GKP-2 parallel box with dual pin-7 buffers

[billbax]

Hi Everyone,

I've been working on a GKP-2 parallel box over the past few weeks, and after many hours of playing and tests, I'm extremely impressed with the results.  The unit also features two buffered pin-7 'normal' guitar inputs, allowing three switchable P7 guitars - GK Hex preamp P7 included.  Furthermore, analog audio switching noise (clicks) between P7 guitars is surprisingly low at around -70dB (you can't hear it), and the high-impedance normal guitar buffers are as quiet as a GK3's P7 short cable - even with a 10 foot guitar cable!

The GKP-2 parallel buffer box will work with most v-guitar units, and only draws about 15-20mA from a v-guitar unit GK power supply. If you are thinking of buying a fragile and pricey Roland GKP-2 parallel cable, then perhaps a robust GKP-2 parallel P7 buffer box is a more practical solution. 

Considering component costs and labor, the GKP-2 asking price is £125.00GB / 165.00USD. This seems about right, and would be happy to hear opinions from forum members.

GKP-2 Parallel Box P7 dual buffer switcher.


GKP-2 Rear.



How it works, basic layout and connecting up.


Internal 13-Pin panel sockets are superior compared to 13-pin pcb sockets.
Even a pcb version with locking wings and brackets will eventually fail, due to left/right, up/down plug to socket rough and tumble.  Superior panel socket grounding is excellent, with tight enclosure ground contacts to panel socket 360-degree ground contacts. Point-to-point wiring is another audio bonus, although this adds several hours to construction time. Frank likes panel sockets


GK3 P7 buffer versus GKP-2 P7 buffer shootout. 
The spectrum analyzer screen capture shows how little there is between a GK3 P7 buffer and GKP-2 P7 buffer, not to mention the need for a well-screened guitar cable.
1. Pickup = Seymour Duncan humbucker at full volume. 
2. Black spectrum trace =  GK3 buffer using the small 11 inch cable.
3. Red spectrum trace = GKP-2 buffer using a 10 foot cable.


Mission creep and other parallel box options explored and rejected:
1. Mono P7 output socket.
2. GK subsonic filter.
3. Normal guitar effects loop.

Considering GKP-2 basic functions, I don't see the need to add any further features.  Doing so will certainly push the price up, and mostly likely have a detrimental affect on the audio.
I'll stop the rant here, and may post a couple of GK3 buffer VS GKP-2 buffer audio examples in my next rant.

Thanks for viewing.

Bill

https://separate-strings.co.uk

[arkieboy]

Spookily hits the exact spec I had in mind for my current pedalboard.


How do I know that the that the guitar signal presented to the external GK devices is the one from the 13-pin input?  Both GTR LEDs off?


If I had to pass a minor criticism I would say that it would be nicer if all of the sockets were on the same edge, which would be much more convenient for a permanent pedalboard installation.


As for price £125 seems fair.  If it could be trimmed to a ton it would be an easier sell to my other half ;-)

[whippinpost91850]

Very nice Bill, maybe a little 2 pricey for me ( I'just retired a few months ago) , but probably a fair price.

Paul

[chrish]

Good price.,The problem I have with the Roland US20 is having to unplug to use with another 13 pin guitar. Also some issues when used with a VG99 high gain and GR300 model sustain to noise,, but the VG99 is sold. Don't have the same issue with its replacement, the SY1000.

But for VG99 owners, have you tested your unit for sustain and noise issues when used with those type patches I mentioned?

[CodeSmart]

Price is highly reasonable. For low volume it's virtually impossible to get anywhere below $150 considering milling, painting, printing, components, assembly, testing and logistics. I know this for a fact. Good work Bill!

[billbax]

Thanks for the feedback, arkieboy, Paul, Chrish, CodeSmart.

arkieboy

How do I know that the that the guitar signal presented to the external GK devices is the one from the 13-pin input?  Both GTR LEDs off?

When the red LED is ON via GK footswitch, GK guitar Pin-7 is output to both GK POWER & GK OUT 2.  With the red LED OFF, the green or blue LED is ON, and GTR 1 or GTR 2 buffers are output to both GK POWER & GK OUT 2 Pin-7.  The GTR SELECT footswitch flips between GTR1/2 buffers. This analog switching arrangement has a maximum of three Pin-7 switching options.
1. GK Guitar Pin-7 (RED LED)
2. GKP-2 Buffer one GTR Pin-7. (Green LED)
3. GKP-2 Buffer two GTR Pin-7. (Blue LED)

arkieboy

If I had to pass a minor criticism I would say that it would be nicer if all of the sockets were on the same edge, which would be much more convenient for a permanent pedalboard installation.

I did think of this very early on, but this would mean a larger case and would up the asking price etc. The case size is good and about right, don't see GKP-2 buffer users have major problems setting up. Btw, you could always use right-angled 1/4" jacks, and slim down the profile.

Chrish

Good price.,The problem I have with the Roland US20 is having to unplug to use with another 13 pin guitar. Also some issues when used with a VG99 high gain and GR300 model sustain to noise,, but the VG99 is sold. Don't have the same issue with its replacement, the SY1000.
But for VG99 owners, have you tested your unit for sustain and noise issues when used with those type patches I mentioned?

I don't have a VG-99, only a GR55, GP10, VG88.  The last part of your comment prompted me to record a short HI-GAIN GR-55 GK3 Pin-7 Versus GKP-2 Buffer audio demo.  The .wav file is below. Tone-wise there is no difference, although GKP-2 P7 buffers are about 3dB quieter due to a fully screened enclosure. Also see spectrum analyzer screen capture gif below.

CodeSmart

Price is highly reasonable. For low volume it's virtually impossible to get anywhere below $150 considering milling, painting, printing, components, assembly, testing and logistics. I know this for a fact. Good work Bill!

Could not put it better myself. Looking into a Star Trek replicator for Christmas, and will replicate one for you

GK3 Pin-7 buffer VS GKP-2 buffer HI-GAIN Patch audio demo.
1. Seymour Duncan 'Little '59' humbucker Pin-7 > GR-55.
2. A 16-bit mono audio file (3.6mB), which should play automatically in your media player.
3. 0-21 seconds GK3 Pin-7.  22-42 seconds GKP-2 buffer. 
There is no apparent difference tone-wise, although under scrutiny, the GKP-2 buffer is about 3dB quieter.
https://separate-strings.co.uk/gk3p7buffervsp7buffer.wav

GK3 Pin-7 Buffer VS GKP-2 Buffer spectrum analyzer screen capture gif.
1. Merged stereo file (silent parts) used in the HI GAIN GR-55 .wav demo.
2. Seymour Duncan 'Little '59' humbucker. 
3. Strings are hand dampened for minimum noise viewable on a spectrum analyzer.
The 3dB difference is due to the GK3 enclosure and short cable not being fully screened.  GKP-2 buffer and 10 foot guitar cable are both fully screened. Actually, the 10 foot GKP-2 buffer mono guitar cable has a double screen. While the short GK3 guitar cable uses a single copper screen. GK3 does not have a ground screen, due to its plastic enclosure.



GR-55 + GKP-2 Buffer Pin-7 re-amping.
I was pleasantly surprised how well this worked and sounded.  With a few DAW output level-matching adjustments, GR-55 normal guitar patch adjustments are a doddle.



Thanks for viewing.

Bill

https://separate-strings.co.uk

[gumtown]

Bill's audio example below
https://separate-strings.co.uk/gk3p7buffervsp7buffer.wav

[billbax]

Thanks Gumtown for posting the GK3 pin-7 versus GKP-2 pin-7 audio file link.  You could have chosen a larger font size

Since my last post before Christmas, I am happy to report a further 2dB improvement with GKP-2 pin-7 buffer versus GK3 pin-7.  Overall this translates into an impressive 5dB improvement over previous tests - how low can you go?

Thanks again,

Bill

[billbax]

Hi everyone,

Here's a simple solution for those interested in DAW to GR-55 normal-guitar re-amping. The mini cable is simple to make, and will certainly open up further GR-55 normal-guitar patch exploration. 

* Cable component costs are around £10.00GBP or less.
* 1/4 in-line coupler is recommended due to standard 1/4 in-line socket reliability problems.
* A Pin-7 circuit buffer is not required for a DAW line-out to GR-55's normal guitar Pin-7. Audio checked with and without a Pin-7 circuit buffer.
* Pin-8 controls 'Synth-Volume' and a 'Phantom power' mod is not required.

Does it sound good? Yes, with a few DAW output adjustments, it sounds near identical to a GK > GR-55 Pin-7 input. 



Thanks for viewing,

Bill

https://separate-strings.co.uk

[billbax]

Hi forum,

GR-55 normal-guitar pin-7 input options?

[billbax]

It's going to be a scary long one.

Hi Everyone,
V-Guitar forum accessory suppliers and forum audiophiles, will be happy to learn the modern OPA1656 FET-input op-amp is now available. 
The op-amp is centre place in my GKP-2 Switcher, and the new single passive guitar input buffer project.  In comparison to most DAW instrument- inputs or a GK3 normal-guitar input preamp, OPA1656 guitar buffer adds audio punch, clarity and micro detail, while stock FET-input buffer op-amps as TL072/TL082 sound artificial and plastic. See 1656 buffer THD+N and noise-floor figures below.

Datasheet quick details. https://www.ti.com/product/OPA1656


Single guitar input buffer project.
*  Easy to make.
*  OPA1656 buffer op-amp. DIL adaptors are web available for project constructors, although placing bypass op-amp ceramic capacitors is a problem.
*  Custom length double-screened output cable directly wired to buffer PCB, minimises resistance and cable capacitance colouration.
*  Diecast case enclosure greatly reduces EMI/RF from the buffer circuit.
*  13-Pin power options with P7 v-guitar unit input. A dual 9V battery supply is an audiophile option, reducing power supply ripple by 2-3dB. Rechargeable PP3 batteries generally last about 15 hours before +/- battery status LEDs dim.
*  Further input buffer options: Passive Hi-Pass filter @ 30Hz. 1Meg pot with variable input-impedance control.  V-Guitar forum suggestions are most welcome on the subject of circuit improvements.
*  Low project bill of material costs - £30.00GBP/34.00USD.

Simple guitar buffer circuit.


Guitar input buffer project.



And the latest GKP-2 switcher with dual buffers.


OPA1656 input buffer audio specifications:
THD+N & dynamic-range spectrum analyser screen captures.

A-Weighted THD+N test with a best figure of 0.00181%.
THD is extremely low @ -100dB.  In comparison a BOSS GP10 normal-guitar output THD+N is about 0.013%. Boss GT1 THD+N is around 0.045%.


Input: Strat bridge & middle pickup, with a best noise-floor figure of -93.08 dB.
The spectrum analyser scaling is 'flat' and not 'A-Weighted'.  A 'flat' setting exposes low-frequency EMI such as a trailing guitar cable, power cables, computer EMI and room EMI/RF. EMI/RF ghost fields are everywhere! A BOSS GP10 normal-guitar output noise-floor (no effects) is around -85 dB, and a BOSS GT-1 noise-floor (no effects) is generally -83 dB. 


13-pin power socket with obvious v-guitar P7 output option.


A budget DAW interface (sub £800.GBP) line-input can be utilised as a superior instrument-input, using a OPA1656 guitar input buffer. Most DAW instrument-inputs require a near maxed-out 5 O' clock setting, while a comfortable 12 O' clock setting can be used with a guitar buffer line-input. The bottom spectrum analyser capture shows an impressive 10 dB noise-floor improvement using a buffered line-input. Having said that, I would never consider recording a normal guitar with a Behringer 1820, as there is too much harmonic distortion.



A guitar input buffer makes for an excellent EMI/RF detector/sniffer/magnifier.
Spectrum trace with electro magnetic interference from a British power cable.
Further scary investigation will show up high frequency EMI from keyboards, mice, and plastic power supply cases etc. You might only be adding a 0.5 dB per item, But total audio EMI  accumulates, and you do not want that infesting your precious guitar audio. Suggest investigating power cables proximity to audio cables first, then work on minor EMI/RF offenders.  A heatshrink ¼ jack probe dust cover is recommended, and be careful what you touch!


Thanks for viewing, assuming you got down this far

Bill
https://separate-strings.co.uk

[paganskins]

Hello Bill   I can't see the P7 switcher on your site, I was interested in pricing? Thanks

[billbax]

Hi paganskins,

The GKP-2 switcher hasn't made it to website yet, as forum interest is quiet and almost  mute.  I see the latest price for a Roland GKP-2 cable is £92.00GBP https://www.andertons.co.uk/roland-gkp-2-parallel-cable
The GKP-2 13-pin distribution buffer unit, with 3-way selectable normal-guitar inputs is £125.00GBP. Track and signed UK shipping is £7.65.

Latest GKP-2 switcher.


GKP-2 rear connections.


Thanks for your message.

Bill
https://separate-strings.co.uk

[paganskins]

Hi paganskins,

The GKP-2 switcher hasn't made it to website yet, as forum interest is quiet and almost  mute.  I see the latest price for a Roland GKP-2 cable is £92.00GBP https://www.andertons.co.uk/roland-gkp-2-parallel-cable
The GKP-2 13-pin distribution buffer unit, with 3-way selectable normal-guitar inputs is £125.00GBP. Track and signed UK shipping is £7.65.

Latest GKP-2 switcher.


GKP-2 rear connections.


Thanks for your message.

Bill
https://separate-strings.co.uk

Thanks Bill, sounds good – I'll contact you through your website to arrange ordering.