DI to VTB1 - feedback

[Scott Collins]

Hi All,

I own a 50-watt Marshall head and a Weber MASS attenuator, which has a DI port on it.

I just decided to try working with the DI output of the MASS, connecting it to a Studio Projects VTB1 preamp. The moment I plug it in to the VTB1 I start getting a squealing kind of feedback. It isn't normal guitar feedback - sounds like some sort of electrical interference squeal. If I disconnect the cord from the DI to the VTB1 (but leave it connected to the MASS) the feedback stops immediately. Any ideas?

The cord connecting the MASS and the VTB1 is good, the VTB1 "line in" switch is selected and both Input & Output controls are on zero.

[Alan Hyatt]

Scott, the Weber MASS is an attenuator. The way attenuators work, is they connect between an amplifier’s output and the speakers where they can then limit the amount of power that goes to the speakers and control the overall volume.

The VTB1 is not meant to do what you are asking it to do. That is why it is squealing like a pig. It thinks you are trying to kill it!

[Scott Collins]

Hi Alan!

Thanks for the notes. Actually, I think the MASS is OK for this purpose - in addition to providing an attenuated, powered line to the speaker/cabinet, its also got a dedicated DI output that I believe is indeed made for this.

Please understand, I know this isn't a VTB1 issue - I was just hoping someone might have some advice. In fact, I get the same problem if I bypass the VTB1 and go straight into the sound card.

After some more checking around, I'm fairly certain the problem is a ground loop issue (whatever that means). I've had a few different electronics folks suggest this. One of them loaned me a Behringer Ultra DI box as a test - it'll take a 1/4 inch input from the MASS DI and allow an XLR output, which I then ran to the VTB1.

That canceled the squeal/feedback thing, but it hosed the tone (at least to my ears). As a confirmation, I clicked the "ground lift" button on the Ultra-DI box and the feedback/squeal thing came right back (thought slightly lower in volume).

Any tips on how to get rid of this problem? I've tried moving things around to different AC outlets in the same room - no go. :(

[Brent Casey]

Hi Scott,

I'll give Ted Weber a call tomorrow - we can compare notes. He's on central time and already headed out.

-Brent Casey

[Scott Collins]

Wow. Brent, that'd be fantastic! I had a short email exchange with Ted and he was of the opinion it was a ground loop issue, too.

Someone suggested this device might work: http://www.ebtechaudio.com/he-2des.html. However, I've also been cautioned that - while that device might work to stop the issues - it might squish the tone the way the Behringer DI box did.

I'm also curious if all else fails this is something an electrician could solve? I'm confident I'm going to be using the MASS DI for quite some time to record electric guitar, so this is something I've got to get sorted. Previously, I've been trying to record the Marshall in a closet, but the comb filtering was an absolute terror. Using the DI of the MASS into a cab impulse or cab simulator appears a much better option for me.

[Scott Collins]

Problem Solved! Per Ted, I tried one of those little gray adapters that converts a 3-prong plug into a 2-prong. This approach works when the adapter is connected to the Marshall head (I also tried it when connected to my Chandler Tube Driver, but that didn't work.)

FYI! Whew... :)