hexaphonic pickup - DI breakout box - mixer

[hp3]

I am experimenting with installing a hexaphonic pickup in a guitar to experiment with computer based signal processing or for use with a conventional mixer to apply different eq and pan settings for each string.

I will be using an ubertar.com or ultramagnetics.com pickup.  These have 6 output wires from the pickup that I will solder into a 7 pin connector (this will be easier than 13 roland type connector).

I will need a break out box to convert the 7 wire cable from the guitar into 6 1/4" jacks to go into a mixer or a USB analog to digital converter then into the computer. As I understand, most USB DAC and mixers have balanced line inputs.  The guitar pickup is unbalanced.

So I assume that a breakout box, for my purposes would, need to transform the impedance from unbalanced guitar pickup output to balanced before going to DAC or mixer. Is this correct?  Since this is what a DI box does, the breakout box would is a 6 channel DI box.  Is this correct?

I would like to find out more about how I would build this type of breakout box.  What issues should I consider for adapting a typical DI box schematic for use with 6 inputs (one for each string) instead of one input.

[earthtonesaudio]

I would rethink the idea of sending 7 unbalanced weak signals through a long cable.  That's inviting lots of noise.

My suggestion would be to bypass everything and do the ADC inside the guitar; give the guitar a USB port.  You can power the device through the USB to mitigate the need for battery power.  Some issues with this approach include the limited space inside the guitar, and the difficulty of finding a performance-worthy USB cable of adequate length.  But I think some DIY ingenuity could solve both of those.

[Mark Hammer]

Stabndard divided pickup electronics for analog guitar synths provide an op-amp gain stage for each string prior to exitting the guitar.

This page has schematics for a lot of the various commercial guitar synths that have cropped up over the years:  http://www.joness.com/gr300/index.htm

[hp3]

thanks for pointing out the issues of noise in the 7 wire line and the possible use of op-amp for gain.  I will contact the pickup manufacturers and find out how they are addressing these issues, if at all.

[ashcat_lt]

I didn't click through or anything, but I know ubertar from the TapeOp Forum.  As I understand, he will provide you with a breakout box - either transformer coupled or otherwise.

[Gurner]

I would rethink the idea of sending 7 unbalanced weak signals through a long cable.  That's inviting lots of noise.

S'cool, but then...

My suggestion would be to bypass everything and do the ADC inside the guitar; give the guitar a USB port.  You can power the device through the USB to mitigate the need for battery power.

Now *that's* inviting lots of noise.

Even the dedicated units that process 6 analogue signals...(they supply +7V & -7V into the guitar via the 13 way cable) woeful amount of noise ....the Terratec design team in particular dept should be manhandled out of their building.

[phector2004]

But isn't the digital signal noise-resistant?

How much noise do you need to change a 0 to a 1? 

[ashcat_lt]

The output of the hex pickup is analog till it gets to whatever box does the conversion.

In the USB scenario, I think the issue is that the 5V is likely to be far less than flat.  If it's used to power a buffer/booster without proper filtering...

Then there's that whole thing with the digital clock very close to the pickups.

And, of course, if it's USB when it leaves the guitar you can't really go patching in your stompboxes, can you?

I haven't heard anybody complain about excessive noise in ubertar's system, and have read a number of testimonials - all from recording engineers who I'd expect to be pretty sensitive to that type of thing.

[Earthscum]

You know it would be fairly simple to use a battery, and 6 opamps worth of IC's (quad and a dual?) to just make 6 buffers out... non-inverting. If you wanted to get fancy-pants about it, each could have its own gain control on-board, and active tone control. Got knob space?

[hp3]

Thanks for all the responses thus far.  I was worried this might be passed off as an odd ball topic...

I emailed with Paul at ubertar and he says he does not experience any noise issues with the 7 din cable from the pickup.  He is using a 7 pin midi cable.  There are recordings on the ubertar.com web site of people playing through the pickup and cable. I do not hear significant noise in these recordings.  This is a subjective way to evaluate noise.

Paul does seem to understand the issues around hexaphonic guitar very well and has a good combination of solutions to work from. He does offer break out boxes, which would probably be fine for my needs but I still wanted to understand what might be involved if I decided to build one myself. 

Should the break out box function like a DI box and convert unbalanced to balanced?

[ashcat_lt]

Balanced outputs - at least on XLRs like a normal DI box - would make it tough to patch in your stompboxes.  I figure that's most of the point of the thing, no?  Different effects on different strings.  I guess balanced on TRS might work?  A standard TS would just short the ring.  Might be some volume drop there.

[hp3]

The goal is to have independent signal processing on each string, or separate pan or eq for each string.  I would most likely be using a computer for the DSP.

I would not need xlr.  I would need TRS balanced and at line level. 

Though, it seems a number of usb ADC devices support unbalanced TRS line inputs too.  So, it might be ok to have unbalanced TRS at line level.

[earthtonesaudio]

I imagine you can do all the functions of a hardware mixer from within your DSP program.  So if I were you I would evaluate how important it is to you to have a hardware mixer.  On one hand the physical control surface is really nice to use, but on the other hand it requires 6 additional cables plus whatever balanced/line level converter circuitry you want to use, which introduces more cost, complexity, and reliability issues.

In my opinion a hardware mixer would really shine if you plan on doing lots of panning/EQ and fader work, but not very much DSP-per-string.  On the other hand if you plan on doing more DSP and not so much pan/EQ then the mixer would tend to just get in the way.

[hp3]

I guess my concern at this point is regardless of whether I go into a Hardware mixer or a USB ADC to computer, I still need to address the issue of going from instrument level to line level, and perhaps unbalanced to balanced. 

Both a mixer and ADC typically have "line level" TRS inputs, but if the guitar is at "instrument level" (the ubertar pickup is passive), do I need to bring it up to line level with a pre-amp?  If the mixer or usb device has balanced line level inputs, do I also need to change the guitar signal impedance from unbalanced to balanced and in addition to pre-amp the guitar signal from instrument level to line level?

[Skruffyhound]

Don't use usb power, Gurner is correct ^, having some issues with that myself right now.
Buffer the outputs inside the guitar whatever else you decide to do, would be my suggestion. Good luck.

[earthtonesaudio]

Don't use usb power, Gurner is correct ^, having some issues with that myself right now.
Buffer the outputs inside the guitar whatever else you decide to do, would be my suggestion. Good luck.

I agree it's good practice to buffer the pickups before going into the ADC.  But I am curious about the power issues.  USB 2.0 defaults to 100mA per port and you can instruct it to deliver up to 500mA, which ought to be enough current to power some voltage followers. 

Of course it would be silly to treat the 5V coming through a 3-5 meter cable (with tiny conductors) as an ideal battery.  That's what regulators are for.

Or, I could be missing something important.  Skruffyhound, would you care to elaborate on the power issues you experienced and why you recommend against using the USB power?

[ashcat_lt]

Generally, guitar levels are not really like way too low for line inputs.  A little low yes, but it can work.  Here I'm talking about the voltage output.  I think at least one of ubertar's versions is specifically noted as being higher output.  Especially if you're recording to 24 bit there's plenty resolution there.  Digital gain is quieter (no added noise because no actually circuitry involved) and cleaner.

Balanced vs Unbalanced has nothing to with impedance.  You probably won't want to plug a guitar directly into any line input, balanced or not, for the tone loss.  Balanced connections can help with the noise picked up in the cable.  If you're not going super far, it probably won't make much difference.  You can usually plug an unbalanced TS plug into a balanced TRS input without hurting anything.

[Gurner]

Or, I could be missing something important.  Skruffyhound, would you care to elaborate on the power issues you experienced and why you recommend against using the USB power?

Why I recommend *not* to take any power from the USB for audio ...is the sheer amount of gunk that can be present on a PC's ground.... PC's aren't known for their great grounding practises & with all the clocks inside a PC, the ground can be very noisy.

I owned a Terratec Axon AX50 midi guitar unit....I sold it because of the horrific noises getting into the guitar's audio signal   ....the path was "noisy PC" ->USB cable (with noisy ground)->Axon Unit->7V DC down the 13 way (with noisy ground)-> grounded inside the guitar   = noise in the analolgue guitar signal ....bad noise.

[hp3]

Generally, guitar levels are not really like way too low for line inputs.  A little low yes, but it can work.  Here I'm talking about the voltage output.  I think at least one of ubertar's versions is specifically noted as being higher output.  Especially if you're recording to 24 bit there's plenty resolution there.  Digital gain is quieter (no added noise because no actually circuitry involved) and cleaner.

Balanced vs Unbalanced has nothing to with impedance.  You probably won't want to plug a guitar directly into any line input, balanced or not, for the tone loss.  Balanced connections can help with the noise picked up in the cable.  If you're not going super far, it probably won't make much difference.  You can usually plug an unbalanced TS plug into a balanced TRS input without hurting anything.

ok, this is helpful.  So, I may not need to worry about instrument level vs. line level and unbalanced/balanced.  It sounds like all my break out box needs to do is just wire a pin from the 7 pin cable to a quarter inch jack.  Is this correct or should do I need to do something else?

[Skruffyhound]

why you recommend against using the USB power?

I built the Meeblip synth which in it's first itteration was usb powered. They quickly moved to a regulated power supply because of noise issues, unfortunately that part of my board has a fault which I haven't fixed yet so I'm on usb power and yes there is noise on the line.