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SVT 4 Pro line out / XLR hum

Trailight

Trailight

Oct 19, 2010
1,751
1,949
Vancouver Island BC
trailight.bandcamp.com
Disclosures
Endorsing Artist: Spector Bass / Darkglass Electronics / Tsunami Cables
#1
Greetings to all.

I've looked through the forum and there seems to be some answers in regards to this problem, but I am not 100% sure that this is what is happening with my amp;

During practice, I connect my 4-Pro with an XLR cable to the mixer and always got a clear sound, no worries.
We had a gig last week in an old theatre and when I connected my 4-Pro to the house board with an XLR all of a sudden it started humming / buzzing like crazy.
We changed cables....nothing, no ground lift on the 4 pro either so we ended up mic'ing the cab.
Got back to the jam space, connected back to the mixer, exatly as before, and now I get that hum.

Did something happen to the XLR out connection in the back? Is this something that one can fix or best to take it to a tech considering it appeared all of a sudden?

Many thanks!
 
#4
RockNRollAl said:
Is it possible the mixer at the gig was running 48V phantom power in all of the mic channels? That would have put up to 48V back into your DI...
Click to expand...

Not 100% sure but I am guessing not since they had other cabs done that way at the show.

Technotitclan said:
Does your amp have volume control on the line out? My B2RE has a -15db on my line out that I was told to use to get rid of hum that I was having.
Click to expand...

It does have a volume control for the line out, but regardless of the levels, I still hear the hum.
 
#7
LegoBass said:
I sort of had the same problem. When I first got my 4 Pro, both of the XLR outs on the back would hum, extremely badly at that. Then one day, nothing, the hum had just disappeared. I dunno, it is really weird.
Click to expand...

Exact opposite for me.... no hum and all of a sudden it started. :eyebrow:
 
#12
The output is transformer isolated on the 4PRO, so 48V is not a factor*.

There isn't a ground lift, and the XLR has no pin 1 connection, so that isn't an issue either. no ground connection means no ground problem (usually).

If you have hum, it most likely is coming from the amp or the signal from the bass itself.

Depending on the input into which the DI is plugged on the mixer, there might be some issue due to the input not working well with the DI out, mismatched levels, poor common mode rejection, etc.

There are two line out level controls, and if they are turned way way down to match a mic input, there might be hum. Better for the sound guy to put in some attenuation to match the signal, and turn up the levels.... but sound guys "vary" in their knowledge and adaptability.

Another possibility is that the amp might be on top of something else with a big power transformer right under the DI isolation transformers. They generally are not shielded as well as mic inputs...... and can pick up external magnetic fields.

*
Actually, 48V is not a factor on ANY of the "DI" outputs on SLM products that I am aware of.... we took that into account even for the DI that do not have transformer isolation. Maybe there are some vulnerable ones that I don't know about, but not this one.
 
#13
Jerrold Tiers said:
If you have hum, it most likely is coming from the amp or the signal from the bass itself.
Click to expand...

IF it were coming from the amp, wouldn't I hear it through the speakers as well, which is not the case...?
I plugged my bass into another amp, and no hum, so the bass is not giving out that signal. (I am using a Spector Forte 4)

Jerrold Tiers said:
Depending on the input into which the DI is plugged on the mixer, there might be some issue due to the input not working well with the DI out, mismatched levels, poor common mode rejection, etc.

There are two line out level controls, and if they are turned way way down to match a mic input, there might be hum. Better for the sound guy to put in some attenuation to match the signal, and turn up the levels.... but sound guys "vary" in their knowledge and adaptability.
Click to expand...

The thing is that it started happening all of a sudden at one gig and now it's happening again at the jam space. It was a very old building that we played at and there was a cacophony of power bars and cables running at the back of the stage to power everything.
I tried working with the line out levels...still the issue persists.

Jerrold Tiers said:
Another possibility is that the amp might be on top of something else with a big power transformer right under the DI isolation transformers. They generally are not shielded as well as mic inputs...... and can pick up external magnetic fields.
Click to expand...

The amp always sits on my cab ( I use two 410 cabs) and the guitarists rig sits next to my rig about a foot or so away.

Thanks for your reply!! :bassist:
 
#14
There is also a chance that the cable is messed up, or that in some way the cable gets grounded when in use.....

or for some reason the jack may have gotten a short to ground that causes hum

Are we talking about one cable?

Or does it hum with any of several cables?

Does moving the other amp affect it?

Does turning the 4PRO affect it?
 
#15
Jerrold Tiers said:
There is also a chance that the cable is messed up, or that in some way the cable gets grounded when in use.....

or for some reason the jack may have gotten a short to ground that causes hum

Are we talking about one cable?

Or does it hum with any of several cables?

Does moving the other amp affect it?

Does turning the 4PRO affect it?
Click to expand...

Does it with different cables as well. I tried a few.
Moving it, turning it, or moving the amp beside it doesn't help either.
 
#16
The signal comes from the effects out. If you don't hear it in the amp output, then it is likely one of these

1) due to some external factor in the wiring, even though "it can't be that"

2) due to an internal short..... if a high voltage between units at the gig damaged the transformer insulation, for instance.

3) due to a bad connection inside..... but if it works, just has hum, that may be less likely.

4) due to some form of induction from some external magnetic field... less likely if turning the unit does nothing
 
#17
Jerrold Tiers said:
The signal comes from the effects out. If you don't hear it in the amp output, then it is likely one of these

1) due to some external factor in the wiring, even though "it can't be that"

2) due to an internal short..... if a high voltage between units at the gig damaged the transformer insulation, for instance.

3) due to a bad connection inside..... but if it works, just has hum, that may be less likely.

4) due to some form of induction from some external magnetic field... less likely if turning the unit does nothing
Click to expand...

For some reason I have a feeling it's either #2 or #4. I will have to open up the top and have a look. I have to wait until next week though...argh.
Although, I am quite content with having the cab Mic'ed as it gives me a fuller sound in my ears :)
 
#18
I'm all for you figuring out what the issue is, and I'll pay attention to this thread, but my easy fix was putting a Sansamp BDDI in the effects loop.

I may not know what the exact problem with my SVT-4Pro's DI output was, but in my case the BDDI is always much quieter. It's a great pedal to have around anyway.
 

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