AB168 PP light

[debzdoodleParticipant]

Had a weird thing occur on the AB168 this past weekend.
Bass player has been using his EBS micro bass pre amp powered with 48v for a few months but on friday he decided to bring his TC amp DI to FOH instead – no 48v necessary.
We were plugging in everything and noticed that the channel on the AB168 that the bass was connected to had the pp light illuminated BUT the 48v was NOT turned on at the mixer – I double checked. The channel was correctly assigned to D Snake.
Not sure if this has happened before but we had never noticed it.
Why might this happen??

[GCumbeeParticipant]

If this was an analog snake I could understand. Backfeeding voltage. But with a digital snake I’m not sure. Something weird. I really can’t see how that channel would be lit. Maybe voltage on the device was somehow triggering the logic circuit in the front end of the console that fooled the AB. Strange.

[ahjeffModerator]

I think this is working as intended. From the user guide, “The Phantom Power indicator detects voltage at the socket whether supplied by the AB168 or received from an external source”.

Hope this helps.

– Jeff, A&H

[GCumbeeParticipant]

Well I guess I was on the right track. Thanks Jeff.

[debzdoodleParticipant]

Thanks guys …. “Received from an external source” – what source?

[airickessParticipant]

The bass player’s DI.

[debzdoodleParticipant]

He wasn’t using a DI box or pedal. The point is he was using his amp head direct to the PA. This is my confusion.

[Mike CParticipant]

I take it unplugging from the amp and the light on the channel goes out.
Is the output on the amp an XLR line out male or a 1/4 inch.
Are you sure it’s a line output and not a speaker out.

What model amp is it.

With a volt meter set on DC volts measure the XLR output on the amp, measuring between pins 2 and 1 and then pins 3 and 1. If the amp is putting out DC voltage on an XLR line
out jack I’m going to say that’s not a good thing!

Do you have an isolation transformer to put inline between the amp and the AB168 input alos you could try a pin 1 lift on the XLR.

[debzdoodleParticipant]

The pp light only showed up when the amp was connected to it. DI output is XLR and was definitely the one used – speaker outs are SpeakOn.
TC Helicon BH550 amp.
There has to be some leakage somewhere that was detected by the AB168. A DI box is probably the answer – strange though.

[Mike CParticipant]

I would say the amp has an issue or a design flaw. Maybe TC was going for the purest signal path to the XLR line out and did not put any DC blocking caps in the circuit but failed to design a circuit that did not have any DC offset.

If you get a change to measure the voltage on the XLR out I would be curious to know
what the voltage is.

I would put a 1 to 1 isolation transformer between the amp and the AB168, a pin 1 lift would break the circuit so the AB168 would not really see the voltage.

[debzdoodleParticipant]

Thanks Mike….Haven’t had a chance to measure yet but I got a 1:1 isolation transformer – a nice small inline one so we will check it out and report back.

[Mike CParticipant]

Thanks Mike….Haven’t had a chance to measure yet but I got a 1:1 isolation transformer – a nice small inline one so we will check it out and report back.

Let us know what you find.

[Mike CParticipant]

What transformer did you get?

[debzdoodleParticipant]

This one
https://www.bhphotovideo.com/c/product/711747-REG/Whirlwind_ISOXL_ISOXL_Line_Level_Isolation_Transformer.html?ap=y&c3api=1876%2C%7Bcreative%7D%2C%7Bkeyword%7D&gclid=EAIaIQobChMIxuGB6bbZ2QIVBK_ICh1bzQXcEAkYASABEgLIDvD_BwE

[Mike CParticipant]

That will work and it’s always handy to have one or two of those handy
for last minute input output interface isolation.

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