DI out - puzzling behaviour

mart

I used a new bass amp today - a Warwick WA300 that I picked up cheap as it was ex-demo.
I plugged an XLR cable into the DI out, and connected it to the snake to the mixing desk just as I would with any bass amp. The amp sounds fine through my cab, but there was no signal at the desk.

Tried switching the DI out to be pre/post EQ, and switched ground lift on and off, but no change - still no signal. Tried different xlr cables, and different channel on snake, but still no bass feed.

But when I unplugged the cables there was the usual crackle through the system, suggesting the cables and snake are fine.

I then tried plugging a guitar cable into the line out, and connecting that to a DI box, and then using xlr cable from there to the snake. That worked.

So this all left me thinking the DI out on the amp is dodgy.

Then I noticed a nearby monitor wedge had an xlr mic input, so I tried plugging an xlr cable from the DI out on the bass amp into the monitor. That worked fine. So xlr out on the amp clearly works!

So now I’m completely baffled. I can’t seem to get a signal from the DI out to the desk, but I can to a monitor. Can somebody explain how is that possible? @ICBM ?

Bridgehouse

Did the desk have phantom power on for that XLR socket?

mart

Bridgehouse said:

Did the desk have phantom power on for that XLR socket?

Don’t think so, but that’s not something I checked. Good thought.

Bridgehouse

mart said:

Bridgehouse said:

Did the desk have phantom power on for that XLR socket?

Don’t think so, but that’s not something I checked. Good thought.

Either that or that channel had a random button pressed to either mute it, route it somewhere else, not point it at FOH etc..

My money is on phantom power tho.

mart

Thing is when I tried different channels on the snake they were feeding to different channels on the desk. My daughter was on the desk soloing the channels to see if there was any signal at all coming through. So pretty sure it wasn’t muted or fed somewhere funny. So phantom power is looking the most likely.

Especially just after Halloween. (Sorry!)

ICBM

I think I've come across something like this before, but I can't remember for sure what it was… I think it was something like half of the balanced output wasn't working, so it would work when going into something with a centre-tapped input (producing half the signal strength, but you wouldn't notice that), but not when going into something with a true isolated balanced input since the circuit is then not completed. Or something like that!

Bridgehouse

ICBM said:

I think I've come across something like this before, but I can't remember for sure what it was… I think it was something like half of the balanced output wasn't working, so it would work when going into something with a centre-tapped input (producing half the signal strength, but you wouldn't notice that), but not when going into something with a true isolated balanced input since the circuit is then not completed. Or something like that!

I'm not saying you're wrong at all - I'm sure this is entirely plausible, but I had a similar thing recently, and it was simply because the desk in question had a universal "phantom power to all xlrs" button on the back which buggered about with the whole connectivity for xlrs as well.

mart

Certainly it’ll be easier to switch off phantom power then sort the wiring on the xlr socket.

Is there an easy way to test the xlr pins in the socket?

Bridgehouse

mart said:

Certainly it’ll be easier to switch off phantom power then sort the wiring on the xlr socket.

Is there an easy way to test the xlr pins in the socket?

You mean on the amp? Now that's where @ICBM really does come in...  

mart

Yes, on the amp, to see if there is a “half” problem with the socket like IC suggested.

ICBM

Phantom power is a possibility, but *in theory*, any properly-designed XLR output shouldn't be affected by it. That doesn't mean this amp necessarily won't be, of course...

ecc83

It is likely that the XLR output is 'Impedance Balanced'. This means pin 2 * is 'hot' but pin 3 has no signal on it but is returned to '0' reff', screen via a resistor .

Now, if '3&2 are swapped over at some point the hot signal is shorted to ground, hence buggerall. If you plug into a balanced XLR, line or mic input all will work fine.

*Possible that the amp's XLR is miss-wired but more likely a homebrew cable.

For the record I do not like XLR 'line' outputs on guitar amps (yup! KNOW that!) . If 'they' are going to fit an XLR make it at MICROPHONE level and properly balanced via a transformer. Yes, I know high Q audio traffs are expensive but something easily good enough for R&R is not. O.EP. make several.  THAT way we ALL know  WTF we are!

Dave.

mart

ecc83 said:

It is likely that the XLR output is 'Impedance Balanced'. This means pin 2 * is 'hot' but pin 3 has no signal on it but is returned to '0' reff', screen via a resistor .
...

Hmm, ok, that makes sense.
What’s the solution if that’s the problem?

ICBM

The best solution is to use a proper outboard DI box.

As Dave says, half-arsed DI outputs on amps are a pain in the bum. It does sound like it's either one of those, or it's faulty.

Actually, I think I have the remains of a Warwick bass amp I scrapped (totally failed power stage, not worth repairing) somewhere. If I've still got the DI output board I may be able to see how they do it.

ecc83

mart said:

ecc83 said:

It is likely that the XLR output is 'Impedance Balanced'. This means pin 2 * is 'hot' but pin 3 has no signal on it but is returned to '0' reff', screen via a resistor .
...

Hmm, ok, that makes sense.
What’s the solution if that’s the problem?

Try different wiring on an XLR lead until you find one that works. Try pin 3 hot and pin 1 shield for starters.

Once you have it working..MARK THE CABLE UP AS A SPECIAL! I am an old fart that has been around group gear an sound kit all my life and you tend to gather a collection of such special leads!

A short list would include,

XLR -XLR rev polarity

XLR-XLR pin 1 dissed one end.

XLR gender changers.

Jack-jack TRS with rev'pol etc..Plus at least one passive high Z (guitar to mic) DI box and one passive 1:1 low Z (line) 'stereo' traff box. (look at Orchid electronics and Art kit)

But! You kids have it easy ! You see NOTHING like the variety of connectors we had to contend with 50 yrs ago..Shoot! There was about SEVEN different mains sockets!  But I am glad I no longer 'have to do it'!

Old F' had had his morning moan.

Dave.