GK MB500 Direct out noise problem

[show-stoppers]

Are there any known noise issues with the Direct out on GK MB500's?

I'm getting substantial hiss and some crackling on the DI out even with nothing plugged into the amp with the EQ bypassed.

Switching in the EQ increases the amount of hiss and noise proportionately, and the ground lift has no effect either way.

The hiss and crackling don't seem to be there at the speaker out into the cabinet. (Eden 210 Nemesis)

Comparing the GK DI to the signal coming off a JDI direct box into another channel, The JDI is dead silent with both channels matched to unity gain, while the GK is full of hiss and crackling. (It's the crackling that makes me think there is something about to give in the amp)

I've already swapped mic cables and mixer channels and the noise is always there.

Thanks for any insight.

 

[Jay Corwin]

It sounds like a problem with the DI on the head to me. Can You get a replacement head under warranty?

 

[tbirdsp]

I have the MB Fusion 500 and the DI is fine, pre or post EQ. I'd say something is wrong.

 

[show-stoppers]

No warranty. 2nd hand unit.

Looking at the block diagram, it doesn't make sense that any noise at the DI out isn't showing up at the speaker out so their has to be more to the signal path from the input to get a Lo-Z Direct output, like a transformer or chip.
I haven't tried the 1/4" line out jack for noise yet, so that's next. If there's no noise there, I'll resolder the XLR connections to eliminate cold solder as a possibility.

 

[B-string]

Typical sound of a failing/defective junction in an opamp. I'm sorry I don't have schematics for the MB500 to go further than that. Contact [email protected] for some help and or guidance.

The block diagrams are simplified signal path, the DI may have it's own opamp.

 

[show-stoppers]

After more testing it's looking less like something broken and just an unfair comparison of the Radial JDI's amazing job at isolation and a low noise floor compared to the GK's noisy direct out.

The crackling seems to be some electronic interference that is louder with the ground lift switch on the GK ON. It's not there on the speaker out or on the Radial JDI. The remaining hiss seems to be the GK's high noise floor compared to the quiet JDI.

Then again, the GK direct out is fairly quiet until the unit heats up to operating temperature.

 

[B-string]

The G-K's DI should be pretty quiet, G-K has been including DI's since 1982 and they are generally pretty good (at least I have never heard complaints on properly working units).

 

[tbirdsp]

I run the DI out of my MB Fusion 500 straight into my IEM transmitter before it goes to the FOH - using Westone triple-driver earbuds - never noticed any crackling or undo hiss/noise.

 

[David Jayne]

I have that amp and mine's very quiet, even once warmed up(not that it actually gets warm).

 

[show-stoppers]

I tried an el cheapo direct box rather than the GK direct out and it has a lot of the same noises, less when the ground is lifted while the Radial JDI is still dead silent, so the solution is obvious, I'll add a second Radial DI and skip using the GK's. (The JDI is for FOH and the GK was going to my IEM rig)