cable that leads to the patchhbay

 

[adamfiasco]

Thanks for all the help people. Bowie - I went through a stage where Gregory didn't return emails ....it was just after I sent him payment for the unit too which made me a little nervous! Keep trying and he will get back to you!

Dustbro - I have ground lifts connected to both the XLR In and XLR Out on the back of the unit - I've tried these in all possible ways and combinations with no success!

Thinking about it, my interface (MR816X) doesn't have the capacity to bypass the preamps so could it be that the interface is massively gaining-up the output of the LA2A? When I apply the -20dB on the input of the interface the hum is inaudible at normal - fairly loud listening levels and I can just use the LA2As gain pot to make up the difference - still no hum! I don't really want to change my interface at this point but in the future if I did and made sure the press could be bypassed do you think this problem would go away?

Adam