Variax - Multiple connection mythology

Started by vtgearhead, January 05, 2016, 04:41:47 AM

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vtgearhead

There have been claims made by one of the Line 6 folks to the effect that using both VDI and analog (1/4") connections simultaneously can damage the guitar's electronics.  There are no specifics provided to explain the failure mode (because then they'd have to kill you).  Personally, I think it's utter nonsense for a number of reasons but I don't want to get into a p*ss*ng contest with a Line 6 employee on their forum.  Just wondering what the more technically inclined folks here think. 


HAMERMAN409

#1
Assuming everything is correct electrically speaking with your houses wiring then I don't see where damage could occur. There is definitely the potential for ground loops so noise could be an issue.

Where you could run into trouble is if you were plugged into two different AC circuits and one of them is wired improperly.

Elantric

#2
What model Variax and what are you connecting your Variax too?

review the Variax VDI thread for details on the VDi interface
https://www.vguitarforums.com/smf/index.php?topic=13484.msg99013#msg99013


There is 7VDC voltage on the VDI Ethercon connector.

And the "Ring" contact on the 1/4" TRS jack can be used as a remote DC Power input for powering the Variax.

Its conceivably possible that damage / strain on the Variax internal electronics may occur if you connect  a standard mono 1/4" TS Guitar cable while also using the VDI cable connection. There are internal voltage steering diodes to prevent shorting these two  power entry points   - but Line-6 has seen these fail, and the result is a smoked / burnt VARIAX DSP board   

If you want to be safe while using the VDi cable, always use a 1/4" TRS cable with "open" Ring contact to connect to the Variax 1/4" TRS jack.

But know the Downfall of using a stock 1/4" TRS cable is that you typically are connecting to a battery powered floor stomp FX, but the floor FX will not "turn on" unless the TRS "Ring" contact is grounded.

So build a custom 1/4" Guitar cable with an open "Ring" connection on the TRS (Variax) side, and a normal TS plug on the Amp / FX side.     


http://line6.com/support/topic/17559-multicore-cable-to-connect-jack-and-cat5-simultaneously/

vtgearhead

Thanks, Steve.  That's the first reasonable technical analysis I've seen, since the Line 6 person (you know who I'm referring to) is fond of scare mongering while providing no details to back it up.  I'm surprised to hear of diode failures at 7V of reverse bias, but stranger things have happened and they may have had a bad batch somewhere.  Your point about a hybrid TRS - TS cable is a good belt + suspenders solution to head off any possibility of unpleasant surprises.

I am not using the guitar in this manner, but was growing irritated with the lack of technical foundation being provided (and not a little with being talked down to).  It's certainly reasonable for Line 6 to protect their trade secrets, but the techniques used to mitigate between two possible power sources is a laughably well-known design practice.  Hard to see what harm there'd be in actually providing some justification for the original claim.

Elantric

#4
QuoteI'm surprised to hear of diode failures at 7V of reverse bias, but stranger things have happened

True - - this could become much higher than 7VDC , particularly if  each cable to the Variax is referenced to a different Ground .   Ive played many gigs on USA  stages in Buildings with a 235VAC Power Entry to the building  which the local electrician split the wiring to the whole club on two separate phases of 117VAC sent randomly  - often an opposing Phase on each side of the bandstand being fed with only  2 prong ungrounded AC outlets.

Very dangerous conditions still exist in many Venues  - so I always use a simple AC Power tester to know my environment and keep me alive.


The situation on the official Line-6 forum is due to the standard operating procedure of most companies , to protect their customers, and maintain the "No User Serviceable Parts Inside" statements to comply with UL/ CE safety compliance, and protect the revenue stream and value for the official  Factory authorized Repair centers due to the nature of non disclosure agreements regarding specific circuit details , with the expected "scare mongering " official public statements.
And it probably saves many Variax owners a repair bill when Line-6 personal on the official Forum simply state "don't do that", which given the risk for potential damage  - remains a valid public stance. 

vtgearhead

Quote from: Elantric on January 06, 2016, 06:17:21 AM
True - - this could become much higher than 7VDC , particularly if  each cable to the Variax is referenced to a different Ground .   Ive played many gigs on USA  stages in Buildings with a 235VAC Power Entry to the building  which the local electrician split the wiring to the whole club on two separate phases of 117VAC sent randomly  - often an opposing Phase on each side of the bandstand being fed with only  2 prong ungrounded AC outlets.

Very dangerous conditions still exist in many Venues  - so I always use a simple AC Power tester to know my environment and keep me alive.

In the top drawer of my desk is an interesting bit of metal sculpture that consists of the tip from a Switchcraft 280 plug arc-welded to the ground ring of a chassis mount 1/4" jack. It was given to me by one of my customers back when I ran a Pro-Audio business.  I guess the event was quick and quite colorful. Fortunately no one was hurt, but Ray told everyone it was responsible for the fact he was bald (never believed that one...)

I never had anything quite that drastic happen to me at a gig.  A few good shocks and Ground Loops from Hell, but no pyrotechnics. 

I should post a photo of my ground checker.  I probably bought it in 1973 and it still occupies a place of honor in my toolbox.  It might be an "Archer" unit from Radio Shack - will have to check.

admin

QuoteI should post a photo of my ground checker.  I probably bought it in 1973 and it still occupies a place of honor in my toolbox.  It might be an "Archer" unit from Radio Shack - will have to check.

http://pdf.textfiles.com/catalogs/RADIOSHACK/

vtgearhead

Well - I was close.  It's a Slater Ground Monitor.  At least they both end in 'er' :-).  But thanks for the pointer to those catalogs.  What a trip down memory lane!

vtgearhead

#8
Truth Or Consequences!



(The plug body had to be cut off from the tip to get the jack out of the panel. Pretty, ain't it?)