Guitars - BrixIT Blog

Reverse engineering the Variax guitar

Martijn Braam — Sun, 04 Oct 2026 16:27:30 -0000

The Variax guitars are an "old" series of modelling guitars from Line6. The early models don't even feature any magnetic pickups since all the sound comes from piezo pickups in the bridge. The guitar actually has a piezo pickup for every string which gives it the ability to do good pitch-shifting and modelling. At least a lot better than the modellers I've used.

I've known about these guitars since getting my first Line6 guitar effects unit that had a very interesting connection on the back. The effects unit I got was a Pod XT (somewhere in 2014) and on the back it had an ethernet jack marked "Variax".

Rear of the Pod XT

According to the manual this allowed connecting a Variax guitar and when using that the entire signal path from the guitar to the Pod was digital instead of using the ADC in the Pod after a long cable. I've always thought it would be a good idea to move the converters into the guitar since the pickups produce a pretty weak signal and converters are cheap now but apparently Line6 actually did that. Sadly the guitars were pretty expensive though.

Now 12 years later I got one secondhand and finally get to play with one of them.

Variax JTV-69

This thing works great as a standalone guitar, for this you'd have to put their propriatary li-ion battery in it and then the analog output jack acts like a regular active guitar. But since I first got my Pod XT I have upgraded to the rackmount Pod X3 Pro so I wanted to finally use these two things together.

For this I got a short EtherCON cable. These are just normal ethernet cables but the plugs have a shell around them to make them lock like XLR connectors instead of having the clip on them to clock the ethernet jack.

All together it worked.... fine...

The X3 does not have the most intuitive user interface and adding a Variax guitar doesn't make it easier. In fact it seems like I need to manually edit all the factory presets to make it actually pick a Variax model for the built-in presets. The Variax capable Line6 modellers also all have the issue of not fitting in the official Line6 Variax guitar carry case so the only solution left is... I build it myself...

The electrical interface

The nice thing about this is that I'm not the first person to mess with this so there's already some documentation around the digital interface. A great reference has been the vguitarforums which has a few great threads with information: https://www.vguitarforums.com/smf/index.php?topic=13484.0

Most importantly there's this diagram that has been posted in many spots talking about the VDI interface:

Variax VDI pinout, from Ben Craven

This combined with https://line6.com/technical-schematics/ which has schematics for various Variax guitars and VDI-capable modellers gave me enough information to build my own interface.

The idea was to build this around the Teensy 4.1 and basically copy most of the FOSDEM audio mixer for the analog parts and add the VDI port to it. The Teensy can easily accept the AES3 digital audio on the s/pdif input pin after it has been unbalanced and the library for the Teensy also has everything I'd need to do the MIDI parts.

I also modified the part of the schematic that normally generated +48V for microphone phantom power to generate the +7.5V power for the guitar instead.

Mini Variax interface

The result is the PCB above, two balanced analog inputs, two balanced analog outputs. The EtherCON jack for the guitar and still unpopulated in the picture above is the optical toslink connector that I want to use to send the signal digitally to my audio interface, and between the XLR jacks there's an unpopulated 3.5mm headphone jack.

Audio hardware mistakes

So getting this hardware running has been quite painful, mainly do to me making a few very stupid mistakes.

The first one is that I forgot the pull-up resistors on the i2c bus so the codec was not communicating. That was fixed easily by putting a few resistors on the back of the Teensy headers to pull up the lines.

The second mistake was more painful, I accidentally swapped the TX and RX pair for the MIDI signal on the guitar. This doesn't seem to have an easy way to work around on this PCB so I ended up not using my EtherCON cable for this at all but instead using a regular ethernet cable where I swapped the midi pairs on.

The biggest mistake though is the messed up i2s bus. I did not route mclk to the codec which causes quite a bit of issues since the clock recovery from bclk on the codec is not great, especially since I also did not get the impedance matching for the bus right so it has a lot of reflections. I now have a bodge wire connecting the 11Mhz mclk signal through a 50Ω resistor to the right pin on the codec and that has made the audio quality bearable at least.

I think with this together I've found all the major hardware issues I've created so that can very easily be fixed for a second revision board if I feel like throwing away $300 again to produce a second one.

Routing audio

The way audio is handled is pretty much the opposite of the Pod series. Instead of picking a patch on the Pod and maybe the guitar following that setting I'm actually making my board switch patches based on the guitar model selected on the Variax.

The plan for how audio is routed is pretty simple, when I'm on an accoustic model the audio should be routed through directly to one of the XLR outputs so it can go to my audio interface and then my headphones (or a full range speaker). I'm also using freeverb on the teensy to add a bit of reverb to this.

When selecting an electric model the audio is routed to the second XLR output instead which then goes through my electric guitar rig and finally returns to the XLR input where it is mixed back into the full range XLR output using the analog passthrough of this codec. In this case my guitar rig is just the ToneX one pedal that apparently can switch presets over USB so that might be something I implement in the future.

Configuration for the Teensy

The current setup in the Teensy is pretty simple. The inputs are routed to mixers so I can select and blend various inputs to the main output. Then there's two EQ banks for the main output, one doing the main EQ and the other removes a bit of bass from the sound before going through the reverb. Finally mixer3 blends the reverb sound and the dry sound together for the XLR output.

What's not pictured here is the sound of XLR in 2 also being mixed into XLR out 1 directly in the codec with the volume for that path controlled by the preset switching logic.

The blue path here visualizes the routing when an accoustic model is selected and the orange path is selected when an electric guitar model is being used. The green path is always active to route in AUX audio into the output, or could be used to also run a microphone through the reverb path.

MIDI Control protocol

For the control part of the guitar I'm going to need to actually reverse engineer a bit more myself. The protocol is MIDI and the basics are available online but I'm interested in a few more complicated features. I want to have the board load in the preset information from the guitar on bootup like the Line6 PC software can so I don't have to hardcode a list of preset mappings in my firmware. I also want to load the custom tunings that are set on the tuning wheel since I just can't remember what I stored in what slot so I need to put it on a display.

The initial thing I did for testing is create an USB midi interface on the teensy and link that to the hardware midi port I've created. Suprisingly this is enough for the Line6 Workbench HD software to recognize the guitar and start the configuration for it. Sadly just like using the X3 the software just doesn't work and crashes the guitar when changing settings...

Variax Workbench HD

I've added a debugger to the Teensy firmware that dumps the midi communication to the serial port so I can see what the Workbench software is doing in realtime. All the communication here is MIDI sysex vendor commands except the initial connection sequence where the Identity Request/Reply messages are used from the MIDI standard.

PC  -> 7E 7F     06 01 (Identity Request)
VDI <- 7E 7F     06 02 00 01 0C 07 00 05 00 30 32 32 33 (Identity Reply)
PC  -> 00 01 0C  03 10 00
PC  -> 00 01 0C  03 0F 00
PC  -> 7E 7F     06 01 (Identity Request)
VDI <- 7E 7F     06 02 00 01 0C 07 00 05 00 30 32 32 33 (Identity Reply)
PC  -> 00 01 0C  07 05 2A 01 (Wakeup?)
PC  -> 00 01 0C  07 05 18 00 00
VDI <- 00 01 0C  07 05 01

Then the software does 60 requests for the 60 presets slots available

PC  -> 00 01 0C  07 05  21  00 09 0C 0C 00 00 00 0B 0C 0C 00 00 00
VDI <- 00 01 0C  07 05  21  00 09 0C 0C 00 00 00 0B 0C 0C 00 00 00 0C 0C 0C 00 00 00 2E 10 03 00 00 00 00 70
PC  -> 00 01 0C  07 05  21  01 09 0C 0C 00 00 00 0B 0C 0C 00 00 00
VDI <- 00 01 0C  07 05  21  01 09 0C 0C 00 00 00 0B 0C 0C 00 00 00 0C 0C 0C 00 00 00 1A 44 33 00 00 00 00 70
PC  -> 00 01 0C  07 05  21  02 09 0C 0C 00 00 00 0B 0C 0C 00 00 00
VDI <- 00 01 0C  07 05  21  02 09 0C 0C 00 00 00 0B 0C 0C 00 00 00 0C 0C 0C 00 00 00 52 09 4E 00 00 00 00 10

According to timing it seems like the data responded here is enough to show both the preset slot name in the software and the details about the guitar but there just doesn't seem to be enough bytes different between these slots to actually encode all that. It might just be a reference into a list of stock presets that are stored.

There's also a big packet sent when you change guitar models in one of the slots that seems large enough that it might actually loading DSP code into it. After increasing my sysex buffer size a few times and then switching to a streaming approach it seems like the models are 6KB large and sending them is the part that make the guitar crash until rebooted. This makes it very hard to actually reverse engineer more of the format.

The other thing I looked into is the custom tunings, which I did manage to modify using the official software. So here's a dump for the response of the various tuning slots:

VDI: 1B 01   00 00 00 00 00 00   00 00 00 00 00 00   00 00 00 00 00 00   00 00 00 00 00 00   00 00 00 00 00 00   00 00 00 00 00 00       01 04 04 05 03 05 01 04 04 04 0E 04 00 02 04 04 02 05 00 02 00 02 00 02 00 02 00 02 00 02 00 02 00 02 00 02
VDI: 1B 02   00 00 00 00 00 00   00 00 00 00 00 00   00 00 00 00 00 00   00 00 00 00 00 00   00 00 00 00 00 00   0B 0A 0A 0A 0A 0E       0F 04 02 05 04 04 04 04 00 02 00 05 00 02 00 02 00 02 00 02 00 02 00 02 00 02 00 02 00 02 00 02 00 02 00 02
VDI: 1B 03   05 05 05 05 05 0F   05 05 05 05 05 0F   05 05 05 05 05 0F   05 05 05 05 05 0F   05 05 05 05 05 0F   05 05 05 05 05 0F       0C 04 01 04 08 04 04 04 00 02 06 04 0E 04 07 05 0F 04 00 02 00 02 00 02 00 02 00 02 00 02 00 02 00 02 00 02
VDI: 1B 04   05 05 05 05 05 0F   05 05 05 05 05 0F   05 05 05 05 05 0F   05 05 05 05 05 0F   05 05 05 05 05 0F   00 00 00 00 00 0E       0F 04 02 05 04 04 04 04 00 02 00 05 00 02 00 02 02 06 00 02 00 02 00 02 00 02 00 02 00 02 00 02 00 02 00 02
VDI: 1B 05   0B 0A 0A 0A 0A 0E   0B 0A 0A 0A 0A 0E   0B 0A 0A 0A 0A 0E   0B 0A 0A 0A 0A 0E   0B 0A 0A 0A 0A 0E   0B 0A 0A 0A 0A 0E       05 04 0E 04 0F 04 0F 04 04 04 00 02 00 02 0E 04 07 05 00 02 00 02 00 02 00 02 00 02 00 02 00 02 00 02 00 02
VDI: 1B 06   0B 0A 0A 0A 0A 0E   0B 0A 0A 0A 0A 0E   00 00 00 00 00 00   00 00 00 00 00 00   00 00 00 00 00 00   0B 0A 0A 0A 0A 0E       04 04 01 04 04 04 04 04 01 04 07 04 00 02 00 02 00 02 00 02 00 02 00 02 00 02 00 02 00 02 00 02 00 02 00 02
VDI: 1B 07   0B 0A 0A 0A 0A 0E   0B 0A 0A 0A 0A 0E   05 05 05 05 05 0F   00 00 00 00 00 00   00 00 00 00 00 00   0B 0A 0A 0A 0A 0E       05 04 00 05 0F 04 04 04 00 02 0E 04 00 02 00 02 00 02 00 02 00 02 00 02 00 02 00 02 00 02 00 02 00 02 00 02
VDI: 1B 08   00 00 00 00 00 00   0B 0A 0A 0A 0A 00   00 00 00 00 00 00   0B 0A 0A 0A 0A 0E   0B 0A 0A 0A 0A 0E   06 05 05 05 05 0D       05 06 00 07 0F 04 03 04 00 02 0E 06 01 06 0D 06 00 02 02 07 0F 06 0A 06 00 02 00 02 00 02 00 02 00 02 00 02
VDI: 1B 09   0A 0A 0A 0A 0A 02   0A 0A 0A 0A 0A 02   0A 0A 0A 0A 0A 02   0A 0A 0A 0A 0A 02   0A 0A 0A 0A 0A 02   05 05 05 05 05 01       00 07 01 06 03 04 04 03 00 02 0F 06 02 07 04 06 00 02 00 02 00 07 0F 06 0E 06 05 07 04 07 07 06 0E 06 09 06
VDI: 1B 0A   00 00 00 00 00 00   0B 0A 0A 0A 0A 0E   00 00 00 00 00 0E   05 05 05 05 05 0F   00 00 00 00 00 00   00 00 00 00 00 00       05 04 00 05 0F 04 01 04 00 02 0E 04 00 02 00 02 00 02 00 02 00 02 00 02 00 02 00 02 00 02 00 02 00 02 00 02
VDI: 1B 0B   06 05 05 05 05 0D   06 05 05 05 05 0D   06 05 05 05 05 0D   06 05 05 05 05 0D   06 05 05 05 05 0D   06 05 05 05 05 0D       03 07 0D 02 03 04 0E 06 01 06 04 07 02 07 01 06 04 06 00 02 00 02 04 06 00 02 00 02 00 02 00 02 00 02 00 02
                                                                                                                                         03 07 00 02 01 04 0E 06 01 06 04 07 02 07 01 06 04 06 00 02 00 02 04 06 00 02 00 02 00 02 00 02 00 02 00 02

The first 36 bytes seem to encode the actual pitch shifting offsets of the string which is a number in the range of -12 to +12 for every string. I can't think of a reason why they decided to encode a number that fits into a single byte into 6 bytes per string instead...

The rest of the data seems to encode the name of the custom tuning although the text encoding scheme is very weird. On the last line the same name is returned again but the second character is changed from to - (I think the stored name was C Standard but as you can see in the comparison it's two bytes that have changed, the third and the fifth one.

03 07 0D 02 03 04 0E 06 01 06 04 07 02 07 01 06 04 06 00 02 00 02 04 06 00 02 00 02 00 02 00 02 00 02 00 02
03 07 00 02 01 04 0E 06 01 06 04 07 02 07 01 06 04 06 00 02 00 02 04 06 00 02 00 02 00 02 00 02 00 02 00 02

The number encoding for the string offsets is also weird. The 25 possible options I can set using the software are:

-12  01 00 00 00 00 08
-11  0B 0A 0A 0A 0A 08 
-10  06 05 05 05 05 09
-9   01 00 00 00 00 0A
-8   0B 0A 0A 0A 0A 0A
-7   06 05 05 05 05 0B
-6   00 00 00 00 00 0C
-5   0B 0A 0A 0A 0A 0C
-4   06 05 05 05 05 0D
-3   00 00 00 00 00 0E
-2   0B 0A 0A 0A 0A 0E
-1   05 05 05 05 05 0F
0    00 00 00 00 00 00
1    0B 0A 0A 0A 0A 00
2    05 05 05 05 05 01
3    00 00 00 00 00 02
4    0A 0A 0A 0A 0A 02
5    05 05 05 05 05 03
6    00 00 00 00 00 04
7    0A 0A 0A 0A 0A 04
8    05 05 05 05 05 05
9    0F 0F 0F 0F 0F 05
10   0A 0A 0A 0A 0A 06
11   05 05 05 05 05 07
12   0F 0F 0F 0F 0F 07

At least there seem to be some sort of pattern in it but I can't figure out what the logic behind it is.

I have enough of the protocol that I can at least show the tuning info on a display and currently I have a hardcoded mapping in the firmware from model preset slot number to model name and routing settings until I figure out more of the protocol.

First I need to make an actual case for the thing so the bodge wires don't break while I'm debugging software. Also getting the actual display and the rest of the connectors will help for this. But I think I will save that for a future blog post.

Harley Benton (Thomann) Power Attenuator test and teardown

Martijn Braam — Fri, 26 Feb 2016 23:00:00 -0000

Thomann just started shipping their new Harley Benton Power Attenuator. It can handle an input power of
80 watts (it contains an 100 watt L-pad, so they added some margin). It is a simple device containing
only passive components. It has a control for cabinet volume and line-out volume but they are not fully
seperated. The line-out level is a signel tapped of after the main cabinet volume knob so you can't
turn the cabinet completely off and use the line out for recording / headset usage.

Measurement

To measure the performance of the attenuator I connected it to a USB DAC/ADC and ran a frequency sweep with the
tool QLoud. QLoud is a tool normally used to measure the frequency response of a speaker using a measurement microphone.

I connected the line out signal of the DAC to the 16 Ohm speaker input of the attenuator and connected the
speaker output of the attenuator to the line in of the USB DAC. I also did a reference measurement by
connecting the input and output of my DAC/ADC directly together. The last set of measurements is connecting
the line signal to the input of my guitar tube amp (a Defender 5H 15 watt all-tube amp) and connecting the
line-out of the attenuator to the USB ADC.

Taking it apart

I'm also curious what is inside a fully passive device for ~100 euro so I took it apart. The case is
very easy to open, 4 screws on the front and 4 on the back and you can just lift the side and top part off.

The answer to my question: there is not a lot in it.

The main part of the attenuator is the 100 watt L-pad (The huge potentiometer behind the cabinet volume knob).
The signal from the amp is put almost directly accross the two outer pins of the potmeter so it will dissipate
most of the energy. The other large part of the circuit is a bunch of 50 watt resistors (The resistors with
orange cooling fins). They are there to match the impedance for the amp. The signal coming from the L-pad pot
and the small potmeter for line-out is put straight into the line-out and speaker-out jack on the PCB. The rest
of the circuit on the PCB is for the MicMod circuit that is supposed to simulate a cabinet and provide a balanced
microphone signal. I haven't tested the MicMod output yet.

Yeah, I'm not so great with CAD software

Modding a Behringer VT911

Martijn Braam — Sun, 03 May 2015 22:00:00 -0000

A few weeks ago I replaced my Line6 Podxt live multieffect pedal with a guitar amp, cabinet and 2 pedals.
The amp is a Kustom Defender 5H head and I use a Kustom 1x12 cab with it.

To replace the multieffect pedal I bought a Behringer VT911 overdrive pedal and a
Digitech Digital Reverb pedal. I chose for the VT911 because it contains a tube for the distortion and it
only cost me € 14,90 at Thomann.

The pedal sounds fine. Not great but it's a great pedal for the price. I searched on the internet
for easy mods to do on the pedal to improve the sound quality and found various suggestions on a few forums.

The first thing I did was adding a resistor and capacitor to the tube board to give the tube some extra
headroom. One of the major problems I had was that it doesn't really have a setting to only distort a little.
In the 0-1 range on the overdrive the pedal is usable but is very quiet even with the volume all the way to 10.
On the 1-10 range the pedal distorts so much that I wouldn't call this a overdrive pedal anymore.

The mod on the tube board increases the headroom so the tube distorts later and has a lot less compression
but the drive knob doesn't have a very large usable range. On the high distortion range the pedal also
starts to sound very muddy. The second mod adds another resistor and capacitor parallel to the drive potmeter
to change it's range and frequency response.

Here is a list of the exact values used in the mod:

Tube board:
  4.7 nF capacitor between pin 1 and 7 on the tube (marked 472)
  470 ohm resistor between pin 7 and ground (yellow purple black black brown)

Main board:
  1.0 nF capacitor connected to the outer pins of the drive pot (marked 102)
  100 Kohm resistor parallel to the same pot (brown black black orange brown)

I've recorded a sample before the mod and after the mod. Judge for yourself:

Some guitar refinishing

Martijn Braam — Sun, 28 Dec 2014 23:00:00 -0000

In the previous post about guitars I told you about building my own guitar. This one is about the finishing
of that guitar and 2 other guitars

The AtlasCaster

I've forgot to take some pictures from the first guitar (The AtlasCaster). The plan was to leave the guitar natural and only apply some glossy finishing. The first thing I did (without reading the manual) is applying some grain filler to the guitar. This obviously made it too white.

The AtlasCaster with grain filler

After actually reading some manuals on the internet I sanded off most of the filler and applied 1 layer of danish oil to the guitar. This really brings out the colour of the wood itself. The manual of the danish oil states that I need a minimum of 3 layers to have it permanently sealed but since I'm applying a high gloss finish after this I've decided that more layers is unnecessary.

The AtlasCaster with danish oil

After letting the danish oil dry I've applied 4 layers of nitrocellulose high gloss lacquer from an aerosol can. This is then sanded down with some 800 grit sanding paper and then polished with some car wax.

The red Stratocaster

The next guitar to receive a finishing upgrade is my brothers stratocaster. This stratocaster is made from a brandless guitar kit and actually the first built. After building this guitar was stained mahogany red.

The guitar kit used for the red Stratocaster

The stained guitar

After finishing the AtlasCaster with a glossy finish my brother decided the guitar should be candy apple metallic red. My brother sanded almost all the stain of and applied the grain filled (this time I was right) and after that some candy apple red metallic nitrocellulose aerosol paint.

The Les Paul model

At the same time as the red Stratocaster I've also refinished my Les Paul guitar kit This guitar was also stained with the mahoney red stain but I've only applied one layer.

I've sanded down this guitar only a little bit and applied some grain filler after. After the filler I've applied a whole can of nitrocellulose black paint and sanded all the texture down.

Since the guitar is completely black and I've painted over the white binding I thought it would be cool to add some gold lines around the edge. To add these lines I've used some gold gel pens. I drew 2 lines around the whole edge of the guitar (excluding the neck) and added some extra lines at the knobs and the switch on the guitar.

After drawing the lines I sprayed on a whole can of high gloss lacquer again and sanded that down with 800 grit sandpaper. After polishing I found that the lacquer looked scratchy so I ordered some 2000 grit sandpaper and sanded and polished it again. Now it only looks slightly scratchy under bright lighting but that seems unavoidable with a black nitro finish.

What's left to do...

The only problem left with the Les Paul model guitar is that it sounds crappy (because it has 2 for 5 dollar grade pickups, it sounds fine acoustic) so I ordered the Seymour Duncan hot rodded set. While browsing the website I also ordered a Fender custom 69
and a Seymour Duncan SSL-5 pickup for my AtlasCaster. Those should arrive in a few days so then I will upload some sound samples from both guitars.

From Atlas Cedar to rocking ear bleeder

Martijn Braam — Tue, 16 Sep 2014 22:00:00 -0000

Because of the urgent need for satellite reception of a friend of mine my brother needed to cut down an Atlas Cedar tree. The wood from this tree was just lying around so my brother and I decided to build a guitar with it.

The first guitar I've got is a Fender Squier Strat (made in India). After opening up the back cavity I found out that its completely made of plywood so the plan was to use all the parts from the Squier (including the neck) and to create a new body to fit it all

My brother owns a chainsaw jig to cut planks from the tree trunks. I've set the jig to the thickness of the guitar (43 mm) and cut out a part big enough for the guitar body. Online I found the design for a 53' model Fender Stratocaster as full-size PDF. I've used this to cut out the basic shape of the guitar.

After cutting out the basic shape I decided that I didn't want it some colour + white parts but instead I want it the original wood colour and everything else black or chrome. It would also be nice if it had some better pickups than the original china-made crap that was in the Squier so I ordered a new black pickguard, pickups, volume/tone knobs and 5-way switch (the old one was crackling and noisy).

While milling the holes for all the parts in the body I made a measurement error for the bridge. The hole was too big and a centimeter off in position. After having the hole filled up with another part of wood I ordered a new bridge (black of course) that was top-loading. That means I don't need to create a new hole for the bridge, its completely mounted on top of the body and doesn't require springs in the back of the guitar.

This is the new bridge, fully toploading

After putting together the parts i soldered the electronics. I reused the old pots and output jack from the Squier guitar. To connect it all I needed to solder 6 grounding wires to the housing of the volume pot (I don't use the tone pots) I found out that I burned the old pot. Also the new 5-way switch was crackling worse than the original one from the Squier guitar. I've decided to order original Fender ones from a local webshop (for a whopping 2 euro more than the knock-offs I got from ebay) and used those. The official Fender one is actually a logarithmic pot instead of the cheap linear ones thus making volume control much more precise in the low volume range.

After building the guitar and using it for a while I decided I would like to know how it sounds with an actual a-grade pickup so I ordered a DiMarzio DP187BK Cruiser Bridge(a rail humbucker) and put that in the bridge position. I've also replaced the tone knobs with two switches. One switches the polarity of the middle pickup and the other one connects the neck pickup to the volume pot so I can enable all three pickups at the same time.

Now the only thing that's left is the output jack and the neck of the guitar. Here are some audio samples recorded through a Line 6 PodXT Live emulating a Fender Reverb Deluxe Blackface