IOS - Uncontrollable feedback with Tonestack and BiasFX - help?

Started by Smash, October 27, 2016, 08:27:25 AM

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Smash

Setup is iPad on iTrack dock being fed by the guitar output from vg99

For some reason I'm getting uncontrollable feedback even at low volumes with high gain patches.

The weird thing is once the feedback gets hold, turning the guitars volume to zero does NOT stop the feedback - only disconnecting the jack lead will stop it. I have noticed that taking reverb out of the chain really calms it down.

The only thing non standard about the iTrack is I have mounted 8 magnets on the base to hold it onto my rack shelf so it's easily removable, although I can't see why this would make any difference though.

Any ideas?

Elantric

For debug Run one IOS App at a time and avoid AUM Mixer

Sounds like a classic signal routing problem with outputs being reintroduced to the input on YONAC TONESTACK or BIAS FX

And baby step each part of the audio signal chain

I would first try normal electric guitar 1/4" out > iTrack Dock input > track dock headphone out to headphones with iPad running Tonestack and verify all works , then reintroduce the VG-99.


Smash

Thanks man - back from Lake District this weekend so will go into fault finding mode...

Elantric

And verify the feedback is not from microphonic coil windings on your non wax potted mag guitar pickups.

Also some GK-3's also suffer from  microphonic coil windings - that exhibit  high pitch squeal only when played at high decibel stage volumes

read - Potting pickups

https://www.vguitarforums.com/smf/index.php?topic=43.msg71#msg71

Headless68

Have wound up volume (gig level ) on bias FX without issue on same setup - seems odd
I will try into mix out to see if that makes any difference

Smash

Using patch Bias FX patch 5 USA Metal as uber gain test

First Ibanez S470 straight into iTrack = no feedback

Next Guitar out of VG99 into iTrack. No other vg99 connection I.e. No outputs etc. Just one lead out of Guitar out. Identical sound no feedback.

Introduce mixer Behringer 1202fx: iTrack monitor outs into mixer no problem

As soon as either left or right VG99 main outs are plugged in to mixer feedback is immediate.

Turning the iTrack monitor out knob down to halfway stops the feedback

???!!




admin

#6
Quote
Turning the iTrack monitor out knob down to halfway stops the feedback

FWIW - Feedback is to be expected if you enable the iTrack DIRECT MONITOR button on the iTrack
Avoid enabling the iTrack DIRECT MONITOR button with your setup.



Smash

Direct monitor is not on - though I did try it for the sake of trying = no difference.

The monitor out knob is a bit of weird name for it as it's basically the volume control for the main 1/4" jacks.

It's connecting vg99 to the same mixer that does ir

Elantric

#8
It really helps to draw a block diagram to identify the source of the feedback

QuoteUsing patch Bias FX patch 5 USA Metal as uber gain test

First Ibanez S470 straight into iTrack = no feedback

Next Guitar out of VG99 into iTrack. No other vg99 connection I.e. No outputs etc. Just one lead out of Guitar out. Identical sound no feedback.

Introduce mixer Behringer 1202fx: iTrack monitor outs into mixer no problem

As soon as either left or right VG99 main outs are plugged in to mixer feedback is immediate.
seems like the mixer allows iPad output to reenter iPad input.


Smash

Ok so narrowed this right down now.

As soon as the shield of the VG99 cable connects to the mixer the feedback starts......

I wonder if one of the iTrack outputs is getting reversed polarity for some reason?

Gonna try the digital output next

*edit* Test meter says no polarity reversal

Smash

So..

Different mixer = still feedbacking

Different GK3 = still feedbacking

Digital out = interesting...will still feedback when pushed vs direct from Guitar which won't. Also as soon as earths of VG are touched to mixer this introduces background hum to the normal pup via digital out of VG99.

This seems to be a problem with my VG99 as best I can tell. I can't find any settings that might cause the problem.

Is it worth doing factory reset on VG?

Would a ground loop isolator on the VG outs be worth trying?

This is driving me nuts

GuitarBuilder

"There's no-one left alive, it must be a draw"  Peter Gabriel 1973

Smash

OK, OK - I got the message lol! Attached

All PSU's are plugged into 4 gang mains extension lead - none of them have earth connections on their mains supply plugs.

Is there any harm in running an earth from the third pin of the mains plug (UK power has 3 pin plug - central = earth) to the VG chassis?




Brent Flash

#13
 Just to be clear are we talking about feedback or ground loop hum?

Headless68

I was running the exact same config but into Art splitmix 4 as the mixer - no feedback - so, at least I can confirm that it 'should' work without feedback

Smash

Quote from: Brent Flash on November 10, 2016, 04:58:00 AM
Just to be clear are we talking about feedback or ground loop on hum?

TBH it's a bit of both! When the shield of either left or right main output from VG99 touches the the mixer then there is instant feedback on very high gain Tonestack sounds i.e. preset 5 USA Metal - but yes there is also an increase in background noise/rumble/hum.

It's only on really serious high gain tones it does it.

Interestingly, if I use the Digital out of VG99 through Fio D/A into iTrack then it is much more resistant to the feedback but can still be induced and again there is also noise/hum introduced when connecting the shield of the VG99 output to the mixer.

I'm going to try lifting the ground tonight and see what happens as I'm becoming increasingly convinced it's a ground loop issue. If the ground lift doesn't do it I'll upload a video.

Phil - perhaps it's the introduction of powered mixer that's the difference? The power supply appears to have three feeds into the mixer.

Elantric

#16
My prediction:

* The Hum / Noise is from a Ground Loop

* The high pitched feedback from high gain patches alone might be indicative of a microphonic GK-3 hex pickup.

I had one of those once,  solved that by wax potting the GK-3 hex PU 

My Roland GK-3 PU has microphonic feedback at High Volume.
https://www.vguitarforums.com/smf/index.php?topic=43.msg6190#msg6190

Headless68


Elantric


Smash

Technology is surely a Schrodinger's cat to me - love and hate it both at the same time!

I've ordered a ground loop isolator - cheap and cheerful and there is clearly something coming through the ground.

It's not GK3 related - the high gain sound is on the ipad whilst the output volume of VG99 is at 0 - it's purely the mag pup signal being affected. The two mixers I've tried both have the same type of power supply so I'm thinking this is where the problem lies and was also probably where I had an issue with my old Variax GK3 git when using the Vax psu - ground loop again.

Phil - thanks for offer but let's see what the ground loop isolator does - I'm confident (ish!)...

The digital out from VG99 works great  - nice to be able to run the COSM models through BIAS or Tonestack - but the FIIO does introduce some noise on higher gain tones even with the VG99 digital output level max'd at 200. I like the concept enough to look for another D/A with better signal to noise ratio (the FIIO is 95db - there are some kicking about with >120db but with a suitably large price tag!)

Elantric

#20
QuoteIt's not GK3 related - it's purely the mag pup signal being affected.

Then check your normal mag pickups for loose windings which promote microphonic feedback.

On my Guitars I wax pot the pickups, so I can play ultra high gain patches at deafening stage volume (over 100 dB), and only source of feedback is the vibrating guitar strings - not my pickup windings. ( a common problem)

Plug normal guitar into a high gain guitar amp on "10" and guitars controls on "10" at stage volume and palm mute all guitar strings.

If you still hear squealing feedback it's your mag pickups require wax potting

Side story?  I purchased my 1966 Vox AC30 dirt cheap in 1977 because the seller was convinced the high pitched feedback was due to a problem with the AC 30 ( even though I told him it was his microphonic pickups on his 1963 Harmony-silvertone Guitar.

Smash

Ground loop isolator cured all!! Hooray!!

Final cable tieing to the new rig - I now have 5 expression pedals lol!

Elantric

QuoteGround loop isolator cured all!! Hooray!!

Which Make / Model Ground Loop isolator did you use?

Which audio connection is this employed?

Smash

Generic can type with rca fly leads - ebay 5 dollar type.



Pulled it apart and put angle jack fly leads on - contents is two 1:1 transformers.

I strapped it onto the itrack output