Standard Guitar to GR-55 Adapter Box Idea

Started by spitfire, December 29, 2012, 02:27:45 PM

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spitfire

I'm interested in being able to run a standard guitar into a GR-55. I'm aware of the need to buffer the pickup signal, as well as the approach to modify the GR-55 itself by essentially adding an FX send-receive by tapping the signal inside the GR-55.

However, before actually modifying my GR-55, I thought I'd build an external box. I'm looking for comments and suggestions on my idea. And if there is something available off-the-shelf, I'm all for that as well.

My basic idea is to have a box with a 13-pin input and a 1/4" input to accept a standard guitar signal. I will buffer this standard signal. Throw in an AB switch so I can easily switch between the two inputs. Furthermore, add a 1/4" FX send and receive into the switched signal before going to a 13-pin output, which goes on to the GR-55.

The idea being I can leave the whole works connected and switch as needed without having to plug and unplug cables. Plus I can use my standard pedal board via the FX send/rec for both 13-pin and standard guitars.

Furthermore, seeing that the 13-pin DIN connectors are less than common, and the receptacles seem to only be available as PCB mount, I'm thinking of NOT using 13-pin on the box. Rather something like a common 25-pin DIN computer connector (old school serial cable connector). These mount to panels (box sides) quite easily.

This of course requires me to modify the 13-pin cable. But since I need to run a short cable from this box to the GR-55, I was thinking I would just cut a standard Roland 13-pin cable into two pieces. One rather short, and one long. Stick a 25-pin connector on the cut end of each cable.

As for the buffer, I would just duplicate the buffer inside the GK-3 system.

I design and build electronic test equipment for a living, so the electronics of all this are not an issue for me. Just looking for a reality check on my approach since I've had a GR-55 for all of 4 days.



gumbo

As previously stated by a couple of others, welcome to the Forum, Spitfire!

I follow your reasoning about using a D25  connector, and this principle has been talked about at various times in the past as a method of circumventing the use of the 13-pin DIN on DIY applications....in fact I played about with it myself in a few ways in the past.

For reasons of my own, I decided a while back to play about with the idea of creating my own panel-mount 13-pin DIN connector within stuff that I was building...   over the past 18-odd (!) months this has now morphed into a series of products that I am shortly putting on the market (been a long rocky road!) which essentially provide panel and guitar-body "mountability" for a 13-pin DIN connector with locking facility and also a variant that includes captive threaded inserts for 'blind' mounting behind panels etc.  Other versions will also be available with pre-soldered tails to all 13 pins and the shielding.

After a few delayed starts in the last few months, these items will definitely (finally!) begin to be available in the early part of the coming year.

You are welcome to PM me for further information, should you feel it could be of interest to you.

Kind regards from Australia!   :)

Read slower!!!   ....I'm typing as fast as I can...

gumtown

Afew ideas to concider, this setup will give you effects and amp/cab only, no modeled instruments or PCM tones.
You might want a second buffer to connect into one of the hex pickup signal lines and do mono modeling and PCM.
Free "GR-55 FloorBoard" editor software from https://sourceforge.net/projects/grfloorboard/

gumtown

Another concideration is the GK volume either loop two wires for 100% level or add a knob/pot,
might want to add S1 and S2 buttons/foot switches too.
Free "GR-55 FloorBoard" editor software from https://sourceforge.net/projects/grfloorboard/

spitfire

Quote from: gumtown on December 29, 2012, 06:00:54 PM
Another concideration is the GK volume either loop two wires for 100% level or add a knob/pot,
might want to add S1 and S2 buttons/foot switches too.

I wasn't really looking at going Full Monty with this idea. But, given the ease of adding the S1 and S2 switches and a volume control, there's really no reason not to. Though I'm still thinking more about something located on the floor next the the unit, so I'm not sure how useful these would actually be for me.

I hadn't considered the possibility of running the mag signal into one of the hex lines. Does this really require a separate buffer? For example, because the required signal levels are different. Or is there some reason I wouldn't want this happening all the time when I have it switched over to the standard guitar?

How well does this work? Any threads on this you can point me to. It's a very intriguing idea.

spitfire

Quote from: gumbo on December 29, 2012, 03:25:38 PM
As previously stated by a couple of others, welcome to the Forum, Spitfire! ...

gumbo, thanks for the kind welcome. At this point I'm looking for a quick and easy solution, rather than something that necessarily fits in a standard rig. I.E., for my personal use, I have no issue using something other than the 13-pin connector. Especially since I need a short one from box to the GR-55. So cannibalizing a cable sort of kills two birds with one stone.

It's interesting, what you're up to concerning a product for adapting (if I understand you correctly) the PCB mount receptacle to a panel. I find it a bit bizarre that Roland uses this sort of odd man out connector. But I'm not too sure what other options they have. There are some great circular MIL-spec connectors, but I suspect they are on the order of 10X the cost of the DIN connector they use.

Of course, I have to figure some day they'll put the A/D converters in the GK circuit on the guitar and just run a digital line to the synth hardware. Go a step further and make it a digital wireless connection. But that's not what we have now. So we must make do.

I'm not sure I have PM privileges yet, but I will be interested in seeing what you have when you come out with it.

musicman65

The simplest solution is to mount the external GK3 wart on something and plug a long cable into it's 1/4" input. Velcro it to the edge sideways on your GR555 :) Heck, I've borrowed the jack on an installed unit with the GK3 guitar sitting on a stand. Use a US20 to A/B two 13pin devices if needed.

bd

spitfire

Quote from: musicman65 on December 29, 2012, 09:13:36 PM
The simplest solution is to mount the external GK3 wart on something and plug a long cable into it's 1/4" input. Velcro it to the edge sideways on your GR555 :) Heck, I've borrowed the jack on an installed unit with the GK3 guitar sitting on a stand. Use a US20 to A/B two 13pin devices if needed.

bd

Simplest, you bet.

But I'm looking for a solution where I don't have to move plugs around to switch between a GK equipped guitar and a standard guitar. This is for my "studio", and I like to be able to grab a guitar, plug a cable into it and go without having chase cables around and figure out what's plugged in and what's not. I also prefer to avoid having to mess with stuff on the floor when possible. More so the older I get.

Also, a little DIY now and again is fun.


OSAR

Has anybody found an alternate buffer to the PC4570G that's readily available in the US?  Thanks and Rock On!

spitfire

Quote from: OSAR on December 31, 2012, 01:54:42 PM
Has anybody found an alternate buffer to the PC4570G that's readily available in the US?  Thanks and Rock On!

I hadn't looked into this at all yet, but your question prompted me to look around. The NJM4580 made by JRC looks to be very close. I also found some references online for it being pretty good for audio applications requiring higher drive current. I don't know if that is a requirement in this application (probably not), but the characteristics are close to the uPC4570.

The NJM4580 datasheet didn't specifically say it was stable at unity gain (that's what these circuits are configured as), but TI seems to have the same part as a RC4580, and it at least gives a slew rate spec at unity gain. So that suggests the TI part is stable at unity gain.

I'll probably give some of these NJM4580's a try. They're dirt cheap at < $1 and available in surface mount or DIP. I'm in no way an audio cork sniffer, so I suspect anything that works at all will be good enough for me. But I'm sure someone on this forum has a suggestion based on actual experience. Hopefully they will pipe in with a suggestion.

billbax

#11
Hi Spitfire,

opa2134 is the op-amp you should be getting your hands on.

Bill

www.separate-strings.co.uk


spitfire

Quote from: billbax on January 04, 2013, 10:02:27 AM
Hi Spitfire,

opa2134 is the op-amp you should be getting your hands on.

Bill

http://billbax.110mb.com

Why? What do you see as being significantly better in this application?

Elantric

#13
QuoteWhat do you see as being significantly better in this application?
Lower Noise, Higher Slew rate, "opa2134 " is a quality Op-Amp for Audio applications

http://www.ti.com/lit/ds/symlink/opa134.pdf

QuoteThe OPA134 series are ultra-low distortion, low noise
operational amplifiers fully specified for audio applications.
A true FET input stage was incorporated to
provide superior sound quality and speed for exceptional
audio performance. This in combination with
high output drive capability and excellent dc performance
allows use in a wide variety of demanding
applications. In addition, the OPA134's wide output
swing, to within 1V of the rails, allows increased
headroom making it ideal for use in any audio circuit.
OPA134 op amps are easy to use and free from phase
inversion and overload problems often found in common
FET-input op amps. They can be operated from
±2.5V to ±18V power supplies. Input cascode circuitry
provides excellent common-mode rejection and
maintains low input bias current over its wide input
voltage range, minimizing distortion. OPA134 series
op amps are unity-gain stable and provide excellent
dynamic behavior over a wide range of load conditions,
including high load capacitance. The dual and
quad versions feature completely independent circuitry
for lowest crosstalk and freedom from interaction,
even when overdriven or overloaded.
Single and dual versions are available in 8-pin DIP
and SO-8 surface-mount packages in standard configurations.
The quad is available in 14-pin DIP and
SO-14 surface mount packages. All are specified for
–40°C to +85°C operation. A SPICE macromodel is
available for design analysis.

http://www.digikey.com/product-detail/en/OPA2134UA%2F2K5/296-22024-2-ND/1649193

But a TL074 Op-Amp will work too, as these are more readily available
http://www.digikey.com/product-detail/en/TL074CDT/497-2204-2-ND/599312



FWIW -
Build a 1/4" Guitar Input > GK 13 pin out adapter cable?
https://www.vguitarforums.com/smf/index.php?topic=5252.0

OSAR

Any suggestions for what type of cap to use for C10, 0.047uf?  Also, any suggestions for a location to grab the supply voltages?  Thanks!

Elantric

#15
Quote" 0.047uf?

Correct 0.047 Microfarad = 47 nanofarad =  47nF

+7VDC is on GK 13 pin #12

-7VDC is on GK 13 pin #13

Ground is on the Cable shell

https://www.vguitarforums.com/smf/index.php?topic=73.0

spitfire

I don't have any reason to promote one op-amp over another. So everything I'm about to bring up is because I have honest questions about this. While I have decades of experience in electronic instrumentation, it has not been in the area of audio signals.

So first off, when comparing the NJM4580 to the OPA2134, I'm just interested in understanding what the important differences are between these two, in order to gain some insight.

Elantric said the OPA2134 had lower noise and a higher slew rate than the NJM4580. The OPA2134 certainly has a higher slew rate spec  at 20 V/us vs 5 V/uS.  But does this matter?

Doing some very quick research, it appears that 5 V/uS is more than enough in this application.

Based on a full range sine wave (music is not a sine wave,  but more on that later), the maximum slew rate needed is:

SR = 2 PI * Fmax * Vmax

If we use the common 20 kHz as the maximum frequency and use 7 V (since the available supply voltage is +/- 7 V, the peak sine wave voltage cannot exceed that), we get a maximum slew rate for a full scale, 20 kHz sine wave as 0.88 V/uS. This is considerable lower than any of the op-amps discussed. And would be even lower when we factor in that the output signal cannot reach 7 V. But it gives an upper bound.

Now, since music is much more complex than a single sine wave, it is technically possible that a combination of many frequencies add up to a slew rate exceeding what a single sine wave needs. Even if that single sine wave represents the maximum frequency of interest. However, from what I can tell, research has been done analyzing recorded music trying to identify the highest known slew rate.

According to work (see link below), the highest equivalent slew rate found in actual music is equivalent to a 12 kHz, full scale sine wave. In other words, actual music is less demanding than the full scale 20 kHz sine wave. The reason, apparently, is that real music doesn't have huge amounts high frequency content relative to what is technically possible.

http://www.stereophile.com/features/404metrics/index1.html

So the point being that it would appear that a slew rate of more than 1 V/uS would be plenty. Note: A part of this is the rather low power supply voltage of this circuit and how it limits the maximum available slew rate required.

One big question, is that published slew rates, for these op-amps ,are at common +/-15 V supplies, I have no idea how either of these amp's slew rates might be affected by the power supply only being +/- 7 V in this application.

Summary higher slew rate, yes, but does that matter? And, does the lower power supply voltage significantly affect the slew rate?

As for noise, the noise specs for the NLM4580 appear to be better, not worse than the OPx134.  It's a bit hard to compare the values directly, but the NLM4580 has 0.8 uVrms over 30 kHz and the OPAx134 1.2 uVrms over 20 kHz.

In terms of spectral density, the voltage noise of the NJM4580 is 2 nV/SQRT(Hz) at 1 kHz and the OPAx134 is 8 nV/SQRT(Hz).

Furthermore the THD of the NJM4580 is given as 0.0005% at 1 kHz with a gain of 20 dB and 5V output. The OPAx134 is given as 0.0008 % (slightly higher) at 1 kHz, 20 db, and 3 V output. Both into 2 kOhm loads.

Now, I'm NOT saying the very small difference in THD makes the NJM4580 significantly better. More like, it's not worse. Also, these are at gains of 20 dB, and this circuit is 0 dB. But the NJM4580 data sheet o gives the 20 dB spec.

I honestly doubt any of this makes any real difference given the levels of distortion etc. we tend to intentionally apply to the guitar signals, but I'm still interested in learning what does and doesn't matter and why.

I've just touched on a few parameters and even these I may be taking out of context. If so, please point out where I'm mistaken.  But, I'd appreciate some details so I can gain some understanding.


Elantric

#17
As I said - a TL074 Op-Amp will work too, as these are more readily available
http://www.digikey.com/product-detail/en/TL074CDT/497-2204-2-ND/599312


But Bill Bax is passionate about audio and GK-13 expanders, has a site which supports his claims of superior performance when using the OPA2134

details here:

http://billbax.110mb.com





Also the title of this thread is "Standard Guitar to GR-55 Adapter Box Idea" and remember we not only talking about providing a path for using your normal guitar to play PCM synths or trigger Guitar to MIDI, but also simple audio interface front end to allow the GR-55 to be used as an Amp /FX Modeling system - like a Boss GT-100 /POD.  In the later case, yes the opamp selection used in this front end buffer amp WILL make a difference in perceptible clarity, hiss, and  noise you will hear in your guitar signal at the final output stage of the GR-55.

Also I will add there are other options -
Heres info on a mod for the US-20 to provide a normal Guitar Input on your GR-55, and allows the use of external stomp FX upstream in front of the GR-55 / VG-99
https://www.vguitarforums.com/smf/index.php?topic=7245.0


gumtown

The buffer has unity gain, but is changing the impedance from 1M ohm to 100 ohm,
I'm guessing the lower impedance has better noise rejection inside the (comparatively) high noise environment of the digital GR-55.
Free "GR-55 FloorBoard" editor software from https://sourceforge.net/projects/grfloorboard/

spitfire

heydenk, yes!

This is exactly what I proposed doing and in fact have started to build. I've simply been too busy to finish it up.

Pin 7 of the 13-pin connector carries the magnetic pickup (standard) guitar signal. So this is the only signal on the 13-pin you need to switch, though you may want to switch others. Now, the standard guitar signal needs to be buffered. In my case I've built a little board with the op-amp on it. In yours you're using the guts of a GK-3.

I think you understand this, but for the sake of any others, I'll provide the details. Using the guts of the GK-3, you will plug your standard guitar into the GK-3 1/4" input. In your case, I think you are proposing to mount this 1/4" jack on your box. Not sure if you are removing the 1/4" from the GK-3 circuit board. If so run wires from your box jack to the matching circuit board points. If not, then I guess you would solder a cable to your box jack and put a 1/4" plug on into the GK-3 1/4" jack. Again, I think you, heydenk, have this figured already.

You will, of course, have two 13-pin jacks on your box (or other, I'll explain later). One of these goes to your GK-55, the other is for your GK-3 equipped guitar to plug into. Most of the connections from one 13-pin jack will go straight to the other. This will allow your GK equipped guitar to work normally.

But at a minimum, you will need to switch the standard or mag guitar signal line (pin 7) of the 13-pin connector that goes out of your  box to the GK-55. In one case this signal will come from the 13-pin jack that your GK equipped guitar plugs in to.

When switched the other way, this signal will come from pin 7 of your internal GK-3 circuit board.

The 13-pin connector of the internal GK-3 board in your box, doesn't need all 13 connections. It only needs power, ground, and the signal pin (minimum). You don't need to switch the power and grounds. You'll want to look at the GK-3 schematic, but power is on pins 12 and 13. Ground is the outside ring or shell of the 13-pin connector.

So the 13-pin connector going to the GK-55, will have pins 12, 13, and the ground going to both the other 13-pin connector (to GK guitar), and to the internal GK-3 board.

Your switch will simply switch the outgoing (to GK-55) pin 7 between the internal GK-3 circuit board pin 7 output and the pin 7 of the 13-pin coming from your GK equipped guitar.

Now, you may also want to switch the volume control pin 8. This is the signal from the GK-3 volume control knob. If you don't switch it, then this signal will be active when using your regular guitar, but it will be coming from your GK equipped guitar. If that guitar is not plugged in, I'm not sure what would happen.

In your case, you can just switch pin 8 to the pin 8 of your internal GK-3 circuit board. You would then need to set the internal volume knob at full and hope it stays, bring it outside your box (giving you a volume control knob on the box), or you could remove the volume pot from the GK-3 board and replace with appropriate jumper.

Or, as I'm doing, I'm providing a 1/4" jack to go to an external expression pedal that will allow my to control the volume that way, when switch to standard guitar.

If switching both pin 7 and pin 8, you will need a DPDT switch. In my case I'm just using a toggle switch, since this isn't something I would need to stomp since it switches between two different guitars. Not something I would do mid song.

Also, the toggle provides a visual queue as to which way the box is set. You could use a push button, but then you would need a 3rd pole so you could switch an LED indicator signal. These exist, I think as the true bypass type switches for stomp box. Available, but not common.

Finally, you can also wire up the S1 and S2 switches. Since these are momentary, your S1 and S2 switches in your box can be wired up directly to the appropriate pins on the outgoing 13-pin connector even while these are also wired to the incoming 13-pin connector.

I hope some of that make sense. But it is a bit like trying to describe something that is best shown in a schematic. Although I haven't finished my build yet (and proven it), and it doesn't use the GK-3 board, my schematic might be helpful. I don't want to post an unproven circuit schematic, but I'll send it directly to anyone who PM's me.

Anyone, looking to do this, very quickly starts to wonder where all these 13-pin connectors are coming from and how to work with them. Here's my solution, and I did mention it earlier in this thread.

I'm NOT using them. There's no law that says I have to use the 13-pin DIN connector on my box. What I'm doing is taking the regular 13-pin cable and cutting it into two pieces. One about 2 ft long to go from the box to the GK-55, the other, much longer, to go to the GK equipped guitar.

I'm attaching 15-pin rectangular DIN connectors. This are just bigger versions of the common 9-pin connectors used for computer RS-232 serial, COM port connectors. These are readily available. So I cut the cable and attach these much more common connectors to it. There are also many versions of the mating connectors to go on the box.

Or of course, you could use anything you want for this. It's just to get around the fact that the only 13-pin DIN receptacles available are the circuit board mount type and these are not well suited for attaching to panels.

I'm using the female 15 pin on the cable going to the GK-55 and male pins on the cable going to the GK guitar. This allows me to plug the cables together if I don't want to use the box.

Obviously there is a disadvantage in having to modify the cable and possibly having to do it again if the cable wears out. But it also gives you a good excuse to shorten the one to the guitar, the standard one is too long for me.




spitfire

Quote from:  heydenkm
Spitfire, thanks very much for the detailed reply!!  I love it.  Sounds like I could just switch the pin 7 signal.  The volume control is just for synth volume, isn't it?  If that's the case, even if the volume was down, it shouldn't effect the 1/4" signal that I have coming from my standard guitar.  I do want to have an S1 and S2 but it doesn't sound like those need to be switched from one source to the other.  That would also allow me to use the footswitches instead of the guitar-mounted S1/S2 switches if I wanted, right?

The volume control is really just a controller and I believe can be assigned to any control parameter in the GR-55. I think it is most commonly an overall volume control (synth and standard guitar). I do think you could ignore it, but you have to make sure your patches don't use it. Since you are likely to have dedicated patches when using a standard guitar, that's probably not that big of a deal.

But if you do hook it up it gives other options. Like mine, I will use an external expression pedal for it.

Also, if your box is roomy enough and you lay it out properly, you could add switching of the volume control later.

gumtown

You could even switch the guitar signal to one of the hex inputs and use the GR-55 as a mono-phonic synth during leads or playing one string on a normal guitar.
Free "GR-55 FloorBoard" editor software from https://sourceforge.net/projects/grfloorboard/

Elantric

As a reference for what exists in the GK-3 and see where improvements could be made, consult the
GK Schematic Library
https://www.vguitarforums.com/smf/index.php?topic=73.0