Ghost Acousti-phonic+Hexpander RF noise?

Started by kknyc, January 05, 2024, 06:04:12 AM

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will do.

Today I actually started dismantling the system to fine-tune the 3d print. With parts still on order, I'll try and clip everything back together and get an mp3 with my phone.

AS it turns out, even the simplest print (cylinder with a bottom plate) is going to take about 15hrs.. so this weekend I'll look for premade parts to make a prototype. (If results are promising , then I print a full design) —I think the simplest cage would be a 4"x 2.5" tin round epoxied to a backplate could be a 'down and dirty prototype'.

PS> finally heard back from GraphTech CS was told (if not using MAG pickups) to jump the Mag input pin to grd.
This doesnt not help reduce the RFI (I had already tried that)

And in reference to my question about "how I might go about shielding the 13pin module", and where/what should/could be shielded: They said "shielding the pickup lead would not help". And no other comment. But Im left wondering, so what WILL help?


just had a thought...

after reading some un/related links here on VGF, maybe this RFI issue is exaggerated with the fact that my bass is Carbon fiber?

Using a DMM, I have continuity on all unfinished interior surfaces of the bass (basically everything).

So I imagine there might be a ground loop happening from the Ghost system's ground to the output jack AND the interior cavity of the body—like with the Ampero ground loop noise issue?

I'll try some heat shrink on the jack or tape on the body hole, or the aforementioned plastic spacer.
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Electrical ground is a confusing topic, even for Electronic Engineers- many have textbook knowledge, but no practical experience, assumming its the least interesting part of the signal chain, when actually its the most important , my HAM radio operator self educated grandpa knew more about this topic than most of my EE co workers


yeah, im really confused by all the topics LOL (im basically just a musician that can wire my basses)  ;D

I would imagine that a hollow-body instrument  (thats made of conductive materials, in contact with the output ground would help the issue—acting like a big cage in-and-of itself. But this combination of materials/system/piezos has me even less likely to make the right decision hah.


Perform an isolated set of tests with the unshielded Graph Tech electronics on a test bench and record the resulting noise at each step.

* Add the Hexpander (measure noise)

*connect the piezo saddes. (measure noise)

* Mock up a grounded shielded cage(measure noise)

All above  without being installed in carbon fibre instrument

Measure noise at each step above

 Document the impact on the noise floor for each step and basically mock up all the physics of your intended implementation


Connecting section by section outside the instrument its the same as I've experienced previously;

The Acoustiphonic system is essentially silent before the Hexpander
RFI is slight with the HExpander + saddles, (0:16)
Then pronounced once the 13pin module is added, (0:25)
and finally with GK13 cable (0:30)

I also tied adding a clip-on ferrite (@ pup leads bundled and 13pin module ribbon bundled) - didn't help

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pic of each step of assembly
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I would also test everything without the saddles connected and maybe everything completelly off the guitar (except saddles).


IMHO  No surprises,  you are witnessing the known high noise floor of the Graphtech Hexpander  electronics,  the 1/4 Output has very little noise, and is the output used by 95%  of their customers , but the 6 channel  analog Opamp Hexpander  boards GK13 output noise floor is not subtle - full of hiss.
 Graphtech management is aware, but they remain stuck in the mindset all GK-13 processors convert incoming signals from GK-13 Input into MIDI - (they confirmed by internet search (*&%$)
. Youll get the long stare when attempting to inform them about Roland/Boss VGuitar /VBass systems , where the string signal is the source  for 6 channel DSP processing - the Guitar GK13 preamp is a crucial part of the audio output signal .


I realize Ive come full-circle with this—to the chagrin of some Veteran VGFers Im sure ;)  —but an issue this obvious seemed like a failing component, not a poorly designed one. I once had an active bass that had a similar RFI issue but I was able to diagnose it down to the failure for a repair.. but this? WOW.

Stunning that a Leading brand company would let an issue like this stand in their product-line . If the Processed signal was the only thing I used, it wouldn't be an issue, but if I want to blend the signal "dry and processed" in the GR-55 its useless!

well, I really thought there was a bad component on the 13pin module as thats where/when the RFI is introduced. I guess theres no board mod/replacement option.
(Cue the "trombone fail")

Heres my final (I promise) post of RFI...

tested with NO pickup attached—
its silent, even with the ribbon cable attached (0:00)
add the 13pin module RFI. (0:05)



...and thanks Elantric and Cags12 for indulging my tenacious 'fix it' itch  ;D
I really appreciate you guys sharing your knowledge and time with me!


sounds very quiet with no pickup attached,
 that would indicate some sort of diode action going on in the piezo pickup if you are receiving radio signals.

Free "GR-55 FloorBoard" editor software from


HI gumtown,
It was also silent with the pickup connected (beginning of mp3 sample in previous post)

I was mentioning earlier in the thread, Ive got a woefully limited electronic education, but eager to experiment and learn :)

Could you offer an explanation of how I could test and implement?
would I add a diode on each pickup lead,  something in series or a parallel connection somewhere?  ;D


It is also possible the fault is not in the instrument, what sort of outboard equipment is connected ?
From what I read, the issue starts when a second grounded lead is plugged into the guitar causing an earth loop,
 so wondering what else is there, as in the whole equipment setup ?
Free "GR-55 FloorBoard" editor software from


The signal chain is GTGhost system <TS cable> MOTU 8pre-ES <THB4 cable> Mac Studio /LogicPro (no issues with my amp or daw, or other gear)

Ghost Acoustiphonic system runs silent/no RFI (both with and without the saddles connected), sounds great in the bass and thru my DAW, and my amps.

RFI is picked up only when I plug in the Hexpander's 13pin module, otherwise the signal is clean/silent/within nominal expectations.

The Ghost system is made up of:
piezo saddles > the Acoustiphonic Board > 1/4" output to AMP/DAW

The Hexpander expansion board adds a 13pin GK13 cable option to GK enabled gear. The signal chain is:
1.piezo saddles > 2.the Acoustiphonic Board + 3.Hexpander board AND 13pin module to either
> 1/4" output to AMP/DAW
> GK cable to GR-55

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The RFI is the same out from the bass' 1/4" output AND the GR-55s Gtr Output
The GR-55s processed signal is clean, no issue

PS) Ive also done these tests 1st with the system installed in the bass, followed by same tests outside of the bass. the results are always the same.

A recent revelation was that my carbon fiber instrument (Emerald Balor 5 bass) has a conductive interior surface (like a shielded cavity).


If you are connected to a computer, I would bet on the RFI source coming from that.
Try unplugging the TB3 cable and see if the noise stops (assuming you are monitoring directly from the MOTU)

Also try grounding the MOTU chassis to a good known earthing point, there is a good chance your whole setup has no physical ground connection to the 'Earth'.
Free "GR-55 FloorBoard" editor software from


thanks, I had previously checked that too. RFI happens if I run into DAW or through an amp (at home and reh room)

as a double check, I unplugged the THB4 cable — no affect on the RFI.
I also removed my snake and plugged directly into the interface.

no change in amount of RFI, all instances same when adding the 13pin module, none of the usual suspects attenuate or change the RFI.

Ironically, the system + the Hexpander runs great... but no 13pin access LOL