STill expericing ground hum using DI in between two mixers.

[wheresgrant3]

I've posted here about this problem in the past. I'm a keyboard/synth player who controls his own submix to the main mixer. I use a Yamaha MG 10/2 because I didn't want to carry a rack mixer. I mix the single from 3 synths (and sometimes a fourth) and send stereo out to the main PA mixer and I use the stereo C/R Out for my monitor mix-two keyboard wedge amps at my feet.

 

 

About a year ago I started experiencing some extreme hum/buzzing in a new room we played. After getting advice on this board I purchased a Samson Active DI with a ground lift. Voila... problem solved. My mix is a clear as filtered water. The only draw back is the constant diet of 9V batteries it consumes.

 

 

A few weeks back the DI took a spill and fell cracking it's case. Rather than taking the chance I choose to replace it... this time with an ART passive DI. Even though it's billed as an Isolation transformer it's back to 'Hum-City'. Anyone know why? Is there something I should be doing or instructing our sound guy to do? I remember with my last post that many said an active DI was not necessary... that a DI is essentially just that... a ground lift... whether active or passive. I use balance TRS cables from my keys to the mixers. I use balanced TRS cables from the mixer to the DI and I use XLR from the DI to the main PA mixer. What in this chain could be causing this hum?

[agedhorse]

First, you don't have the power supply for the mixer sitting on the DI do you? That will hum like a hive of bees.

 

Second, which model DI? does it have a ground lift? Do not use TRS cables between the mixer out and the DI in, some DI's use the ring terminal to isolate the ground.

 

Is the hum there regardless of the sub-mixer's output level setting. Does the hum go away if the output is truned down?

[wheresgrant3]

Originally posted by agedhorse

First, you don't have the power supply for the mixer sitting on the DI do you? That will hum like a hive of bees.

 

No but it's quite close... I will move it away from the mixer altogether.

 

Second, which model DI? does it have a ground lift? Do not use TRS cables between the mixer out and the DI in, some DI's use the ring terminal to isolate the ground.

So they should be just regular TS cables? No problem.

 

The Unit is a ART DTI .

 

I was sold this at a local shop as a DI box. I was in a rush, they didn't have the box or paperwork and I was late for 'load in'. I just took their word for it. It isn't a DI, just an isolation transformer. I'm always the furthest away from the PA board (the XLR's I use are 30 feet and they tie into a 100 ft snake) so maybe that's why my signal sounded like dirt on Saturday? I assume this might be part of the re-emerging problem?

 

Is the hum there regardless of the sub-mixer's output level setting. Does the hum go away if the output is truned down?

 

I don't know but I will test this out tomorrow.

 

 

Aged! Thanks for the input!

[SoundMan]

This is one of the downfalls of the small mixers with the external lump. There is no connection to earth ground through the lump, therefore the mixer gets it's ground through whatever equipment happens to be plugged into it.

 

This is probably not normally a problem, if you are close to the other equipment plugged into the mixer. That extra distance between the equipment increases the probablity of hum problems.

 

I'm not sure this helps you at all, but it might explain things a bit.

 

SoundMan

[where02190]

 

The only draw back is the constant diet of 9V batteries it consumes.

 

 

Turn on the phantom power to the mixer the DI is feeding to(the XLR end) and remove the battery from the DI.

[agedhorse]

 

Originally posted by SoundMan This is one of the downfalls of the small mixers with the external lump. There is no connection to earth ground through the lump, therefore the mixer gets it's ground through whatever equipment happens to be plugged into it. This is probably not normally a problem, if you are close to the other equipment plugged into the mixer. That extra distance between the equipment increases the probablity of hum problems. I'm not sure this helps you at all, but it might explain things a bit. SoundMan

 

 

Jim, this was exactly what I was thinking. There is no (solid) ground reference at the mixer and the ART isolator floats pin 1, maybe to an electrotatic shield within the transformer.

 

Assuming it's not the line lump inducing hum into the transformer, the problem is not that the 1:1 transformer is the WRONG device but that the pin 1 connection to the input shield is missing. The box could be modified but it may be tricky depending on how it's built, or just get the passive DI that has a ground lift switch and use it in the ground connected position. You don't have a problem with too much signal at the input to the main console do you? This is another possible problem with 1:1 transformers used as inputs to a console. They are really designed for line level use in most cases as you are outputting a line level from your submixer but your main mixer wants something a bit less (unless it's a real pro console).

 

This is one reason why we include a ground lift switch on the DI's of our bass amps. Under some conditions, like yours, the ground must be connected.

[wheresgrant3]

 

Originally posted by SoundMan This is one of the downfalls of the small mixers with the external lump. There is no connection to earth ground through the lump, therefore the mixer gets it's ground through whatever equipment happens to be plugged into it. This is probably not normally a problem, if you are close to the other equipment plugged into the mixer. That extra distance between the equipment increases the probablity of hum problems. I'm not sure this helps you at all, but it might explain things a bit. SoundMan

 

 

Actually it explains alot! Thanks!

[wheresgrant3]

 

Originally posted by agedhorse Jim, this was exactly what I was thinking. There is no (solid) ground reference at the mixer and the ART isolator floats pin 1, maybe to an electrotatic shield within the transformer. Assuming it's not the line lump inducing hum into the transformer, the problem is not that the 1:1 transformer is the WRONG device but that the pin 1 connection to the input shield is missing. The box could be modified but it may be tricky depending on how it's built, or just get the passive DI that has a ground lift switch and use it in the ground connected position. You don't have a problem with too much signal at the input to the main console do you? This is another possible problem with 1:1 transformers used as inputs to a console. They are really designed for line level use in most cases as you are outputting a line level from your submixer but your main mixer wants something a bit less (unless it's a real pro console). This is one reason why we include a ground lift switch on the DI's of our bass amps. Under some conditions, like yours, the ground must be connected.

 

 

Thanks man! To answer your question I have heard from many sound men that my signal is SUPER hot. I like to play with the channel volumes set so they are match (use the Balanced stereo 7/8 & 9/10) and I like it so I can see the minumum LED gets lit on moderate stuff and gets lit to 1/4 way up for stabs or loud program patches. Regardless of what I hear I know approximately the level the board is setting.

 

So that leaves my individual channel vols set between 6-10 (depending on the synth).

 

The master volume on 5-6....

 

I'm usually told by every sound to turn the mixer way down... that the signal is too strong or hot.

 

So maybe this contributes as well. I know that my mixer is the central cause of the hum. Depending on the room I can hear it through my keys monitors at my feet.

 

BY the way... some things to note. This is the second MG10/2 mixer I've used. The first fell during teradown and went kaput shortly afterward. Also the pwr supply is proprietary... ungrounded two prong at one end.... block in the middle and a 3 pin input into the mixer. It's only a $100 mixer so I'm not expecting that much, but you would hope it's not a design flaw on Yammies part.

[agedhorse]

A standard passive DI will solve your level problem as they use a 6:1 - 10:1 ratio transformer.

[boomerweps]

While it may or may not be related, you mentioned using TRS as patch cables.

 

Are you sure the keyboards' outputs are TRS? Many have stereo outs using a TRS and others are just TS.

 

As was mentioned, DI's usually are set up for a TS input from a guitar or combo amp output.

 

Last question on cables, are you sure the mixer has TRS balanced outputs? The majority of small mixers do not.

 

Anyways, all those short runs means TS cables are more than adaquate.

 

Do you still have the cracked case DI? I'd just tape it up and see if it works. Most DIs are pretty sturdy & can handle a few drops. Worse case (pun intended) might involve a little soldering IF there isn't a printed circuit board carck.

 

Boomerweps

[agedhorse]

 

Originally posted by boomerweps While it may or may not be related, you mentioned using TRS as patch cables. Are you sure the keyboards' outputs are TRS? Many have stereo outs using a TRS and others are just TS. As was mentioned, DI's usually are set up for a TS input from a guitar or combo amp output. Boomerweps

 

 

Yes, good info.

[GigMan]

Hey Grant -

Why don't you try that Rolls RM203x mixer that you acquired from me in our recent equipment trade/summit meeting...

 

Maybe it will eliminate the hum - although you'd have to rig up another setup for it, different that the nice little kit you created for the Yam. MG/10, since the Rolls is a rackmount and the Yammie is not.

[GigMan]

Originally posted by Eddie Kramer Yeah the Rolls is staying put... right next to my PC. It's a great line mixer but I prefer having the EQ control, easy access to channel volume and a display LED right next to me. And ... no rack.

 

Ok but Grant - why are you now posting under the name "Eddie Kramer"

[wheresgrant3]

Originally posted by GigMan Hey Grant - Why don't you try that Rolls RM203x mixer that you acquired from me in our recent equipment trade/summit meeting... Maybe it will eliminate the hum - although you'd have to rig up another setup for it, different that the nice little kit you created for the Yam. MG/10, since the Rolls is a rackmount and the Yammie is not.

 

Yeah the Rolls is staying put... right next to my PC. It's a great line mixer but I prefer having the EQ control, easy access to channel volume and a display LED right next to me.

 

 

And ... no rack.

[wheresgrant3]

Originally posted by boomerweps While it may or may not be related, you mentioned using TRS as patch cables. Are you sure the keyboards' outputs are TRS? Many have stereo outs using a TRS and others are just TS. As was mentioned, DI's usually are set up for a TS input from a guitar or combo amp output. Last question on cables, are you sure the mixer has TRS balanced outputs? The majority of small mixers do not. Anyways, all those short runs means TS cables are more than adaquate. Do you still have the cracked case DI? I'd just tape it up and see if it works. Most DIs are pretty sturdy & can handle a few drops. Worse case (pun intended) might involve a little soldering IF there isn't a printed circuit board carck. Boomerweps

 

Hmmmm... I don't know. I always assumed they are TRS output. Just about everything I've read recommends to use balanced cables out to any mixer. I never thought to check if the outputs on my synths were balanced or not (which would suck since I've invested $$$ in cables over the years).

 

The Yamamha does support balanced stereo out.

 

 

The DI is toast.... remember it was a Samson

[wheresgrant3]

Originally posted by GigMan Ok but Grant - why are you now posting under the name "Eddie Kramer"

 

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