Feedback When using Amp as Monitor

thebasssponge · #1 10-12-2009, 09:15 PM

I am going to start playing at my Church soon + I am learning to run the sound board and equipment (large church w/ alot of sound equipment) for when I am not playing and other activities.

So anyway I take my equipment w/ me tonight to play there for the 1st time and I plug into the DI input w/ my Bass and everything sounds great - hook output from the DI to my Amp using instrument cable and got an instant audible hum from my amp and through sound system - guitar straight to amp is fine - try diff. wall socket for amp, no go - I have 2 basses (1 passive and 1 active), no diff. - diff. cables still no dice - switch all settings on DI, no change - changes on mixing board, nope - diff. DI box, nope - remove amp from equation and its fine

DI Boxes : DOD 275 Active Direct Box
http://www.dod.com/accessories/accessories.htm

Amp : Acoustic B20
http://www.guitarcenter.com/Acoustic...67-i1383305.gc

main reason I would like to use amp as my monitor is to have it close and its what I am used to hearing - stage monitors are used for vocals + we have an organ, clavinova and piano on stage as well and they have their own monitors

I intend to get a diff. amp w/ a XLR output soon and that would eliminate the DI, but for the time being I am stuck w/ what I have

I am very new to all this and I may be missing something here thats obvious to some so please be easy on me

Nick Kay · #2 10-12-2009, 09:37 PM

That's not feedback, that's a ground loop. You need to lift the ground on the line going to the board - it'll fix it right up.

thebasssponge · #3 10-13-2009, 09:59 PM

ill have to check that out - from the schematics of the DI box there is no ground pin lift at the box, I will have to check at the board

any suggestion for an inexpensive solution if no ground lift at the board ?

something like this possibly ?
http://www.sweetwater.com/store/detail/GLT255/

GregShadoan · #4 10-13-2009, 10:59 PM

Quote:

Originally Posted by thebasssponge

ill have to check that out - from the schematics of the DI box there is no ground pin lift at the box, I will have to check at the board

any suggestion for an inexpensive solution if no ground lift at the board ?

something like this possibly ?
http://www.sweetwater.com/store/detail/GLT255/

Well, you could always open the box and install a SPT switch on the ground wire, effectivly installing a ground lift on it.

Or get a DI that has one. For now Throw a 57 in front of it :-)

TimmyP · #5 10-15-2009, 01:54 AM

Quote:

Originally Posted by GregShadoan

Well, you could always open the box and install a SPT switch on the ground wire, effectivly installing a ground lift on it.

Or get a DI that has one. For now Throw a 57 in front of it :-)

If the DI relies on phantom power - as most active DIs do - opening the ground will kill the phantom, and thus the DI.

thebasssponge · #6 10-15-2009, 10:06 AM

Quote:

Originally Posted by TimmyP

If the DI relies on phantom power - as most active DIs do - opening the ground will kill the phantom, and thus the DI.

this DI has phantom and battery power (1-9volt)

thebasssponge · #7 10-23-2009, 03:16 PM

ended up buying a new DI

http://www.behringer.de/EN/Products/DI600P.aspx

works like a charm w/ the ground lift, thanks for the help

TimmyP · #8 10-24-2009, 04:43 AM

Quote:

Originally Posted by thebasssponge

ended up buying a new DI

http://www.behringer.de/EN/Products/DI600P.aspx

works like a charm w/ the ground lift, thanks for the help

The first time it hums because it's crummy transformer picks up an EM field from something, you'll wish you'd returned it, and bought something at least of this quality: http://www.audiopile.net/products/DI...cutsheet.shtml

walterw · #9 10-25-2009, 10:36 PM

Quote:

Originally Posted by TimmyP

The first time it hums because it's crummy transformer picks up an EM field from something, you'll wish you'd returned it, and bought something at least of this quality: http://www.audiopile.net/products/DI...cutsheet.shtml

50k input impedance? yikes! i hope the bass is active, or at least has a buffered tuner pedal or something in line before hitting a box like that.

TimmyP · #10 10-26-2009, 03:41 AM

Quote:

Originally Posted by walterw

50k input impedance? yikes! i hope the bass is active, or at least has a buffered tuner pedal or something in line before hitting a box like that.

I think you'll find that to be pretty common for passive DIs. Some active DIs are only 100K. I agree that it's not good for passive instruments.

thebasssponge · #11 10-26-2009, 07:45 PM

I have an active and passive bass

here are the specs to the DI I bought ...........

Frequency response 40 Hz (-3) to 20 Khz
THD +N (distortion) 0.0003% @ 1kHz input level +4dBu
Input 1/4" TS (unbalanced)
Level change (input to output) -20 dB
Output Balanced XLR
Impedence ratio (input to output) 110:1
Inst/amp switch 30 dB pad
Filter switch low pass @ 7.5 kHz (-3 dB)

should I be only using my active bass and not the passive ?
I also have a boss me-50b that I had not planned on using at church but will if deemed necessary

and can you explain to me what you are talking about by the input impedance on the one Timmy listed and it not being good for passive instruments

TimmyP · #12 10-26-2009, 09:54 PM

Passive instruments should be used into loads of 250k or greater. The tone can be greatly affected if the load gets too low.

The Behringer's spec is "impedance ratio 110:1" But what does that tell us? The spec on a Jensen DI transformer is 200k:1.5k (which is 133.33k:1). So is the Behringer 100k? And if so, does that change when the DI is loaded? Transformer makers say that a transformer "reflects its load impedance". So does a 2k mic pre raise or lower that 100k? I've yet to see it fully explained.

thebasssponge · #13 10-27-2009, 05:44 AM

ok that makes since

so ...............
*the active bass should be no problem

*the passive bass should not be used unless I run it through my Boss ME-50B ? Or not ?

TimmyP · #14 10-27-2009, 09:13 AM

Quote:

Originally Posted by thebasssponge

ok that makes since

so ...............
*the active bass should be no problem

*the passive bass should not be used unless I run it through my Boss ME-50B ? Or not ?

Depends on whether you like the sound. You won't hurt anything.

thebasssponge · #15 10-27-2009, 12:30 PM

Quote:

Originally Posted by TimmyP

Depends on whether you like the sound. You won't hurt anything.

thats what i was concerned with, was hurting something

the DI box is for temporary use till I get a bigger/better amp w/ XLR output which hopefully will be by the 1st of the year

thank you so much for all your help